Influence of post welding heat treatments on sensitization of AISI 347 stainless steel welded joints

Austenitic stainless steels can undergo intergranular corrosion attack in some service conditions and to reduce this possibility, some Ti (AISI 321 SS) and Nb (AISI 347 SS) stabilized austenitic stainless steels are used in extreme conditions, such as oil refineries, thermo electrical and petrochemical plants industrial furnaces operating at high temperatures (between 500 °C and 675 °C). Even these ones, if they do not receive sufficient attention in order to avoid sensitization during manufacture or service, can fail during operation. The manufacture standards prescribe solution heat treatment, but, on the other hand, following solution treatment, stabilization is an optional procedure that is adopted only when requested by the client. After welding stabilized pieces, requirements of solution and stabilization heat treatments can be followed, but not always, because sometimes this procedure can be impossible or doubtful. This work presents the influence of the post weld thermal cycles on the AISI 347 SS, with the purpose of establishing recommendations for the obligatory or optional use of these treatments, as a function of temperatures and other operational conditions. The absence of post welding heat treatment (PWHT) in welded components of AISI 347 SS, either in the base metal (BM) or in the heat affected zone (HAZ) is allowed when the service temperature is lower than 450 °C, since the original stabilization heat treatment in the material avoids the sensitization in the piece, while above 450 °C, the local application of stabilization PWHT is more effective to protect this material.