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Discussion Forums - The Hendrix Group
HomeHomeDiscussionsDiscussionsOil Refinery Co...Oil Refinery Co...PWHT of SS321 welds in prosess with hydrogenPWHT of SS321 welds in prosess with hydrogen
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12/13/2004 9:00 AM
 
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Dear Sir, I read your answer to the previously asked question by Mr. Kumar on Sep. 05-04. We have the same problem in Diesel/Kerosene Dehydration Unit Construction. I will ask the question specifying the conditions more detailly. Material: SS 321 Thickness range:above 19 mm Service:Process with hydrogen Temperature:-29 to 482 C Flange rating: 900-1500 PWHT: 900C, min 30min, rapid cooling by pressurized air Discoloring on the outer surface will be cleaned after PWHT by pickling. In the refinery specification it is specified that PWHT shall be applied for the thicknesses above 19 mm for SS321 materials, without refering any referance. Now, we have some doubt about this heat treatment. Is there any application like this? Will PWHT cause any problem (metallurgical or internal stress between heated band and cold area after sudden cooling etc) in service later? Thank you very much for your kind answers.
 
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3/15/2005 9:00 AM
 
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This is basically a stabilizing heat treatment required for stabilized grades like SS 321 & SS 347. We have done it before. Metallurgically speaking ( If I remember correctly)) - During welding, there is chance of Cr carbide formation as Cr-carbide is more stable in that temp. range. During the PWHT, Cr-crbide etc. will disslve & srabilizing element like Niobium etc. will combine with C (as these are more stable at this 900 C range over Cr carbides) and hence less C will be available for formation of Cr-carbide. This will ensure no sensitization. Regarding mechanical stresses - these materials are supposed to take that much stress. Remember, no stress relieving is necessary for any SS weld joint even if these are welded/repaired/rewelded due to its ductile nature (austenitic -- FCC structure --ductile) 'Hope this helps Regards PLC
 
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3/22/2005 9:00 AM
 
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Only in cold forming activities for the AISI TP 321 I would recommend a stress relieving. For the situation you describe the benefit of PWHT welds will be less than the malefic. Regards Luis Marques
 
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