Carbon and Nitrogen Effects on Sensitization Resistance of Type 347 Stainless Steels

The sensitization resistance of the stabilized Type 347 (UNS S34700) stainless steels (SS) having different carbon and nitrogen contents was evaluated using the double-loop electrochemical potentiokinetic reactivation (DL-EPR) and ASTM A262, practice E methods. Samples were heat-treated to simulate the coarse grain region of the heat-affected zone (HAZ) during welding and subsequently aged at 948 K for different times. For comparison, DL-EPR tests also were conducted for the unstabilized steels, Type 316 (UNS S31600) and Type 304 (UNS S30400). For Type 347 SS, a criterion for sensitization in accordance with the ASTM A262, practice E test corresponded to a condition of > 10% reactivation ratio (peak reactivation current [I_r]/peak activation current [I_a]) from DL-EPR test. The carbon and nitrogen effects on the sensitization resistance of the Type 347 SS were explained in view of the observed microstructures. By introducing a resistance parameter based on the chemical composition, sensitization resistance of the stabilized Type 347 SS and the unstabilized steels was expressed as a unified function.