Elimination of Intergranular Corrosion Susceptibility of Cold-Worked and Sensitized AISI 316 SS by Laser Surface Melting

Abstract

Susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) due to sensitization is one of the major problems associated with austenitic stainless steels. Thermal exposures encountered during fabrication (welding, hot working, etc.) and elevated temperature service may lead to sensitization of components of austenitic stainless steels. Laser surface melting (LSM) is an in-situ method to increase the life of a sensitized component by modifying the surface microstructure without affecting the bulk properties. In this paper, the results obtained in the attempt to improve IGC resistance of coldworked and sensitized 316 SS by LSM are presented. Type 316 SS specimens cold worked to various degrees ranging from 5 to 25% reduction in thickness and sensitized to different degrees by exposing at 898 K for different durations were laser surface melted using continuous wave (cw) CO2 laser. ASTM standard A262 practice A, optical metallography, and ASTM standard G108 were used to characterize the specimens before and after LSM. Influence of prior deformation on the desensitization behavior was evaluated for the laser melting conditions adopted during the investigation. Complete dissolution of M23C6 due to laser melting and suppression of re-precipitation due to rapid quenching result in a desensitized homogenous microstructure, which is immune to IGC. Under identical laser melting conditions, the extent of desensitization decreases with an increase in the degree of cold work, and hence, higher power levels and an extended interaction time must be adopted to homogenize the sensitized microstructure with prior cold work.