PWHT Temp for 1.25 Cr - 0.5 Mo

[roniprabowo]

Dear all,
I have an Heat Exchanger made by 1.25 Cr - 0.5 Mo (SA 387). AS per ASME VIII PWHT for P4 min temp is 593 C. But Our project spec stated that PWHT temp min is 670 C. I also read from KOBE cataloque temp min PWHT is 670 C.
My question what make these different value ? Is there any problem if my min temp PWHT is 593 ? Is there any mettalurgical reason ?

 

[metengr]

The post weld heat treatment specified by ASME Boiler and Pressure Vessel Code in Section VIII, Div 1 is considered a sub-critical post weld heat treatment when performed at 593 deg C. What this means is that after welding is completed, the component is heated to a temperature below the lower critical transformation temperature to provide stress relief and to temper the weld and base metal heat affected zones. The weld metal mechanical properties will be equal to or greater than the base plate mechanical properties for this type of post weld heat treatment operation.

Post weld heat treatment can also be specified as a normalize and temper. In this case, the welded component is heated (at higher temperature) above the lower critical transformation temperature to eliminate the weld heat affected zone.

The normalize and temper is commonly specified during seam welded pipe and vessel component fabrication. However, this type of post weld heat treatment is typically not performed in the field or when installing nozzles or attachments to finished pressure parts because of cost and potential distortion concerns. The normalize and temper post weld heat treatment provides for uniform mechanical properties across the weld and base metal.

 

[stanweld]

670 C is also a subcritical anneal. While the ASME Section VIII, Div. 1 requirement will provide stress relief, it will not achieve full stress relief. ASME B31.1 and B31.3 require PWHT at 700 C (1300 F) minimum. If a minimum hardness requirement is mandated by the Customer, you may not be able to achieve it with the 593 C (1100 F) minimum permitted by Section VIII.

 

[metengr]

Just to clarify on a point made by stanweld. The ASME code committees have their own agenda's and conservatisms. I want to be perfectly clear that the PWHT minimum temperature specification in ASME Section VIII, Div 1 is quite acceptable for industry, and has been proven for use. The slightly higher minimum subcritical post weld heat treatment temperature in the Piping Codes have more to deal with added conservatisms because of external piping exposure. REMEMBER, the temperature requirements in Section VIII, Div 1 are MINIMUM, as long as the lower critical transformation temperature is NOT exceeded. As a customer you can specify a higher sub-critical PWHT temperature. The fact that the piping code committees chose to have a higher minimum temperature is their prerogative.

The sub-critical PWHT minimum temperature differences will not result in a longer lasting weld joint OR will outperform one over the other.