Technology Development for Thick Section of Aerospace-Grade MDN 250 Weldment with Higher Weld Strength and Toughness by Suppressing Reverted Austenite Phase

Maraging steels are precipitation hardened steels with high-strength and excellent toughness. Maraging steels being the key player in the strategic sector, demands welding of thick sections up to 12 mm for critical applications. They are readily weldable in the soft solution-annealed condition and further strengthened by post-weld heat-treatment (PWHT). However, welding of thick sections, on the other hand, necessitates a longer welding time, more number of passes and a higher heat input. This paper elucidates the influence of different PWHTs on the metallurgical and mechanical behavior of 12-mm-thick plates of MDN 250 grade maraging steel by using multi-pass gas metal-arc welding (GMAW). The different PWHTs adopted for the study include; Direct Aging (DA), Solutionizing + Aging (SA) and Homogenizing + Solutionizing + Aging (HSA). The microstructures of the fusion zone (FZ) with DA and SA condition reveal the presence of reverted austenite (RA) along the cell boundaries. However, in the weldment with HSA treatment was free from RA. Metallographic analysis of the as-welded FZ showed nickel, molybdenum and titanium segregation along the cell boundaries. This resulted in the formation of RA on subsequent aging. The SA treatment was not effective in complete elimination of reversion. The HSA treatment, on the other hand, had completely eliminated both elemental segregation and reversion. The welded joint with HSA treatment had an ultimate tensile strength (UTS) of 1582 MPa and a fracture toughness of 92.9 MPa√m, respectively. HSA treatment also shows a remarkable improvement in fracture toughness compared to other PWHTs. The present study underscores the fact that multi-pass GMAW with HSA treatment provides optimal mechanical properties.