Welding Grade P91Steel: Consumable Selection as per ASME

No material in ASME pressure vessel fabrication demands more precision in consumable selection and procedure control than Grade 91 chrome-moly steel. Specify the wrong electrode. Miss the PWHT window by 40°C. Start post-weld heat treatment before the weld has fully cooled. Any of these seemingly minor deviations can produce a joint that looks perfect on radiographic examination — and fails catastrophically by creep within years of entering service.

SFA-5.5 and SFA-5.28 were both revised in the 2025 edition of ASME BPVC Section II Part C. This article extracts every P91-specific requirement from the specifications and their Annex A guidance — including details that most electrode data sheets and welding procedures omit.

What Is Grade 91 Steel and Why Is It Difficult to Weld?

Grade 91 (SA-335 P91 pipe, SA-387 Grade 91 plate, SA-213 T91 tube) is a 9% chromium, 1% molybdenum steel modified with deliberate additions of vanadium (0.18–0.25%), niobium/columbium (0.06–0.10%), and nitrogen (0.03–0.07%). These microalloying additions form fine, thermally stable VN, V(C,N), and Nb(C,N) precipitates that pin grain boundaries and resist coarsening at elevated temperature — the mechanism responsible for P91’s superior long-term creep strength compared to unmodified 9Cr steels.

The welding difficulty arises from two intrinsic material properties. First, P91 transforms to martensite on cooling from the austenitising temperature — it air-hardens. This means the weld metal and HAZ are fully martensitic after welding, with hardness potentially exceeding 400 HV — far above the 250 HV maximum permitted by most piping codes. Second, the Ac1 lower transformation temperature of P91 weld metal is comparably low (approximately 820–830°C for B91 weld metal) — only 30–90°C above the required PWHT temperature range. This narrow margin means any thermocouple calibration error or temperature overshoot risks catastrophic re-austenitisation.

ASME SFA-5.5: Complete P91 Electrode Classification

ASME SFA-5.5 (Specification for Low-Alloy Steel Electrodes for Shielded Metal Arc Welding) classifies three P91 SMAW electrode designations. Per SFA-5.5 Annex A7.1.2.6, these were formerly classified as E90XX-B9 and redesignated E90XX-B91 to better conform to industry standards and practices — the material and chemical requirements are unchanged.

Weld Metal Chemistry — What SFA-5.5 Table 2 Actually Specifies

The chemistry of E9015/9016/9018-B91 weld metal is defined in SFA-5.5 Table 2. The most operationally important requirements are not the Cr and Mo limits — it is the combination of limits on microalloying elements (V, Nb, N) and the combined Mn+Ni restriction (Table 2, Note g).

The Mn+Ni Combined Restriction — Table 2 Note g Explained

SFA-5.5 Table 2 Note g states that for B91 classifications, the combined Mn + Ni content must not exceed 1.00 wt%. This restriction exists for a precise metallurgical reason explained in Annex A7.1.2.6:

“The combination of Mn and Ni tends to lower the Ac1 temperature to the point where the PWHT temperature approaches the Ac1, possibly causing partial transformation of the microstructure. By restricting the Mn + Ni, the PWHT temperature will be sufficiently below the Ac1 to avoid this partial transformation.”

— ASME SFA-5.5 Annex A7.1.2.6, 2025 Edition

In practical terms: if an electrode manufacturer supplies B91 electrodes with Mn = 0.90% and Ni = 0.60% (both within individual limits but combined = 1.50%), the actual Ac1 of the resulting weld metal may drop to 790–800°C — putting it within or below the PWHT temperature range. PWHT at 790°C would then partially re-austenitise the weld metal, which transforms to fresh untempered martensite on cooling, with hardness potentially exceeding 450 HV and near-zero creep ductility.

SFA-5.28: ER90S-B91 Bare Wire for GTAW and GMAW

ASME SFA-5.28 (Specification for Low-Alloy Steel Filler Metals for Gas Shielded Arc Welding) classifies ER90S-B91 as the matching bare wire for P91 GTAW (TIG) and GMAW applications. The chemistry requirements are essentially identical to E9015-B91 weld metal, with the same Mn+Ni restriction applied.

SFA-5.28 also classifies ER90S-B91C (with higher C range), ER90S-B91CMn (with higher Mn for creep strength enhancement), and ER90S-B92 (for Grade 92 base metal). For standard P91 GTAW pipe root passes, ER90S-B91 is the standard specification. The mechanical property requirements after PWHT are: minimum tensile strength 90,000 psi (620 MPa), minimum yield strength 60,000 psi (410 MPa), minimum elongation 16%.

PWHT Requirements — The Most Critical Operation in P91 Fabrication

PWHT for P91 is not simply a stress-relief operation. It is a full tempering treatment that converts the hard as-welded martensite (400+ HV) into tempered martensite with the required combination of creep strength, toughness, and hardness (<250 HV). Every parameter of the PWHT cycle matters.

Pre-Weld Requirements: Preheat and Hydrogen Control

P91 requires minimum 200°C (392°F) preheat before welding begins — not just before the first pass. This temperature must be maintained throughout welding (interpass temperature minimum of 200°C by most codes). The preheat serves three purposes for P91 specifically:

1. Hydrogen diffusion: Slows cooling, extending time for hydrogen to diffuse out before the weld cools to martensite transformation temperatures where hydrogen-induced cracking risk peaks.
2. Austenite stability: Slows transformation rate, preventing quench-cracking as the high-Cr weld metal transforms to martensite on cooling.
3. Thermal gradient reduction: Reduces stress concentration at the weld/HAZ interface by minimising temperature differential between the weld zone and surrounding material.

WPS Documentation: What Must Be Recorded for P91

For P91 welding under ASME Section IX, the WPS must record more than the standard minimum. The following are essential variable requirements that are frequently missed in P91 procedure documentation:

• SFA specification: SFA-5.5 (SMAW) or SFA-5.28 (GTAW/GMAW) — with 2025 edition noted
• AWS classification: E9015-B91 or E9016-B91 or E9018-B91 — full classification including B91 (not the old B9 unless edition year is pre-2025)
• Supplemental designator: H4 minimum — H4R recommended — this MUST appear on the WPS per QW-404.12
• F-Number: F-4 (SFA-5.5 low-hydrogen covered electrode) or F-6 (SFA-5.28 bare wire)
• A-Number: A-4 (2.25–10.5% Cr) for B91 weld metal per QW-442
• Preheat range: 200–300°C (record both minimum and maximum interpass)
• PWHT range: 730–790°C with hold time — both temperature AND time are essential variables under QW-407
• Shielding gas (GTAW): SFA-5.32 classification for argon — SG-A per SFA-5.32

P91 Dissimilar Joints: The Nickel-Alloy Buffer Layer Strategy

One of the most complex applications involving P91 consumables is the dissimilar joint between P91 and austenitic stainless steel (most commonly 304H or 321H in power plant steam circuits). Direct welding of P91 to austenitic SS with either material’s matching filler creates an interface subject to carbon migration during service — chromium carbides dissolve on the P91 side and precipitate on the SS side, creating a carbon-depleted soft zone adjacent to the P91 fusion boundary.

The industry solution is a nickel-alloy barrier layer, most commonly using ERNiCrFe-7 (Alloy 52M) or ERNiCr-3 (Alloy 82) as a butter deposit on the P91 side before joining to the SS. The nickel-alloy acts as a carbon diffusion barrier due to thermodynamic activity differences. This dissimilar joint is a separate, complex WPS qualification in itself. See our article on Nickel Alloy Consumables Series Part 1 for the ENiCrFe-3 classification details.

The Six Most Common P91 Consumable and PWHT Mistakes