Research On Microstructure And Mechanical Properties Of Ultra-low Carbon ADI With Sphere Carbide

The ultra-low carbon ductile cast iron contained sphere carbide was a new type of wear resistant material based on the austempered ductile iron(ADI).At present,extensive efforts had been devoted to investigating the traditional ADI,unfortunately,little attention has been paied to the ultra-low carbon ductile cast iron contained sphere carbide.In this paper,the formation mechanism of the sphere carbide in the ultra-low carbon ductile cast iron had been investigated.The results demonstrated that the sphere carbide was part of eutectic carbide which crystallized in the isolated eutectic liquid area.The formation of the sphere carbide related to the contact and intersection between the primary dendrite and the secondary dendrite of the austenite.The oxides of magnesium,rare earth and other elements can be acted as heterogeneous nucleation site for the sphere carbide.It was also found that the number of the sphere carbide increased with the increase of the carbon content.The cooling rate had an important influence on the sphere carbide at the same composition condition.The influence of heat treatment process on microstructures and properties of ultra-low carbon ductile cast iron contained sphere carbide were systematically investigated.The results showed that the sphere carbide began to dissolve in alloy matrix at 860? and dissolved completely at 920?.Sphere carbide in the cast microstructure was partly dissolved during the austenitizing process,and the remianing carbide continued to exist in the matrix.The largest hardness value was obtained in the temperature of 900?.The hardness of the ultra-low carbon ductile cast iron contained sphere carbide decreased first and then increased at the austenitizing temperature from 860? to 940?.At the same time,the impact toughness increased first and then decreased.The hardness of the material decreased first and then increased and the impact toughness increased first and then decreased in the range of time from 0.5h to 4.5h.The hardness of the material decreased first and then increased and the impact toughness increased first and then decreased during the isothermal temperature from 240? to 340?.According to the research resultes the optimal heat treatment process was austenization at 880?~900? for 1h,rapidly quenched into salt bath at 280? for 3.5h and then cooled to the room temperature.The impact toughness value could reach to 24 J/cm2 at this condition,and the hardness value was at the HRC of 49.The effect of the heat treatment condition on the wear resistance of the alloy was systematically studied.The results demonstrated that the weight loss of the alloy decreased first and then increased with the increasing of the austemper and the austenitizing temperature.The wear mechanisms during the wear process were mainly abrasive wear and fatigue wear,and the wear surface morphology showed the furrows and fatigue spalling pits features.The wear properties of the alloys in high speed and high temperature condition were also explored.The results showed that the friction coefficient decreased with the increase of the load and the temperature.The wear rate increased with the increase of the temperature,however it increased first and then keep in a certain range with the increase of the load.The mixed wear mechanism during the process of the high temperature wear mainly included abrasive wear,adhesive wear and oxidation wear.And the wear morphology was mainly large plow and floc oxide.In the wearing process of the different load,the wear mechanism was mainly about abrasive wear and fatigue wear.And the wear morphology was furrow and lamellar spalling pits.