Influence Of Composition And Process On The Microstructure And Properties Of Dual Phase Austempered Ductile Iron

The dual phase austempered ductile iron (Dual Phase ADI) is austempered ductileiron with eutectoid ferrite, which is formed by the partial austenitizing process. Due to theexistence of eutectoid ferrite, the machinability of ADI is improved and the sensitivity ofADI for defects is reduced. The heat treatment and then machining is carried out for highaccuracy in dimension.In this paper, the effects of silicon on the microstructure as-cast andthe eutectoid three-phase region of ductile iron, the effects of heat treatment process on themicrostructure, mechanical properties and wear properties under oil lubricating of dualphase ADI, the effects of copper on the microstructure as-cast of ductile iron and themicrostructure, mechanical properties and wear properties under oil lubricating of the dualphase ADI, and microstructure formation mechanism of ADI and the dual phase ADI wereinvestigated. In the test,the metallographic microscope, scanning electron microscope(SEM), X ray diffraction (XRD) were used for the characterization of the composition,morphology and microstructure; the mechanical properties were measured by Brinellhardness tester and impact tester; the wear resistance was detected by MM-200friction andwear testing machine; the corrosion resistance of the samples was tested by electrochemicalworkstation. The research results show that:(1) The upper line and lower line and range of eutectoid phase temperature are increased;the spheroidization rate of ductile iron is increased, the pearlite content is decreasedgradually with the increasing of silicon content of ductile iron.(2) The content of eutectoid ferrite in matrix is reduced and the content of ausferrite isincreased, residual austenite and its carbon content are increased with the increasing ofaustenitizing temperature. The mechanical properties are showed higher hardness andtensile strength, lower elongation and impact toughness. Wear resistance under oillubricating and corrosion resistance are improved.(3) With the increasing of austempered temperature, the ausferrite morphology is changedfrom the fine needle into the featheriness, the residual austenite and its carbon content areincreased; the hardness and tensile strength are decreased, impact toughness and elongationare increased, the corrosion resistance is decreased first and then is increased, wear massloss is increased. Due to the stress induced, the phase transformation is happened fromaustenite to martensite, wear mass loss is decreased. However, when the austempered temperature reaches395℃, the emergence of a large number of pearlite in the matrixresults in decrease of the residual austenite content and mechanical properties, wear massloss increase and corrosion resistance decrease.(4) With the increasing of copper content, the spheroidization rate of ductile iron andthe content of pearlite are increased gradually. The effects of copper on matrix morphology,mechanical properties, wear resistance of dual phase ADI are not obvious. The effect ofcopper on dual phase ADI is mainly reflected in the enhancing hardenability, the corrosionresistance of dual phase ADI is improved for the addition of copper.(5) The transformation mechanism of austeniting of ADI follows the transformationfrom pearlite to austenite. In the process of austenite growth, carbon is diffused from thedistal austenite of the graphite to the near of it. In the process of isothermal transformation,carbon content is fluctuate in austenite inner. The acicular ferrite is nucleated in poorcarbon region. The supersaturated carbon of ferrite is diffused to austenite to obtain the richcarbon residual austenite.