A novel 2.1 GPa martensitic stainless steel manufactured by laser powder bed fusion and post treatment

Ultra-high strength martensitic stainless steel prepared by traditional methods often has to go through forging or rolling process. In the work, A novel 20Cr13Co12Mo4Ni2 martensitic stainless steel was developed and manufactured by laser powder bed fusion (LPBF) and cryogenic + heat treatment. The ultimate tensile strength of martensitic stainless steel is 2121 MPa, and the elongation of fracture is 9.2 %. Due to cryogenic + heat treatment converts a large amount of austenite to martensite, and the high density dislocation generated by the rapid heating and cooling process of LPBF is retained, the sample has a dislocation density of 1.85 x 1016 m-2, which is comparable to forging and rolling. The high dislocation density can promote the nucleation of the second phase and make the precipitated phase small and dispersed, therefore, there are a large number of nanoscale rod-like (Fe,Cr)2Mo laves phase and M2C carbide precipitated in the heat-treated specimens. In addition, the cellular heterostructure formed by LPBF process can improve the ductility. Through theoretical calculation, the second phase strengthening is the main strengthening mechanism. The steels in this work have significant advantages in mechanical properties, and the trade-off between strength and ductility is well avoided.