Handling of 9 Cr 1 Mo Material

[Ittichet]

Hi, I working on HRSG Fabrication with 9 Cr material, I have customer specification state that "If the assembly is not PWHT within 24 hours of the completion of welding, it shall be enclosed or covered and be stored 5 degrees above ambient temperature" I wonder what is the reason to store work piece at 5 degrees above ambient temperature? Also, spec require to Hold at 200 F for minimum 1 hrs before PWHT, What's the reason for that? Is all of this for allow hydrogen escape and keep hydrogen (Humid) away from weld?

Thanks for all answer.
Pook

 

[Dhurjati Sen]

Dear Ittichet,

Is it for some Gulf customer namely Kuwait?

Actually the requirement was there to prevent hydrogen ingress as you have correctly thought of.

But I do feel the code needs to be changed as enclosing and covering is enough. No point in holding 5 degree above ambient.

Regards.

DHURJATI SEN
Kolkata, India

 

[Ittichet]

Dhurjati Sen.

No, This is Vogt Power spec. Thanks for the answer!

Pook

 

[bobjustbob]

My understanding is the concern is hydrogen. We made valve bodies to the cast spec ASTM A217 Grade C12A. Upon completion of a weld repair we had to wrap each casting in heated blankets and KO wool to perform the bake out before letting them cool from the preheat temperature. We were on the production side not the user, so I am not overly familiar with the failure mode. From what I was told by engineers from the valve manufacturers, after this material was put into use they started having failures. To prevent the failures they added requirements such as a microstructure check for blocky ferrite. And then added the weld repair requirements.

Bob

 

[kingnero]

Somebody might prove me wrong, but I doubt a hydrogen bake out has much effect if performed >= 24h after welding...

 

[Ittichet]

Thanks for all answer.

 

I presume you are talking about P91 and not conventional P9 steel.
Hydrogen per se is not the problem. Stress corrosion cracking has been encountered in welded P91 parts allowed to sit at room temperature after welding and before PWHT. I am not up on the very latest research but I am not sure the mechanism is well understood yet. It does seem to involve moisture, and empirically it is known that the problem can be avoided by PWHT as soon as possible after cooling to below a sufficiently low temperature (200°F; to ensure full metallurgical transformations are completed).
EPRI and CCJ websites will have information about this phenomenon.
p.s., do not confuse post-weld soaking with heat treatment, which has specific Code meanings.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[weldstan]

It's all about moisture control; that is, keeping the welded parts at a temperature where moisture will not condense on the parts to mitigate the potential for stress corrosion cracking. Vogt has had this requirement for over a decade.