This study was undertaken to gain a better understanding of microstructures obtained by multipass gas tungsten arc welding in maraging steel grade 250. Metallography and microhardness measurements were carried out on sheet and welded joints in as-welded and post-weld aged conditions. It was found that there was a significant amount of reverted austenite formed on cell boundaries of weld metal after aging at 758-823 K for 3-5 h, and was stable at room temperature. Aging at higher temperatures led to an increase in the continuous network of patchy austenite along the cell boundaries. The reason for the above, in our opinion, is the concentrational heterogeneity which characterizes the microstructure of maraging steel welds. No reverted austenite was observed in as-welded specimens. Solution annealing at 1093 K for 1 h did not completely eliminate the chemical heterogeneity associated with weld structures. However, homogenizing at 1373 K produced homogenous structure that on subsequent aging produces austenite-free lath martensitic structure.

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The effects of thermal cyclic aging on mechanical properties and microstructure of maraging steel 250 were studied using hardness tester, tensile testing machine, impact tester, optical, scanning electron, and stereo microscopy. Samples were solution annealed at 1093 K for 1 h followed by air cooling to form bcc martensite. Cyclic aging treatments were carried out at 753 and 773 K for varying time periods. Increase in hardness and strength with corresponding decrease in ductility and impact strength was observed with increasing aging cycles. Reverted austenite was detected by x-ray diffraction technique formed as a result of cyclic aging. The presence of reverted c was also confirmed by EDX-SEM analysis and attributed to the formation of Mo-and Ni-rich regions which transformed to c on cooling. Heterogeneity in composition and amount of reverted c was found to increase with increase in aging cycles and aging time. Fractography reveals the change in fracture mode from ductile dimple-like to brittle cleavage with increase in hardness and strength due to cyclic aging.

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