Oct 6, 2026
What Welding Wire Is Used for P91 Steel? ER90S-B9 vs B91
What welding wire is used for P91 steel? Should you use ER90S-B9 or ER90S-B91, and why are preheat and PWHT important?
What Welding Wire Is Used for P91 Steel?
For P91/T91 steel, the typical solid welding wire is ER90S-B9 or ER90S-B91.
These Cr-Mo welding wires are intended for P91/T91 applications where the weld must meet the required strength and high-temperature service requirements.
The correct classification should always be confirmed against the project specification, WPS/PQR, applicable AWS edition, and customer requirements.
ER90S-B9 vs ER90S-B91
ER90S-B9 is widely found in existing P91 welding specifications, WPS documents, and purchasing records. ER90S-B91 is the newer AWS classification.
This does not mean that every B9 specification can simply be changed to B91. Before purchasing, check:
- Applicable AWS specification and edition
- Project requirements
- Approved WPS/PQR
- Customer or EPC requirements
Why Does P91 Welding Need Preheat?
P91 has high hardenability. If the weld cools too quickly, the weld metal and HAZ can develop excessive hardness, increasing the risk of cracking.
Preheat and interpass temperature control help manage the cooling rate and welding thermal cycle.
The required preheat depends on factors such as material thickness, joint restraint, welding process, heat input, applicable code, and the qualified WPS/PQR.
Therefore, a generic preheat value should not replace the project welding procedure.
Why Is PWHT Important for P91?
After welding, P91 can contain hard transformed microstructures and high residual stresses.
PWHT is used to temper the weld and HAZ and reduce residual stresses, helping the joint achieve the required metallurgical and mechanical condition.
Incorrect PWHT can result in excessive hardness, unsuitable microstructure, reduced toughness, or increased service risk.
For P91 welding, PWHT should therefore be controlled according to the applicable code and qualified WPS/PQR.
Can ER80S-B8 Be Used for P91?
Generally, do not assume that ER80S-B8 can replace ER90S-B9/B91.
The usual material-to-wire relationship is:
P9/T9 → ER80S-B8 P91/T91 → ER90S-B9 / ER90S-B91
Although both are Cr-Mo steels, their alloy systems and strength requirements are different. Any substitution should be technically evaluated and qualified.
What If a P91 Weld Cracks or Is Too Hard?
If a P91 weld cracks or shows excessive hardness, changing the welding wire alone may not solve the problem.
Check the complete welding procedure:
Consumable → Preheat → Interpass → Heat Input → Cooling → PWHT → Hardness
Problems can result from inadequate thermal control, hydrogen, excessive cooling, unsuitable heat input, or incorrect PWHT.
FAQ
What welding wire is used for P91 steel?
ER90S-B9 or ER90S-B91 is typically used for P91/T91 steel, subject to the applicable specification and qualified WPS/PQR.
Does P91 welding require preheat?
Yes. Preheat and interpass control are important because P91 has high hardenability and is sensitive to rapid cooling.
Does P91 require PWHT?
For many code-controlled P91 applications, PWHT is required according to the applicable code and qualified welding procedure.
Can B8 replace B9/B91 for P91?
Do not assume so. B8 is normally associated with P9/T9, while B9/B91 is associated with P91/T91.
Conclusion
For P91/T91 steel, ER90S-B9 or ER90S-B91 is the typical solid welding wire choice.
However, successful P91 welding depends on more than consumable selection. Preheat, interpass control, heat input, cooling, and PWHT must all be controlled according to the qualified welding procedure.
