Effect of Sub-zero Treatment on the Microstructural Characteristics and Mechanical Properties of Maraging Stainless Steel

The evolution of the microstructural and mechanical properties of maraging stainless steel specimens with or without sub-zero treatment was examined in the present study. Through various microstructural analysis techniques, the primary microstructural characterization after various heat treatments can be summarized as follows: First, the amount of reversed austenite increased monotonically as the aging temperature increased. Second, the lattice strain of the martensite matrix was increased by sub-zero treatment but later relieved by the aging process due to the recovery effect of the martensite matrix. Furthermore, the fraction of high-angle grain boundaries (HAGBs) grew when the aging treatment was executed at higher temperature, which was ascribed to the merging of laths and blocks. Third, numerous η -Ni 3 Ti particles having a hexagonal closed-packed (HCP) crystal structure and adopting a Burgers orientation relationship with martensite matrix precipitated with homogeneous distribution in the steels. In addition, the mechanical properties of the tested maraging stainless steel were also consistent with the microstructural features of the specimens. For instance, in comparison with specimens without sub-zero treatment, specimens with sub-zero treatment had higher ultimate tensile strength (UTS) and yield strength (YS). These differences can be primarily ascribed to lattice strain, i.e., dislocations, introduced in the matrix upon sub-zero treatment. That lattice strain led to precipitation strengthening in the steel due to the greater number of nucleation sites facilitating the precipitation of η -Ni 3 Ti particles.