P91 Steel Welding — Preheat 200°C, Interpass ≤300°C & Mandatory PWHT 730–760°C
P91 steel — full designation ASME SA-335 Grade P91, composition 9% Chromium – 1% Molybdenum – Vanadium — is a creep-resistant alloy used in main steam lines, hot reheat piping, and headers in thermal power plants, as well as in high-pressure process piping in refineries. Its tempered martensite microstructure with finely dispersed carbides enables service at up to 600–625°C under high pressure. That same microstructure makes P91 one of the most demanding steels to weld and one of the most unforgiving of temperature control errors on site.
Why P91 is More Difficult to Weld Than Carbon Steel
During welding, the heat-affected zone (HAZ) and weld metal of P91 undergo a complex phase transformation sequence. As the joint heats and cools, the austenite structure transforms to martensite — a hard, brittle phase. The critical transformation temperatures for P91 are:
- Martensite start (Ms): approximately 400°C
- Martensite finish (Mf): approximately 200°C
This means the joint must be cooled below 200°C — close to ambient temperature — before PWHT is applied. The weld cannot be taken directly from the welding temperature into the PWHT furnace.
Three Mandatory Temperature Parameters for P91 Welding
1. Minimum Preheat: 200°C (392°F)
ASME B31.1 and B31.3 require a minimum preheat of 200°C (392°F) for P91 (P-Number 5B). The purpose of preheat for P91 is not only to prevent hydrogen-induced cold cracking — it is also to reduce the thermal gradient between the weld zone and the surrounding base metal, which is especially important because P91 has lower thermal conductivity than plain carbon steel.
Tempilstik® part number for preheat verification: #28327 (392°F / 200°C).
Measure at a minimum of 4 evenly distributed points around the full pipe circumference, at least 75 mm from the weld edge on each side. A single-point check is not acceptable.
2. Maximum Interpass Temperature: 300°C (572°F)
While preheat is a minimum threshold, interpass temperature is a maximum limit that must not be exceeded. For P91, the standard maximum interpass temperature is ≤ 300°C (572°F).
If the surface temperature in the weld zone exceeds this limit before depositing the next pass, welding must stop and the joint must be allowed to cool. Excessive interpass temperature causes austenite grain coarsening in the HAZ, reducing Charpy impact toughness — P91 welds in main steam systems must meet strict impact energy requirements per ASME.
Tempilstik® for interpass maximum check: #28047 (600°F / 316°C).
This crayon melts at 316°C. If it liquefies on contact, the surface has exceeded 300°C — stop welding, allow natural cooling, re-measure before continuing.
3. Cool Below 200°C Before PWHT, Then PWHT at 730–760°C (1350–1400°F)
After the weld is complete and the joint has cooled below 200°C (to allow complete martensite transformation), PWHT must be performed in the range 730–760°C (1350–1400°F). Minimum soaking time per ASME: 1 hour per 25 mm of wall thickness, minimum 1 hour regardless of thickness.
PWHT is performed using electric resistance heating or a furnace, with thermocouples continuously recording temperature via a data logger. Tempilstik® is used for supplementary spot-check confirmation at locations not directly covered by thermocouples:
- #28065 (1450°F / 788°C) — confirms surface has exceeded 788°C at spot-check locations during the heating phase; when melted uniformly at all check points, the mass is above the PWHT lower bound
- #28057 (1022°F / 550°C) — intermediate checkpoint during heating for P22 and supplementary check for P91
Tempilstik® supplements the thermocouple record — it does not replace it. The thermocouple data logger is the mandatory primary QA document for PWHT.
Summary Table — P91 Temperature Requirements & Part Numbers
| Stage | Requirement | Temperature | Tempilstik® Part No. | Applicable Standard |
|---|---|---|---|---|
| Preheat | Minimum | 200°C / 392°F | #28327 | ASME B31.1, B31.3 |
| Interpass | Maximum — must not exceed | 300°C / 572°F | #28047 (melts at 316°C) | ASME B31.1, B31.3, EN 1011-2 |
| Pre-PWHT cool-down | Must reach below this point | < 200°C / 392°F | #28327 (used as indicator) | Metallurgical requirement |
| PWHT — lower bound | Minimum soak temperature | 730°C / 1346°F | #28065 (788°C spot-check) | ASME B31.1 Table 132 |
| PWHT — upper bound | Maximum — must not exceed | 760°C / 1400°F | Thermocouple data logger | ASME B31.1 Table 132 |
Consequences of Non-Compliance — Documented Cases
Creep failure in P91 welds is a well-documented failure mode that has occurred at multiple power plants worldwide, particularly from the early 2000s when P91 adoption expanded rapidly but welding and PWHT procedures were not yet fully standardized. Failure typically initiates at the CG-HAZ (Coarse Grain HAZ) and FG-HAZ (Fine Grain HAZ) — zones that develop a weak "Type IV" microstructure when PWHT is performed below the minimum temperature or for insufficient duration.
Documented contributing factors to premature P91 creep failure:
- PWHT performed below 730°C — carbides do not dissolve and redistribute correctly
- Insufficient soaking time — residual stress not fully relieved
- Excessive interpass temperature — coarse austenite grains, reduced HAZ toughness
- Joint not cooled below 200°C before PWHT — martensite transformation incomplete
- Repair welds performed without full re-PWHT of the entire joint
Applicable Standards for P91 Welding
- ASME B31.1 (Power Piping) — Table 132.1 for preheat; Table 132 for PWHT by P-Number and wall thickness
- ASME B31.3 (Process Piping) — Table 330.1.1 for preheat; Table 331.1 for PWHT
- EN 1011-2:2001 — Annex B and Section 8 for preheat by CET method; Section 9 for PWHT — applicable on European-specification EPC projects
- ASME Boiler and Pressure Vessel Code, Section I — for boiler applications
- AWS D10.10 — Local heat treatment of welds in piping — supplementary reference for field heating procedures
See also: PWHT — Post Weld Heat Treatment: full ASME requirements and Tempilstik® role.
Frequently Asked Questions
Need Tempilstik® #28327, #28047, or #28065 for P91 projects?
Fast Group Engineering — authorized Tempil® distributor in Vietnam. Direct import from USA, C/O + C/Q + VAT invoice. Individual crayons or box of 10. Stock in HCMC and Vung Tau.
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