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 precipi-tated 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 martens-ite,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 sec-ondary 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.
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