Studies on Improvement of Toughness in Modified 9Cr-1Mo Steel A-TIG Weld Joint

Abstract

Modified 9Cr-1Mo steel plates of 10 mm thickness have been fabricated by double side welding procedure employing A-TIG welding process. Even after regular post weld heat treatment (PWHT), the impact toughness of A-TIG weld joint was found to be lower compared to that of conventional TIG weld joint. Therefore, post weld heat treatments of longer durations were considered for improving the toughness. Accordingly, in the present study, PWHT was carried out on A-TIG weld joints at 760 °C for 4, 6 and 8 h duration to optimize the tempering time for improving the toughness by careful study of the microstructures. It was observed that the A-TIG weld joint subjected to PWHT at 760 °C for 4 h exhibited higher hardness (410 VHN) and lower toughness (39 J) resulting from partial tempering of as-welded martensitic microstructure. The weld joint that was tempered at 760 °C for 6 h exhibited fully tempered martensite microstructure with extensive precipitation of M23C6 type carbide. The joint in this condition exhibits good impact toughness to a value of 96 J. After 8 h of tempering at 760 °C, the hardness decreased to a low value of 170 VHN and the toughness value increased beyond 200 J. It was attributed to over tempering of matensite resulting in coarsening of M23C6 precipitates and reduction in dislocation density. A-TIG weld joint with fully tempered martensite microstructure exhibit an average toughness value of 96 J and minimum single value of 75 J. Therefore, PWHT at 760 °C for 6 h was identified as the optimum tempering treatment for improving the impact toughness in mod. 9Cr-1Mo steel A-TIG weld joint.