Effect of Boron on the Microstructure and Strength-Toughness of Fe-Cr-B-C Alloy after Heat Treatment

In order to explore new methods for improving the strength-toughness matching of Fe-Cr-B-C alloy， the effect of B content on its microstructure， hardness， and impact toughness after heat treatment were investigated. The results showed that after air-cooling quenching at 1 050 ℃， a large amount of extremely fine secondary carbon-borides precipitated in the interdendritic region of low-B （0.000 6%） alloy， and fine dispersed secondary carbon-borides precipitated in the matrix of intermediate-B （0.51%） and high-B （2.89%） alloys. As the B content increased， the size of secondary carbon-borides increased but the particle density decreased. After air-cooling quenching at 1 050 ℃ plus tempering at 500 ℃， the number of secondary carbon-borides increased and the precipitation region expanded. For the low-B alloy， the dendrite arm was composed of coarse lath-like martensite， and the interdendritic region was composed of acicular martensite， high-density secondary carbon-borides and a small amount of solidification precipitates. For the intermediate-B alloy， the microstructure was composed of acicular martensite matrix with dispersed secondary carbon-borides， and network boron-carbides. For the high-B alloy， the microstructure was composed of γ matrix with a small amount of dispersed secondary carbon-borides， and denser network boron-carbides. The low-B alloy possesses the highest impact toughness and moderate Rockwell hardness. The intermediate-B alloy possesses the highest Rockwell hardness and moderate impact toughness. The high-B alloy possesses the lowest Rockwell hardness and impact toughness. Compared with the as-cast state， the solidification precipitates dissolved slightly after the quenching and tempering treatment， which resulted in the local rupture of netlike boron-carbides in intermediate-B and high-B alloys. Thus， after air-cooling quenching at 1 050 ℃ plus tempering at 500 ℃， the impact toughness of intermediate-B and high-B alloys improved obviously.