Effect of repeated laser scanning on the microstructure evolution of carbon-bearing martensitic steel manufactured by laser powder bed fusion: Quenching-partitioning drives carbon-stabilized austenite formation

Abstract

Carbon-bearing martensitic steels have gained interest for their use in aerospace and defense applications due to their ultra-high strength and reasonable ductility. In addition, additive manufacturing (AM) is an effective manufacturing process for a high degree of control over geometry. Microstructure evolution during additive manufacturing should be importantly considered because it is closely related to the formation of defects, mechanical properties, and conditions of post-heat treatments. This study investigated the microstructure evolution of carbon-bearing martensitic steel during laser powder bed fusion (LPBF) to identify the effect of rapid solidification, solid-state transformation, and in-situ heat treatments on the formation of the as-built microstructure. As a result of investigation of a reconstructed prior austenite map by the Kurdjumov-Sachs orientation relationship, it was confirmed that the austenite is generated mainly at the columnar, martensite block, and lath boundaries. A Scheil-Gulliver model represented that carbon and molybdenum are segregated to the columnar boundary, and this could be the main reason why austenite showed high fraction at room temperature. The carbon concentration in the austenite is much higher than that of the martensite interior, which is significantly caused by carbon partitioning derived by repeated laser scanning. A constrained carbon equilibrium model was used to estimate the final fraction of austenite, and the results showed that the characteristics of the as-built microstructure can be reasonably estimated. The microstructural evolution mechanism introduced here is expected to allow successful LPBF of carbon-bearing martensitic steels.