Microstructure And Properties Of Short-process Austempered Ductile Iron Based On Lost Foam Casting

Austempered ductile iron(referred to as ADI)is widely used in mechanical parts such as gears and crankshafts because of its excellent mechanical properties and wear resistance.At present,the heat treatment process of ADI is mainly divided into two stages,austenitization and isothermal treatment.In this thesis,ADI was prepared by short-process austempering,which omits the austenitizing process of castings in the traditional austempering.The microstructure and properties of traditional ADI and short process ADI were compared.In this thesis,a traditional ADI and two short-process ADIs were prepared.The heat treatment process of traditional ADI(hereafter referred to as ADI-1)was 900?x2h+300?xlh.The heat treatment process of short-process ADIs is that the eastings are quenched into the molten salt at 300? for 1h when the castings are cooled to 900? and 950?respectively(hereinafter referred to as ADI-2 and ADI-3).Among the three different ADIs,the size distribution of graphite spheres in short-process ADIs are uniform,and the growth of graphite spheres precipitated above the liquidus temperature was inhibited.The size distribution of graphite spheres in traditional ADI is not uniform,the growth of graphite spheres precipitated above the liquidus temperature was not inhibited.The microstructures of the three kinds of ADI are pearlite,acicular ferrite and retained austenite.The content of ADI-1 is more than ADI-2 and ADI-3.In ADI-1,the pearlite is dispersed in matrix,which reduces the continuity of acicular ferrite and retained austenite,but the content of pearlite makes the continuity of acicular ferrite and retained austenite is relatively strong in ADI-2 and ADI-3.Among the three different ADIs,the tensile strength of ADI-1 is the lowest,and the tensile strength of ADI-3 is the highest,the values of which are 919MPa and 1031MPa,respectively.The elongation of ADI-1 is the lowest and the elongation of ADI-2 is the highest,which are 6.1%and 7.4%,respectively.The impact toughness of ADI-1 is the lowest and the impact toughness of ADI-2 is the highest,which are 5.17J and 6.56J,respectively.The hardness of ADI-1 is the lowest and the hardness of ADI-3 is the highest,that the valus are 344HV and 381HV,respectively.In terms of friction performance,the friction coefficients of ADI-1 are less than ADI-2 and ADI-3 under different loads.Under the same load,the friction coefficient of the same ADI increases first and then decreases with the increase of the applied load.For the same ADI,the friction coefficient increased firstly and then decreased with the increase of load.The mechanical properties of ADI-1 are the worst because of the high content of pearlite and the poor continuity of acicular ferrite and retained austenite,while the mechanical properties of ADI-2 and ADI-3 are better than ADI-1.Due to the influence of graphite spheres lubrication,surface roughness and hardness,ADI-1 has a minimum coefficient of friction during friction and wear.On account of the influence of microstructure,the wear volume of ADI-3 is the smallest,and the wear volume of ADI-1 is the largest.