Microstructure And Mechanical Properties Of H13 Steel Processed By Austempering And Tempering

The comprehensive mechanical properties of H13 steel after conventional heat treatment are insufficient to meet the daily productive needs.In this dissertation,as research object,H13 steel was subjected to preliminary heat treatments and final heat treatments(including austempering and austempering + tempering),and the microstructure and mechanical properties were studied.After three different preliminary heat treatments,i.e.spheroidizing annealing,quenching and tempering,and spheroidizing annealing + normalizing + spheroidizing annealing,H13 steel was subjected to final heat treatments,i.e.austenitizing at 1030 °C for 20 min and austempering in salt bath at 310 °C,345 °C and 355 °C and tempering.The mechanical properties of the samples were tested by Rockwell hardness tester,Shimazu micro-tensile testing machine and Charpy impact tester.the Microstructure,phase composition and impact fracture morphology were examined and analyzed by scanning electron microscopy,transmission electron microscopy and X-ray diffractometry.Lower bainite,residual austenite,small amount of martensite and small amount of undissolved carbide were obtained in sample after preliminary heat treatments and austenitizing.The average width of the lower bainitic was 97±22 nm~126±32 nm.The residual austenite was existed two forms,one is thin filmaustenite between bainite laths and the other is block austenite between bainite lath bundle.For the same preliminary heat treatment,with increas the isothermal temperature,the thickness of bainite laths gradually increases,the residual austenite content gradually decreases,the hardness and strength gradually increase,and the impact energy and elongation first decrease and then increase.For the same austempering,the bainite lath size of the sample pretreated by spheroidizing annealing + normalizing + spheroidizing annealing is less than that of the sample pretreated by quenching and tempering,the impact energy,elongation and strength of pretreatment of spheroidizing annealing + normalizing + spheroidizing annealing of isothermal quenching specimen is greater than pretreatment for solid solution treatment and the isothermal quenching sample.After single tempering,most of the residual austenite in the sample was transformed into bainite and martensite.During the tempering process,carbides were precipitated,the hardness of the sample increased and the impact energy decreased.And the hardness of tempering samples decreases with the increase of tempering times.The best mechanical properties were obtained by the following optimum process: preliminary heat treatment with spheroidizing annealing + normalizing + spheroidizing annealing,and final heat treatment with austempring at 310 °C.By the optimum process,the hardness,tensile strength,Charpy impact energy and elongation reached to 51 HRC,2002 MPa,40 J and 16.2 %,respectively.The comprehensive mechanical properties are better than H13 steel processed by conventional heat treatment.