P91 P15E 200f prior to PWHT is this req?

[INSPECTACON]

I was wondering if some one could give me some reference with regards to why you would need to bring P15E (p91) material down to 200F prior to PWHT all I could find was that most shop purchased P91 filler has more nickel than does the parent metal which implies the weld must be cooled down to 200 F prior to PWHT ( in order to form 95% martensite prior to PWHT).The only exception may be welds thicker than 4" thk and true matching filler. The higher nickel filler is more ductile than matching filler and thus reduces the likelihood of cracking in the shop prior to PWHT due to handling stresses ( ie pushing the bundle into the oven). If we are welding in the shop 8" sch 160 and we are equipped with setting up to wrap each joint with controlled electric resistance or induction heating methods with monitoring from the start of welding to finish why could we not go right into PWHT if we are not moving it. The WPS states to bring it down too 200F prior to PWHT but I think that is for the reason if you have large spools that you want to put into the frurnace and have to add stress to the spools to lift them.
I would agree that as long as we are not handling the piping or putting any stresses on it immediately after weld completion there is no reason to cool to 200 before PWHT
can some let me know there thoughts.

Thanks

 

[metengr]

Grade 91 is a creep strength, martensitic steel. Cooling to 200 deg F after welding ensures any retained austenite formed during welding has been transformed to martensite before PWHT. Otherwise, the retained austenite can be transformed to fresh martensite upon cooling from the PWHT temperature range.

 

[INSPECTACON]

so to retained austenite that can be transformed to fresh martensite upon cooling from the PWHT temperature range is not a recommended practice?

 

[weldstan]

It is not a recommended practice. When the retained austenite is transformed to martensite, that martensite does not have the strengthening precipitates formed during the tempering process.

 

[Guest102023]

The 200°F figure is considered to be the temperature at which P91 welds have essentially completely transformed to martensite, which is always desired for hardenable steels before tempering (PWHT essentially performs both a tempering and stress relief function in this case). Otherwise, untransformed austenite will transform to untempered (and hard) martensite upon cooling from PWHT. Ni and Mn are the alloy elements that suppress martensite transformation, and there has been a well documented painful learning curve about how these influence the minimum PWHT hold temperature.

On a related question, don't leave a P91 weldment sitting around @ RT any longer than necessary before commencing PWHT. It is vulnerable to a form of stress corrosion cracking that to my knowledge is not yet well understood.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"