Microstructure and mechanical properties characterization of aged and repair welded Alloy 617M

A chemistry-controlled variant of nickel-based Alloy 617 (UNS N06617) designated as Alloy 617M is the candidate structural material for superheater header and tubing and high pressure and intermediate pressure steam turbines for Indian advanced ultra-supercritical (AUSC) coal-fired thermal power plant mission program. This study evaluates the effect of ageing due to service exposure on microstructure, mechanical properties and repair welding behaviour of Alloy 617M boiler tube material. Initially, Alloy 617M tube weld joints (52 mm diameter and 11.9 mm thick) were fabricated by multi-pass manual gas tungsten arc welding (GTAW) process using ERNiCrCoMo-1 welding wire. Service-exposed material is simulated by subjecting the base metal and welded tubes to ageing heat treatment at 700 degrees C and 750 degrees C for 1000 h and 4000 h. Extensive carbide precipitation along the grain boundaries and gamma '-Ni-3(Al, Ti) within the grains were observed in the aged base metal and weld metal. The yield strength and tensile strength of Alloy 617M base metal and weld metal after ageing heat treatment increased by 150-200 MPa in comparison to as-received/as-welded condition. In contrast, the Charpy V-notch toughness showed significant reduction after ageing, viz. weld metal toughness reduced from 125 J in the as-welded condition to 50 J after ageing. Partial penetration repair weld joints fabricated from the aged welded tubes showed presence of micro-fissures in the heat-affected zone (HAZ) and are intergranular in nature. However, the application of solution annealing heat treatment at 1160 degrees C for 1 h to aged Alloy 617M before repair welding could help in producing repair welds without micro-fissures.