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　alpha-Fe,　eutectic　borocarbide　M-2(B,　C),　and　boron-cementite　M-3(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　degrees　C,　1100　degrees　C,　and　1150　degrees　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　degrees　C.　The　red-hardness　is　defined　as　the　alloy　hardness　after　four　rounds　of　tempering　at　600　degrees　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.　(C)　The　Minerals,　Metals　&　Materials　Society　and　ASM　International　2018.