Mr Rao,
Ferrite in austenitic weld metal had been the topic of numerous research articles. IIW-IX-1695-1992 (Guidance on Specification of Ferrite in Stainless Weld Metal) is an excellent source book on this topic. Typically ferrite in weld metal is expressed as:-
§ % Ferrite—Metallographic method
§ FN-Magna Gauge Method.
As per this IIW document, the typical heat-treatments of austenitic stainless steels are categorized in to:-
1. Low temperature- for dimensional stability- between-400-5250C
2. Medium temperature- for stress relief- between-550-8200C
3. High temperature -for solution anneal- above 9000C.
In all 3 cases, a drop in Ferrite No ( FN) can be expected. Longer the soak, better is the drop. Note there are 2 types of ferrite in case of Steel. For austenitic s.s it's delta ferrite (a high temperature phase, which essentially precipitates during welding) and conventional alpha-ferrite which may occur through any temperature ranges through diffusion and nucleation process.
.
1. For the first case, typically 4750 C embrittlement may occur for 15FN or more. Exposure at this temperature range would not harm the weld metal and may cause the Ferrite No (FN) to drop. This change may not be detected by Metallographic method.
2. For the second case, typically between 550-9000C- depending on composition and soaking time, ferrite may transform partially or completely into sigma phase (major) and secondary austenite. This detectable both in Ferrite No ( through drop in Ferrite No-FN) and measured percentage ferrite (metallographic reading).
3. Above 9000C –when exposed to solution annealing temperature, delta ferrite would go in to solution, transform into austenite any causing almost an elimination of ferrite(both in FN and %Ferrite)
Should S.S weld metal or overlay have to undergo PWHT , 525-5500 C is the recommended temperature range.
I have attached the relevant highlighted extract form this document, with notes which would be of help to you.
Thanks.
Pradip Goswami,P.Eng.
Welding & Metallurgical Engineer/Specialist
Ontario Power Generation Inc.
Email-pgoswami@sympatico.ca,
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of claes.gillenius@esab.se
Sent: Monday, June 21, 2010 3:54 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:0] Ferrite content variation with PWHT time
The delta ferrite content will decrease with PWHT time. Depending on composition and ferrite content, the ferrite will transform into brittle sigma-phase. Impact properties of the weld metal will decrease, tensile strength increase. This is especially pronounced for higher chromium contents, like 309-type weld metal.
Regards,
Claes Gillenius
Development Engineer, MMA. (Spec. stainless and non Fe-alloys)
| "P.Rao" <vasantharao2009@rediffmail.com> 2010-06-20 14:39
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Dear all,
Is it Delta ferrite number/value in stainless welds/Overlay will varies with PWHT soaking time?.
regards
P.Rao.
0966543636485
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