Development of Hot Wire GTAW Process for 18% Ni Maraging Steel Used for Rocket Motor Casing

Maraging steels possess ultrahigh strength and excellent fracture toughness which makes it suitable for rocket motor casing and other applications. Though it has good weldability and effectively welded by conventional or cold wire Gas Tungsten Arc Welding (CW-GTAW), defects like entrapped slag, lack of fusion, porosity, etc., are encountered. The weld pool is also sluggish and does not flow freely as with carbon steel or stainless steels and has limitation of low speed and hence lower deposition rates. The hot wire GTAW (HW-GTAW) process is ideally suited for increasing the weld fluidity which helps the slag to afloat easily. It also provides the benefit of elimination of porosity from the weld deposit along with increased weld speed and deposition. In present work, HW-GTAW process was used to weld 8 mm thick maraging steel to reduce slag formation and other types of defects. Welding was done with higher welding speed and higher wire feed rate resulting in higher deposition. Bead on trials with HW-GTAW were conducted to establish the relation between travel speed, wire feed rate and hot wire current. Transverse tensile properties and fracture toughness of weldments were evaluated. Macro and microstructure analysis of weldment was carried out to analyse the various zones. Microhardness across the weld was measured. UTS of 1720–1749 MPa, 0.2% PS of 1687–1713 MPa and FT upto 89 MPa√m was achieved with joint efficiency between 93 and 90% on tensile and fracture toughness, respectively, using HW-GTAW process. Microstructure of weld fusion zone showed dendritic structure having martensite with narrow dark HAZ containing reverted austenite. Reduction in heat input, minimal defects and improved productivity was achieved by using HW-GTAW process.