Microstructure engineering during wire-arc additive manufacturing of high-strength martensitic steels

Abstract

Martensitic steel components produced using wire-arc additive manufacturing (WAAM) can contain coarse, highly textured columnar prior austenite grain (PAG) structures in the as-built state, which may exacerbate cracking issues during building and are difficult to remove with conventional post-build heat treatments. Here, we demonstrate that fine PAG structures can be achieved in WAAM’d martensitic steel by using a sufficiently low interpass temperature and ensuring a large enough heat-affected zone (HAZ) depth relative to the deposited layer height. It was found that controlling these two conditions during the building of 300M steel ensured that each layer of steel deposited was repeatedly cycled between austenite and martensite, refining the PAGs since they were in the HAZs of subsequent layers. The interpass temperature and HAZ depth required for 300M were found using dilatometry followed by electron backscatter diffraction (EBSD) mapping and validated through the microstructural characterization of three 300M WAAM walls built with different interpass temperatures and HAZ overlaps. EBSD characterization revealed that, when austenitised, 300M exhibited a memory effect that was followed by recrystallisation without applied external deformation. Importantly, since the grain refinement was achieved through austenite-martensite cycling, it is highly likely that the principles exploited here could be applied to achieve similar grain refinement in other martensitic steels.