Microstructural evolution and mechanical properties of the aluminum-alloyed Fe-1.50 wt%B-0.40 wt%C high-speed steel

The microstructure and properties of Fe-1.50 wt%B-0.40 wt%C high-speed steel (i.e. High boron high-speed steel, HBHSS) containing various Al contents have been investigated. The results show that the microstructure of HBHSS is composed of martensite, a little M₆(C,B) and a large number of eutectic M₂B borides. With the increase of aluminum, the martensite can reduce and lots of ferrite and pearlite occur, while M₂B boride is gradually refined and isolated. Meanwhile, most of aluminum is mainly segregated within the ferrite grains. The room tensile strength of the steel begins to decrease when Al content exceeds 0.6 wt%, whereas the fracture and impact toughness are remarkably enhanced. A little aluminum can maintain high-temperature tensile stress of the steel at 500 °C and simultaneously improve its elongation. The high-density dislocations can be formed within Al-segregated ferrite zone to exhibit the strengthening and toughening roles, and a possible orientation relationship between (Fe,Cr)₂B and multi-component M₂B is (110)_(Fe,Cr)2B ∕∕ (110)_M2B in the steels.