Effect Of Heat Treatment On Microstructure And Properties Of Al-Bearing High-Boron High-Speed Steel

High-boron high-speed steel is a new wear-resistant material developed on the basis of common high-speed tool steel with boron as the main alloy element.Low-cost element boron can replace expensive alloying elements,such as W and Mo and other elements,to reduce costs and boride is formed to improve the hardness and wear resistance of materials.Aluminum can improve the tempering stability and red hardness of steel.Based on this,a new type of Al-bearing high-boron high-speed steel?AB-HSS?was designed in this paper.The microstructures of as-cast and heat-treated alloy were analyzed by optical microscope?OM?,scanning electron microscope?SEM?,X-ray diffractometer?XRD?and energy spectrometer?EDS?.The hardness and wear resistance of the alloy under different heat treatment conditions were tested by Rockwell hardness tester,Vickers hardness tester and wear tester.The influence of quenching temperature and tempering temperature on the microstructure and properties were emphatically analyzed.The experimental results show that:?1?As-cast microstructure of AB-HSS is mainly composed of pearlite and ferrite and a large number of eutectic hard phases,and the eutectic hard phases consist of network M₂B type eutectic boride and a small amount of M₂₃?C,B?₆ type borocarbide with granular morphology.?2?CCT curves and TTT curves of AB-HSS after austenitizing at different temperatures?1030?,1050?,1070?,1090?and 1100??were simulated and the ferrite precipitation temperature,pearlite precipitation temperature,bainite precipitation temperature,martensite transformation starting temperature?Ms?,martensite transformation finishing temperature?Mf?and critical cooling rate with austenitizing temperatures were obtained.Combining with the experience of previous research group on quenching temperature,the quenching temperatures are determined to be 950?,990?,1030?,1070?,1110?and 1150?.By simulating the phase microstructure and mechanical properties of the alloys at different cooling rates for 0.1?/s,1.0?/s,10.0?/s and 100?/s,it is found that different cooling rates have great influences on the properties of the alloys.Overall,the mechanical properties of the alloy are the best when the cooling rate is 100?/s.?3?After quenching treatment at 950¹110?,the alloy matrix gradually transforms from ferrite and pearlite to martensite,and its hard phases are still M₂B and M₂₃?C,B?₆.However,with the increase of quenching temperature,the continuous network boride gradually breaks and develops into isolated blocks.When quenching temperature is 1150?,the matrix structure is obviously spheroidized and coarsened,and a small amount of residual austenite is produced.?4?After quenching,the macro-hardness of the alloy is improved significantly.With the increase of quenching temperature,the macro-hardness increases gradually from 40.1 HRC in as-cast to 65.1 HRC after oil quenching at 1110?.At this time,the corresponding micro-hardness of matrix also reaches the maximum and is about 856 HV,while the hardness of the hard phase is almost the same as that of the as-cast,and keeps at 1450 HV.The macro-hardness and matrix hardness decrease slightly when the temperature exceeds 1110?.The alloy hardness increases slightly after quenching in air at different temperatures.The maximum macro-hardness is only 58.6 HRC,and the hardness matrix hardness is also significantly lower than that of alloy cooled in oil.?5?The wear resistance of the alloy is the best after oil quenching at 1110?.When the quenching temperature is increased from 950?to1110?,the wear resistance increases.When the quenching temperature is 1150?,the wear resistance decreases slightly.Finally,the alloy was quenched at 1110?and then tempered at 400⁶00?,and the effect of tempering temperature on the microstructure and properties of the alloy was studied.The results show that:?1?The tempered structure of alloy consists of tempered martensite,a small amount of ferrite and hard phase.The tempered martensite is lath-like and has excellent strength and toughness.The type of hard phase has not changed.It still consists of M₂B type boride and M₂₃?C,B?₆ type borocarbide.?2?With the increase of tempering temperature,tempering hardness first increases and then decreases.The hardness of the alloy reaches the highest value of 60.2 HRC at 450?,and the micro-hardness of matrix is 784.7 HV.This is mainly due to the precipitation of a large number of fine borocarbide from martensite at 450?.The dispersive strengthening effect is strong.The retained austenite transforms into martensite,secondary hardening occurs and the hardness reaches its peak value.?3?With the increase of tempering temperature,the wear resistance of the alloy increases first and then decreases.When the tempering temperature is 450?,the wear resistance of the alloy reaches the best.When tempering temperature is 400?and 450?,borocarbide distributed in fine dispersed with high hardness are continuously precipitated from martensite matrix with increasing tempering temperature,which can be used as effective particles to resist wear and improve wear resistance.In addition,secondary hardening also increases wear resistance.When tempering temperature is more than 450?,the precipitated borocarbide gradually aggregate and grow up with the tempering temperature increasing.The long-term heat preservation provides conditions for the restoration of network structure,leading to the decrease of the hardness and toughness,and the wear resistance also decreases.