Evaluation of metallurgical and mechanical properties of 12 mm thick aerospace grade MDN 250 steel by plasma ARC welding technique

Abstract

Carbon-free nickel alloy maraging steel has high demand in manufacturing aerospace vehicles and hypersonic missiles. Due to the absence of carbon and the strengthening of intermetallic precipitates, maraging steel can attain high strength and toughness with ductility. This material has enormous weldability in both annealed and post-weld heat-treated (PWHT) conditions. During high-thickness sections, the heat flow rate and the number of passes were found to be redundant. The existence of reverted austenite (RA) has been identified because prolonged aging also has an impact on the mechanical characteristics of the weldment. The present research demonstrates the characterization of plasma arc welded (PAW) maraging steel MDN 250 in welded and PWHT conditions. The weldments were subjected to three types of heat treatment, namely, (i) Subsequent Aging (TA), (ii) Solution treating + Subsequent Aging (STA), and (iii) Homogenizing + Solution treating + Subsequent Aging (HTA). This study also emphasizes the importance of precipitates and RA in affecting the strength and toughness of welded joints. The HTA treatment solely helps to eliminate the RA from the weldment. The microstructural study depicts the formation of reverted austenite in the TA condition. The reverted austenite has not been found in the microstructure under HTA conditions. The EBSD (Electron backscatter diffraction) analysis has resulted that the RA has been eliminated. The EDS (Electron Dispersive Spectroscopy) analysis shows that segregation of the enriched alloys has been effectively lowered in HTA condition. Compared to the other weldments’ mechanical characterization, the HTA condition gives supremacy results. The average ultimate tensile strength of 1830 MPa has been achieved in HTA condition, with a yield strength of 1809 MPa. The average fracture toughness results of 88.3 MPa√ m was obtained in HTA condition. The work display of PAW (Plasma Arc Welding) is capable of meeting all of the requirements for welding maraging steel in high-thickness sections.