Post-heat treatment design for high-strength low-alloy steels processed by laser powder bed fusion

A novel post-heat treatment design is implemented for additively manufactured copper-bearing high-strength low-alloy (HSLA) steels by understanding the processing-structure-property relationships. Hot isostatic pressing (HIP) is adopted to reduce the porosity from 3% to less than 1% for the HSLA-100 steel processed using laser powder bed fusion (LPBF). Quenching dilatometry is employed to design the parameters for the HIP cycle with an optimized cooling process. In order to achieve the maximum amount of martensite, drop cooling after HIP is found to be more suitable than controlled cooling. A subsequent cyclic re-austenitization is introduced to achieve effective grain refinement to compensate for the coarsened microstructure after HIP. The re-austenitization effectively leads to a 60% reduction in the prior austenite grain (PAG) size. The microstructure of the as-built and HIP HSLA steels before and after cyclic re-austenitization consists of martensite, bainite and martensite/ retained austenite (M/A) islands. Tempering heat treatment is applied after HIP, to induce strengthening due to precipitation hardening, that is optimized through microhardness and microstructure characterization. The peak hardness is achieved at 5 h of aging and the microstructure consists of tempered martensite, bainite, and M/A islands. A significant increase in the tensile yield strength, as well as the ductility, is achieved in comparison with the as-built alloy with the tailored microstructure obtained after the application of the designed post-heat treatment to HSLA-100 steels processed using LPBF. As LPBF of HSLA steel is attempted for the first time, this work demostrates the significance of post-heat treatment design for the AM technique.