Tailoring ?/? Phase Of Ultrafine Spheroidal Grap Hite Iron And Its Effects On Mechanical Properties

In this paper,the ultrafine spheroidal graphite iron casting with homogeneously distributed high graphite density(>400/mm2)was prepared by vertical continuous casting technology.The asutempering treatment and carbon partitioning treatment were conducted to tailor the microstructure,including the size,volume fraction and carbon distribution of ?/?phase.The relationship between microstructure and mechanical properties was established.Additionally,the wear behavior was explored.The main results are as follows:After austempering treatment,the ultrafine spheroidal graphite iron is composed of spherical graphite,? phase and ? phase.With the increase of austenitizing temperature,the volume fraction of ? phase and its carbon content increase obviously,and its dimension is coarsening significantly.Additionally,thin needle-shaped a phase formed at high austenitizing temperature,which tends to form slender needle shape.When the austenitizing temperature is lower than 850?,the austenitization is incomplete and part of untransformed ferrite is retained.On the other hand,with the increase of austempering temperature,the content of y phase and its carbon content are increased.? phase with thick lath-shaped forms at high austempering temperature.When the austenitizing temperature is between 850?-950?,it is found that the strength of the ultrafine spheroidal graphite iron decreases and the ductility increases with the increase of the ? phase.However,when the austenitizing temperature is further increased,? phase is rapidly corarsened,which deteriorate the mechanical properties of the ultrafine spheroidal graphite iron.On the other hand,the y phase is increased with the austempering temperature,which results in the decrease of strength and the increase of ductility.The friction and wear experiments show that the graphite in ultrafine spheroidal graphite iron can be dragged to the surface and form graphite transfer film,which reduces the friction coefficient.The friction coefficient of ultrafine spheroidal graphite iron is lower than that of GCr15.Additionally,low hardness can promote to form graphite transfer film.Additionally,as the load and sliding speed increase,the friction surface temperature increases and begins to oxidize,which results in the change of the wear mechanism.With the increase of load and sliding speed,the wear mechanism changes from delamination wear at low load and low speed to slight oxidation wear,and then to severe oxidation wear at high load and high speed.For the austempered samples,partitioning treatment promotes the diffusion of carbon atoms from ? phase to ? phase,which results in carbon-depleted ? phase and supersaturated ? phase.Additionally,no carbide is formed.This microstructure can significantly improve ductility without reducing strength.The elongation of the partitioned sample is increased by about 50%compared to the austempered sample.