P91 weld procedure

[ageometry]

Hi All,

The 2015 Epri best practice guideline for P91 (2nd edition) has removed the recommendation for cooling the weld down to 200F after welding and before bake out (as compare to first edition issued in 2011). It has tied the requirement to the nickel + manganese content of the filler. If it is below 1.2%, it only needs to cool to preheat temperature provided that preheat temp is less than 400F. For filler material with nickel + manganese content above 1.2%, it states that cooling to 200F may be required, but also note the significant risk of fracture for field welds with high applied loads. I like to know what the experts thoughts on this. Does the risk of fracture outweighs the risk of incomplete transformation? Is controlling the nickel + manganese content of the filler enough to ensure complete transformation at 400F instead of 200F?

Thanks in advance for all replies.

 

[metengr]

Yes. Controlling the Ni and Mn content of the weld metal is absolutely critical to avoid the formation of fresh martensite after welding and before cooling prior to tempering from retained austenite. Fresh martensite results in poor notch toughness and creep properties.

You will also see other variables evolving with Grade 91 from EPRI especially with a tighter control on residual elements. ASME B&PV Code recently passed a Type 2 chemical composition versus the original Type 1 to promote improved creep rupture ductility and slightly higher allowable stresses.

 

[ageometry]

Thanks for the quick response.

So you do not see any harm from not cooling to 200F after welding as long as the Ni+Mn content of the filler is below 1.2%?

Conversely, if we still cool to 200F after welding, what is my risk of fracture? and would this fracture of the weld show up during the PWHT process, or afterwards in operation?

Thanks again for your expertise.

 

[metengr]

Correct.
Conversely, I do not see any harm in cooling to 200 deg F to ensure complete transformation of austenite to martensite prior to tempering. Proper handling to avoid hydrogen pickup will avoid fracture from delayed or cold cracking.

 

[EdStainless]

If you cool remember that you need to PWHT promptly.
And you still need to cool to the preheat or 400F, whichever is lower.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

Often RT will be done prior to PWHT to assure soundness. If so, perform a dehydrogenation heat treatment preferably after cooling below 200F before RT.

 

[ageometry]

We are doing weld, cool to 200F, followed by bake out. PWHT the next day. Thanks for all the help.