INFLUENCE OF THERMAL TREATMENTS ON SENSITIZATION IN Cr-Mn-Cu AUSTENITIC STAINLESS STEEL WELDS

Abstract

In an earlier work, Cr-Mn-Cu austenitic stainless steel was established to be susceptible to chromium carbide precipitation in the as-welded condition in the heat input range 207-302 J/mm due to the formation of sensitized structure. Whereas, literature indicate that pre or post-weld thermal treatment has been effective in minimizing carbide precipitation in stainless steel welds; such has not been reported for the welds of the new Cr-Mn-Cu austenitic stainless steel variety. Thus, in this work, sensitization behaviour in Cr-Mn-Cu austenitic stainless steel welds subjected to either pre weld or post-weld thermal treatment was investigated. Pre-weld thermal treatment was conducted with oxy-acetylene flame torch to pre-heat temperatures of 180, 220, 260, 300 and 350⁰C, respectively for 30 min each and then welded at a heat input of 293 J/mm. Post-weld thermal treatment was conducted on welded coupons at a fixed temperature of 650^oC for varying times of 30, 60., 120, 180 and 240 min, respectively. Microstructural characterisation using 10% oxalic acid etch established that post-weld thermal treatment could not prevent the occurrence of sensitization irrespective of the treatment time. On the other hand, pre-weld heat treatment of Cr-Mn-Cu austenitic stainless steel was only effective for eliminating sensitization at preheat temperature around 300^oC and above. The work further showed that the mechanism of chromium back diffusion known to be responsible for the effectiveness of post-weld thermal treatment in the control of carbide precipitation in other grades of stainless steel is not applicable to Cr-Mn-Cu austenitic stainless steel grade.