Understanding the Toughness Mechanism of High-Boron Stainless Steel: Impact of Boron Content

The increase of boron content leads to decreased toughness of high-boron stainless steel, which limits its application in the nuclear field. Hot isostatic pressing (HIP) sintering can effectively control the size and distribution of borides, improving plasticity and toughness. However, boron content significantly impacts microstructure and mechanical properties, particularly toughness. This study prepared high-boron steel samples with 2.06 wt% and 3.32 wt% boron using HIP, and systematically investigated the specific mechanism by which boron content affects toughness through microstructural observation, phase analysis, and comparative analysis of microstructure and texture before and after tensile deformation. Results show that when the boron content exceeds a critical proportion, the connection between boride grains causes an increase in boride grain size and phase fraction from 23.54% to 50.12%. The tensile strength remains comparable at 819 ± 1 MPa and 839 ± 23 MPa. However, the elongation decreases dramatically from 22.1 ± 2.6% to 1.2 ± 0.2%. In 2 wt sample, the interconnected austenite matrix accommodates coordinated deformation through dislocation cross-grain slip and grain rotation, resulting in ductile dimple fracture. In the 3.3 wt sample, the interconnected boride network restricts cross-grain dislocation motion and grain rotation, leading to brittle intergranular fracture with minimal plastic deformation. This study provides theoretical support for further optimizing the microstructure and mechanical properties of high-boron stainless steel materials.