A weld that meets all applicable requirements of the governing code, specification, or standard. Acceptability is determined by the designated examination methods and acceptance criteria — not by visual impression alone. A weld may pass visual examination but fail radiographic or ultrasonic examination and therefore not be acceptable.

A flux used in submerged arc welding that intentionally modifies the weld metal chemical composition by adding elements such as manganese or silicon to the weld pool. Active fluxes are restricted to single-pass or minimal multi-pass applications because their cumulative alloying effect in multi-pass welds can cause composition to drift outside the specified range.

The shortest distance between the weld root and the face of a fillet weld, measured through the cross-section of the weld. For a convex fillet weld with equal legs on a 90-degree joint, the actual throat = 0.707 × leg size. Distinguished from Effective Throat, which is used for design strength calculations.

A thermal cutting and gouging process in which the metal is melted by an electric arc struck between a carbon-graphite electrode and the workpiece, and the molten metal is continuously blown clear by a high-velocity jet of compressed air directed parallel to the electrode. Used for joint preparation, back-gouging of root passes, and removal of defective weld metal.

A tensile or other mechanical test specimen machined so that its gauge length is composed entirely of deposited weld metal, with no base metal or HAZ in the test zone. Used to determine the intrinsic mechanical properties of the filler metal independent of the base metal. Required by ASME Section IX in certain procedure qualification tests.

A substance that has metallic properties and is composed of two or more chemical elements, of which at least one is an elemental metal. Steel is an alloy of iron and carbon; stainless steel adds chromium and often nickel and molybdenum. Filler metal alloy composition is specified by its AWS classification (SFA designation) or UNS number.

Electric current that reverses its direction of flow at regular intervals, typically at 50 or 60 Hz (cycles per second). In GTAW, AC is selected for aluminium welding because the electrode-positive half-cycle provides cathodic cleaning of the aluminium oxide surface film, while the electrode-negative half-cycle concentrates heat in the workpiece for efficient melting.

An electrical measuring instrument used to determine the current flowing in a welding circuit, measured in amperes. In welding, amperage is monitored continuously during qualification testing to verify that actual welding parameters are within the essential variable range of the WPS.

The quantity of electric current flowing in the welding circuit per unit time, measured in amperes (A). Amperage is the primary parameter controlling heat input, depth of fusion, bead geometry, and deposition rate. It is an essential variable in virtually all WPS qualification tests. Also called welding current.

A heat treatment in which a metal is heated to and held at a suitable temperature, then cooled slowly (typically within the furnace) to reduce hardness, relieve internal stresses, refine grain structure, and improve ductility and machinability. Full annealing of steel involves heating above the Ac3 temperature followed by slow furnace cooling. Contrast with Normalising and Stress Relief.

A sustained electrical discharge through a gas or vapour column between two electrodes, or between an electrode and a workpiece, that produces intense heat, light, and ionised plasma. In arc welding, the arc temperature may exceed 6,000 °C at its core. Arc stability is critical for consistent weld quality and is affected by current type, polarity, arc length, shielding gas composition, and electrode condition.

The deflection of a welding arc from its normal path due to magnetic forces. Forward arc blow deflects the arc in the direction of travel; backward arc blow deflects it opposite to travel. Arc blow is most severe when using DC on ferromagnetic materials near ends of joints, at changes in section, or when the work return clamp is poorly positioned. Remedies include switching to AC, re-positioning the work return connection, reducing current, or changing electrode angle.

A feature of constant-current welding power sources that temporarily increases output current when the arc voltage drops (short arc length), preventing electrode stubbing. Arc force setting is sometimes labelled “dig” or “arc control” on SMAW machines. Excessive arc force increases spatter; insufficient arc force causes sticking, particularly with low-hydrogen electrodes at short arc lengths.

The distance from the tip of the welding electrode to the nearest surface of the weld pool. Arc length directly influences arc voltage, bead width, and shielding gas effectiveness. A short arc produces a narrow, higher bead; a long arc widens the bead, increases porosity risk, and reduces penetration. In SMAW, the rule of thumb is that arc length should approximate the electrode core wire diameter.

A discontinuity resulting from an arc initiated at a location on the base metal surface other than the weld zone. Arc strikes create localised hardened zones (sometimes with cracks) in the HAZ due to rapid heating and cooling of a very small area. Under ASME and AWS D1.1 codes, arc strikes outside the weld area must be removed by grinding and the area examined by magnetic particle (MT) or liquid penetrant (PT) testing.

A group of welding processes that produce coalescence of workpieces by heating with an electric arc, with or without the application of pressure, and with or without the use of filler metal. Arc welding processes include SMAW, GMAW, GTAW, SAW, FCAW, PAW, CAW, and ESW. They collectively account for the vast majority of welding performed in industrial fabrication.

The condition of weld metal, the HAZ, and the base metal in a welded joint after welding but prior to any subsequent thermal, mechanical, or chemical treatment. As-welded hardness is highest in the HAZ and weld metal before any tempering or stress relief occurs. Mechanical property testing may be performed on as-welded specimens or on post-weld heat treated specimens, depending on the production condition being qualified.

Welding performed with equipment that performs the welding operation without adjustment of the controls by a welding operator. The equipment is set up and started by a welding operator but, once running, controls all variables including travel speed, arc initiation, and shielding gas flow without manual intervention. Distinguished from Machine Welding (where an operator makes adjustments during the weld) and Semiautomatic Welding.

A fusion weld made without the addition of filler metal. The weld metal is derived entirely from the melting of the base metal. Autogenous welds are common in GTAW of thin-gauge stainless steel, in electron beam welding, and in laser welding of precision joints. The weld metal composition is essentially identical to that of the base metal.

A line through the length of a weld, perpendicular to and at the geometric centre of its cross-section. The axis of the weld is used to define weld orientation in relation to loading direction (longitudinal or transverse), and to describe the weld position (welding position designations are defined relative to the weld axis).

A classification system in ASME Section IX (Table QW-442) that groups deposited weld metals by chemical composition for procedure qualification purposes. A-Numbers ensure that a WPS qualified with one filler metal covers similar filler metals with the same general composition. For example, A-No. 1 covers plain carbon steel weld metal; A-No. 8 covers austenitic stainless weld metal.

The face-centred cubic (FCC) crystal structure of iron and its solid solutions, stable above the upper critical temperature in carbon steels. Austenite has greater solid solubility for carbon than ferrite and is the starting microstructure for all heat treatment transformations in steel. In austenitic stainless steels (300 series), the austenite phase is stabilised to room temperature by the addition of nickel, making these steels non-magnetic and non-hardenable by quenching.

A weld bead resulting from a back welding pass, applied to the root side of a joint after the main groove weld has been completed from the face side. Prior to back bead deposition, the root must be back-gouged to clean, sound metal. Distinct from a Root Pass (the first pass from the face side).

The removal of weld metal and base metal from the weld root side of a welded joint to a depth that ensures sound metal before depositing a back bead. Performed using air carbon arc cutting, grinding, or plasma cutting. Required when full-penetration quality cannot be guaranteed by welding from one side only, particularly on joints with a tight root gap or difficult positional access.

Material placed at the root of a weld joint to support the molten weld pool and facilitate complete root penetration without burn-through. Types of backing include: steel backing strip (permanent), copper backing bar (temporary, with chill effect), ceramic backing tape, flux backing, and inert gas (root purging for austenitic stainless and titanium). Under ASME Section IX, the use or removal of backing is an essential variable for WPQ and WPS qualification.

A ring of material placed inside a pipe joint at the root to serve as backing. Backing rings are used in pipe welding where internal access is limited or to ensure complete root penetration on socket-weld or butt-weld pipe joints. Consumable insert rings (pre-placed filler metal rings) serve both as backing and as a controlled source of additional filler metal at the root.

A weld bead deposited at the back of a single-groove weld to serve as backing for the subsequent groove weld passes. The backing weld is made from the root side before the main weld is begun from the face side. Distinct from a Back Bead, which is deposited after the groove weld is completed.

The metal or alloy that is welded, brazed, soldered, or cut. Also termed parent metal in international usage (ISO terminology). The base metal P-Number and Group Number govern the scope of WPS qualification under ASME Section IX. Base metal chemistry and heat treatment condition determine preheat requirements, HAZ microstructure, and post-weld heat treatment needs. See our P-Number and base metal grouping guide.

A weld consisting of one or more string or weave beads deposited on an unbroken surface. A Stringer Bead is deposited without transverse oscillation; a Weave Bead involves side-to-side oscillation of the electrode during travel. Weave beads increase bead width and interpass temperature; stringer beads minimise heat input and HAZ width.

An angular joint preparation at the edge of a workpiece that forms the groove face. The bevel angle is the angle between the groove face and a reference plane (perpendicular to the surface). A double-bevel preparation produces a groove on both sides without a compound angle. Bevel preparation is produced by thermal cutting, machining, or grinding.

The angle formed between the prepared groove face of a member and a plane perpendicular to the surface of the member. The bevel angle is half the groove angle in a symmetrical double-V joint. Typical bevel angles for SMAW are 30–35° per side (giving a 60–70° included groove angle), but these vary by process, joint thickness, and access requirements.

A group of joining processes that produce coalescence using a filler metal with a liquidus temperature above 450 °C (840 °F) but below the solidus temperature of the base metal. In brazing, the base metal is not melted; the filler metal flows into the joint by capillary action. Brazing differs from soldering only by the filler metal melting temperature threshold (soldering uses filler metals with liquidus below 450 °C).

A welding method in which a filler metal with a liquidus above 450 °C is used to fill a prepared groove or fillet joint without relying on capillary action. Unlike brazing, braze welding does not require a close-fitting joint for filler metal flow. The base metal is not melted. Bronze filler metals (e.g., RBCuZn) are commonly used for braze welding of cast iron.

A joint between two workpieces aligned approximately in the same plane. The most common joint type in pressure vessel and piping fabrication. Butt joints may be welded with groove welds (V, U, bevel, square) from one or both sides. CJP butt joints develop the full strength of the thinner member in tension.

A condition in which the welding arc penetrates completely through the base metal, creating a hole or void. Burn-through is caused by excessive heat input (too-high current, too-low travel speed), excessive root gap, or insufficient root face. It is a weld defect under all fabrication codes and requires removal and repair.

The deposition of one or more layers of weld metal on the groove face or fillet weld surface of one member of a joint, before the member is assembled into the joint for welding. Buttering is used to build up a transition layer of a different composition on a base metal prior to joining, typically to allow dissimilar metal welding or to provide a weldable surface on a difficult alloy.

The final weld pass or passes that complete the face of a multi-pass groove weld to its finished contour. The cap pass determines the final bead profile, reinforcement height, and surface finish. It is the pass most visible to visual inspection and must meet all surface quality criteria of the applicable code.

A formula-based index expressing the combined effect of carbon and alloying elements on the hardenability and cold-cracking susceptibility of a steel, in terms of an equivalent carbon content. The IIW formula (most common in ASME and AWS applications) is: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Higher CE indicates greater hardenability and a greater need for preheat. Calculate using our Carbon Equivalent calculator.

A standardised dynamic impact test in which a notched specimen is struck by a swinging pendulum hammer; the energy absorbed in fracturing the specimen is measured in joules. Used to assess fracture toughness (notch toughness) of weld metal and HAZ, particularly at sub-ambient temperatures. Required by ASME Section VIII UG-84 for pressure vessels in low-temperature service. See our UG-84 Charpy impact testing guide.

The growing together or growth into one body of the materials being welded. Coalescence is the fundamental objective of all welding processes, whether achieved by fusion (melting and solidification), diffusion bonding (solid-state), or a combination of heat and pressure. The term appears in the formal AWS A3.0 definitions of every welding process.

A crack in the weld metal or HAZ that develops after the joint has cooled to near-ambient temperature, typically caused by the simultaneous presence of: (1) diffusible hydrogen, (2) a susceptible hard microstructure (martensite), and (3) residual or applied tensile stress. Also known as hydrogen-induced cracking (HIC) or delayed cracking. Cold cracks may not appear until hours or days after welding. Prevention: preheat, low-hydrogen consumables (F-4 / F-5), hydrogen bake-out heat treatment.

A groove weld condition in which weld metal extends through the full thickness of the joint. CJP is required in critical structural and pressure applications where the joint must develop the full strength of the thinner connected member in tension. The term refers to the weld condition, not to a specific weld type. Contrast with Partial Joint Penetration (PJP).

The maximum distance from the face of a concave fillet weld perpendicular to a line joining the weld toes. A concave fillet weld has a face that curves inward between the toes, reducing the effective throat below the theoretical 0.707 × leg value. Concavity is typically caused by excessive voltage, insufficient current, or wrong electrode angle. Most codes limit allowable concavity.

A welding power source with a volt-ampere characteristic (drooping characteristic) in which the output current remains approximately constant despite significant changes in arc voltage. CC sources are used for SMAW and GTAW, where the welder controls arc length manually. The drooping characteristic limits short-circuit current and prevents electrode welding-on.

A welding power source with a relatively flat volt-ampere characteristic in which the output voltage remains approximately constant despite large changes in output current. CV sources are used for GMAW, SAW, and FCAW, where wire is fed at a constant speed and the arc is self-regulating: if the arc shortens, current rises (increasing burn-off rate) until equilibrium is restored.

Filler metal that is preplaced within the joint and is melted into the weld as part of the first (root) pass. Consumable inserts are used in GTAW root passes of pipe joints to ensure consistent root penetration and bead geometry without requiring an internal root bead or back-gouging. They are particularly valuable in critical alloy piping where internal access is impossible.

The conducting tube or tip in a GMAW or FCAW gun through which the electrode wire passes, and from which the welding current is transferred to the wire by sliding electrical contact. Also called the contact tip or contact nozzle. Contact tube bore diameter is matched to the wire diameter; bore wear increases contact resistance and causes arc instability.

The maximum distance from the face of a convex fillet weld, perpendicular to a line joining the weld toes. Excessive convexity increases the effective throat above the theoretical value but creates a stress concentration at the weld toe and is wasteful of filler metal. AWS D1.1 limits convexity based on weld leg size.

A joint between two workpieces located approximately at right angles to each other in an L-shape. Corner joints are welded with fillet welds on the inside, groove welds at the edge, or both. They are common in structural box sections, pressure vessel nozzle pads, and equipment frames.

The chemical or electrochemical reaction between a metal and its environment that results in the degradation of the metal’s properties. Weld joints are often preferentially attacked due to compositional differences between the weld metal, HAZ, and base metal. Types of corrosion relevant to welding include galvanic corrosion, intergranular corrosion (sensitisation of stainless steels), crevice corrosion, and stress corrosion cracking. See our corrosion types guide.

A depression in the weld face at the point where welding is terminated. Craters form because the weld pool shrinks and solidifies after the heat source is removed. If crater fill technique is not used, the resulting crater may contain cracks (crater cracks) caused by solidification shrinkage. Crater cracks are a nucleation site for fatigue crack propagation in cyclic loading applications.

A crack formed in the crater of a weld bead upon termination of the welding arc. Crater cracks typically radiate from the crater centre in a star pattern and are caused by solidification shrinkage. Prevention requires filling the crater completely before breaking the arc, using a crater-fill current ramp-down, or back-stepping at each weld termination.

A form of localised corrosion that occurs in confined spaces (crevices) formed at weld root defects, backing rings, incomplete-penetration joints, or poorly fitted connections. The restricted environment within the crevice becomes depleted in oxygen, creating a concentration cell that drives accelerated metal dissolution. Relevant to austenitic stainless steels and nickel alloys in chloride-containing environments.

The flow of electric charge through a conductor, measured in amperes (A). In welding, current is the primary control parameter for the generation of arc heat, bead geometry, and deposition rate. The type of current (AC or DC) and polarity (DCEP or DCEN) are specified in the WPS as essential or non-essential variables depending on the process and code.

A DC welding circuit polarity in which the electrode is connected to the negative terminal and the workpiece to the positive terminal. Also called straight polarity (DCSP). Approximately one-third of the arc heat is generated at the electrode end; two-thirds at the workpiece. DCEN gives a wide, shallower weld bead. GTAW on carbon steel and stainless steel uses DCEN as standard.

A DC welding circuit polarity in which the electrode is connected to the positive terminal and the workpiece to the negative terminal. Also called reverse polarity (DCRP). Approximately two-thirds of arc heat is generated at the electrode; one-third at the workpiece. DCEP gives deeper penetration and faster electrode melting. SMAW with low-hydrogen electrodes (E7016, E7018) uses DCEP.

One or more discontinuities whose type, size, orientation, location, or accumulated effect renders a weld or part unable to meet the minimum applicable acceptance criteria or specification requirements. The term designates rejectability. A defect requires repair or removal. Critically, a “defect” in one code may be an acceptable “discontinuity” under another code with different acceptance limits.

The ratio of the weight of deposited weld metal to the net weight of filler metal consumed, expressed as a percentage: DE% = (weld metal deposited / filler metal consumed) × 100. SMAW efficiency: 55–70% (stub loss, spatter, slag). GMAW solid wire: 90–95%. SAW: up to 99%. Flux-cored wire: 80–90%. Higher efficiency reduces material cost and welding time per unit of weld metal deposited.

The weight of deposited weld metal per unit of time, typically expressed in kg/h (or lb/h). Deposition rate is a function of welding current, wire feed speed (GMAW), and electrode type. SAW and FCAW offer the highest deposition rates; GTAW the lowest. Deposition rate is distinct from deposition efficiency, which accounts for how much of the consumed electrode becomes deposited weld metal.

The distance that fusion extends into the base metal or previous weld pass from the surface melted during welding. Insufficient depth of fusion results in incomplete fusion, a planar weld discontinuity. Depth of fusion is primarily controlled by welding current and travel speed; it is verified by macro examination of weld test coupons during procedure qualification.

The change in the chemical composition of a welding filler metal caused by admixture of base metal in the molten weld pool, expressed as the percentage of base metal in the completed weld bead. High dilution (common in SAW and single-pass welds) can significantly alter weld metal properties. In corrosion-resistant overlay welding, excessive dilution reduces the alloying element concentration below the minimum required for corrosion resistance.

Electric current that flows continuously in one direction without reversing. DC welding provides a stable, consistent arc with smooth metal transfer. The choice of DCEP or DCEN polarity determines how heat is distributed between the electrode and the workpiece. Most modern inverter power sources produce DC output.

Any interruption in the typical structure of a weld, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity is not necessarily a defect; it becomes a defect only when it exceeds the acceptance criteria of the governing code. Common weld discontinuities include porosity, slag inclusions, cracks, incomplete fusion, undercut, and arc strikes.

The change in shape, size, or alignment of a weldment caused by non-uniform thermal expansion and contraction during welding. Types include angular distortion (rotation of flanges or plates around the weld axis), longitudinal distortion (bowing along the weld length), and transverse distortion (shrinkage across the weld). Distortion control methods include pre-setting, back-step welding, balanced welding sequences, clamping, and the use of fixtures.

The angle between the electrode axis and a reference plane perpendicular to the weld axis, when the electrode is inclined in the direction opposite to travel (the electrode tilts away from the direction of travel). A drag angle causes the arc to push slag ahead and is used in backhand welding. Contrast with Push Angle (electrode tilted toward the direction of travel in forehand welding).

A powered roll used in a wire feeder to pull wire from the spool and push it toward the welding gun. Drive roll groove profiles are matched to wire type: V-groove for solid wire, knurled V-groove for hard flux-cored wire, and U-groove for soft aluminium wire. Using the incorrect drive roll profile can deform the wire, creating feeding irregularities and arc instability.

A non-standard colloquial term for gas-shielded flux-cored arc welding (FCAW-G), in which the wire contains internal flux AND an external shielding gas is also used. The two sources of protection give higher quality weld metal with lower hydrogen content than self-shielded FCAW and higher productivity than SMAW. The term is not used in AWS A3.0.

The percentage of a 10-minute period that a welding power source can operate at its rated output without overheating. A power source rated at 300 A / 60% duty cycle can operate continuously at 300 A for 6 minutes per 10-minute period. Exceeding the duty cycle activates thermal protection. Duty cycle rating is defined at a specific ambient temperature (usually 40 °C).

A joint between the edges of two or more parallel or nearly parallel workpieces. Edge joints are typically used in sheet metal fabrication and are welded with a flange weld. They carry relatively low structural loads and are not generally used in pressure-retaining applications.

The minimum distance from the root to the face of a fillet weld, minus any convexity. For equal-leg fillet welds on 90-degree joints: effective throat = 0.707 × leg size. For deep-penetration fillet welds, the effective throat may be increased by demonstrated root penetration. All fillet weld strength calculations in AWS D1.1 and ASME use effective throat, not leg size.

A component of the welding circuit through which current is conducted and that terminates at the arc. In SMAW, the flux-coated electrode provides the arc, shields the weld pool, and adds filler metal. In GTAW, the tungsten electrode is non-consumable; the filler rod is a separate component. In GMAW/FCAW, the wire electrode is consumable. See our electrode nomenclature guide.

In GMAW and FCAW, the length of electrode wire extending from the contact tube tip to the arc. Also called electrical stick-out or contact-tip-to-work distance (CTWD). Longer electrode extension increases resistive heating of the wire, allowing higher deposition at the same current or the same deposition at lower current. Typical CTWD: 10–20 mm for GMAW solid wire; 20–35 mm for FCAW.

A welding process that produces coalescence of metals with molten slag that melts the filler metal and the surfaces of the workpieces to be welded. The process begins with an arc, which is then extinguished once a molten slag bath is established; thereafter, heat is generated by electrical resistance of the conductive slag rather than by an arc. ESW is used for thick-section vertical welds (50–900 mm) in one pass. The very large heat input results in a wide HAZ with coarse grain structure.

Under ASME Section IX, a condition in a welding procedure or performance qualification whose change beyond specified limits requires requalification (new PQR test or new WPQ test). Essential variables for procedure qualification are listed in QW-251 through QW-289; for performance qualification in QW-350 through QW-367. Changes to non-essential variables only require revision of the WPS, not new testing.

The increase in dimension of a metal when heated, caused by increased atomic vibration. Linear thermal expansion is described by the coefficient of thermal expansion (α). Differential thermal expansion between the weld metal, HAZ, and surrounding base metal during post-weld cooling is the primary source of welding residual stress and distortion.

A low-hydrogen covered electrode that delivers less than 4 mL of diffusible hydrogen per 100 g of deposited weld metal, designated by the suffix H4 in the AWS classification (e.g., E7018-H4). Extra-low hydrogen is specified for high-strength steels, thick sections, or sour-service applications where even standard low-hydrogen levels (H8 or H16) are considered insufficient to prevent cold cracking.

Protective equipment required during all welding operations to shield the eyes from arc radiation (UV, visible, and IR), spatter, and weld flash. Lens shade selection depends on the process and current level. AWS Z49.1 provides the minimum lens shade numbers: SMAW 3/32–1/4 in. electrode requires shade 10; GMAW requires shade 10–11; GTAW shade 8–10. Auto-darkening helmets combine a clear view for set-up with instant darkening at arc initiation.

A chemical reaction that releases energy in the form of heat. In welding, the flux-metal reactions that produce shielding gas and protective slag in SMAW and SAW are exothermic. Thermite (aluminothermic) welding is entirely driven by an exothermic reaction between aluminium powder and iron oxide, reaching temperatures high enough to produce molten steel without an external energy source.

A bend test in which the face of the weld is placed in tension. The test specimen is bent around a mandrel of specified diameter; acceptance requires no cracks exceeding 3 mm in any direction after bending. Used in WPQ and WPS qualification testing per ASME Section IX QW-160 for thinner coupons where side bend testing is not applicable.

A grouping of filler metals in ASME Section IX (Table QW-432) based on usability characteristics — primarily their ability to produce satisfactory welds in given positions. F-Numbers 1 through 4 group carbon steel electrodes by increasing usability restriction; F-5 groups stainless steel electrodes. Qualification with a higher F-Number covers only certain lower F-Numbers; it does not automatically cover F-5 from F-4.

A standardised index of the ferrite content in austenitic and duplex stainless steel weld metals, determined by calibrated magnetic instruments or image analysis. Ferrite content is critical: too little ferrite risks solidification (hot) cracking; too much reduces corrosion resistance and impact toughness. Typical required FN ranges: 3–8 FN for austenitic SS welds; much higher for duplex. See our delta ferrite importance guide.

Metal added to the weld pool from an external source during a fusion welding operation. Filler metal is consumed in the weld and becomes part of the weld metal. It is classified by the AWS SFA (Structural Filler Metal) designation system. Filler metal is selected based on base metal P-Number, required mechanical properties, service conditions, and welding process.

A weld of approximately triangular cross-section joining two surfaces that are approximately perpendicular to each other, in a lap joint, T-joint, or corner joint. The fillet weld is specified by its leg size (L) and weld length. Effective throat = 0.707L. Fillet welds are the most common weld type in structural fabrication. Use our fillet weld consumable calculator to estimate material requirements.

The distance from the joint root to the weld toe along the fusion face of a fillet weld. For equal-leg fillet welds both legs are the same size (L × L). Unequal-leg fillet welds have different leg dimensions specified separately. Leg size is the primary dimension used on engineering drawings to specify fillet weld size.

A non-standard but widely used term for oxyfuel gas cutting (OFC), in which steel is cut by the exothermic reaction between preheated iron and a stream of high-purity oxygen. The cut edge quality and HAZ width depend on the preheat flame temperature, oxygen pressure, and travel speed. Not applicable to stainless steel, aluminium, or other non-ferrous metals that form refractory oxides.

The welding position in which the welding is performed from the upper side of the joint and the weld face is approximately horizontal. The most productive and easiest position, allowing the highest travel speeds and widest electrode operating range. Designated 1G for groove welds on plate; 1F for fillet welds; 1G (rotated) for pipe welded while being continuously rotated.

A material used to prevent, dissolve, or facilitate the removal of oxides and other unwanted substances during welding, brazing, or soldering. In SMAW, the electrode coating burns to produce shielding gas, slag formers, deoxidisers, and alloying elements. In SAW, granular flux completely covers the arc and weld pool, providing full atmospheric shielding. In FCAW, flux is contained within the tubular wire core.

An arc welding process that uses a continuously fed tubular electrode containing flux in its core. Available in two variants: FCAW-G (gas-shielded), which uses an external shielding gas in addition to the core flux, and FCAW-S (self-shielded), which relies entirely on the core flux for shielding. FCAW combines the productivity of GMAW with the metallurgical benefits of flux shielding, making it popular for structural and heavy fabrication.

A welding technique in which the torch or electrode is directed toward the unwelded portion of the joint (pointing in the direction of travel). Also called push welding. Forehand technique gives better joint visibility, lower penetration, and wider, flatter bead profiles. Contrast with Backhand Welding, where the electrode points back toward the completed weld.

A solid-state welding process in which a rotating non-consumable tool with a profiled pin is plunged into the joint between two workpieces and traversed along the joint. Frictional and deformation heat softens the material without melting it; the rotating pin mechanically stirs and consolidates the material behind it. FSW produces joints free from solidification defects and is widely used for aluminium alloys in aerospace, rail, and marine applications.

The melting together of filler metal and base metal, or of base metal only, to produce a weld. Fusion is the mechanism by which metallurgical bonding is achieved in all arc, gas, laser, and electron beam welding processes. Complete fusion — throughout the interface between weld metal and base metal and between successive weld passes — is required by all fabrication codes.

The area of base metal that was melted as determined on the cross-section of a completed weld. The fusion zone, combined with the deposited filler metal, constitutes the total weld metal cross-sectional area. Its extent depends on current, travel speed, and groove geometry. The fusion zone boundary is the fusion line; beyond it lies the HAZ.

Corrosion that results from the electrical current generated when two dissimilar metals are in electrical contact in the presence of an electrolyte. The less noble metal (anode) corrodes preferentially. In welding, galvanic corrosion can occur at dissimilar metal weld joints, at zinc-coated (galvanised) steel welds where zinc and iron are in contact with moisture, or where carbon steel and stainless steel are welded together.

The extent to which the shielding gas protects the weld pool and surrounding hot metal from atmospheric contamination. Adequate gas coverage requires correct nozzle diameter, flow rate, and nozzle-to-work distance. Draughts, incorrect standoff, blocked nozzle, or turbulent flow can cause loss of coverage, resulting in oxidation, porosity, and nitrogen pickup in the weld metal.

An arc welding process that uses an arc between a continuously fed consumable wire electrode and the workpiece. Shielding is provided entirely by an externally supplied gas or gas mixture. Three primary metal transfer modes exist: short-circuit (low heat input, out-of-position), globular (large irregular droplets), and spray transfer (high current, flat/horizontal only, high deposition). Pulse-spray is a fourth mode providing spray characteristics at lower average current. See our GMAW process guide.

An arc welding process that uses an arc between a non-consumable tungsten electrode and the workpiece. Shielding is provided by an inert gas (argon, helium, or mixture). Filler metal, when used, is fed separately as a rod or wire. GTAW produces the highest weld quality of any arc process and is essential for root passes in critical pipe welding, thin-gauge materials, and reactive metals. See our GTAW guide and TIG settings calculator.

The total included angle of the groove between workpieces. For a single-V groove, the groove angle is the sum of both bevel angles. Typical groove angles for SMAW: 60–70° for single-V; 45° for single-bevel. Narrower grooves (as in narrow-gap welding) reduce heat input, distortion, and filler metal consumption but require precise control.

The surface of a member included in the groove of a groove weld. The groove face is the surface that forms the side of the prepared groove. Cleanliness and preparation quality of the groove face directly affect fusion and freedom from inclusions in the completed weld.

A weld made in the groove between two workpieces. Groove weld types defined in AWS A3.0 include: square groove, single-V, double-V, single-bevel, double-bevel, single-U, double-U, single-J, double-J, and flare-V. Selection depends on material thickness, joint accessibility, welding process, and required weld quality.

A bend test in which the specimen is bent to a controlled radius and angle using a mandrel-and-die fixture. Guided bend tests are performed on face, root, and side bend specimens during WPS and WPQ qualification testing per ASME Section IX. The mandrel diameter controls the bend radius, which is a function of the filler metal classification and weld thickness.

The forming of a bevel or groove by thermal (arc or flame) or mechanical means. Thermal gouging methods include air carbon arc cutting, plasma arc gouging, and oxyfuel gouging. Gouging is used for back-gouging root passes, removing defective weld metal, and preparing joint bevels in the field where machining is impractical.

The increase in the average size of grains in a metal that occurs when the metal is held at elevated temperatures for prolonged periods, particularly above the grain coarsening temperature. In the HAZ immediately adjacent to the fusion boundary (the grain-coarsened HAZ or CGHAZ), peak temperatures approaching the solidus cause severe grain growth, reducing toughness. Minimising heat input limits the grain-coarsened HAZ width.

A surfacing operation in which a wear-resistant alloy is deposited on a substrate to reduce material loss from abrasion, impact, erosion, galling, or cavitation. Applied by SMAW, GMAW, GTAW, or PTA processes. Hard facing alloys include tungsten carbide composites, cobalt-based (Stellite) alloys, martensitic chromium steels, and tool steel compositions. Dilution control is critical because excessive base metal dilution reduces the alloy content below that needed for wear resistance.

That portion of the base metal whose mechanical properties and microstructure have been altered by the heat of welding, brazing, soldering, or thermal cutting, but which has not been melted. The HAZ extends from the fusion line to the point where the peak temperature was too low to cause measurable microstructural change. In hardenable steels, the HAZ may contain martensite (brittle), pearlite, and bainite in distinct sub-zones depending on the local peak temperature and cooling rate.

The energy supplied by the welding arc to the workpiece per unit length of weld, calculated as: Heat Input (kJ/mm) = [Arc Voltage (V) × Welding Current (A) × 60] / [Travel Speed (mm/min) × 1,000]. A thermal efficiency factor (k) may be applied depending on the process. Heat input governs HAZ width, cooling rate, microstructure, and distortion. It is an essential variable in many WPS qualifications, particularly for steels requiring notch toughness.

A protective device for arc welding that shields the eyes, face, and neck from arc flash, UV/IR radiation, spatter, and heat. Helmets use a filter plate (passive or auto-darkening) to reduce arc luminance to a safe level. Filter shade selection follows AWS Z49.1 recommendations based on process and current. Auto-darkening helmets have become standard, switching from clear (light state) to the selected shade within 1/25,000 of a second at arc initiation.

The welding position in which the weld axis is approximately horizontal and the weld face lies in an approximately vertical plane. Designated 2G for groove welds and 2F for fillet welds. Gravity tends to cause the weld pool to sag downward; the electrode must be angled slightly upward and travel speed adjusted to maintain bead geometry.

A crack that forms at temperatures near the completion of solidification of the weld metal or in the HAZ during the welding thermal cycle, while the material is in a partially solidified or high-temperature solid state. Hot cracks include solidification cracks (in the weld metal centreline), liquation cracks (in the HAZ grain boundaries), and ductility dip cracks (in some nickel alloys). Driven by solidification shrinkage stresses and low-melting-point eutectic films at grain boundaries.

The second weld pass in a pipeline weld, deposited immediately after the root pass while the joint is still hot. The hot pass burns out any trapped slag or porosity from the root pass, improving root soundness. It is welded at higher current than the root pass and must be completed before the joint cools below the minimum interpass temperature.

Cracking resulting from the combined effect of diffusible hydrogen in the weld or HAZ, a susceptible microstructure (typically martensite with hardness >350 HV), and residual or applied tensile stress. HIC may be delayed by hours or days after welding. Prevention requires: reducing hydrogen source (dry electrodes, clean joint), preheating to slow cooling and promote hydrogen diffusion out of the joint, and PWHT to temper martensite. Critical in sour service; see our sour service guide.

A welding process that simultaneously combines two or more welding energy sources in the same weld pool. The most common combination is laser-arc hybrid welding (LAHW), which combines a laser beam with a GMAW or GTAW arc. Hybrid welding provides the deep penetration and narrow HAZ of laser welding with the gap-bridging ability and deposition rate of arc welding, allowing single-pass welding of thicker sections at higher travel speeds than either process alone.

A measure of a material’s resistance to permanent indentation or scratching. In welding quality control, hardness testing (Vickers HV or Brinell HB) of the weld metal, HAZ, and base metal is performed on macro sections during WPS qualification or production surveillance. Maximum HAZ hardness limits are specified in codes for sour service (typically 248 HV per NACE MR0175/ISO 15156) and hydrogen service.

A weld discontinuity in which fusion did not occur between the weld metal and the groove faces, the weld root, or the surface of a previously deposited weld bead. Incomplete fusion is a planar (two-dimensional) discontinuity and is therefore more severe than volumetric discontinuities such as porosity for the same projected area. Causes include insufficient heat input, incorrect electrode angle, contamination, or inadequate groove preparation.

A joint root condition in a groove weld in which weld metal does not extend through the joint thickness. Leaves a root notch that concentrates stress. In CJP joints, IJP is always a defect. In PJP joints, a defined amount of incomplete penetration is accepted by design, but the actual penetration must meet the minimum specified depth. Not to be confused with Incomplete Fusion.

In multipass welding, the temperature of the weld area immediately before each subsequent pass is deposited. A minimum interpass temperature (equal to the preheat requirement) prevents rapid cooling between passes. A maximum interpass temperature (e.g., 250 °C for C-Mn steel; 175 °C for some austenitic SS grades) protects against overheating, grain coarsening, and sensitisation. Interpass temperature is measured by contact thermocouple or infrared thermometer at a specified distance from the weld joint.

A welding power source that uses solid-state electronics to convert incoming AC supply (50/60 Hz) to DC, invert it to a high frequency (20,000–100,000 Hz), then transform and rectify it again to the required output. Inverter technology enables dramatic reductions in transformer core size and weight compared with conventional transformer-based machines, while providing improved arc dynamics and faster response to arc length changes.

A gas that normally does not combine chemically with the base metal or filler metal. Argon and helium are the primary inert gases used in welding. Argon is denser than air, providing good coverage at lower flow rates; helium is lighter and provides higher arc voltages and wider bead profiles. Mixtures of argon and helium (or argon with small percentages of active gases for GMAW) tailor the arc characteristics to specific applications.

Corrosion that preferentially attacks grain boundaries rather than the grain interiors. In austenitic stainless steels, sensitisation causes chromium depletion adjacent to grain boundaries, making them susceptible to intergranular attack by acids or chlorides. Tested per ASTM A262 (e.g., Huey test, Strauss test) to verify that the material or weld is free from sensitisation. See our weld decay guide.

The removal of slag, spatter, oxidation products, and contaminants from each weld pass before depositing the next pass. Interpass cleaning is mandatory for flux-shielded processes (SMAW, SAW, FCAW) to prevent slag inclusions. Methods include chipping, wire brushing, grinding, and air carbon arc cleaning. Inadequate interpass cleaning is a leading cause of slag inclusions in multi-pass welds.

Dynamic mechanical testing that measures the energy absorbed when a notched specimen is fractured by a single blow from a swinging pendulum (Charpy test) or falling weight. Impact testing quantifies notch toughness, which is the ability of a material to resist brittle fracture under suddenly applied loads at specified temperatures. Required by ASME Section VIII UG-84 for low-temperature service applications.

A groove weld in which the cross-section of the groove is a J-shape on one side. The J-groove preparation provides a smooth curved root face that minimises weld metal volume and distortion compared with a bevel (flat-face) preparation. Used in thick-section vessels and pipe joints where reduced groove volume is beneficial. J-grooves require machine cutting; they cannot be produced by flame cutting alone.

The junction of two or more members that are to be joined or have been joined. Five basic joint types are defined in AWS A3.0: butt joint, corner joint, edge joint, lap joint, and T-joint. Each joint type accommodates specific weld types and is suited to different loading conditions and accessibility requirements. See our weld joint types guide.

The distance between the faying (mating) surfaces of a brazed or soldered joint. Correct joint clearance is critical in brazing: it must be small enough for capillary action to draw the molten filler metal through the joint (typically 0.05–0.1 mm for furnace brazing), but not so tight that the filler metal cannot flow. Excessive clearance results in joints with insufficient filler metal fill and reduced strength.

The shape, dimensions, and configuration of a weld joint as specified on engineering drawings or in the WPS. Parameters include joint type, groove configuration (angle, root face, root opening), backing details, and weld access dimensions. Joint design influences penetration requirements, accessibility, distortion, residual stress, and fatigue performance.

A weld symbol on an engineering drawing that designates a J-groove weld preparation. Shown as a reversed J on the arrow or other side of the reference line in the AWS A2.4 weld symbol convention. The symbol specifies the groove depth and root opening. See our welding symbols guide.

The width of the cut produced by a cutting process. In thermal cutting (plasma, oxyfuel, laser), the kerf width determines the material loss and the dimensional accuracy of the cut part. Laser cutting produces the narrowest kerf (0.1–0.5 mm); oxyfuel cutting the widest (1.5–5 mm depending on tip size and plate thickness). Kerf width must be accounted for when dimensioning cut parts.

A hole produced through the full thickness of the base metal at the leading edge of the weld pool in plasma arc welding (PAW) operating in keyhole mode and in laser welding. The keyhole is maintained by the balance between arc plasma or laser pressure and surface tension of the molten metal. As the heat source advances, the metal flows around and behind the keyhole to fill and solidify. Keyhole welding enables single-pass full-penetration welds in thick material.

An informal term (not standard AWS A3.0 terminology) for a low-hydrogen covered electrode that has been re-dried in an oven after moisture absorption to restore its hydrogen classification. Covered electrodes absorb atmospheric moisture if left exposed; redrying at 260–430 °C (depending on classification) for 1–2 hours restores the H4 or H8 hydrogen level. Dried electrodes must be stored in heated ovens (60–120 °C) when not in use.

See Incomplete Fusion. “Lack of fusion” is a widely used colloquial term for the same discontinuity. AWS A3.0 prefers the term “incomplete fusion” as the standard term.

A subsurface stepped cracking phenomenon that occurs in the base metal beneath welded joints, caused by through-thickness tensile stresses combined with low through-thickness ductility due to non-metallic inclusions (typically sulphide stringers) in rolled plate. Lamellar tears form parallel to the plate surface and are found in the base metal, not the weld or HAZ. Prevention: low-sulphur or Z-quality plate (guaranteed minimum through-thickness reduction of area), joint redesign to reduce through-thickness stress.

A joint between two overlapping workpieces in which the surfaces of the workpieces are approximately parallel. Typically welded with fillet welds on one or both sides of the overlap. Lap joints are common in structural connections, tank construction, and sheet metal fabrication. The fillet weld leg size and length are specified to carry the required shear load.

A stratum of weld metal consisting of one or more weld beads deposited at approximately the same elevation in a multi-pass groove weld. The number of layers and passes per layer are specified in the WPS. Minimum three-layer deposits are required under certain ASME Section IX rules to qualify a welder for unlimited thickness.

A surface or near-surface indication detected by magnetic particle testing (MT) or liquid penetrant testing (PT) whose length is three or more times its width. Linear indications are more severe than rounded indications because they are characteristic of planar discontinuities (cracks, incomplete fusion, cold laps) rather than volumetric ones (porosity). Most codes have significantly lower acceptance limits for linear than for rounded indications.

A form of hot cracking that occurs in the HAZ of the base metal or in previously deposited weld metal, at the partially melted zone adjacent to the fusion boundary. It is caused by the liquation of low-melting-point constituents (e.g., NbC, sulphide films) at grain boundaries during the welding thermal cycle, followed by grain boundary separation under solidification shrinkage stress. Common in nickel alloys, high-alloy steels, and cast irons.

A weld whose axis is parallel to the longitudinal axis of the member being welded. In a pressure vessel shell, the longitudinal seam is the weld running along the length of the cylinder. Longitudinal welds in pressure vessels carry circumferential (hoop) stress and are thus subject to the full design pressure loading. Contrast with Circumferential Weld.

A covered electrode designed to produce weld deposits with a low diffusible hydrogen content. The electrode coating contains no organic materials (which would introduce hydrogen) and uses a lime-fluorspar formulation. Designated by the suffix H4, H8, or H16 in the AWS classification (e.g., E7018-H8 delivers less than 8 mL H per 100 g weld metal). Low-hydrogen electrodes must be stored dry; moisture absorption increases hydrogen content and must be corrected by oven redrying.

Welding with equipment that performs the welding operation under the continuous manual control and observation of a welding operator. The operator may make adjustments to variables during the weld. Distinguished from Automatic Welding (no operator adjustments during the weld) and Semiautomatic Welding (where only the electrode wire feeding is mechanised).

Examination of a prepared and etched cross-section of a weld at low magnification (typically 1×–10×) to reveal pass sequence, penetration depth, HAZ extent, and the presence of macro-level discontinuities such as cracks, slag inclusions, porosity, and incomplete fusion. Required by ASME Section IX for fillet weld procedure and performance qualification tests (QW-183, QW-184).

A hard, body-centred tetragonal (BCT) phase formed in steel when austenite is cooled faster than the critical cooling rate, suppressing diffusion-controlled decomposition. Martensite hardness increases with carbon content and is the primary microstructure responsible for cold cracking susceptibility in the HAZ. Hardness above ~350 HV is a common rejection criterion in sour service and hydrogen-environment applications.

The group of tests performed to determine the mechanical properties of a weld or base metal, including tensile strength, yield strength, ductility (elongation, reduction of area), hardness, and impact toughness. Mechanical testing of PQR test coupons is required by ASME Section IX; test types and acceptance criteria are tabulated in QW-451 through QW-453. See our mechanical testing guide.

The movement of molten filler metal from the electrode to the workpiece across the arc. In GMAW, four primary transfer modes are recognised: short-circuit (globule bridges gap then short-circuits), globular (large irregular drops fall into the pool), spray (fine droplets accelerated axially into the pool), and pulse-spray (alternating peak and base current controls transfer). Transfer mode selection controls spatter, heat input, penetration, and positional capability.

Visible root reinforcement produced on the back side of a single-side weld. Acceptable melt-through is specifically permitted in certain pipe welding codes where internal root bead visibility confirms complete root fusion. Excessive melt-through that forms an interior projection or crevice exceeding specified limits is a defect requiring removal.

A weld consisting of two or more passes deposited in sequence to fill a groove or build up a fillet. Multipass welding allows weld metal microstructure to be refined (each pass tempers the previous pass’s HAZ), controls interpass temperature, and provides the opportunity for inter-pass cleaning. WPS control of pass sequence, bead size, and interpass temperature is critical to achieve required mechanical properties throughout the weld cross-section.

Welding performed entirely by a welder without mechanical assistance. The welder controls the arc, travel speed, electrode angle, and wire feeding (if applicable) entirely by hand. SMAW and GTAW with manual filler feed are classic examples of manual welding. Manual welding flexibility makes it suitable for complex joint geometries and field repairs where mechanised equipment cannot be deployed.

A destructive test used to assess the internal soundness of fillet welds. The specimen is notched at each end of the weld, then fractured along the weld axis by hammer blows or in a press. The fracture surface is examined visually for the presence and distribution of porosity, slag inclusions, incomplete fusion, and cracks. Used in AWS D1.1 for fillet weld qualification testing.

The examination of a weld or material for discontinuities using methods that do not alter or damage the item being examined. Standard NDE methods: Visual Testing (VT), Radiographic Testing (RT), Ultrasonic Testing (UT), Magnetic Particle Testing (MT), Liquid Penetrant Testing (PT), Phased Array UT (PAUT), Time-of-Flight Diffraction (TOFD), and Eddy Current Testing (ET). Method selection depends on discontinuity type sought, material, geometry, and applicable code.

Metals that contain no iron as the principal constituent. Aluminium, copper, nickel, titanium, cobalt, and their alloys are nonferrous. Nonferrous metals cannot be tested by magnetic particle inspection and do not harden by quenching from austenite. Their welding requires specific filler metals, shielding gases, and often AC (for aluminium GTAW) or DC negative (for GTAW of copper and nickel alloys).

A heat treatment in which iron-base alloys are heated to approximately 50–100 °C above the upper critical temperature (Ac3), held until fully austenitised, then cooled in still air. Normalising refines grain structure, removes the effects of prior rolling or forging, and restores a more uniform microstructure with improved toughness compared to the as-rolled condition. Normalised steels have slightly lower strength but better notch toughness than as-rolled material.

The ability of a material to absorb energy during fracture in the presence of a stress concentrator (notch). Measured by Charpy V-notch impact testing at specified temperatures, reported in joules (or ft-lb). Notch toughness is critical in pressure vessels, offshore structures, and low-temperature piping systems where brittle fracture must be prevented. Both weld metal and HAZ notch toughness must meet specified minimums after WPS qualification testing.

The weld metal of a resistance spot weld, seam weld, or projection weld that is formed between the electrodes and solidifies within the joint. Nugget diameter is the primary quality measure for resistance spot welds; minimum nugget diameters are specified by AWS D8.1 for automotive applications and ISO 14273 for general resistance welding.

A device through which shielding gas is directed at the weld zone. In GTAW, the nozzle surrounds the tungsten electrode and directs argon or helium over the arc and weld pool. In GMAW, the gas nozzle concentrically surrounds the contact tube and directs shielding gas. Nozzle diameter, shape (cylindrical vs. conical), and standoff distance affect gas coverage quality and the ability to access confined joints.

The voltage between the output terminals of a welding power source when no welding current is flowing (before the arc is struck). OCV must be sufficient to ionise the arc gap and initiate the arc. CC power sources for SMAW typically have OCV of 55–80 V. OCV above 113 V peak (80 V RMS AC) is considered a shock hazard under IEC 60974-1 in damp or confined spaces.

The welding position in which welding is performed from the underside of the joint. The weld face is approximately horizontal and faces downward. Overhead welding is the most difficult position due to gravity acting against the molten pool; smaller electrodes, lower currents, and faster travel speeds are used to maintain bead shape. Designated 4G for groove welds; 4F for fillet welds.

The protrusion of weld metal beyond the weld toe or weld root, lying on the base metal surface but not fused to it. Overlap creates a notch on the reverse side of the toe interface that acts as a stress concentrator in fatigue-loaded joints. Caused by too-low travel speed, excessive amperage, or wrong electrode angle. Overlap is a defect under AWS D1.1 and ASME codes.

A group of thermal cutting processes that use the heat of a combustion flame and a stream of oxygen to cut ferrous metals. The cutting reaction is exothermic oxidation of iron: the preheating flame brings the steel to ignition temperature (~870 °C), then a high-purity oxygen jet oxidises the iron and blows away the iron oxide. Not applicable to stainless steel, aluminium, or other metals that form refractory oxides preventing continuous cutting.

See Overhead Position. Overhead welding requires specific technique modifications: electrode angle adjustment to push the pool against gravity, reduced bead size, faster travel speed, and often a lower current than for the same electrode in flat position.

Any welding performed in a position other than the flat (1G/1F) position, where gravity does not act favourably on the molten pool. Out-of-position positions include horizontal (2G/2F), vertical (3G/3F), and overhead (4G/4F). Welding processes and electrode types must be qualified for out-of-position use; not all electrodes or transfer modes are suitable for vertical and overhead welding.

A base metal grouping in ASME Section IX (Table QW/QB-422) that assigns materials with similar weldability, composition, and mechanical properties to the same group, reducing the total number of procedure qualification tests needed. P-No. 1: carbon steel; P-No. 8: austenitic stainless steel; P-No. 9A/9B: 2.5% nickel steels; P-No. 15E: Grade 91 CrMoV alloy. See our P-Number reference guide.

A groove weld in which the weld metal intentionally does not extend through the full joint thickness. The design accounts for the effective throat reduction. PJP is acceptable where the full joint cross-section is not required for load transfer, but the unwelded root constitutes a notch and PJP joints are generally avoided in fatigue-critical or impact-dominated applications.

A single progression of welding or surfacing along a joint, from start to finish. A weld pass may be a stringer bead or a weave bead. Multiple passes are deposited in sequence (root pass, hot pass, fill passes, cap pass) to complete a groove weld. Pass sequence is specified in the WPS because it affects interpass temperature, residual stress distribution, and microstructure in previous passes.

The distance that the weld metal extends below the surface of the workpiece into the base metal beyond the original groove bottom (joint penetration) or the depth to which the weld metal fuses into the base metal (depth of fusion). Joint penetration is critical in CJP and PJP groove welds. Inadequate penetration leaves a root notch; excessive penetration (melt-through) creates an interior protrusion.

An arc welding process that uses a constricted arc between a non-consumable electrode and the workpiece (transferred arc) or between the electrode and the constricting nozzle (non-transferred arc). The plasma gas is forced through the constricting orifice, creating a high-energy, high-velocity plasma jet that can weld at higher speeds and greater thicknesses than GTAW in a single pass. PAW can operate in melt-in mode (similar to GTAW) or keyhole mode (full-penetration single-pass welding of 6–10 mm stainless steel).

Cavity-type discontinuities formed by gas trapped in the solidifying weld metal. Forms include: uniformly scattered porosity, cluster porosity, linear porosity, and elongated porosity (piping or wormholes). Primary causes: moisture in consumables or on base metal, contaminated shielding gas, excessive arc length, draught disrupting gas coverage, or oil/grease contamination. Porosity is a volumetric discontinuity and less severe per unit size than planar discontinuities (cracks, IF) under code acceptance criteria.

Any heat treatment applied to a weldment after welding. In most code contexts, PWHT refers specifically to thermal stress relief: heating the completed weldment to a temperature below Ac1, holding for a time dependent on thickness, and cooling at a controlled rate. PWHT reduces residual welding stresses, tempers martensite in the HAZ, and improves corrosion resistance in certain alloy systems. ASME VIII UCS-56 mandates PWHT for carbon steel joints above threshold thicknesses.

The application of heat to the base metal immediately before welding begins, raising it to a specified minimum preheat temperature. Preheat slows the post-weld cooling rate, reduces thermal gradient, promotes hydrogen diffusion out of the weld, and minimises martensite formation in the HAZ. Preheat requirements are calculated from carbon equivalent (CE) values or specified in tables within AWS D1.1, ASME B31.3, and similar codes.

A record of the welding variables used to produce an acceptable test weld and the results of all tests conducted on the weld to qualify a welding procedure specification (WPS). The PQR is certified by the manufacturer or contractor and forms the technical basis for the supporting WPS. Multiple WPSs may be supported by a single PQR if the WPS variables fall within the ranges demonstrated by the PQR test.

A welding variation in which the current is cyclically varied between a higher peak level and a lower background level. During the peak, metal is melted and transferred; during the background, the pool solidifies slightly. Pulsed current reduces overall heat input, improves positional capability (GMAW-P gives spray transfer at lower average current), and can improve weld microstructure by promoting grain refinement through cyclic solidification.

The displacement of air from the root side of a pipe or vessel weld joint by inert gas (argon, nitrogen) prior to and during welding. Purging prevents oxidation of the weld root and inner bead surface in stainless steels, titanium, and nickel alloys where even trace oxygen or nitrogen would cause discolouration, oxide inclusions, and loss of corrosion resistance. The purge is maintained until the completed root pass has cooled below approximately 300 °C.

An index used to rank the resistance of stainless steels and nickel alloys to pitting corrosion in chloride-containing environments. The most common formula: PREN = %Cr + 3.3%Mo + 16%N. Higher PREN indicates better pitting resistance. Duplex stainless steels typically have PREN of 34–45; super duplex 40–48. Use our PREN calculator to evaluate specific alloy compositions.

A weld made in accordance with a qualified welding procedure specification (WPS) by a qualified welder or welding operator, and verified to meet the applicable acceptance criteria. Qualification involves both the procedure (PQR testing) and the individual (WPQ testing). A weld may be made per a qualified WPS by an unqualified welder and still fail the code requirement.

Rapid cooling of a metal from an elevated temperature, typically by immersion in water, oil, polymer solution, or air blast. In carbon and alloy steels, quenching from above the Ac3 temperature suppresses diffusion-controlled decomposition of austenite and promotes martensite formation. Quenched steels have high hardness and strength but low ductility and toughness, requiring subsequent tempering. In welding, unintentional quenching of the HAZ by the base metal mass (thermal mass effect) can produce undesirable hard microstructures.

A document specifying the quality management activities, responsibilities, procedures, and resources to be applied to a specific project or product. For welded fabrication, a quality plan typically identifies the applicable codes, all WPS and WPQ document references, NDE methods and extent, hold points for inspection, and requirements for PWHT documentation. Required by ISO 3834 and many client specifications for critical welded structures.

A two-step heat treatment sequence in which steel is first quenched (rapidly cooled from above Ac3 to form martensite) and then tempered (reheated below Ac1 to reduce brittleness and improve toughness). Q&T steels such as ASTM A517 and API 5L X70 achieve high strength-to-weight ratios. Welding Q&T steels requires careful WPS control of heat input and interpass temperature to avoid over-tempering (softening) the HAZ.

A non-destructive examination method in which X-radiation or gamma radiation is passed through the weld and recorded on film or a digital detector. Variations in radiation absorption produce image density differences that reveal internal discontinuities such as porosity, slag inclusions, cracks, and incomplete penetration. RT can qualify a welder for WPQ under ASME Section IX; it is the volumetric NDE method specified in QW-191 for groove welds.

A tensile test specimen machined from a weld test coupon with a reduced cross-sectional area in the gauge length to ensure fracture occurs within the test section. Used in WPS qualification testing (ASME Section IX QW-150) to demonstrate that the weld joint meets the minimum tensile strength of the base metal or weld filler metal classification, whichever is lower.

Stress that exists in a material or structure without any external load applied. In welded joints, residual stresses arise from differential thermal expansion and contraction during the welding thermal cycle. Residual tensile stresses in the weld and adjacent HAZ can reach yield-stress magnitude and promote stress corrosion cracking, fatigue crack growth, and hydrogen-induced cracking. PWHT reduces residual stresses through creep relaxation at elevated temperature.

Weld metal in excess of the quantity required to fill the joint. Face reinforcement is the excess on the weld face; root reinforcement is the excess on the root side. Excessive reinforcement creates a stress concentration at the toe and is wasteful of filler metal. Most codes specify maximum allowable reinforcement heights (e.g., ASME B31.3 limits face reinforcement to 3 mm for pipe welds).

A bend test in which the root of the weld is placed in tension. Used with face bend tests in WPS and WPQ qualification for thinner coupons where side bend testing (10 mm and above) is not applicable. Root bend tests are particularly sensitive to incomplete root fusion and root cracks, as the tensile stress concentrates at the root during bending.

The portion of the groove face adjacent to the weld root, left flat or parallel (not bevelled). Also called the land. Root face dimension controls burn-through risk and root penetration consistency. Too thin a root face: burn-through risk. Too thick: incomplete root penetration. Typical root face for single-V SMAW: 1.5–3 mm. In GTAW pipe welding, a tighter root face (0.8–1.5 mm) with precise gap control is standard.

The separation between the faying surfaces of a butt joint at the weld root. Also called root gap. Root opening provides access for the arc and filler metal to achieve complete root penetration. Must be controlled within the WPS tolerance; excessive gap increases distortion and consumable consumption; insufficient gap risks incomplete root penetration. Typical SMAW root opening: 1.5–3 mm; GTAW: 1–2.5 mm.

The first weld bead deposited in a multi-pass groove weld, filling the root area of the joint. The root pass is the most critical pass with respect to cold cracking, root penetration quality, and structural integrity. In pipe welding, the root pass is typically made by GTAW or open-root SMAW. After root pass deposition, the root is often inspected by RT or UT before fill passes proceed.

Weld metal on the root side of a single-sided groove weld that projects beyond the original joint root surface. Acceptable root reinforcement confirms complete root penetration; excessive root reinforcement (excessive melt-through) creates an internal protrusion that may reduce flow area in piping, cause turbulence, and create a crevice susceptible to corrosion or fatigue cracking at the inner root toe.

Welding with equipment that controls only the electrode feed. The travel speed, gun angle, and gun position are controlled manually by the welder. GMAW and FCAW are the most common semiautomatic processes; the wire feeder controls electrode feed rate while the welder moves the gun along the joint. Semiautomatic welding is the most productive manually-controlled welding method.

The precipitation of chromium carbides (primarily Cr23C6) at grain boundaries in austenitic stainless steels when heated in the range 425–815 °C, causing chromium depletion in the adjacent zones and loss of corrosion resistance. The sensitised microstructure is susceptible to intergranular corrosion. Prevention: use low-carbon grades (304L, 316L), stabilised grades (321 with Ti, 347 with Nb), solution anneal after welding, or minimise heat input. See our stainless weld decay article.

An arc welding process that produces coalescence of metals by heating with an arc between a covered (flux-coated) consumable electrode and the workpiece. The arc and molten pool are shielded from the atmosphere by gases and slag produced from the decomposition and melting of the electrode coating. Known colloquially as stick welding or manual metal arc (MMA). The most versatile and portable arc welding process. See our SMAW guide.

A bend test in which the side of the weld cross-section is placed in tension. The specimen is bent edgewise (the weld cross-section is on the convex side of the bend). Side bend tests are performed on coupons 10 mm (3/8 in.) or thicker in WPS and WPQ qualification testing per ASME Section IX. Side bends test the full weld cross-section simultaneously and are particularly effective at revealing incomplete fusion between weld beads.

Nonmetallic material resulting from the dissolution and interaction of flux and non-metallic impurities during welding, brazing, or cutting. In SMAW, SAW, and FCAW, slag solidifies on the weld bead surface and must be completely removed by chipping and wire brushing before depositing the next pass. Slag inclusions occur when solidified slag is trapped within the weld metal during multi-pass welding.

Solid non-metallic material entrapped in the weld metal or between the weld metal and base metal. Slag inclusions result from inadequate interpass cleaning, undercut in a prior pass trapping slag, wrong electrode angle preventing slag from floating to the surface, or excessive slag fluidity at high heat input. Slag inclusions are volumetric discontinuities visible on RT and UT examination.

Hot cracking that occurs in the weld metal during solidification, caused by the development of tensile stresses across partially solidified regions containing low-melting-point eutectic films at grain boundaries or dendrite boundaries. Solidification cracks typically propagate along the weld centreline (centreline cracks) or at columnar dendrite boundaries. Risk increases with high sulphur content, high carbon, and high heat input promoting a wide columnar dendritic structure.

Metal particles expelled from the welding arc and weld pool during welding that do not form part of the completed weld. Spatter adheres to adjacent base metal surfaces and must be removed before NDE. It is most prevalent in GMAW short-circuit and globular transfer modes and in SMAW with excessive arc length. Spatter is reduced by anti-spatter coatings, optimised arc parameters, and pulse-transfer GMAW.

Cracking produced by the combined action of corrosive environment and tensile stress (applied or residual). SCC requires all three conditions simultaneously: a susceptible material, a specific corrosive medium, and tensile stress above a threshold. Common examples: austenitic stainless steel in chloride environments, copper alloys in ammonia, and carbon steel in caustic solutions or hydrogen-sulphide (sour service). Residual welding stresses are a primary driver of SCC in welded equipment; PWHT to reduce residual stress is a key mitigation.

A heat treatment performed below the lower critical temperature to reduce residual stresses in a weldment by creep relaxation. Not to be confused with full PWHT; stress relief is one objective of PWHT. For carbon steel, stress relief is typically performed at 595–650 °C. The holding time (typically 1 h per 25 mm of thickness, minimum 1 h) must be sufficient to allow homogeneous temperature distribution and relaxation throughout the section.

An arc welding process that produces coalescence of metals by heating with an arc or arcs between a bare metal electrode or electrodes and the workpieces. The arc and molten metal are submerged in a blanket of granular fusible flux. SAW produces very high deposition rates (3–10 times SMAW), excellent weld quality, and virtually no arc flash or fumes. Limited to flat and horizontal fillet positions. See our SAW guide.

The application by welding, brazing, or thermal spraying of a layer or layers of material to a surface to obtain desired properties or dimensions. Types of surfacing include: hard facing (wear resistance), cladding or overlay (corrosion resistance), build-up (dimensional restoration), and buttering (transition layer). Surfacing weld metal composition and dilution control are critical quality parameters.

A soldering method in which filler metal is pre-applied to one or both faying surfaces of a joint, the assembly is brought together, and heat is applied to flow the pre-applied solder and unite the surfaces. Sweat soldering is used for close-fitting joints in HVAC and plumbing where capillary flow of separately applied solder would be insufficient.

A weld made to hold the parts of a weldment in proper alignment until the final welds are made. Under most fabrication codes, tack welds incorporated into the final weld must meet the same quality requirements (including preheat) as production welds. Tack welds that will not be incorporated must be completely removed; their removal locations must be ground smooth and inspected.

A welding technique used to refine the grain structure and reduce hardness in the HAZ of prior weld passes by depositing subsequent passes that apply controlled heat cycles to the previous pass’s HAZ. Used as an alternative to PWHT in repair welding of pressure-retaining equipment where full PWHT is impractical. Requires careful control of heat input and bead placement sequence. Specifically addressed in ASME Section IX QW-290.

A heat treatment applied after quenching in which a hardened steel is reheated to a temperature below Ac1 and held, allowing the decomposition of martensite into tempered martensite with improved toughness at some sacrifice of hardness. Critical for Q&T steels; also the objective of PWHT for low-alloy steel weld joints where martensite has formed in the HAZ. Tempering temperature must be carefully controlled to avoid over-tempering (excessive softening in high-strength steels) or re-hardening.

A joint between a workpiece whose edge is attached to the surface of another workpiece approximately at right angles, forming a T-shape. The most common joint in structural fabrication. Typically welded with fillet welds on one or both sides, or with groove welds when higher strength or fatigue performance is required. Through-thickness tensile stresses in heavy T-joints can cause lamellar tearing in susceptible base metals.

The distance from the weld root to the weld face, measured through the weld cross-section. Different throat dimensions apply depending on measurement method: actual throat (as measured), effective throat (used for strength calculations), and theoretical throat (geometric minimum for an equal-leg fillet = 0.707 × L). Throat is the controlling dimension for fillet weld strength in structural and pressure welding codes.

The angle between the electrode axis and a reference plane perpendicular to the weld axis. A positive travel angle (electrode tilted forward, in the direction of travel) is used in forehand or push welding; a negative travel angle (electrode tilted backward, opposite to travel) is used in backhand or drag welding. Travel angle affects penetration, bead width, and slag control.

The rate of progression of the welding arc along the joint axis, expressed in mm/min (or in/min). Travel speed is a primary component in the heat input calculation. Increasing travel speed reduces heat input, bead cross-sectional area, and reinforcement; it also reduces penetration below a threshold. Travel speed is an essential variable in most WPS qualifications.

A non-consumable electrode used in GTAW and PAW, composed of pure tungsten or tungsten alloyed with thoria, ceria, lanthania, or zirconia. Electrode composition and diameter are selected based on current type and level. Ceriated (grey, 2% CeO2) and lanthanated (gold/black) electrodes are increasingly preferred over thoriated (red, radioactive) electrodes for DC welding. Zirconiated (white/brown) electrodes are preferred for AC aluminium welding.

A weld joining the end of a heat exchanger tube to the tubesheet (the plate through which the tube passes). Special WPS and WPQ qualification requirements apply under ASME Section IX QW-193; separate testing using tube-to-tubesheet mock-ups is required because the joint geometry, accessibility, and technique are fundamentally different from plate or pipe groove welds. See our tube-to-tubesheet qualification guide.

The maximum stress a material can withstand while being stretched or pulled before fracturing, expressed in MPa (or psi). In WPS qualification testing, the reduced-section tensile test specimen must fracture at a load corresponding to at least the specified minimum tensile strength of the base metal (ASME Section IX QW-153). If fracture occurs in the weld metal at a lower value, the procedure fails unless the code permits lower weld metal strength.

A groove weld in which the cross-section of the groove before welding has a U-shape on one or both sides (single-U or double-U). U-grooves require smaller included angles than V-grooves for the same depth, reducing weld metal volume, heat input, and distortion in thick sections. They require machine preparation and are common in heavy-wall pressure vessel and nozzle joints.

A non-destructive examination method in which high-frequency sound waves (typically 2–10 MHz) are introduced into the weld and reflections from discontinuities are detected, displayed, and evaluated. UT can detect internal planar discontinuities (cracks, incomplete fusion) more reliably than RT, and is the preferred method for thick-section welds. Phased Array UT (PAUT) and Time-of-Flight Diffraction (TOFD) are advanced variants providing improved defect characterisation and sizing. UT is accepted for WPQ qualification (ASME Section IX QW-191) for thicknesses ≥ 6 mm.

A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal. Undercut reduces the effective load-carrying cross-section of the base metal and acts as a sharp notch, concentrating stress at the weld toe under fatigue loading. AWS D1.1 limits undercut depth to 1 mm for static loading and 0.25 mm for cyclic (fatigue) loading. Caused by excessive current, too-long arc, wrong electrode angle, or excessively high travel speed.

A depression on the weld face or weld root where the weld metal surface is below the adjacent surface of the base metal. Underfill reduces the joint cross-sectional area below the base metal cross-section and is a defect under all structural and pressure codes. Caused by insufficient weld metal deposition in the cap pass or root pass.

A resistance welding process that produces coalescence simultaneously over the entire area of abutting surfaces, or progressively along a joint, by the heat obtained from resistance to the flow of electric current through the area of contact of those surfaces. Force is applied before heating begins and is maintained throughout the heating period. Used for flash butt welding of bars, rails, and pipe ends.

A condition in which the weld metal yield strength or tensile strength is lower than that of the base metal. Undermatching is generally avoided in structural applications because it concentrates plastic strain in the weld and can limit joint efficiency below 100%. Intentional undermatching is occasionally used in high-strength steels to improve weld toughness, with structural design adjusted accordingly.

A groove weld in which the cross-section of the groove before welding has a V-shape (single-V) or double-V shape. The most common groove preparation type; producible by flame cutting, grinding, or machining. Single-V groove angles typically 60–70° included. Double-V reduces total weld metal volume and distortion compared with single-V for thick sections welded from both sides.

The welding position in which the weld axis is approximately vertical. The weld can be progressed upward (vertical up, 3G-up) or downward (vertical down, 3G-down). Vertical-up provides better penetration and fusion at lower travel speed; vertical-down gives shallower penetration but higher travel speed. Most codes restrict or prohibit vertical-down for groove welds on structural applications due to the risk of incomplete root fusion.

The most fundamental and widely applied NDE method. VT detects surface-accessible discontinuities including cracks, porosity, undercut, overlap, underfill, incorrect bead profile, spatter, arc strikes, and dimensional non-conformance. VT is mandatory for all welds before any other NDE method is applied (AWS D1.1) and must be performed by a qualified inspector (ASME Section V Article 9). Tools include adequate illumination, magnification (10× maximum per ASME V), weld gauges, and mirror/borescope for root inspection.

The electrical potential difference that drives current through the welding circuit, measured in volts (V). Arc voltage is measured across the arc gap and primarily controls bead width, arc length, and the shape of the arc column. In GMAW (CV process), voltage setting is the primary operator control for arc length. In SMAW (CC process), arc voltage is determined primarily by the arc length that the welder maintains manually.

The standardised notch machined in a Charpy impact test specimen to concentrate stress and initiate fracture at a specific location. The Charpy V-notch (CVN) has a 45° included angle, 2 mm depth, and 0.25 mm root radius per ISO 148-1 and ASTM E23. The V-notch location (weld metal centre, fusion line, FL+2, FL+5 mm) is specified when toughness at specific zones of the weld is to be evaluated.

A microhardness or macrohardness test in which a diamond pyramid indenter is pressed into the test surface under a specified load, and the diagonal of the resulting indentation is measured. Vickers hardness (HV) is used for weld cross-section hardness surveys in WPS qualification and production surveillance. It provides a continuous scale from very soft (10 HV) to very hard (1,000 HV) without the range limitations of Rockwell scales, making it suitable for comparing base metal, HAZ, and weld metal hardness in one test series.

A weld bead made with transverse oscillation (side-to-side movement) of the electrode during travel. Weave beads are wider than stringer beads and deposit more metal per pass, reducing the number of passes needed to fill a groove. However, they increase heat input, interpass temperature, and HAZ width. Many codes limit weave bead width (e.g., AWS D1.1 limits weave width to 3 times the electrode diameter for SMAW in certain applications).

One who performs manual or semiautomatic welding. Under ASME Section IX, a welder is a person who manually controls or guides the arc, torch, or gun during welding. Welders must be individually qualified by performance qualification testing (WPQ) for each process and essential variable range used in production. Distinct from a Welding Operator.

The qualification of a welder by testing to demonstrate skill to produce sound weld metal, documented per ASME Section IX Article III. The WPQ certificate (Form QW-484A) records all essential variables tested and the qualification ranges for thickness (QW-452) and pipe diameter (QW-452.3). WPQ qualification lapses if the welder has not used the qualified process in production within any 6-month period.

One who operates machine or automatic welding equipment. The welding operator does not directly manipulate the arc or weld pool. Under ASME Section IX, welding operators are qualified separately from welders; different essential variable tables and a different record format (QW-484B) apply. A welder cannot serve as a welding operator without a separate qualification test, even in the same process.

The orientation of the weld axis and weld face relative to the horizontal plane, categorised to define accessibility and technique requirements for qualification and production purposes. Standard positions: 1G/1F (flat), 2G/2F (horizontal), 3G/3F (vertical), 4G/4F (overhead), 5G (pipe horizontal fixed), 6G (pipe 45° inclined fixed), 6GR (restricted access). A 6G qualification covers all other pipe and plate positions. See our welding positions guide.

A document that provides direction for making production welds to the required code. The WPS specifies ranges for all essential, supplementary essential, and non-essential variables. It is supported by one or more PQRs that demonstrate that welds made within those variable ranges meet the required mechanical properties. The WPS must be available at the point of work during production welding.

Standardised graphical symbols on engineering drawings that communicate weld type, size, length, location, process, and inspection requirements without written notes. Defined in AWS A2.4 (US) and ISO 2553 (international). The weld symbol system uses a reference line, arrow, and weld symbol placed on the arrow or other side, with supplementary information for contour, finish, and nondestructive testing requirements. See our complete welding symbols guide.

The rate at which electrode wire is fed from the wire spool through the welding cable to the contact tube, expressed in mm/min or m/min. In GMAW and FCAW with a CV power source, WFS is the primary control for welding current: higher WFS increases current (the arc self-regulates to maintain the set voltage). WFS is an essential variable in GMAW and FCAW WPS qualifications.

The angle between the electrode axis and a reference plane perpendicular to the weld axis, measured in a plane perpendicular to the weld axis. Work angle determines where heat and filler metal are directed relative to the joint members. For equal-leg fillet welds on T-joints, the work angle is typically 45° to bisect the joint angle. Incorrect work angle causes unequal leg sizes and may direct heat away from the root, causing incomplete fusion.

Elongated, tubular-shaped voids in weld metal formed by gas evolution during solidification, oriented approximately perpendicular or at an angle to the weld surface. Also called piping porosity. Wormhole porosity is more severe than spherical porosity because the elongated shape concentrates stress more effectively. Often caused by contamination from galvanised or coated base metal, hydrogen sources, or excessive moisture in the shielding gas or consumable.

A widely used informal term for radiographic testing (RT) using X-radiation generated by an industrial X-ray tube. X-ray machines produce radiation by accelerating electrons through a high voltage and directing them at a tungsten target. Unlike radioactive isotope sources (Ir-192, Co-60), X-ray machines can be switched off, offering a safety advantage. X-ray RT provides superior image resolution and energy flexibility compared with gamma-ray sources for the same material thickness.

An informal term for a double-V groove weld (also called a double-bevel groove when the bevels are on the same member). The cross-section of the prepared joint resembles the letter X. Double-V and X-groove preparations are used for thick-section joints welded from both sides to reduce total weld metal volume and distortion compared with a single-V on one side only.

The stress at which a material begins to deform plastically (permanently). Below the yield point, deformation is elastic and fully recoverable. The specified minimum yield strength (SMYS) of the base metal governs allowable stress in pressure and structural design. Weld metal yield strength must meet or exceed the base metal SMYS in over-matching designs. Yield strength reduces at elevated service temperatures; codes provide temperature-dependent allowable stress tables for design.

A structural joint in which a branch member meets a main (chord) member at an oblique angle, resembling the letter Y. Y-joints are common in offshore jacket tubular structures, truss nodes, and piping branches. The varying included angle around the joint perimeter creates regions of varying groove geometry and accessibility, making welding and inspection more complex than simple T- or butt joints.

Structural steel with guaranteed minimum through-thickness (Z-direction) ductility, specified to resist lamellar tearing in heavy welded T-connections. EN 10164 designations Z15, Z25, and Z35 indicate minimum percentage reduction of area in the through-thickness tensile test. Z-grade steel is specified for nozzle-to-shell connections, heavy flange-to-web welds, and other joints where welding shrinkage imposes significant through-thickness tensile strain on the plate.

The region in a welded joint where the properties, composition, or microstructure change from the weld metal to the unaffected base metal. This zone encompasses the fusion zone, the HAZ, and the partially melted zone. It is the region where most weld-related discontinuities and metallurgical changes occur and where mechanical testing (hardness traverses, impact testing, macro examination) focuses during WPS qualification.

A GTAW tungsten electrode containing approximately 0.15–0.40% zirconium oxide (ZrO2), typically colour-coded white or grey-white. Zirconiated electrodes are preferred for AC GTAW of aluminium and magnesium alloys because they form a clean hemispherical ball at the tip during the AC electrode-positive half-cycle, maintain a stable arc, and tolerate higher current levels than pure tungsten without splitting. They have lower electron emissivity than thoriated or ceriated grades and are not recommended for DCEN welding.

A process of joining or solidifying materials, usually layer upon layer, to make objects from 3D model data. Wire Arc Additive Manufacturing (WAAM) uses GMAW or GTAW to deposit weld metal layer by layer, building up near-net-shape metal components. AM was added to AWS A3.0 in the 2020 edition reflecting the technology’s growing industrial importance. The metallurgical principles governing solidification, HAZ, and interpass temperature in AM closely parallel those in conventional multipass welding.

The ratio of the heat transferred to the workpiece to the total electrical power consumed by the welding arc, expressed as a fraction. Arc efficiency (η) is used in the heat input formula as a process-dependent multiplier: effective heat input = (V × I × 60 × η) / travel speed. Typical values: SMAW 0.80, GMAW 0.85, SAW 0.90–0.99, GTAW 0.60–0.80. Arc efficiency is incorporated into heat input calculations per EN 1011-1 and ASME practice.

The thermal gouging of metal using an arc process, most commonly air carbon arc cutting (CAC-A) or plasma arc gouging (PAG). Arc gouging is used for back-gouging weld roots, removing defective weld metal, and preparing joint bevels in field environments where machining is impractical. The gouge depth and profile are controlled by electrode angle, current level, and travel speed. Preheating may be required after CAC-A gouging on hardenable steels to prevent re-hardening.

A spot weld made by an arc welding process, typically GMAW or GTAW, in which one workpiece is melted through to fuse to the underlying member without repositioning the electrode between workpieces. Used in sheet metal and thin structural fabrication as an alternative to resistance spot welding when only one-sided access is available. The weld nugget forms at the interface of the two sheets.

A welding sequence technique in which individual bead increments are deposited in the direction opposite to the overall direction of weld progression. Each successive increment starts ahead of the previous increment’s start point and ends at the start point of the previous increment. Back-step welding reduces overall distortion and balances the thermal gradient along the joint, because each increment preheats the zone into which the subsequent increment will be deposited.

A welding sequence or technique that distributes weld heat symmetrically about the neutral axis of a weldment to minimise angular distortion and longitudinal bowing. For double-sided groove welds, balanced welding alternates passes between the two sides rather than completing one side before starting the other. Critical in the fabrication of I-beams, box sections, and pressure vessel shells where distortion tolerance is tight.

A weld test in which a bead is deposited on the surface of a plate without making a joint. Used to evaluate electrode performance, determine optimum welding parameters, assess weld metal chemical composition, and check arc stability. The cross-section of a bead-on-plate weld reveals penetration depth, fusion line geometry, and HAZ extent for a given set of welding conditions.

The filler metal used in brazing, having a liquidus above 450 °C (840 °F) and below the solidus of the base metal. AWS A5.8 classifies braze filler metals by composition group: BAg (silver-base), BAu (gold-base), BCu (copper-base), BCuP (copper-phosphorus), BNi (nickel-base), and others. The filler metal is selected based on service temperature, base metal compatibility, joint gap, and required strength.

A relatively thick layer of weld metal applied by surfacing for the purpose of providing a corrosion-resistant or wear-resistant surface. Strip cladding (SAW or ESW process variant) deposits wide, shallow beads of stainless steel or nickel alloy on pressure vessel shells efficiently. Weld overlay cladding must achieve a minimum dilution-adjusted alloy content (typically 2.5–5 mm minimum thickness after machining) to meet corrosion resistance requirements.

A surface discontinuity in weld metal in which the molten pool failed to fuse with the base metal or a previous weld pass at a toe or edge, forming a smooth, curled, unfused overlap. Cold laps are similar to overlap but specifically describe the condition where the weld metal rolled over without fusing. They are detectable by VT or PT/MT and are a defect under all structural codes.

The chemical analysis of deposited weld metal, expressed as weight percentages of each alloying element. Weld metal composition results from the combined contribution of the filler metal and diluted base metal. Composition is controlled to meet the A-Number classification (ASME Section IX), the corrosion-resistance requirements of the service, or the mechanical property requirements of the design code. Composition is verified by chemical analysis of an all-weld-metal test specimen.

A standardised test for assessing the cold cracking susceptibility of a steel in a restrained fillet weld joint. The CTS test uses a specific plate geometry and fillet weld configuration that subjects the HAZ to a defined level of restraint while providing the ability to measure cracking. Results guide preheat requirements for production welding. Similar tests include the Implant test and the Tekken (Y-groove) test for crack susceptibility evaluation of specific base metal–consumable–preheat combinations.

A substance used to remove oxygen from the molten weld metal pool to prevent porosity and oxide formation. Common deoxidisers in welding electrodes and wires include manganese, silicon, aluminium, titanium, and zirconium. In GMAW solid wire, silicon and manganese are the primary deoxidisers; their levels are balanced to give adequate deoxidation while controlling weld metal toughness. Higher deoxidiser content produces cleaner, more porosity-resistant weld metal, particularly on rusty or mill-scaled base metal.

The ability of a material to deform plastically without fracturing, measured by elongation (%) or reduction of area (%) in a tensile test. Weld metal ductility is evaluated in all-weld-metal tensile tests during WPS qualification. Adequate ductility (typically minimum 20% elongation for carbon steel welds) is required to allow stress redistribution around notches and to prevent brittle fracture initiation at weld discontinuities under static overload.

A weld joint between two base metals of different nominal composition, P-Number, or metallurgical classification. Common DMW combinations in industry include: carbon steel to austenitic stainless steel (using ERNiCr-3 or E309 filler), ferritic to austenitic stainless, and carbon steel to nickel alloy. DMW design must account for differences in thermal expansion coefficient, carbon migration at elevated temperature, and galvanic potential differences in aqueous service.

A welding process that produces coalescence by heating with a concentrated beam of high-velocity electrons impinging upon the joint. EBW is performed in a vacuum (hard or soft vacuum variants) to prevent beam scattering. The extremely high energy density (up to 10&sup7; W/cm²) produces a very narrow, deep penetrating weld (keyhole mode) with a minimal HAZ, making EBW ideal for heat-sensitive components, precision assemblies, and dissimilar metal joints in aerospace and nuclear applications.

An arc welding process that uses a consumable electrode to produce coalescence between two workpieces in a vertical position. Unlike electroslag welding (which uses a resistance-heated slag bath), electrogas welding maintains an arc throughout. Molding shoes (copper dams) confine the weld pool on both sides. EGW is used for welding thick vertical plates in one pass, such as ship hull panels and structural members. Travel is vertically upward; deposition rates are high.

The progressive, localised, permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point(s) and that may culminate in cracks or complete fracture after a sufficient number of fluctuations. Weld toe geometry (the weld toe notch), weld surface roughness, and residual tensile stress all reduce fatigue life significantly below that of the unwelded base metal. Fatigue design of welded joints follows fatigue classification curves (S-N curves) in AWS D1.1, BS 7608, or IIW recommendations.

A small crack-like discontinuity with only a slight separation of the fracture surfaces. Fissures may be found in weld metal or the HAZ and are often associated with high hydrogen content or solidification cracking. They are particularly difficult to detect by visual examination and typically require magnetic particle testing (MT) or dye penetrant testing (PT) to reveal. In service, fissures can propagate to full cracks under cyclic loading.

A covered electrode in which the flux coating is applied by extrusion, dipping, or wrapping onto the core wire. The coating formulation determines the electrode’s classification, operational characteristics, and the shielding gas and slag composition it produces. Cellulosic coatings (E6010) produce a high-hydrogen, gaseous-shielded weld suitable for root passes in pipelines. Basic (low-hydrogen) coatings (E7018) use calcium carbonate and calcium fluoride, producing minimal hydrogen and excellent toughness properties.

A solid-state welding process that produces coalescence of metals by heating and then applying pressure sufficient to cause permanent plastic deformation at the interface, expelling oxide films and bringing clean metal into intimate contact. Forge welding is the oldest welding process, historically used by blacksmiths. Modern applications include pipeline flash butt welding and some thermite welding variants where forge pressure is applied to the joint.

Steel coated with a layer of zinc by hot-dipping or electrogalvanising to provide cathodic protection against corrosion. Welding galvanised steel presents hazards (zinc oxide fumes requiring adequate ventilation) and technical challenges (porosity from zinc vapourisation, loss of corrosion protection at the weld). AWS Z49.1 requires specific precautions for welding zinc-coated materials; EN ISO 10882-1 governs fume exposure limits.

A GTAW torch component consisting of a porous diffuser element that produces a laminar, uniform shielding gas flow around the tungsten electrode. A gas lens allows the electrode to protrude further from the nozzle (increased electrode extension) without turbulent gas flow, improving shielding quality in difficult-access joints. Gas lenses reduce shielding gas consumption and virtually eliminate porosity caused by turbulent gas flow.

A WPS requirement specifying a maximum heat input per unit length of weld, applied to protect toughness in the HAZ of impact-tested steels, to control grain coarsening in heat-treated alloys (Q&T steels, P91 CrMoV), or to limit HAZ width in sensitisation-prone stainless steels. Heat input limits are expressed as kJ/mm and are calculated from voltage, amperage, travel speed, and the applicable arc efficiency factor for the process.

See Hard Facing. Both spellings are used in industry; AWS A3.0 uses “hard facing” as the standard term. The non-standard single-word variant “hardfacing” is widely used in consumable supplier catalogues and trade literature.

A brazing process in which the heat required is furnished by infrared radiation. Infrared brazing is used for sensitive electronic assemblies where precise, localised heating without contact is required. It provides good temperature control, rapid heating, and minimal disturbance to adjacent components.

Non-destructive examination performed during the welding process rather than only after completion. In-process NDE includes visual examination of each weld pass, interpass temperature measurement, and inter-run liquid penetrant or magnetic particle testing to detect early-stage cracking. In-process controls are particularly important in multi-layer, high-restraint joints where defects in early passes are difficult to access and repair after completion.

A covered electrode in which the flux coating contains iron powder that increases the deposition rate and efficiency. The iron powder in the coating melts and adds to the weld pool alongside the core wire, increasing the electrode’s deposition efficiency to 140–200% (E7024, E7028). Iron powder electrodes require the arc to be maintained in contact with the coating (drag technique) and are used primarily in flat and horizontal positions.

A welding process that produces coalescence of materials by heating with the application of a concentrated coherent light beam impinging upon the joint. LBW can be performed in air unlike EBW, which requires a vacuum. The high energy density enables deep penetration welds with narrow HAZ and low distortion. Applications include automotive body panels, stainless steel tubing, medical devices, and electronics. CO₂ and Nd:YAG (fibre) lasers are the most common types in industrial welding.

The distance from the joint root to the weld toe along a fusion face of a fillet weld. For equal-leg fillet welds, both legs have the same size. For unequal-leg fillet welds, each leg is specified separately. Leg size is the dimension used on engineering drawings to define fillet weld size; it is converted to effective throat for structural strength calculations. The minimum leg size is specified in AWS D1.1 Table 5.8 based on base metal thickness.

The weight of electrode melted per unit time, expressed in kg/h. Melting rate is always greater than deposition rate because not all melted electrode becomes deposited weld metal (stub loss, spatter, and slag account for the difference). Melting rate is a function of welding current and electrode composition; it is used with deposition efficiency to calculate actual deposition rate for productivity estimates.

The structure of a material as revealed by microscopic examination of a polished and etched section. Welding microstructures of interest include: martensite, bainite, ferrite, pearlite, Widmanstätten ferrite, and austenite in steels; columnar and equiaxed dendritic structures in weld metal solidification. HAZ microstructure controls hardness, toughness, and susceptibility to hydrogen cracking and is the primary output characterised during WPS qualification macro and hardness testing.

The lowest temperature to which the base metal in the vicinity of the weld must be raised and maintained prior to and during welding. Minimum preheat is determined by the carbon equivalent of the steel, the hydrogen content of the consumable, the base metal thickness (thermal mass), and the level of restraint. It is specified in the WPS as an essential variable under most codes; a temperature below the minimum preheat is not permissible without requalification.

A welding technique in which thick-section joints are prepared with a very narrow groove (typically 8–16 mm width) and filled using GMAW, SAW, or GTAW with specially adapted torches or wire guides. Narrow-gap welding significantly reduces the total volume of weld metal required, decreasing heat input, distortion, and consumable cost for sections above approximately 50 mm. Joint geometry and inter-run inspection are critical quality control points.

The international standard governing materials requirements for equipment used in H₂S-containing (sour) oil and gas production environments. It specifies maximum hardness limits for weld metal and HAZ (typically 248 HV Vickers or 22 HRC Rockwell C for carbon and low-alloy steels) to prevent sulphide stress cracking (SSC). PWHT is a key mitigation measure for weld joints in sour service to reduce HAZ hardness below the threshold. See our sour service guide.

Automatic or mechanised GTAW in which the welding head rotates (orbits) around a fixed pipe or tube joint. Orbital welding is used in pharmaceutical, semiconductor, food processing, and power plant applications where weld quality and consistency requirements are extremely high and manual welding variation is unacceptable. The orbital welding head controls all arc parameters (current, voltage, travel speed, wire feed) through a programmable controller for each angular segment of the joint.

A dimensional distortion in which a welded structure deviates from its specified right-angle geometry due to differential thermal contraction during welding. Also called angular distortion or racking. Out-of-square is particularly problematic in structural box sections and flanged components. It is controlled by pre-setting (pre-positioning members to the opposite distortion direction before welding), clamping, or corrective post-weld straightening.

The mechanical working of metals using impact blows. In welding, inter-pass peening of weld beads can reduce tensile residual stress, refine grain structure, and reduce distortion. However, peening is generally prohibited on root passes (risk of introducing cracks) and cap passes (may mask surface discontinuities). Peening is not a substitute for proper heat treatment in reducing HAZ hardness or hydrogen levels.

Welding operations on circular cross-section hollow members (pipes or tubes). Pipe welding is distinguished from plate welding by the requirement for the welder to weld in multiple positions around the circumference within a single weld. Fixed-position pipe welds (5G, 6G) require the welder to continuously change technique and electrode angle as they progress around the joint. Pipe welding qualifications are among the most demanding performance qualifications under ASME Section IX.

A thermal cutting process in which metals are severed by melting a localised area with a constricted arc and removing the molten material with a high-velocity jet of hot ionised gas (plasma). PAC can cut all electrically conductive metals including stainless steel and aluminium (which cannot be cut by oxyfuel processes) and produces a narrower kerf and cleaner cut than oxyfuel cutting. Water injection and water shroud variants further improve cut quality.

A group of welding processes that produce coalescence using the heat obtained from resistance to electric current flow in a circuit of which the workpieces are a part, and by the application of pressure. Resistance welding processes include spot welding (RSW), seam welding (RSEW), projection welding (RPW), and flash welding (FW). RSW is the dominant joining method in automotive body manufacturing, producing thousands of spot welds per vehicle.

A groove or depression in the root bead of a single-side weld, caused by the bead face on the root side being below the surrounding base metal surface. Root concavity reduces the effective throat of the root pass and creates a notch that concentrates stress at the root. It is caused by insufficient filler metal, too-fast travel speed, or inadequate back purging in GTAW root passes. Most codes specify maximum acceptable root concavity depth.

An inward-facing angle or concavity in a weld joint or weldment cross-section that creates a stress concentration point. Added to AWS A3.0 in the 2020 edition to address geometrical features common in additive manufacturing and complex fabricated structures. Reentrant angles at weld toes, weld roots, or structural transitions are primary fatigue crack initiation sites and are controlled by weld toe dressing (grinding, TIG dressing) or design modification.

A group of joining processes that produce coalescence by heating to a suitable temperature and using a filler metal (solder) having a liquidus not exceeding 450 °C (840 °F). Soldering relies on capillary action to draw the molten solder into the joint gap. The base metal is not melted. Applications include electronics circuit boards, plumbing (lead-free solder), and jewellery. Distinguished from brazing solely by the liquidus temperature threshold of the filler metal.

A type of weld bead made with little or no transverse oscillation (weaving). The electrode is moved straight along the joint axis without side-to-side movement. Stringer beads minimise heat input per bead, HAZ width, and interpass temperature buildup. They are preferred for multi-pass welds in toughness-critical applications (offshore structures, low-temperature pressure vessels) and for austenitic stainless steels where minimising heat input reduces sensitisation risk.

A weld made between or upon overlapping members in which coalescence may start and occur on the faying surfaces or may proceed from the outer surface of one member. In resistance spot welding (RSW), the weld nugget forms at the interface of the two sheets. In arc spot welding, the arc burns through the top sheet to fuse to the lower sheet. Spot weld quality is evaluated by peel test, chisel test, or cross-tension test per AWS D8.1.

A welding procedure specification qualified by an organisation other than the manufacturer (such as the AWS or Lincoln Electric), backed by published test data meeting the requirements of ASME Section IX. An SWPS may be used by a manufacturer or contractor without conducting their own PQR tests, provided the SWPS conditions are met and the manufacturer assumes responsibility for the welds. AWS B2.1 series SWPSs are the most widely used.

A group of coating processes in which finely divided metallic or non-metallic materials are deposited in a molten or semi-molten condition to form a coating. The coating material may be in the form of powder, rod, or wire. Processes include flame spraying (FS), arc spraying (ASP), plasma spraying (PSP), and high-velocity oxyfuel spraying (HVOF). Unlike fusion welding, the base metal is not melted; thermal spray coatings are mechanically (not metallurgically) bonded to the substrate.

A non-standard but widely used colloquial term for Gas Tungsten Arc Welding (GTAW). TIG stands for Tungsten Inert Gas. The AWS standard term is GTAW, but TIG remains universally understood in shops and on job sites. AWS A3.0 lists TIG as a non-preferred term and directs users to GTAW as the standard designation.

A welding process that produces coalescence of metals by heating with superheated liquid metal from a chemical reaction between a metal oxide and aluminium, with or without the application of pressure. The thermit reaction (between aluminium powder and iron oxide) generates temperatures of approximately 2,500 °C, producing molten steel that flows into a mould around the joint. Thermit welding is widely used for joining railway rails in the field.

The maximum engineering stress a material can sustain in a tensile test before necking begins, calculated as the maximum load divided by the original cross-sectional area. In weld procedure qualification, reduced-section tensile specimens must fracture at a load equivalent to at least the UTS of the base metal. If fracture occurs in the weld metal below the base metal UTS, the procedure qualification fails under ASME Section IX QW-153 (unless the code allows weld metal fracture above 95% of base UTS for certain applications).

See Ultrasonic Testing. The terms “examination” and “testing” are used interchangeably in most welding codes, though ASME Section V and many engineering standards prefer “examination” (NDE) rather than “testing” (NDT) to avoid confusion with mechanical testing. ASME Section V Article 4 governs ultrasonic examination procedures, equipment qualification, and personnel certification for pressure equipment.

A three-dimensional (blob-like) weld discontinuity such as porosity or a slag inclusion, in which all three dimensions are of comparable magnitude. Volumetric discontinuities are generally less severe than planar (two-dimensional) discontinuities such as cracks or incomplete fusion for the same cross-sectional area, because they do not present a sharp crack-like geometry. Codes typically accept larger total projected areas of volumetric discontinuities than of planar discontinuities.

A groove weld preparation in which both sides of the joint are given a V-bevel (one V-bevel on each member), producing a joint with a V groove on both the face and root sides of the workpieces. A double-V preparation reduces total weld metal volume by approximately 50% compared with a single-V preparation of the same joint thickness, and reduces distortion by welding from alternating sides. Requires access to both sides of the joint.

GMAW and related processes in which the power source uses a precisely programmed output waveform (rapid variations in voltage and current as a function of time) to control metal transfer mode, arc stability, and heat delivery in ways not achievable with conventional CV or CC output. Examples include Lincoln Electric’s Surface Tension Transfer (STT), Miller’s Regulated Metal Deposition (RMD), and Fronius CMT (Cold Metal Transfer). These technologies enable low-spatter, low-heat-input root pass welding in all positions.

In pressure vessel and piping design, a factor (E) applied to the allowable stress of the base metal to account for the potential that a welded joint may have lower integrity than the unwelded base metal. A joint efficiency of 1.0 (full radiography) indicates that the weld is treated as equivalent to the base metal. Lower efficiency values (0.85, 0.70) reflect spot-radiographed or visually-inspected joints. Joint efficiency directly determines the required wall thickness of a pressure vessel shell per ASME Section VIII UG-27.

Intergranular corrosion in austenitic stainless steels occurring in the HAZ as a result of sensitisation (chromium carbide precipitation at grain boundaries during welding). The term “weld decay” is a colloquial rather than AWS A3.0 standard term for what is correctly described as sensitisation-induced intergranular corrosion. It typically appears as a band of corrosion on either side of the weld. See our dedicated stainless steel weld decay guide.

The order in which individual weld beads are deposited to complete a weld joint. Pass sequence is specified in the WPS and affects: interpass temperature accumulation, residual stress distribution, distortion, hydrogen diffusion, and the tempering effect of subsequent passes on the HAZ of prior passes. A carefully planned sequence can use the tempering effect of each pass to improve HAZ toughness in hardenable steels without requiring PWHT.