PWHT for CrMo Pipe Fittings: Temperature Ranges by Grade
Post-weld heat treatment is mandatory for all CrMo alloy steel pipe fittings — without exception. The temperature range varies by grade, and getting it wrong permanently damages the material.
Why PWHT Is Mandatory
CrMo steels transform to martensite in the heat-affected zone during welding. Weld martensite is hard (potentially >350 HBW), brittle, and susceptible to hydrogen-induced cracking. PWHT tempers the martensite: it restores toughness, relieves residual stress, and creates the stabilised microstructure required for long-term creep service. Without PWHT, the fitting will crack — either immediately during cool-down or in early service.
PWHT Temperature Ranges by Grade
| Grade | EN W.Nr. | PWHT Temp (°C) | Min. Hold Time |
|---|---|---|---|
| P11 / 1¼Cr-½Mo | 1.7335 | 690–730 | ≥1h/25mm, min 1h |
| P22 / 2¼Cr-1Mo | 1.7380 | 700–750 | ≥1h/25mm, min 1h |
| P5 / 5Cr-½Mo | 1.7362 | 720–760 | ≥1h/25mm, min 2h |
| P9 / 9Cr-1Mo | 1.7386 | 720–760 | ≥1h/25mm, min 2h |
| P91 / 9Cr-1Mo-V | 1.4903 | 745–775 | ≥1h/25mm, min 2h |
| P92 / 9Cr-2W | 1.4901 | 750–780 | ≥1h/25mm, min 2h |
Common PWHT Errors
- Temperature too low: Retained hard martensite, HAZ hardness exceeding 265 HBW — the most common non-conformance on CrMo weld inspection.
- Exceeding the maximum: For P91 and P92, the Ac1 transformation temperature is approximately 800°C. Exceeding the PWHT upper limit risks re-austenitising part of the HAZ, which then re-transforms to fresh martensite on cooling — worse than no PWHT at all.
- Using P91 procedure on P92: The temperature windows overlap (745–775 vs 750–780), but P91's ER90S-B9 filler is incompatible with P92 base metal. Separate WPS qualification is mandatory.
- No chart recording: PWHT without thermocouple attachment and chart recording is not accepted under any major code. The chart is a permanent QC document.
Preheat Requirements
All CrMo grades require preheat before welding and maintenance of inter-pass temperature throughout: P11/P22 — typically 150–200°C; P5/P9 — 175–230°C; P91/P92 — 200–250°C. The joint must not be allowed to cool below preheat temperature during the welding sequence.