Zhefeng Xu

As is well known, reconstruction of the initial microstructure before an inter-critical annealing (IA) can optimize the final microstructure as well as improve mechanical performance. In this study, the initial microstructure with "pre-ferrite + martensite" was designed via two-stage cooling in the rolling process. The final microstructure observation showed that bimodal size distributed austenite and double morphologies of austenite. Moreover, lath-shaped austenite existed in the martensitic matrix; blocky austenite occurred at the boundaries of pre-ferrite. According to the numerical simulations of the IA process, blocky austenite tended to be more stable with smaller size and higher Mn concentrations than lath-shaped austenite. The characteristics of austenite were similar at different IA temperatures, but the lower IA temperatures, the smaller size of austenite. An excellent combination of strength (1308 MPa) and ductility (34%) was obtained at 928 K, which was ascribed to a positive TRIP effect induced by the austenite with a large stability gradient.

In this paper, VGCF/304L stainless steel composite was prepared using spark plasma sintering (SPS) with 2 vol.% nanocarbon fiber. Then, phase content, grain size, and mechanical properties were evaluated. The results showed that the Vickers hardness of 304L sintered stainless steel and the composite prepared by SPS under the condition of 950 °C for 10 min were 244 HV and 310 HV, respectively, which is an increase of about 27.0%. The yield strength ascended from 587.1 MPa to 890.2 MPa, registering an increase of approximately 51.6%. Concurrently, the ultimate tensile strength rose from 821.5 MPa to 1064.7 MPa, with an approximate increase of 29.6%. This method for improving the mechanical properties of stainless steel is expected to be widely adopted for producing fiber-reinforced metal matrix composites with excellent overall performance.

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