Requirements of Post Heating in Welding P91 to T91 using GTAW

[Pipoy04]

Good day,

In GTAW welding of P91/T91 tube to header weld, is post heating a mandatory requirements? What are codes to support this query. Thank you.

 

[metengr]

Yes, post weld heat treatment is required based on requirements in the original code of construction (ASME Boiler and Pressure Vessel Code, Section I). Why original code of construction? Because the in-service inspection code for boilers is the National Board Inspection Code, and this code refers users and repair organizations back to PWHT requirements from the code of construction. At present time, there are no alternative welding methods or repair options at least in the US for handling Grade 91 tube to header attachment welds with no PWHT.

 

[Pipoy04]

Good day metengr!

I know post weld heat treatment is required on P No. 15E base on ASME but my concern is about post heating or post baking. Post bake temperature probably higher than that of the preheat maintenance (200C to 300C) base on EN1011-2. Is there other code mention that when welding P No. 15E materials with a GTAW process, a mandatory post bake is required?

Thanks!

 

[metengr]

No. Typically post weld baking to remove hydrogen is a process detail. I would add that if you intend to delay PWHT, I would perform a post weld bake as a belt and suspenders approach for the GTAW process. If you perform PWHT immediately after the cool down to 200 deg F, I see no reason to add a post weld bake prior to PWHT.

 

[weldstan]

Whereas some tempering of the martensitic structure is provided by the dehydrogenation heat treatment, I would recommend it after GTAW welding if Code PWHT will not be performed until some days later. As per metengr, if PWHT will be performed within a few hours (to permit NDE prior to PWHT), dehydrogenation treatments could be deleted.