Improved microstructures and mechanical properties of boron-containing 304L austenitic stainless steel after liquid phase sintering and heat treatment

Powder metallurgy (PM) 304L austenitic stainless steel alloyed with boron exhibits enhanced densification via liquid phase sintering (LPS). However, the formation of continuous intergranular borides often deteriorates its mechanical properties. This study investigates the effects of post-sintering heat treatments at temperatures ranging from 1000 °C to 1200 °C on the microstructure, mechanical properties, and fracture mechanisms of PM 304L+0.4B (wt%) stainless steel. The results show that after sintering at 1300 °C, the density was much increased from 6.36 g/cm 3 to 7.75 g/cm 3 . Microstructural analyses revealed that heat treatment at 1200 °C for 6 hours (HT1200-6) significantly altered the morphology of Fe 2 B borides. The continuous eutectic boride network along austenitic grain boundaries was partially dissolved and transformed into a discontinuous distribution. Due to the above microstructural modification, the HT1200-6 treatment significantly enhanced the tensile elongation by approximately 50 % (from 24.5 % to 36.5 %). Meanwhile, the apparent hardness, yield strength, and ultimate tensile strength were slightly improved from 71 HRB to 78 HRB, from 170 MPa to 176 MPa, and from 452 MPa to 491 MPa, respectively. In situ tensile testing coupled with digital image correlation (DIC) demonstrated that this microstructural modification effectively promoted a more homogeneous strain partition of austenitic grains and delayed intergranular fracture. These findings indicate that combining a controlled B content with an optimal heat treatment provides a viable strategy for developing boron-containing PM stainless steels with a superior combination of strength and ductility.