Hi Abby,
9Cr-1Mo-V, or P-91 steel should be of fully martensitic structure to impart the best mechanical properties as recommended by various literature and other established welding practices. However there had been reported cases on formation of “delta ferrite” (termed as residual , as being unwanted) in this steel. Delta ferrite reduces toughness and creep resistance and may transform to “sigma phase” at high temperature.
By keeping Chromium Equivalent (Cr eq) below 10, the tendency to form delta ferrite is reduced. This number is not absolute, but provides a good guideline since elevated delta ferrite in this material reduces its toughness. Even materials with Cr eq between 10 and 12 may exhibit adequate toughness when the delta ferrite does not exceed five (5) percent.
· Cr eq= Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb + 9Ti + 12Al - 40C –30N - 4Ni - 2Mn-1Cu. ( Ref API-938B, Use of 9Cr-1Mo-V (Grade 91) Steel in the Oil Refining Industry.
The delta ferrite formation tendency for P-91 steel would be quite similar to austenitic stainless steels, as delta ferrite is a high temperature phase, which does not get enough time to disperse during weld solidification. If the weld chemistry is properly balanced this reaction could be avoided to a good extent.
The advanced CSEF (Creep Strength Enhanced Ferritic) steels such as P-92 ( Similar to P91, but with 0.5Mo-1.7W ), P-911(Similar to P91, but with 1%W), may have more tendency on formation of ferrite and resultant sigma phase and would require more careful control of weld chemistry.
The attached paper sheds good light on the same topic.
Thanks.
Pradip Goswami,P.Eng.
Welding & Metallurgical Engineer/Specialist
Ontario Power Generation Inc.
Email-pgoswami@sympatico.ca,
pgoswami@quickclic.net
-----Original Message-----
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Bathula Raghuram (Mumbai - PIPING)
Sent: Monday, August 30, 2010 6:31 AM
To: 'materials-welding@googlegroups.com'
Subject: RE: [MW:6782] 9Cr-1Mo/9Cr-1Mo-V steel welds
To my limited knowledge, it is also depend on time at temperatures, there are studies demonstrated that formation of σ at 480 °C in alloy with less than 12% Cr. However it also states σ forms with long time exposure in the range of 565-980 °C. The composition of σ in austenitic stainless steels is more complex than it is for simple iron-chromium ferritic grades. Carbon also decrease σ formation by forming chromium carbides.
Source: ASM Handbook
-----Original Message-----
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Kathalingam Babu
Sent: Monday, August 30, 2010 3:29 PM
To: materials-welding@googlegroups.com
Subject: Re: [MW:6780] 9Cr-1Mo/9Cr-1Mo-V steel welds
Hi Abby
Pls note that , 9Cr-1Mo/9Cr-1Mo-V Steels are the Ferritic Steels.
Don't confuse Sigma embitterment of ASS with Ferritic Steels.
Regards,
K.Babu
Singapore
----- Original Message -----
From: "Abby Joseph" <abbyjoseph@gmail.com>
To: "Materials & Welding" <materials-welding@googlegroups.com>
Sent: Monday, August 30, 2010 4:37 PM
Subject: [MW:6775] 9Cr-1Mo/9Cr-1Mo-V steel welds
Do 9Cr-1Mo/9Cr-1Mo-V steel welds also develop sigma phase during service, like some stainless steels?
(Note- these alloys also contain substantial wt% of Cr, Fe and Mo. They are also exposed to the high temp at which sigma phase forms.)
If not, why?
Regards,
Abby Joseph
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