Effect of heat treatment on microstructure and mechanical properties of Fe-5Cr-1.4B alloy

The microstructures and mechanical properties of a Fe-Cr-B alloy containing 1.4 wt.-% B and 5.0 wt.-% Cr were researched after different heat treatment processes. The results indicate that the as-cast microstructure of the Fe-5Cr-1.4B alloy consists of M₂B (M = Fe, Cr), M₇(C,B)₃, α-Fe and γ-Fe. The netlike eutectic M₂B phase is distributed in the matrix. After water quenching, the netlike eutectic M₂B phase begins to fracture, the metallic matrix transforms into martensite and secondary M₂₃(C,B)₆ borocarbides are precipitated. When the quenching temperature exceeds 1050 °C, the number of precipitated secondary borocarbides is reduced. After quenching, the macro- and microhardness of the samples increase with the increase of temperature, and the hardness reaches a maximum value at 1050 °C. When the temperature exceeds 1050 °C, the hardness value will slightly decrease. After tempering treatment, the matrix precipitates as miniature secondary borocarbides, and the martensite begins to soften, and the hardness is lower compared to that quenched by heat treatment. The decreasing hardness tendency becomes more pronounced with the increase of the tempering temperature. Moreover, the impact toughness reaches its maximum value (7.35 J × cm²) when the tempering temperature ranges at 500 °C.