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POST WELD HEAT TREATMENT [STRESS RELIEVING]

POST WELD HEAT TREATMENT [STRESS RELIEVING]

(OP)
Good day!

I have been struggling what temperature to use for our PWHT using stress relieving.

I saw the definition of stress relieving which says that material is to be heat treated below its critical temperature.
In Phase Diagram of Heat Treatment, critical temperature is 727 degrees celcius.

In ASME SECTION V111 , normal holding temperature for PWHT is 595 degrees celcius which is below 727 degrees celcius.

So meaning, any temperature lower than 727 degrees celcius will do?

Our product is transportable pressure vessel having a thickness of 6mm only and total height of 350mm.

I am planning to use 200 degrees celcius only since the thickness is too thin. Is it possible?

By the way we used low carbon steel material.

Thank you.

RE: POST WELD HEAT TREATMENT [STRESS RELIEVING]

Post weld heat treatment in ASME code refers to subcritcal treatment where the temperature selected is below the lower critical transformation temperature to reduce harmful residual stresses and to temper (soften). PWHT is a thermal activation process which means the lower the temperature the slower the reaction and vice versa.

If you need to PWHT, follow the requirements in ASME Section VIII, Div 1 Part UCS for carbon and low alloy steels. These are minimum values so you must perform at or above this temperature. Thickness has nothing to do with temperature, it is for holding time at temperature. At 200 deg C this is too low for adequate PWHT, you must be at least 595 deg C or higher but do not exceed the lower critical transformation temperature of 720 deg C.

RE: POST WELD HEAT TREATMENT [STRESS RELIEVING]

But don't go too long, nor at too high a temperature. Your metal can (will!) sag, distort, or change structure.

RE: POST WELD HEAT TREATMENT [STRESS RELIEVING]

Why are you contemplating the need for PWHT for this 6 mm walk vessel? 200C PWHT will do nothing to relieve stress and there is little weld stress anyway. The distortion caused by welding about the weld joint will be the only concern regarding possible cyclic service and that distortion will not be alleviated by PWHT.

RE: POST WELD HEAT TREATMENT [STRESS RELIEVING]

(OP)
Thanks a lot for your prompt and great answers!
Now, it is confirmed. The auditor wanted us to do PWHT for all sizes.

RE: POST WELD HEAT TREATMENT [STRESS RELIEVING]

So the auditor required the 200C PWHT. One would assume that the auditor stated where it was written in the contract for this requirement. If not, you should ask. And if not in the contract, you may consider it an "add on" and charge extra for the additional work should you desire.

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