Dear Farajiazzeddine,
Stabilizing anneal is a recommended practice to restore the lost corrosion resistance in austenitic stainless steels, which happens during welding and fabrication(typically through sensitization).
Stabilizing anneal is a relatively low temperature(wrt to solution anneal) heat treatment process, which involves heating stabilized austenitic steels e.g. grade 321 to 840-900 deg C and hold for a period of 2-4 hrs(depending on pipe thickness).Through this process any precipitated chromium carbide would dissolve back to the solution. However preferentially Ti or Nb carbides would form and precipitate (that's what is desired).
Usually stabilization heat treatment is performed on an austenitic stainless steel component which has already been solution annealed, i.e. it's a post fabrication technique.
NACE RP-170-2004- Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment, describes Stabilizing anneal in details.
One may order the S.S tubes and pipes with Solution anneal + Stabilizing anneal from the mill.
ASME Sec-II, Part-A, Requirements:-
· ASTM/ASME provide an option of stabilizing anneal as supplementary requirements to many product requirements. See SA-213-2007,Clause S2 , SA-312-2007,clause- S6). The HT cycle has to be mutually agreed upon.
The requirement for stabilising anneal for 321/347 S.S in refinery services had been there since long. The attached extract from Old Mobil Oils spec call for the same operation, but in a slightly different language.
MOBIL ENGINEERING GUIDE :-EGE 55-B-2 — 1990, MATERIALS APPLICATION NAPHTHA PRETREATERS
7. MATERIAL CONSIDERATIONS DURING FABRICATION
7.1 To minimize the possibility of intergranular corrosion, stabilized grades (Type 321 or Type 347) of austenitic stainless steel shall be used in welded construction. Stabilized austenitic stainless steels especially Type 321 piping, may become sensitized after several years of service at temperatures over425ºC (800ºF). To prevent intergranular stress corrosion cracking in the heat-affected zones (HAZ) of welds, a stabilizing anneal heat treatment should be performed after welding. This treatment consists of heating to a range of 870ºC to 900ºC (1600ºF to 1650ºF) and holding for four hours, followed by rapid cooling.
I hope this answers to your query well.
Thanks.
Pradip Goswami,P.Eng.
Welding & Metallurgical Engineer/Specialist
Ontario Power Generation Inc.
From: pgoswami@sympatico.ca [mailto:pgoswami@sympatico.ca]
Sent: Tuesday, May 25, 2010 12:25 PM
To: GOSWAMI Pradip -THERMAL
Subject: FW: [MW:5245] PWHT for SS grade 321
Date: Tue, 25 May 2010 06:39:20 -0700
Subject: [MW:5245] PWHT for SS grade 321
From: farajiazzeddine@yahoo.fr
To: materials-welding@googlegroups.com
My question is can someone advise me how to proceed for the Post weld heat treatment for SS 321 as showing below:
The licenser requirement order to perform PWHT for piping in ANSI grade 321 to minimize the risk of chloride SCC.
The recommendation is to stress relieve fields weld on the stabilized SS for the above mentioned grade by thermal stabilization.
best regards
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