| High chromium cast iron is widely used in metallurgy,mining,military,building materials,energy and other fields because of its excellent abrasion resistance and high temperature plasticity.However,due to the common problems of coarse carbide size and serious segregation of components in cast high chromium cast iron,the strength and toughness of the alloy are insufficient,which severely limits the full use of its wear resistance potential,and it is difficult to meet the performance requirements under severe working conditions.Especially the 25Cr series high chromium cast iron has limited practical application because of its high cost and high brittleness.In recent years,the sintered high chromium cast iron,prepared by metallurgy powder,has been systematically studied.Because of its simple carbide morphology and uniform distribution,the bending strength and impact toughness have been doubled,showing good application prospects.However,the current research work of sintered high chromium cast iron is mainly focused on the 15Cr and 20Cr series.In fact,the 25Cr series of high chromium cast iron contains more carbides,so the hardness and wear resistance are more prominent.The use of powder metallurgy preparation technology is not only expected to significantly improve its strength and toughness,but also to further improve its hardness and wear resistance,making it an excellent high-performance wear-resistant candidate alloy.In addition,the preparation of high-performance sintered high-chromium cast iron requires the use of supersolidius liquid phase sintering(SLPS)technology.However,during the SLPS process,there are some problems,such as narrow sintering temperature windows,difficulty in structure control and poor stability of mechanical properties.Solving this technical problem is of great significance to the actual production and application of sintered high-chromium cast iron alloys.This thesis takes 25Cr series hypoeutectic,near-eutectic and hypereutectic high chromium cast iron as the research object,systematically studies the relationship among composition-SLPS process-densification-microstructure evolution-mechanical properties,In view of the technical limitation of SLPS with narrow effective sintering temperature window,a new method of variable temperature segmental SLPS was explored,which can effectively widen the sintering temperature window and significantly improve the mechanical property stability of the alloy.On this basis,the effects of quenching/tempering treatment on microstructure evolution and mechanical properties were further studied,and the wear behavior and wear resistance of the alloy under impact abrasive wear and sliding wear conditions were systematically evaluated and analyzed.The author also carried out a systematic analysis and in-depth discussion on the scientific issues and theoretical mechanisms involved in the research,and put forward some new models.The main research content and conclusions of the thesis are as follows:(1)The morphology,particle size distribution and solid-liquid two-phase temperature range of the three raw materials were analyzed by scanning electron microscopy(SEM),laser particle size distribution analyzer and thermal gravimetric analyzer.The effects of sintering temperature on the densification behavior,microstructure and mechanical properties of the alloy prepared by SLPS at constant temperature were studied.The results show that the stability of the powder decreases with the increase of carbon content,which leads to the left shift of temperature in the solid-liquid two-phase region.The effective sintering temperature ranges of the three alloy powders(hypoeutectic,near-eutectic and hypereutectic components)are 1234～1255°C,1240～1250°C and 1225～1235°C,respectively.During the sintering process,the densification rate of alloy billet increases rapidly at first and then gently and stably with the increase of sintering temperature,and the carbide coarsening rate is proportional to the temperature.In the effective sintering temperature window,relative densities of the sintered hypoeutectic,near-eutectic and hypereutectic alloys are more than 98.5%,and the maximum hardness of the sintered hypoeutectic,near-eutectic and hypereutectic alloys is 57.1HRC,61.7HRC and 64.4HRC,respectively,compared with the real density of the as-cast alloy of the same composition.With the increase of carbon content,the flexural strength and impact toughness of the alloy decrease significantly.When the optimized sintering temperature is 1250°C,the flexural strength and impact toughness of the alloy with sub-eutectic and near-eutectic components are 2164 MPa and 7.3J/cm~2,respectively,and 1655 MPa and 4.4J/cm~2,respectively.The bending strength and impact toughness of the hypereutectic alloy are 1302 MPa and 2.5J/cm~2 at optimized sintering temperature of 1230°C.(2)High chromium cast iron was prepared by variable temperature SLPS process.The effect of sintering process on microstructure and mechanical properties was studied.The results show that the relative density of the alloy prepared by SLPS at is above 98.5%,and its effective sintering window widens to about 30°C.Compared with the high chromium cast iron prepared by constant temperature SLPS,the mechanical properties and stability of the alloy are improved by increasing the amount of equiaxial carbides in the alloy prepared by variable temperature SLPS.The hardness of the hypoeutectic alloy reaches 59.5HRC,the bending strength and impact toughness are 2260 MPa and 7.8J/cm~2 respectively.The hardness of the near eutectic alloy is 61.0HRC,the bending strength is 1817 MPa,the impact toughness is4.7J/cm~2.The hardness of the hypereutectic alloy reaches 65.4HRC,the bending strength and impact toughness are 1394 MPa and 2.5J/cm~2,respectively.(3)The growth morphology and evolution process of primary carbides in sintered high chromium cast iron were investigated by means of Thermo-Calc software,SEM and EBSD.The results show that:the bulk primary carbides in the alloy are bonded by polygonal prismatic carbides with incoherent interfaces.The degree of grain coarsening increases with the increase of carbon content.The growth mechanism of carbides conforms to the theory of periodic bond chain,and grows irregularly in the form of cellular curved surface to the liquid phase,and interconnects with each other to form U-shaped or L-shaped.(4)The hardness of sintered high chromium cast iron was further improved by quenching and tempering treatment.The effect of heat treatment process on microstructure and mechanical properties of the alloy was studied.The results show that with the increase of quenching temperature,the secondary carbide precipitates first,then aggregates,grows up and then dissolves,and the hardness of the alloy increases first and then decreases.However,the effect of quenching and tempering treatment on the hardness improvement is very limited.The maximum hardness(1000°C×2h quenching)of the hypoeutectic and near-eutectic alloys is63.3HRC and 65.1HRC,respectively.While the peak hardness of the hypereutectic alloy is 67.4HRC(960°C×2h quenching).When the tempering temperature is higher than 550°C,the secondary carbides are aggregated and grow up,and the matrix lattice recombination results in the decrease of hardness.After tempering(450°C/2h),the maximum hardness of hypoeutectic and near-eutectic alloys is 61.5HRC and62.9HRC,and the maximum hardness of hypereutectic alloys is 65.8HRC after tempering(500°C×2h).When the tempering temperature is about 250°C and 500°C,the alloy appears the phenomenon of tempering brittleness.The optimal impact toughness of hypoeutectic and near-eutectic alloys after tempering at 300°C×2h and 400°C×2h are 3.7J/cm~2 and 3.9J/cm~2,respectively.The optimal impact toughness of hypereutectic alloys after tempering at 200°C×2h is2.4J/cm~2.(5)The wear resistance behavior and wear mechanism of sintered high chromium cast iron were systematically evaluated and studied by impact abrasive wear test.The results show that in the abrasive wear process,the wear mechanism includes cutting wear,fatigue wear and brittle peeling.Under the condition of low impact load,cutting wear is the main wear mechanism,and the wear resistance of the material increases with the increase of hardness.Compared with high chromium cast iron prepared by constant temperature SLPS,the wear resistance of hypoeutectic and near-eutectic alloys prepared by variable temperature SLPS is increased by 20～30%due to higher hardness.With the increase of impact load,fatigue wear and brittleness spalling become the main wear mechanism,and the reasonable combination of hardness/strength and toughness becomes the key to obtain good wear resistance.As the morphology of the carbides in the hypoeutectic and near-eutectic alloys prepared by SLPS is improved,the strength and toughness of the alloys are improved,and the wear resistance of the alloys is increased by 50%and 33%,respectively.(6)The wear behavior and wear mechanism of hypereutectic high chromium cast iron were evaluated and studied by sliding wear test under static load.The results show that the main wear mechanisms of the alloy are cutting wear and fatigue brittle spalling during static load wear.Under the condition of small load,the wear resistance of the alloy increases with the increase of hardness,mainly due to cutting wear.Compared with the same composition of high chromium cast iron prepared by constant temperature SLPS,the wear resistance of the hypereutectic alloy prepared by quenching and variable temperature SLPS increases by 32.6%and 13.2%,respectively,due to the higher hardness(67.4HRC and 65.4HRC).Under higher load,the proportion of fatigue brittle wear mechanism increases,and the appropriate combination of hardness/strength and toughness is favorable for the improvement of wear resistance.Compared with the constant temperature SLPS,the wear resistance of hypereutectic alloy prepared by quenching treatment and variable temperature SLPS increases by 50%and 33%,respectively. |
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