Effect of boron addition on Nb(C, N) refinement and dissolution behavior in Super 304H austenitic heat-resistant steel

The formation of coarse primary niobium carbonitrides Nb(C, N) during the solidification of niobium-containing heat-resistant steel poses significant challenges for high-temperature homogenization and subsequent mechanical processing. These precipitates exhibit high thermal stability and are difficult to dissolve during homogenization, resulting in non-uniform microstructures and reduced mechanical properties. This study systematically investigates the effect of boron (B) addition on the precipitation and dissolution behavior of Nb(C, N) in Super 304H austenitic heat-resistant steel. The trace B addition (0.012 wt%) reduces the volume fraction of Nb(C, N) from 1.6 % to 0.9 % and decreases its size, which effectively inhibits the formation of coarse primary Nb(C, N) during solidification. However, excessive B addition (0.03 wt%) promotes the formation of detrimental Cr-rich borides, negatively impacting corrosion and oxidation resistance. B addition (0.012 wt%) also enhances the dissolution rate of primary Nb(C, N) during homogenization at 1240 °C, increasing the dissolution fraction of Nb(C, N) from 38.7 % to 54.9 % within 30 min. First-principles calculations demonstrate that B addition increases the binding energy of Nb(C, N), thereby reducing its structural stability. The kinetic analysis using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model also shows that B addition decreases the activation energy for Nb(C, N) dissolution from 324.1 kJ/mol to 294.7 kJ/mol, which significantly accelerates its dissolution rate. By refining primary Nb(C, N) and accelerating its dissolution, B addition significantly reduces the required homogenization time and improves microstructural uniformity. The B alloying method provides a novel and efficient strategy for enhancing the processing efficiency and high-temperature performance of Nb-containing heat-resistant steels.