Stainless steel also known as inox, corrosion-resistant steel(CRES), and rustless steel, is an alloy of iron which is resistant to corrosion and rust. It contains iron with chromium, nickel and other elements like molybdenum, carbon, manganese and nitrogen depending on specific grade. Stainless steel’s corrosion resistant property results from presence of chromium 10.5 % or more which forms a passive layer of chromium oxide that protects the material from corrosion. “SS” is common abbreviation used for stainless steel in general.
What is Stainless Steel
Stainless steel is an iron based alloy containing at least 10.5% of chromium. The rich chromium percentage forms a thin layer of chromium oxide on the surface of steel which acts as a protective layer from other elements present in atmosphere, Whenever the chromium-rich oxide layer is damaged due to any reason, a new layer is formed quickly as chromium reacts with oxygen present in atmosphere. Hence, the stainless steel will remain rust-free. However, the rate of development chromium oxide passive film depends on chromium content present.
In addition to chromium, it may contain other elements such as nickel, molybdenum, and manganese, which enhances its other properties. The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength. Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure. Corrosion resistance can be increased further by the following means:
adding molybdenum (which also improves resistance to pitting corrosion
increasing chromium content to more than 11%
adding nickel to at least 8%
The alloy’s properties, such as luster and resistance to corrosion, are useful in many applications. Stainless steel can be rolled into sheets, plates, bars, wire, and tubing. These can be used in cookware, cutlery, surgical instruments, major appliances, vehicles, construction material in large buildings, industrial equipment (e.g., in paper mills, chemical plants, water treatment), and storage tanks and tankers for chemicals and food products. Some grades are also suitable for forging and casting.
Mechanical Properties of stainless Steel
The most common type of stainless steel, 304, has a tensile yield strength around 210 MPa (30,000 psi) in the annealed condition. It can be strengthened by cold working to a strength of 1,050 MPa (153,000 psi) in the full-hard condition.
The strongest commonly available stainless steels are precipitation hardening alloys such as 17-4-PH and Custom 465. These can be heat treated to have tensile yield strengths up to 1,730 MPa (251,000 psi).
Stainless Steel melting point is near that of ordinary steel, ranges from 1,400 to 1,530 °C depending on the grade. SS are relatively poor conductors of electricity, with significantly lower electrical conductivity than copper. Nevertheless, SS connectors are employed in situations where ECR poses a lower design criteria and corrosion resistance is required, for example in high temperatures and oxidizing environments. The density of SS ranges from 7.5 to 8.0 g/cm3 (0.27 to 0.29 lb/cu in) depending on the alloy.
Types of Stainless Steel
SS is classified into five main families that are primarily differentiated by their crystalline structure:
- Austenitic stainless steel – FCC (face-centered cubic)
- Ferritic stainless steel – BCC (body-centered cubic)
- Martensitic stainless steel – BCT (body-centered tetragonal)
- Duplex stainless steel – FCC + BCC i.e. Contains both Austenite and Ferrite
- Precipitation-hardening (PH) stainless steel
The first four i.e. Austenitic, Ferritic, Martensitic, and duplex are categorized as per their crystal structure and if they are strengthened by the precipitation hardening then the product obtained is known as Precipitation-Hardening (PH) stainless steel.
- Austenitic SS containing Chromium and Nickel as the major alloying elements (In addition to Iron) and are identified as AISI 300 Series types.
- Austenitic SS containing Chromium, Nickel, and Manganese as the major alloying elements (In addition to Iron) and are identified as AISI 200 Series types.
- Ferritic SS contain chromium as the major alloying element and are identified as AISI 400 series types.
- Martensitic SS contain chromium as the main alloying element (In addition to Iron and Carbon) and are identified as AISI 400 series types.
1. Austenitic Stainless Steel
Austenitic SS is most commonly used stainless steel all around the world. They posses an austenitic microstructure, which is face-centered cubic (FCC) crystal structure. This microstructure is achieved by adding required amount of nickel, manganese and nitrogen (Austenitic stabilizer) to maintain an austenitic structure. Thus, austenitic stainless steels are not hardenable by heat treatment since they possess the same microstructure at all temperatures and do not require PWHT.
Austenitic stainless steel sub-groups, 200 series and 300 series:
200 series are chromium-manganese-nickel alloys that maximize the use of manganese and nitrogen to minimize the use of nickel. Due to their nitrogen addition, they possess approximately 50% higher yield strength than 300-series stainless sheets of steel.
- Type 201 is hardenable through cold working.
- Type 202 is general-purpose stainless steel. Decreasing nickel content and increasing manganese results in weak corrosion resistance.
300 series are chromium-nickel alloys that achieve their austenitic microstructure by nickel alloying; some highly alloyed grades include some nitrogen to reduce nickel. 300 series is the largest group and the most widely used.
- Type 304: The most common is type 304, also known as 18/8 and 18/10 for its composition of 18% chromium and 8% or 10% nickel, respectively.
- Type 316: The second most common austenitic stainless steel is type 316. The addition of 2% molybdenum provides greater pitting and crevice corrosion resistance.
Weldability of austenitic stainless steel:
Austenitic stainless steels possess higher thermal expansion than ferritic or martensitic stainless steels. Distortion or warping occurs during the welding of austenitic stainless steel due to it’s high coefficient of thermal expansion and low thermal conductivity.
Austenitic stainless steel is susceptible to solidification and liquation cracking. Hence, proper care to be given while selecting filler material and welding process.
Submerged arc welding (SAW) is not preferred when a fully austenitic stainless steel or low ferrite content weld deposit is required.
2. Ferritic Stainless Steel
Ferritic stainless steels has ferrite microstructure like carbon steel, which is a body-centered cubic crystal structure, and contain between 10.5% and 27% chromium with very limited or no nickel.
This microstructure is present at all temperatures due to the chromium percentage, so they are not capable of being hardened by heat treatment. They cannot be strengthened by cold work to the same degree as austenitic stainless steels. They are magnetic. Additions of niobium (Nb), titanium (Ti), and zirconium (Zr) to type 430 allow good weldability.
These steels exhibit good ductility and have good resistance to stress corrosion cracking, pitting, and crevice corrosion.
Types 430, 442, and 446 are referred to as the first-generation ferritic stainless steels. These grades contain mainly chromium as a ferrite stabilizer along with relatively high carbon content. These grade have low toughness and generally require PWHT otherwise intergranular corrosion may occur.
Whereas, Types 405 and 409 are referred to as the second-generation ferritic stainless steels. These grades have lower chromium and carbon content but contain ferrite formers. These steels are also referred to as pseudoferritic because they require other ferrite formers in addition to chromium. They are comparatively less costly, possess good fabrication characteristics, and have useful corrosion resistance than the first-generation ferritic stainless steels but they often possess low toughness.
Weldability of ferritic stainless steel:
Generally, fewer precautions are required during welding because they cannot be hardened by quenching. Hence, the chances of martensite formation are less during the cooling of weld metal. However, Types 430, 434, 442, and 446 are exceptional cases due to the presence of both high chromium and high carbon content. The risk of hydrogen-induced cracking during cooling is more in these alloys especially when welding is carried out under high restraint conditions such as heavy weldments or surfacing welds on carbon steel. To minimize residual stresses that contribute to weld, preheating of 150°C (300°F) or higher can be used.
Chances of Hydrogen embrittlement increases in ferrites stainless steel when martensite is present along ferrite grain boundaries in the weld metal or HAZ. However, Ferritic stainless steels are less susceptible to hydrogen embrittlement if compared to martensitic stainless steel.
The risk of solidification cracking in ferritic stainless steels is comparatively very less because the primary solidification phase is ferrite. However, Alloys with additional alloying elements like titanium and niobium or high impurity levels are more susceptible to solidification cracking.
Preheat and PWHT requirements: for Ferritic SS :
The preheating requirements are determined largely by job thickness, chemical composition, desired mechanical properties, and restraint conditions. Ferritic stainless steels with low chromium or high-carbon content can be preheated within the range of 150°C to 230°C (300°F to 450°F).
PWHT for first-generation ferritic stainless steels (Types 430, 442, and 446) can be conducted at temperatures ranging from 700°C to 840°C (1300°F to 1550°F). These temperature ranges help prevent further grain coarsening.
Whereas, PWHT for Second-generation ferritic stainless steels (Types 405 and 409) can be conducted at higher temperatures up to at least 1040°C (1900°F).
3. Martensitic Stainless Steel
Martensitic stainless steels have a body-centered cubic crystal structure (BCC), and offer a wide range of properties and are used as stainless engineering steels, stainless tool steels, and creep-resistant steels. They are essentially iron-chromium-carbon alloys with a nominal of 11.5% to 18% chromium. They are magnetic, and not as corrosion-resistant as ferritic and austenitic stainless steels due to their low chromium content. It can be hardened and tempered through aging and heat treatment.
Martensitic stainless steel can be transformed into austenite when heated beyond 1010°C (1850°F). However, rapid cooling from this temperature will again result in a martensitic microstructure.
Martensitic stainless steels are used to fabricate a variety of products, for example, Low and medium carbon martensitic stainless steels are typically used in jet engines, steam turbines, and gas turbines. High carbon martensitic stainless steels are used for gears, shafts, cams, ball bearings, and valves, etc.
Weldability of Martensitic stainless steel:
Martensitic stainless steels often produce hardened HAZs, and as the hardness of HAZ increases, it’s toughness decreases, and susceptibility to Hydrogen induced cracking increases. As a general practice, post weld heat treatment (PWHT) is given to martensitic stainless steel welded joints, to improve the weld properties.
Since Martensitic stainless steels are subject to hydrogen-induced cracking hence proper precautions must be taken in the selection of welding process, handling, and storage of the filler metal and cleanliness to avoid hydrogen from entering into the weld metal.
Preheat and PWHT requirements:
Preheat and post weld heat treatment (PWHT) requirements for martensitic stainless steel are given below
| Carbon content (%) | Preheat temperature (minimum) | Requirements for PWHT | |
| °C | °F | ||
| <0.05 | 121 | 250 | Optional |
| 0.05–0.15 | 204 | 400 | Recommended |
| >0.15 | 316 | 600 | Mandatory |
4. Duplex Stainless Steel
Duplex stainless steels (DSS steels) have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60. They are characterized by higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.
Duplex stainless steels have roughly twice the yield strength of austenitic stainless steel. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steel types 304 and 316.
Duplex grades are usually divided into three sub-groups based on their corrosion resistance: lean duplex, standard duplex, and super duplex. The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications.
Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterized by the pitting resistance equivalence number, PREN = %Cr + 3.3 %(Mo+ 0.5W) + 16 %N.
Standard duplex (PREN range: 28–38) : Typically Grade EN 1.4462 (also called 2205). It is typical of the mid-range of properties and is perhaps the most used today.
Super-duplex (PREN range: 38–45) : so-called hyper duplex grades (PREN: >45) developed later to meet specific demands of the oil and gas as well as those of the chemical industries. They offer a superior corrosion resistance and strength but are more difficult to process because the higher contents of Cr, Mo, N and even W promote the formation of intermetallic phases, which reduce drastically the impact resistance of the steel.
Lean duplex grades (PREN range: 22–27) : Typically grade EN 1.4362, have been developed more recently for less demanding applications, particularly in the building and construction industry. Their corrosion resistance is closer to that of the standard austenitic grade EN 1.4401 (with a plus on resistance to stress corrosion cracking) and their mechanical properties are higher.
Weldability of Duplex Stainless Steel
Duplex SS applications are not to be welded with matching filler wire composition or without filler metal because it will result in excessive ferrite and will disturb the ferrite and austenite ratio. Most available duplex filler metals are added with 3-4% with more Nickel to generate more austenite. Minimum Nickel content of filler recommended is 8% for duplex and 9% for super duplex.
Tight chemistry control for parent metal and filler metal shall be specified as minor variations may cause significant effect, especially in HAZ. Parent metal shall be in solution annealed condition.
Nitrogen content of parent metal shall be on upper range of chemical composition to achieve balance austenite – ferrite ratio in HAZ as there is no outside assistance from overmatch filler metal and welding gas.
As most stainless steel, even more so for duplex, post weld heat treatment shall be avoided, alternatively, full solution annealing with cooling rate control may be performed after proper qualification.