Submerged arc additive manufacturing (SAAM) of low-carbon steel: Effect of in-situ intrinsic heat treatment (IHT) on microstructure and mechanical properties

Abstract

In some traditional formative and additive manufacturing processes widely used in the fabrication of steel products, stress conditions and metallurgy typically result in a strong texture and mechanical anisotropy in both rolling high-strength low-alloy steel and additive manufacturing stainless steel products. To resolve this problem, we have investigated the effectiveness of a novel wire-arc additive manufacturing method in which the heat source is substituted by high-efficiency submerged arc plasma i.e., submerged arc additive manufacturing. An isotropic microstructure can be obtained in the bulk of large-scale low-carbon steel components by ensuring a complete columnar-to-equiaxed transition. We found that the top zone of the as-deposited component comprised columnar α-Fe grains with a typical preferential < 001 > α orientation. Multiple allotropic transformations were triggered by an in-situ intrinsic heat treatment (mainly the combination of multi-layer-penetration normalizing, full-layer-penetration inter-critical annealing, and long-duration tempering—i.e., NIT). Consequently, when the net bead height was no greater than the width of the fine grain zone affected by the previous layer of deposition, the microstructures in each new layer could be progressively refined and homogenized. Therefore, the intermediate zone was predominantly composed of fully equiaxed α-Fe and displayed homogeneous characteristics along the build direction. During full-layer NIT treatment, the pearlite was spheroidized slightly, reduced both in size (from 4 µm to 2 µm) and in area fraction (from ~5% to ~1%), thereby mitigating cracking susceptibility. Moreover, not only were the intracrystalline dislocations significantly reduced, but also their morphology evolved from tangled to movable lines. These changes, along with finer grains and the formation of high-angle grain boundaries, led to lower and more dispersed internal strain in the α-Fe matrix. Consequently, excellent Charpy toughness (over 300 J at − 60 °C) with minimal deterioration in strength was achieved. Impact, tensile, and hardness tests revealed nearly isotropic mechanical characteristics. We believe that this novel method shows tremendous promise in large-scale additive manufacturing.