Development of Multipass Gas Tungsten Arc Welding Technique for Aerospace Grade 18% Ni-Co-Mo Based Maraging Steel to Improve the Metallurgical and Mechanical Properties by Reducing Austenite Pools

Abstract

The maraging steel is an impeccable choice for aerospace applications due to its high strength and excellent toughness. Welding is importantly involved in fabricating various structures out of maraging steel. Gas tungsten arc welding (GTAW) is the most commonly adopted technique for joining maraging steel. However, the major concern regarding the welding of maraging steel is the formation of the reverted austenite phase in the fusion zone and the heat affected zone. This reverted austenite deteriorates the mechanical property performance of welded joints. The present study focuses on GTAW of 12 mm thick MDN 250 maraging steel. An attempt was made to suppress the reverted austenite by employing suitable post-weld heat treatments. Three different types of post-weld heat treatments were adopted, i.e, (i) direct aging (W-DA), (ii) solutionizing + aging (W-SA), and (iii) homogenizing + solutionizing + aging (W-HSA). The micrograph and XRD analysis of the fusion zone with W-DA and W-SA conditions reveal the presence of reverted austenite. The SEM/EDAX examination of the fusion zone of as-welded, W-DA, and W-SA conditions revealed micro-segregation of Ni, Mo, and Ti elements from the matrix to the grain boundaries. On the other hand, the W-HSA condition was free from micro-segregation and austenite reversion. Electron backscatter diffraction (EBSD) analysis was used to estimate the percentage of reverted austenite in the fusion zone. The tensile test shows the highest strength of 1721 MPa (UTS) in W-HSA conditions. Similarly, the microhardness of the W-HSA conditions depicts a higher hardness with an even distribution in the microhardness values across the weldment. Hence, considering both metallurgical and mechanical examination of the weldment, the W-HSA heat treatments give superior properties in the case of GTA welded MDN 250 grade maraging steel.