Titanium welding has exactly one overriding requirement that governs every other decision: keep oxygen and nitrogen away from the hot metal. Everything else — consumable selection, filler wire composition, procedure parameters — is secondary to this fundamental constraint. ASME SFA-5.16 Annex A6 states it precisely: titanium must be shielded from the atmosphere until it cools below 430°C. That single sentence defines the entire discipline of titanium GTAW.
This article extracts all classification, shielding, and procedural requirements from SFA-5.16 and its 2025 Annex A guidance — covering ERTi-2 (commercially pure), ERTi-5 (Ti-6Al-4V), the weld colour acceptance chart, and the ASME Section IX P-number and F-number framework.
- ERTi-2 = Commercially pure titanium (CP-Ti Grade 2): oxygen 0.08–0.16%, 62–80 ksi tensile — for chemical plant, heat exchangers, marine
- ERTi-5 = Ti-6Al-4V (6%Al, 4%V): ~130 ksi (896 MPa) tensile — for aerospace, biomedical, high-strength pressure vessels
- Shield ALL hot titanium until below 430°C (800°F) — this is a hard requirement per SFA-5.16 A6.1, not a guideline
- Weld colour chart: Silver = accept | Gold/straw = marginal | Blue = reject | Grey/white powdery = always reject
- Pure argon (SFA-5.32 SG-A, ≥99.995% purity) is the only permitted shielding gas — no CO₂, no N₂, no O₂ additions
- ASME IX P-numbers: P-51 (CP titanium grades) and P-52 (titanium alloys including Ti-6Al-4V) — separate WPS qualification for each
- F-Number: F-51 for all SFA-5.16 titanium filler metals — one PQR qualifies all F-51 classifications for the same P-number group
Why Titanium Demands Total Shielding — The Embrittlement Mechanism
Titanium is classed as a reactive metal alongside zirconium, hafnium, and niobium. At room temperature it is passive and highly corrosion resistant. Above approximately 430°C, the kinetics of oxygen and nitrogen absorption change dramatically — titanium actively absorbs both gases, incorporating them into the titanium crystal lattice as interstitial solute atoms.
The result of oxygen and nitrogen pickup in titanium is not surface rust (as with steel) — it is irreversible interstitial hardening. The dissolved O and N atoms pin titanium’s hexagonal close-packed crystal structure, dramatically reducing ductility. A titanium weld that looked acceptable in the welder’s booth may have been exposed to air during cooling from 650°C to 430°C — the resulting embrittlement is invisible to visual and radiographic inspection yet can cause brittle fracture under service loads. There is no heat treatment that reverses this embrittlement short of full hot isostatic pressing or melting.
The Weld Colour Chart: Your Real-Time Shielding Quality Indicator
The colour of a titanium weld surface after solidification and cooling is the most direct and immediate indicator of shielding quality. Titanium forms TiO₂ oxide films on its surface — and TiO₂ thin films produce interference colours exactly like oil slick colours on water, with the colour depending on film thickness. Thicker films = more oxidation = more oxygen absorption = more embrittlement.
The Three-Zone Shielding System
Achieving consistent silver weld colour requires three simultaneous shielding zones, each protecting a different region of the hot titanium:
| Shield Zone | Purpose | Equipment | Gas | Requirement |
|---|---|---|---|---|
| ① Primary (torch cup) | Shields arc, molten pool, and just-solidified weld metal directly under the torch | Standard GTAW torch with large-diameter gas lens cup (19–25mm recommended) | Argon SG-A, 10–15 L/min | Always required — standard GTAW shielding |
| ② Trailing shield | Shields solidified but still hot weld metal behind the torch as travel continues | Purpose-made trailing shoe or drag shield extending 150–300mm behind torch | Argon SG-A, 5–10 L/min | Required for all production titanium GTAW |
| ③ Back purge / root shield | Shields root bead and inside pipe wall from atmospheric contamination on the reverse side | Argon fill inside pipe section with dams; tape-sealed end caps with inlet/outlet | Argon SG-A, 3–8 L/min; purge to O₂ <50 ppm before welding | Required for all pipe and tube joints; verify O₂ level with analyser |
SFA-5.16 Classification Guide: ERTi-2 vs ERTi-5 and the Full Family
| Classification | Alloy Group | Composition | Min UTS | Base Metal Target | Primary Application |
|---|---|---|---|---|---|
| ERTi-1 | Group 01 (0100) | CP-Ti, O: 0.03–0.08% | ~62 ksi (425 MPa) | ASTM Grade 1 CP-Ti | Lowest strength; maximum ductility; cryogenic |
| ERTi-2 | Group 01 (0120) | CP-Ti, O: 0.08–0.16% | ~70 ksi (480 MPa) | ASTM Grade 2 CP-Ti | Most common CP-Ti; chemical plant; marine; heat exchangers |
| ERTi-3 | Group 01 (0125) | CP-Ti, O: 0.13–0.20% | ~80 ksi (550 MPa) | ASTM Grade 3 CP-Ti | Medium strength CP-Ti; structural chemical service |
| ERTi-4 | Group 01 (0130) | CP-Ti, O: 0.18–0.32% | ~95 ksi (655 MPa) | ASTM Grade 4 CP-Ti | Highest strength CP-Ti; structural applications |
| ERTi-5 | Group 64 (6402) | Ti-6Al-4V | ~130 ksi (896 MPa) | ASTM Grade 5 Ti-6Al-4V | Aerospace; biomedical; high-strength pressure vessels |
| ERTi-5ELI | Group 64 (6408) | Ti-6Al-4V-ELI (low O, N, Fe) | ~120 ksi (827 MPa) | Grade 23 Ti-6Al-4V ELI | Biomedical implants; cryogenic; fracture critical aerospace |
| ERTi-7 | Group 24 (2401) | CP-Ti + 0.12–0.25% Pd | ~70 ksi (480 MPa) | Grade 7 Ti-Pd | Reducing acid environments; seawater; HCl service |
ASME Section IX P-Numbers and F-Numbers for Titanium
| Classification | ASME IX Base Metal Group | F-Number | WPS Qualification Scope | Notes |
|---|---|---|---|---|
| ERTi-1, ERTi-2, ERTi-3, ERTi-4 | P-No. 51 (CP titanium) | F-51 | Qualifies all F-51 fillers for P-51 base metal | CP titanium grades qualify together under P-51 |
| ERTi-5, ERTi-5ELI, ERTi-23 | P-No. 52 (titanium alloys) | F-51 | Qualifies all F-51 fillers for P-52 base metal | P-52 requires separate qualification from P-51 |
| ERTi-7, ERTi-9 (Pd/Ru bearing) | P-No. 51 | F-51 | Qualifies for P-51 base metal group | Palladium alloys group with CP titanium |
WPS Documentation Checklist for Titanium GTAW
- SFA Specification: SFA-5.16 (note edition year)
- AWS Classification: ERTi-2, ERTi-5, etc. — full designation
- F-Number: F-51 (all SFA-5.16 classifications)
- Base metal P-number: P-51 (CP titanium) or P-52 (titanium alloys) — separate WPS for each
- Shielding gas: SFA-5.32, SG-A (argon ≥99.995%) — specify both primary and trailing shield
- Back purge: SFA-5.32, SG-A — specify flow rate and maximum O₂ concentration (<50 ppm)
- Shielding temperature limit: State explicitly — “maintain gas shielding until weld cools below 430°C (800°F) per SFA-5.16 A6.1”
- Weld colour acceptance: State criterion — e.g., “silver to light straw acceptable; blue or grey unacceptable”
- Preheat: None required — state “ambient temperature; no preheat”
Frequently Asked Questions
What is the difference between ERTi-2 and ERTi-5 titanium welding wire?
ERTi-2 per SFA-5.16 is commercially pure titanium (CP-Ti, Group 01) with oxygen content 0.08–0.16%. It matches Grade 2 titanium base metal and is used for chemical plant equipment, heat exchangers, and marine applications where maximum corrosion resistance and ductility are required. ERTi-5 is Ti-6Al-4V (6% aluminum, 4% vanadium, Group 64 alloy) with minimum tensile strength around 130 ksi (896 MPa) — used for aerospace structures, biomedical implants, and high-strength pressure vessel applications. They cannot be interchanged — ERTi-5 is not suitable for pure titanium base metal due to alloy mismatch.
Why must titanium be shielded until it cools below 430°C during welding?
Per SFA-5.16 Annex A6.1, titanium is sensitive to embrittlement by oxygen, nitrogen, and hydrogen at elevated temperatures. At temperatures above approximately 430°C (800°F), titanium will absorb oxygen and nitrogen from air at a rate that irreversibly embrittles the weld metal and HAZ. Unlike steel, where surface oxidation is mostly cosmetic, oxygen and nitrogen dissolved in titanium dramatically raise hardness and reduce ductility — there is no recovery without full solution annealing. Trailing shields (gas coverage extending behind the torch) and back-purge covers must maintain inert gas protection until the metal cools below 430°C.
What does weld colour indicate about titanium weld quality?
Titanium weld colour is a direct indicator of surface contamination and gas shield effectiveness. The industry-standard colour acceptance chart: bright silver = excellent shielding, fully acceptable; light straw/gold = light oxidation, acceptable for many applications with examination; dark straw/brown = moderate oxidation, marginal — evaluate application; blue = significant oxidation, generally rejected for structural/aerospace applications; grey/white powdery = severe contamination, always rejected. The colour comes from oxide film thickness on the titanium surface — exactly analogous to temper colours on steel but with much lower acceptance thresholds.
What P-number and F-number do titanium consumables have in ASME Section IX?
Titanium base metals are grouped as P-No. 51 (commercially pure titanium grades) and P-No. 52 (titanium alloys including Ti-6Al-4V). SFA-5.16 titanium filler metals are F-Number 51 in ASME Section IX QW-432. This means a WPS qualified with any SFA-5.16 F-51 filler metal qualifies all other F-51 filler metals for the same P-number base metal group. CP titanium (P-51) and Ti-6Al-4V (P-52) are different P-number groups — a separate WPS qualification is required for each.
What shielding gas is used for titanium GTAW welding?
Per SFA-5.16 Annex A6.1, pure argon (SFA-5.32 classification SG-A, argon ≥99.995% purity — welding grade) is the standard shielding gas for titanium GTAW. Helium or argon-helium mixtures may be used for improved penetration on thicker sections. CO₂, nitrogen, and oxygen-containing shielding gases must never be used for titanium — these reactive gases cause immediate contamination and embrittlement. Purity is critical: even small amounts of moisture or oxygen in the shielding gas cause contamination.
Is preheat required for titanium welding?
No preheat is required or recommended for titanium welding. Titanium does not transform to martensite on cooling and is not susceptible to hydrogen-induced cracking (the primary reason for preheat on steels). The metallurgical concern for titanium is entirely different — contamination by atmospheric gases rather than hydrogen cracking. The key thermal control is the post-weld shielding until below 430°C, not pre-weld preheat.
Can ERTi-5 (Ti-6Al-4V) wire be used to weld commercially pure titanium Grade 2 base metal?
Generally not recommended. ERTi-5 (Ti-6Al-4V) filler wire has significantly higher strength (130+ ksi / 896 MPa) than ERTi-2 (CP titanium, ~60–80 ksi / 425–550 MPa). Using ERTi-5 on CP titanium base metal creates a strength overmatch at the weld zone and introduces Al+V alloying elements foreign to the base metal chemistry. For CP titanium base metal, ERTi-2 matching wire provides the correct composition, ductility, and corrosion resistance. ERTi-5ELI (extra low interstitials, SFA-5.16) is preferred for cryogenic and biomedical applications of Ti-6Al-4V where maximum ductility and toughness are required.
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