PWHT heating and cooling rates for P91 material welds

[qualitypro]

I understand ASME B31.1 Para 132.5 gives the rates of heating and cooling rate that should not be exceeded in general for all materials requiring PWHT. We have a pipe fabricator doing some heavy wall P91 welding for our power project and he is almost ready to PWHT. His PQR does not specify the heating/cooling rates.

1.Is there any particular requirement/recommendation for maintaining these rates while doing PWHT of P91 welds?
2.Does it make a difference if the rate of heating above 600F is any xxx F/hr. as long as it does not exceed 600F per B31.1.
3.Does this have to be specified when a fabricator qualifies his P91 welding procedures?

Appreciate yor inputs.

 

[stanweld]

Some new guidelines would impose cooling rates not to exceed 300F/hr down to 600 F. Heating rates would remain per 132.5 of B31.1.

 

[metengr]

The heating and cooling rates as currently defined in ASME B31.1 are appropriate for Grade 91 material. No, the WPS does not require mention of heating and cooling rates, these are driven by code of construction requirements.

 

[davefitz]

The 2010 version of B31.1 par 132.5 states: PWHT Heating and Cooling requirements

"Above 600 F the rate of heating and cooling shall not exceed 600 F/hr divided by one half the max thickness of the material in inches at the weld , but in no case shall the rate exceed 600F/hr"

- I am not aware of the 300F/hr limit stated by stanweld

- theoretically the rate of cooling should be governed by the square of the wall thickness, if one is trying to limit the developement of residual stresses caused by thermal stresses- but maybe the code has a different objective.

-please note the 600 F/hr limit for PWHT is not the same requirement as the cooling rate for P91 when cooling down from a N+T operation- if you cooled down too slowly from 1940F normalization, you would form ferrite- the minimum rate of cooling following N+T is 600 F/hr, not the maximum rate. Refer to the CCT curves for P91.

 

[qualitypro]

Thanks for your responses. The fabricator says he will follow B31.1 but neither does he nor his WPS clearly state what will be the rate of heating or cooling. I presume as long as 600F/hr is not exceeded the PWHT should be good then per B31.1.

 

[Mr168]

That would be correct, assuming the material in question is under 2" in thickness at the weld. As long as he complied with the statement quoted by davefitz above, he would meet the code requirement.