PWHT as per B31.3

[allab]

Gentlemen:

We have a 24-inch x 19.05mm thick alloy pipe, A691, Grade Cr. 1-1/4, Class 32. Table 331.1.1 of B31.3 indicates that heat treatment is required for alloy steels with Cr content greater than 1/2% and less than or equal to 2%. Does this table 331.1.1 supersedes the B31 (.1 or .3) PWHT requirement for pipe wall thickness greater than three fourths of an inch as our pipe is an alloy? Appreciate your valued confirmation.

 

[DekDee]

allab,
Your material is P4 so PWHT is required for thicknesses greater than 13 mm,
Regards,
DD

 

[weldstan]

And per B31.1, the max thickness permitted without PWHT is 16 mm. In any event, PWHT is required for your alloy pipe.

 

[allab]

Thank you so much for the replies, Sir DD and Sir Stan.

If you may allow me, please, to ask another clarification:
Table 331.1.1 of B31.3, for pipes with WT less than or equal to 13mm, PWHT is not required if the minimum tensile strength of the pipe material is equal to or less than 71 ksi. Under Table A-1, the specified minimum strength of our alloy pipe steel is 60 ksi. It is also noted here that the material is P4, as per earlier comment of Sir DD. Under Table 331.1.1, where the nominal WT is greater than 13mm, tensile strength is no longer a limitation, hence PWHT is required.

What is the significance of this tensile strength vis our requirement for a PWHT?
When the alloy pipe is a P4, does that mean PWHT is required regardless of tensile strength and/or wall thickness?
Is Table 331.1.1 the correct table for qualifying a PWHT or is this just a table for 'Holding Time'?

Appreciate very much your continued patience.

 

[weldstan]

When T is > 13 mm, PWHT is required regardless of SMTS and carbon content < or = 0.15%. The requirement is related to hardenability of the alloy and the cooling rate based on thickness. At approximately 13 mm 16 mm the cooling transitions from 2 dimensional (X/Y axis) to 3 dimensional (XYZ) which increases the cooling rate to where martensite will surely be formed.