Studies On Wear Resistance Of Quench-Partition Head Treated Ductile Cast Iron

As the research object,ductile cast iron was melted and prepared.Then Q&P（Quenching and Partitioning）heat treatment is carried out.The mechanical properties of as-cast and heat-treated specimens were tested by universal tensile tester and Rockwell hardness tester.Using Al₂O₃ ceramic ball as friction pair,the dry sliding friction and wear experiments of different samples were carried out with the reciprocating electrochemical corrosion friction and wear tester（MPT-EC4000）.The microstructure of the samples was characterized by Optical Microscope（OM）,Scanning Electron Microscope（SEM）,Transmission Electron Microscope（TEM）and X-Ray Diffraction（XRD）.The influences of partitioning time on the mechanical properties and wear resistance of ductile cast iron,as well as the influences of different friction parameters on the wear resistance were discussed.And the wear mechanisms of ductile iron in different states were analyzed.In addition,based on the alloy element copper can effectively improve the mechanical properties of as-cast ductile cast iron,the influence of copper content on the wear resistance was studied.The main conclusions are as follows:（1）The content of retained austenite changes non-monotonously with the increase of the time,first increasing and then decreasing.The content of retained austenite reached a maximum value of about 18.1%with the partitioning time of 10 minutes.In the early stage of the partitioning treatment,the carbon in the supersaturated martensite diffuses to the adjacent austenite,which improves the carbon content in the austenite and enhances its stability.Hence,the probability of transformation from austenite to martensite is reduced in the final cooling stage.As a result,the content of retained austenite is increased.When the incubation period of bainite transformation is exceeded,part of austenite will be transformed into bainite and the total austenite content will be reduced.Although the content of retained austenite is non-monotonic,the carbon content of retained austenite increases with the increase of partitioning time.The tensile strength and elongation increase with the increase of partitioning time,but the hardness decreases due to the decrease of martensite content.The results of friction and wear experiments show that the wear resistance increases with the increase of austenite content.When the content of bainite increases,although the strength and toughness increase,the wear resistance decreases,indicating that the wear resistance of bainite is lower than that of martensite.（2）The wear rate of ductile cast iron in different state increases with the increase of friction time.In the early stage of friction process,the friction work is so small that it does not reach the fatigue limit of specimen surface deformation.Therefore,the friction coefficient is relatively low.With the increase of friction time,the matrix of the sample peels off due to fatigue wear and the friction coefficient rises sharply.As a result,the wear rate rises.In addition,the wear rate decreases with the increase of friction load and/or frequency.With the increase of friction load and/or frequency,much heat and energy were generated and cannot be completely absorbed by the matrix,which result the damage of matrix and formation of a large number of wear debris and oxidation.These oxide films and oxidation particles can effectively prevent the contact between friction pairs and reduce wear.Q&P treatment of ductile iron tends to fatigue wear under higher friction load and/or frequency,but tends to abrasive wear under lower load.When the friction load is fixed,the higher the strength of ductile iron is,the more it tends to abrasive wear,and the lower the strength is,the more it tends to fatigue wear.（3）Alloy element Cu can effectively increase the volume content of pearlite in as-cast ductile iron,so as to improve its tensile strength and hardness.The results of friction test show that Cu can effectively improve the wear resistance of as-cast ductile iron.When the matrix structure is mainly composed of soft ferrite,the main wear mechanism is plastic deformation.With the increase of pearlite content,the abrasive wear is intensified.For the only quenched samples,the comprehensive mechanical properties and wear resistance improve with the increase of Cu content due to the effect of Cu on stabilizing austenite.However,for the Q&P treated samples,due to the complex effect of Cu on solution strengthening and phase transformation,the effect of Cu on the wear resistance is not obvious,which needs to be revealed by further experiments.