PAN;
The interpass is definitely higher than what I would use for austentic stainless steels. However, in dealing with 321 stainless, this is a stabilized grade, and a little more tolerant for higher interpass temperature. I would probably accept it.

metengr,

The filler metal for the welding is from AVESTA 347. In my understanding, the limit of maximum interpass temperature is 150 Dec.C.

Please advise me about the reference which indicates the higher interpass temperature limit in case of welding Type 321 or Type 347.

PAN;
The suggested limit of a maximum 150 deg C interpass temperature applies to all austenitic stainless steel welding to reduce susceptibility to sensitization during welding, not "crack resistibility" as you reported. This interpass temperature limit is conservative to keep heat input during welding to a minimum value or as low as practicable.  

Your manufacturer had unfortunately worked outside this suggested range. My comment regarding tolerance of interpass temperature above applies to the stabilized grades of austenitic stainless steels (like 321 and 347). Do I have a specific reference to allow for higher interpass temperature, no, not off the top of my head. If the NDT was performed after welding, and no indications were reported, the only concern is the amount of sensitization from the higher interpass temperature. If you need to disposition this nonconformance, have a coupon welded under the same conditions as the fitting was welded and evaluate the degree of sensitization using one of the ASTM test methods.  

metengr,

My reference is Kobelco Welding Today www.kobelco.co.jp/english/welding/files/v4n1.7.pdf .

Would a hardness tell more than NDT in regards to the issue of to high of an interpass temp?