post heating of P91

[fazel2]

Hi,
After welding P91 steel (P No. 5B), we perform post heat treatment at 300 deg.C for 4hr and then PWHT at 745deg.C for 2hr. If we perform PWHT exactly after welding (without any delay), post heat is required or not?
Why shall we perform post heat treatment?
Regards

 

[metengr]

If you are referring to a bake-out treatment for the weld joint, this is done as a precaution to remove hydrogen, however, this is not required nor necessary.

Typically for Grade 91 steels, welding is performed and the joint is cooled to below 100 deg C. Once this operation is completed, you PWHT. All welding consumables should be low hydrogen and follow proper bake-out procedure before welding.

 

[davefitz]

Meteng is correct. The cooldown to 100C following welding ( and prior to PWHT) is required to ensure nearly all austenite is coverted to martensite prior to PWHT. This is espescially neccesary if high nickel weld filler is used . Likewise, accurate control of PWHT ( surface thermocouples, logged to a recorder)and adjustment of PWHT setpoint based on Nickle content of weld filler is recommended. refer to papers by Eurweld, Vallourec-Mannesman, Jeff Henry.

 

[stanweld]

An intermediate heat treatment is only required for interuptions of welding per 131.6.1 (C) of ASME B31.1.

 

[tc7]

Question for Davefitz - or anyone:

Why would a weld filler with high nickel content make it more crucial to allow martensite conversion? Is martensite not strictly an iron base material phenomenon?

Thanks.

 

[davefitz]

mfr's often increase the nickel content over the P91 matching spec to ensure a ductile weld , one which will not crack while the piece is being manhandled from the weld site to the heat treat oven. This add'l Nickel lowers the temp at which 95% of austentite converts to martensite, down to about 100 C ( 212 F). If you neglect to cool the piece to 21F prior to PWHT, then the ultimate cooling to room temp following PWHT will result in a formation of more martensite and increased hardness above spec.

Hi nickel also lowers the 1st and 2nd critical temp, so if you PWHT at too high a temp you may overtemper the piece and lose creep stength properties, so the max PWHT will be lowered by about 30 F compared to the optimum PWHT for a matching electrode. See papers by Jeff Henry and Eur-weld

 

[cloa]

BTW There are many non-ferrous materials with martensite transformations including Cu-Al-Ni alloys and a lot of proteins.