Microstructural evolution and martensitic transformation in FeCrV alloy fabricated via additive manufacturing

Abstract

FeCrV alloy (high-wear-resistance steel or HWS) has been extensively used in additive manufacturing for hardfacing and repairing owing to its excellent hardness, superior weldability, and wear resistance. This alloy must possess superior mechanical property under harsh environments, such as friction and wear, to facilitate its application in repairing gears and strengthening the surface of a die locally. HWS deposited by directed energy deposition was subjected to heat treatment by quenching and tempering (QT) to improve the mechanical property and investigate the influence of martensitic transformation. During the solidification of the molten pool, the as-deposited HWS exhibited compressive residual stress (−102 (±72) MPa) on the surface, resulting from martensitic transformation. In contrast, the surface of the heat-treated HWS (HT-HWS) had tensile residual stress (150 (±34) MPa), which was induced by released carbon from the matrix and decarburization at the surface. QT heat treatment precipitated spherical chromium carbide (W0·07Mo0·07Cr1·31V0·31Fe1·38C) and rod-like vanadium carbide (W0·07Mo0·08Cr0·48V0·56Fe1·08C) on the grain boundaries and nanosized-vanadium carbide on the martensite lath boundaries. During heat treatment, the austenite of the as-deposited HWS transformed into martensite, and only 4.80% of the austenite remained. Despite heat treatment, the dislocation density did not decrease because the martensitic transformation caused plastic deformation of the adjacent grains. In addition, the precipitation of fine carbide inhibited grain growth during the heat treatment. For these reasons, the hardness of the as-deposited HWS increased by 17% (i.e., from 659 (±18.7) HV to 773 (±3.33) HV), corresponding to the hardness of carburized SCM420 (774 (±4.31) HV). The tensile strength of the as-deposited HWS increased from 570.6 (±21.5) MPa to 848.1 (±47.7) MPa by removing the melt pool boundary and needle-shaped austenite which are the crack propagation paths of the QT-heat-treated alloy. The carbide that precipitated during heat treatment changed the fracture mechanism from intergranular with dimples to cleavage.