Preheat and PWHT of P91

[gerdmil]

For a project under the scope of ASME B31.1, I have been asked by the client to prepare ''preheat and PWHT curves'' for the material P91. Is there anyone having knowledge and experience for such an issue?

[wgadmin]

The SA-335 Grade P91 belongs to the special category of the creep strength enhanced ferritic steels (CSEF) that are intended for use in high temperature services where maintaining strength and creep resistance is required. Relating to the welding of such a material, P91 maintains its CSEF properties provided that proper welding consumables are used, preheat and welding are implemented by skilled and qualified personnel and PWHT is strictly performed based on specific instructions. Especially for the PWHT, P91 obtains its mechanical properties by exhibiting a specific microstructure and thus it could be achieved by proper heat treatment of welds (improper PWHT could significantly alter the CSEF properties of such a material to the point that it may perform significantly different than intended).

Control of preheat and interpass temperature as well as postweld hydrogen bake-out (hydrogen diffusion) are necessary to avoid hydrogen cracking. Temperature monitoring and control of thermal gradients is extremely important and thus local flame heating should be avoided and instead furnace heating, electrical resistance and electrical induction heating shall be used.

In attached file you could find a suggested heating curve for pipe thickness higher than 13mm. For pipe thickness lower than 13mm the postweld hydrogen bake-out could be eliminated based on pipe thickness and the extent of low hydrogen practices used (no flux type or low hydrogen welding consumables, etc).