Mechanical properties and fracture mechanism of boron-containing 304L austenitic stainless steel densified by liquid phase sintering

摘要

Boron (B) is an effective element for facilitating liquid phase sintering (LPS) of powder metallurgy (PM) steels. However, the roles of B in the mechanical properties and fracture behaviors of PM steels have not yet been fully clarified. The purpose of this study was to investigate the effects of B on the LPS, microstructure, mechanical properties, and fracture mechanism of 304L PM stainless steel. The deformation and fracture behaviors were analyzed by in-situ tensile tests, digital image correction, and observation of the fracture surfaces. The results showed that adding 0.6 wt% B to 304L significantly increased the sintered density from 6.86 g/cm~3 to 7.47 g/cm~3 and facilitated pore spheroidization after 1250 °C sintering. Networked eutectics consisting of austenite and M_2B borides rich in chromium formed along the grain boundaries, as identified by electron backscatter diffraction and electron probe micro-analysis. Moreover, the apparent hardness improved by 86%, the yield strength by 64%, and the ultimate tensile strength by 63% without obvious sacrifices to tensile elongation. The fracture analyses in this study indicated that, in pure 304L, the main fracture mode was ductile fracture at sintered necks. In contrast, in 304L+0.6B, the predominant fracture mode was transgranular cleavage through the austenite grains. Intergranular eutectics and pores were not the primary fracture sites. The inter-granular eutectics were not fully continuous along the grain boundaries and did not dominate the fracture, although the eutectics were the regions with strain concentration or cracking before fracture. The correlations among the microstructure, mechanical properties, and fracture mechanisms of B-alloyed 304L stainless steel are discussed in this study.