PWHT of Carbon Steel Vessel Materials

[Penrymmj]

A client specification states that "Materails with properties enhanced by heat treatment cycles such as Tempering, Intermediate Stress Relief and the final post weld heat treatment shall be tested to verify that their mechanical properties have been retained after all heat treatment cycles."

I had considered that this clause relates to materials covered by ASME VIII Part UHT “Requirements for Pressure Vessels Constructed of Ferritic Materials with Tensile Properties Enhanced by Heat Treatment” but am now not so sure.

We are using A516-Gr.70N plate, A106-Gr.B pipe and A105N forgings which do not come under the requirments of Part UHT. Maximum shell thickness is 20mm and the vessel is subject to post weld heat treatment to reduce hardness in DGA duty.

For these materials, what material properties are likely to change during PWHT?

 

[PAN]

Penrymmj,
What is the requirement of test in your client specification? In general, I will perform only hardness test after PWHT.

What is the operating and design temperature of your DGA service?

 

[Penrymmj]

The specification doesn't define the required tests other than to prove that the mechanical properties are retained. I assume that to mean a full set of mechanical tests.

The tests have to included to additional PWHT cycles to account for future repairs or alterations.

Final welds and material will be subject to hardness testing.

 

[Penrymmj]

Also, Design Temp = 260°F
Operating Temp = 140°F

 

[stanweld]

In general, the meaning is to have the mechanical tests as required by the materials specifications and documented on the MTR to be performed after the material has been fully heat treated as for the final produced vessel; e.g., normalized followed by tempering at the PWHT cycle.

 

[metengr]

For SA-516 Grade 70 and the other materials you have stated, the properties that can be affected by PWHT cycle(s) -especially notch toughness. The web site below is one that provides good information on the effects of PWHT on plate steels;

http://www.intlsteel.com/content/products/prod_plate_lit.aspx

 

[106coral]

The eventual intent of your client is to very that weldment would be obtained the same level of properties as that of base material through PWHT.
Accordingly, PWHT will increase to the resistance to brittle fracture due to hardness reduction and relaxing residual stresss on weldmetal.
For your reference PWHT is not necessary most applications.
The need of PWHT assumes a potential hydrogen cracking problem exists in due to a sensitive base metal microstructure, high level of hydrogen, and/or high stresses.
Neverthless, as PWHT may be a code requirement, so it has to be carried out as per governing code.
For SA-516-Gr.70N notch toughness could be affected depending on plate thickness.

 

[PAN]

Penrymmj,
How many PWHT cycles in your concern? This should be a factor in your consideration during PQR preparation.