PWHT

[priyam07]

Hello all,
I have a question on the PWHT after welding for 15B30 materials. History: water quenched and tempered at 400F then welded.
My question is what is the correct PWHT temperature, read somewhere it should be lower than the initial tempering temperature and it makes sense as otherwise PWHT might cause unwanted microstructural change.
Cheers

 

[metengr]

Typically, the PWHT should be below the tempering temperature (if Q&T or N&T) or below the transformation temperature to avoid changes in bulk mechanical properties. With that said, you should be not performing any PWHT nor welding should have been performed on this part with tempering only at 400 deg F, because welding or PWHT will soften the material.

 

[priyam07]

Say it gets welded by the customer with a pre heat procedure being followed (may soften a bit by 2-5 Rockwell points). So does PWHT below 400F makes any sense? what happens if you don't do PWHT?

 

[metengr]

Since PWHT will do nothing at 400 deg F to relieve residual stresses from welding, you can at least temper the base metal HAZ, which was hardened from welding, if you heat the component between 350 to 400 deg F. Residual tensile stresses can be very harmful, which is why PWHT is typically performed at elevated temperatures where substantial softening and stress relief occur simultaneously. Having a high level of residual stresses will impact service life of the component - reduced fatigue life and SCC susceptibility.

PS; I would strongly recommend you perform surface nondestructive testing to ensure a defect-free weld region if you leave as-is or perform a 350-400 deg F re-temper of the weld region.

 

[priyam07]

Sounds good thanks metengr. On another note why not weld first and do the whole heat treatment after if that makes sense, am I missing something?

 

[metengr]

Welding followed by heat treatment would have been a better approach because it would have removed the weld heat affected zone in the base material.

 

[priyam07]

I thought so, thanks for your fast response!