Evolution of Microstructure and Elements Distribution of Powder Metallurgy Borated Stainless Steel during Hot Isostatic Pressing

Prepared by powder metallurgy process incorporating atomization and hot isostatic pressing (HIP) sintering at six different temperatures from 600 to 1160 °C, borated stainless steel (BSS) containing boron content of 1.86 wt% was studied. The phase of BSS, relative density of different temperature, microstructure, elemental distribution, and mechanical properties were tested and analyzed. The phases of the alloy were calculated by the Thermo-Calc (2021a, Thermo-Calc Software, Solna, Sweden) and studied by quantitative X-ray diffraction phase analysis. The distributions of boron, chromium, and iron in grains of the alloy were analyzed by scanning electron microscopy and transmission electron microscope. The grain size distributions and average grain sizes were calculated for the boron-containing phases at 900, 1000, 1100, and 1160 °C, as well as the average grain size of the austenite phase at 700 and 1160 °C. After undergoing HIP sintering at 900, 1000, 1100, and 1160 °C, respectively, the tensile strength and ductility of the alloy were tested, and the fracture surfaces were analyzed. It was found that the alloy consisted of two phases (austenite and boron-containing phase) when HIP sintering temperature was higher than 900 °C, and the relative density of the prepared alloys was higher than 99% when HIP temperature was higher than 1000 °C. According to the boron-containing phase grain size distribution and microstructure analysis, the boron-containing phase precipitated both inside the austenite matrix and at the grain boundaries and its growth mechanism was divided into four steps. The tensile strength and elongation of alloy were up to 776 MPa and 19% respectively when the HIP sintering was at 1000 °C.