9Cr : why maintaining preheat? 1

[aburafdi]

Dear all,
It's already known to everybody that welding of 9Cr material need to maintain preheat temperature during welding. In other word: it's not allowed to drop the temperature of weldment to go below preheat temperature until weld is finished. Simple question : what is the reason behind it ?

 

[stanweld]

The high probability of hydrogen induced "cold or delayed" cracking.

Interuption of preheat may be performed with adequate intermediate (dehydrogenation) heat treatment.

 

[metengr]

The second reason for elevated preheat is to allow the weld region to remain at or slightly above the lower critical transformation temperature (which is the temperature where austenite formed on heating transforms to martensite). While welding, the weld remains austenitic until post weld cooling results in proper martensite formation.

 

[Guest102023]

It is always good practice to maintain preheat through to the completion of welding, but more so with low alloy steels. Hydrogen cracking happens at or near room temperature; you also might overstress partially completed joints.

metengr,
What is the temperature that 9Cr welds should be cooled to?
I am assuming conventional 9Cr.

 

[aburafdi]

Thanks to all.
Best regards,
aburafdi

 

[metengr]

brimstoner;
There is an excellent AWS paper titled "Welding of Martensitic Creep-Resistant Steels" by L.BERES, A. BALOGH, and W. IRMER . This paper describes the proper selection of preheat and interpass temperature maintenance for 9% Cr and higher steels to avoid post weld cracking concerns related to an improper ratio of austenite to martensite.

Cooling rate was held constant for all specimens evaluated based on preheat near or below the calculated MS value for the steel. The evaluation focused on crack sensitivity after welding.

 

[aburafdi]

Hello Metengr,

Do you mean MS as Martensite Start? For preheat, it's near or below MF, not MS, isn't it ?

 

[metengr]

aburafdi;
Yes, I was referring to the martensite transformation temperature and not the lower critical. Thanks