When welding these two materials the P-91 material will govern when it comes to pre-heat and PWHT. When we weld P5B (P91) to P5A (P22) materials with the GTAW process, we require ER90S-B9 SFA 5.28, for SMAW we use E9016-B9, or E9018-B9 SFA 5.5. For P5B material we require that only GTAW be used for the root pass with a backpurge gas of Argon for a least the first two passes.

Pre-heat for these two materials is going to depend on what code you are using (i.e, ASME Section I, B31.1). We require a minimum pre-heat of between 430 and 450 degrees F which is above what code requires, and a max interpass temperature of 600 degrees F. Pre-heat must be maintained throughout the welding process. If welding is interrupted for any reason, the weld and adjacent area will be uniformly heated from the pre-heat temperature to 600-700 degrees F for one hour then wrapped with insulation and allowed to cool. We then require NDE on the joint before welding on the part again. We also require that electric resistance pre-heat be used. We do not allow open flame pre-heat for P5B materials.

For PWHT, we require a temperature range of 1375 – 1425 Degrees F. we require a ramp up of 200 degrees per hour and a ramp down of 200 degrees per hour. Soak time will be dependent on thickness. We require a 2-hr min. for up to 2 inches.

Hope this helps and good luck.