Study On Strengthening And Toughening Control Of H13 Steel For High Strength Aluminum Alloy Extrusion Die

This thesis is composed to find the best solution in terms of the problems of early failure and short service life including deformation and cracking of H13 Hot Work Die Steel in the hot extrusion production process of 7 series aluminum alloy.By means of optical microscope,SEM,EDS,Rockwell hardness tester,pendulum impact tester and other equipments,the type and evolution rule of carbides are analyzed through the observation of microstructure characteristics,combined with hardness and impact performance.By doing so,the effects of the heat treatment process of quenching+multiple tempering,alloying elements including Mo,Si,V and surface nitriding treatment on the microstructure and properties of H13 hot work die steel with three different compositions were studied,analyzed,discussed and summarized in order to find out the best matching process and effectively improve the life of H13 aluminum alloy extrusion die in the process of production and use.The results show that:(1)With the increase of tempering temperature,the martensite characteristics in the structure gradually weaken,and the sorbite characteristics gradually appear,the number of carbides increases,the homogeneity of the structure increases,the hardness decreases,the toughness increases,and the impact fracture characteristics gradually change from brittleness to dimple fracture;(2)V element has a significant effect on the grain size of the material.After quenching,the grain size of high-V H13 steel is small.At the same time,V element can improve the tempering stability and secondary hardening ability of the steel to a certain extent.The high content of Si element is not conducive to the integration of alloy elements into the matrix and can prevent the aggregation and growth of carbides.It can effectively improve the tempering stability of the steel,but to a certain extent,it can reduce the toughness of the steel.The increase of Mo content may increase the peak value of the temperature of secondary hardening,raise the temperature range of secondary hardening,and enhance the effectiveness of secondary hardening;(3)After nitriding at 520°C×5 h+560°C×5 h,the nitriding layer of the three experimental steels has high uniformity and low brittleness.Among them,the nitriding samples with low Si,low V and high Mo type H13 steel 48 HRC hardness show excellent toughness,uniform penetration layer,and the thickness can be maintained above 120 ?m;(4)The H13 steel with low Si,low V and high Mo in three kinds of experimental steels is quenched at 1020°C,tempered at 560°C×1 h for one time+tempered at 580°C×1 h for three times,and nitrided at 520°C×5 h+560°C×5 h,which can achieve the best hardness,toughness matching and surface strengthening effect.