KEY-HOLE PLASMA ARC WELDING OF 8 MM THICK MARAGING STEEL - A COMPARISON WITH MULTI-PASS GTAW

Maraging steel is an iron nickel alloy with typical 18Ni-8Co-5Mo composition. It has low C martensitic structure which is strengthened by precipitation of inter-metallic compounds through ageing. Due to good fracture toughness in addition to high strength to weight ratio, the steel is widely used in aerospace applications. Conventionally we have been welding 8 mm-thick maraging steel with multi-pass automatic GTAW process. The steel shows "austenite reversion" phenomenon when heated to two phase (austenite + ferrite) region. Due to this phenomenon, maraging steel weld has two peculiar heat-affected zones; zone A nearest to the weld which is heated to fully austenitic region and zone B adjacent to it is heated to two phase region. Zone B contains reverted austenite which is not hardened by ageing. In conventional GTAW weld, zone B is quite wide, due to cumulative effect of multipasses. As zone B is not hardened fully by aging, it affects the mechanical properties of weldments. To reduce the width of the HAZ (zone B), single pass key-hole Plasma Arc Welding (PAW) was evaluated for 8 mm-thick plate. This paper describes the effect of weld passes and heat input on weld properties as a comparative study of single pass PAW and multi-pass GTAW. Mechanical properties (hardness, tensile properties, and fracture toughness) and macroscopic observations have been discussed. Also filler wire consumption & arcing time has being compared which are important aspects for production. The effect of single pass from multi-pass is very much visible in macroscopic observations. PAW weld shows significant reduction in width of HAZ (zone B) as compared to multi-pass GTAW. In aged condition in PAW weld does not show sudden drop in hardness in HAZ which is a common phenomenal in GTAW & the Improvement has been observed in tensile properties of PAW weld.