Research On Microstructure Of AL-bearing High-boron High Speed Steel

Al-bearing high-boron high speed（AB-HSS）is a new type of hot roll material with excellent wear resistance and thermal and economy.The high content of boron significantly increases the hardenability of the material,reducing the amount of expensive alloy elements,and formed a lot of borocarbides.Aluminum can improve the tempering stability and red hardness of steel.The borocarbides is relatively coarse and distributed as a continuous network along the grain boundary in the usual AB-HSS which destroys the continuity of the matrix.Aluminum inclines the matrix toward the Ferrite and reduces the macroscopic hardness of the material.They aren’t conducive to give full use to excellent wear resistance,and modification treatment and heat treatment are needed to optimize the microstructure of AB-HSS.Therefore,it is of great significance to research the microstructure in different states and deeply characterize borocarbides of A-BHSS.In this paper,it is studied the microstructure of a new type of A-BHSS with excellent mechanical properties provided by Fu Hanguang’s research group.The morphology and types of borocarbides are characterized by various material characterization methods,and analyzed on boron carbide by the modification and heat treatment.The main results are as follows:The As-cast microstructure of AB-HSS is a near-eutectic structure.The matrix is mainly Ferrite and a litter martensite,and the borocarbides are a continuous coarse network shape.The composition and phase are analyzed of borocarbides by XRD,EBSD/EDS.The network borocarbide is roughly divided into three types:rod-shaped or fish-bonded Fe₂B type Cr-rich borocarbide,large bulk or dense spherical Fe₃C type Cr-Mo borocarbide and dense network or granular Fe Mo₂B₂ type Mo-rich boron carbide,and a small amount of V-rich granular VC type borocarbide.The accuracy of the crystal structure and composition were determined by transmission electron microscopy.Fe₃C type borocarbide is the main component of the coarse network,and surrounds or wraps Fe₂B type borocarbide and Fe Mo₂B₂ type borocarbide.After RE-Mg-Ca modified treatment,the microstructure of cast AB-HSS is obviously purified and refined,which is hypoeutectic.The metamorphic treatment don’t change the type of borocarbides,but the fish-boned and dense spherical borocarbides are significantly reduced,and the volume fraction of borocarbides is slightly reduced.The microstructure of AB-HSS spheroidized after heat treatment,and the network shaped borocarbides obviously borken.Component analysis and phase identification of borocarbides can be divided into rod-shaped or fish-bone Fe₂B type Cr-rich borocarbide,small bulk（Fe,Cr）₂₃C₆ type Cr-Mo borocarbide,and dense network and granular Fe Mo₂B₂ type Mo-rich borocarbide.The accuracy of the crystal structure and composition were determined by transmission electron microscopy.The composition of Fe₃C and（Fe,Cr）₂₃C₆ borocarbides are similar.The volume fraction of granular Fe Mo₂B₂ type borocarbide is obviously increased.The fracture of network borocarbides during heat treatment is due to the decomposition of metastable Fe₃C borocarbide and precipitation of granular Fe Mo₂B₂ boron carbide and conversion into bulk（Fe,Cr）₂₃C₆ borocarbide.The microstructure of the modified AB-HSS is more uniform,and the borocarbides are dispersive in the matrix.the material toughness is significantly improved.The wear resistance is more excellent.