Mr. Gandhi,
You may follow this earlier discussion on the same subject in this forum.
Stabilized /Non-stabilized Austenitic stainless steels experience Polythionic Acid Stress Corrosion Cracking (PASCC) in� refinery service.� PASCC is a type of stress corrosion cracking which occurs in refinery or even in coal fired plant( burning high sulfur coal). PASCC normally occurs during shutdowns, start-ups or during operation when air and moisture are present. Cracking is due to sulfur acids forming from sulfide scale, air and moisture acting on sensitized austenitic stainless steels. Sensitization is common in austentic stainless steels following welding or after prolonged exposure to service.
1. It usually occurs adjacent to welds or high stress areas.
2. Cracking may propagate rapidly (in a matter of minutes or hours) through the wall thickness of piping and component
The critical levels of sensitization and tensile stress required to initiate PTA SCC are not well understood. Therefore, austenitic stainless steel and other austenitic alloy process equipment that may be exposed to PTA should be protected using one or more of the PTA SCC mitigation methods presented in� NACE-SP-170-2012(see below).
Equipments which will� operates below the sensitizing temperature range and the austenitic material has not been sensitized by any prior fabrication practices (e.g., hot forming, welding, heat treatment) would not be the suspects for PASCC.
A thermal stabilization heat treatment at 900�C (1650�F) may be applied to chemically stabilized austenitic stainless steels after welding to reduce sensitization and PASCC susceptibility.
• Thermal stabilization heat treatment is recommended for heater tubes and welds.
• Between 321h AND 347, the later performs better in service following stabilization.
As per ASTM(SA 213, S2),� the stabilization cycle is subjected to mutual agreement between the purchaser and the supplier. However the following is recommended cycle to follow( as per one of the oil giants)
Apply the following heat treatments:
a) Solution annealing at 1040 – 1070�C for � - � hour per inch followed by water quenching for 321/321H and 347/347H, and
b) Stabilization 850 – 900�C for 4 hours followed by rapid air cooling.
c) The adequacy of the above heat treatments must be verified by an intergranular corrosion/sensitization test, i.e. ASTM A262 Practice C. or A 262 Practice E(as minimum). If not specified earlier, meeting A 262 practice C would be really tough, while practice E could be relatively easy.
3) For 321 stainless steels, the Ti:C stabilization ratio shall be greater than or equal to 8.-- important
4) For 347 stainless steels, the Nb:C stabilization ratio shall be within the range 10 to 12.-important
Stabilizing anneal fixes Ti (in 321H) or Nb (in 347) through formation of stable carbides e.g. TiC or NbC. If leaves Cr in the solution, thus without any deterioration of overall corrosion resistance of 321 & 347 S.S.
It would be advisable to get material with stabilization anneal done in the mill with the MTR. Otherwise it may be a problem later.
Pradip Goswami, P.Eng,IWE
Welding & Metallurgical Specialist
Ontario, Canada
https://groups.google.com/forum/#!topic/materials-welding/QRV0tdBM_UE
http://materials-welding.blogspot.ca/2013/07/mw18218-re-18216-heat-treatment-for-tp.html
API-RP571-2011-Extract of PASCC
Dye penetrant inspection showing extensive O.D. cracking around welds
Dear Experts,
Which cooling cycle shell be followed? Rapid water quenching or slow cooling in furnace up to 300� C. Is it required to trail mock up tests (IGC, tensile and flattening) after simulation HT i.e. Solution Annealing + stabilization HT? Will these tests satisfactorily pass?
Thanks & regards,
C. R. GANDHI
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of CHINTUKUMAR GANDHI
Sent: Sunday, February 16, 2014 5:11 AM
To: materials-welding@googlegroups.com
Subject: [MW:20112] Stabilizing Heat Treatment of SS 321
Dear Experts,
In one of our client project spec., it is specified that SS 321 pipes shall be in a stabilized HT condition. Stabilizing HT shall be carried out subsequent to the normal solution annealing. Soaking temp. & holding time for stabilizing HT shall be 900� C & 4 hrs respectively.
My questions are (1) what is the purpose of stabilizing HT (2) This stabilized HT pipe shall be solution annealed after bending in coil form. Is it allowed or again we shall have to carry out stabilized HT after solution annealing? Client Spec. asks for solution annealing HT only after bending in coil form.
Your prompt response on the above shall be highly appreciated.
Thanks & regards,
C. R. GANDHI
(QCM)
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