Effect of Aluminum on Borocarbides and Temper Softening Resistance of High-Boron High-Speed Steel

The effects of aluminum content on the morphology of eutectic borocarbide and temper softening resistance of high-boron high-speed steel with 2.0 wt pct B-0.4 wt pct C-6.0 wt pct Cr-4.0 wt pct Mo-x wt pct Al-1.0 wt pct Si-1.0 wt pct V-0.5 wt pct Mn (x = 0.0, 1.0, 1.5, 2.0) have been investigated in the present work. The experimental results indicate that aluminum not only promotes the refining and nodulizing of borocarbide, but also improves the red-hardness of the alloy. The as-cast microstructure of high-boron high-speed steel (with different aluminum contents) consists of matrix α-Fe, eutectic borocarbide M2(B,C), and boron-cementite M3(B,C) (M = Fe, Cr, Mo, V, Mn). Borocarbide presents a continuous network structure in the microstructure of alloy without aluminum addition. With increasing aluminum content, borocarbide is spheroidized, and its size decreases. Furthermore, the variation in aluminum content barely affects the phase type. The hardness testing results of heat-treated samples at 1050 °C, 1100 °C, and 1150 °C reveal that the quenching temperature for obtaining the martensite matrix rises with an increase of aluminum content. However, the alloy matrix with 2.0 wt pct aluminum cannot transform to a martensite matrix through quenching, even at 1150 °C. The red-hardness is defined as the alloy hardness after four rounds of tempering at 600 °C for 1 hour. The hardness of alloy without aluminum addition reduces significantly after tempering, while the hardness of alloy with 1.0 wt pct aluminum exhibited the highest value. Moreover, no apparent change in borocarbide morphology occurred after tempering, indicating that the alloy microstructure renders good tempering stability.