Effect Of QPQT Treatment On The Microstructure And Thermal Fatigue Properties Of H13 Hot Working Die Stee

H13 steel is widely used in hot forging die,die casting die,hot extrusion die and other fields.Because of its good hardenability,toughness and thermal fatigue properties,it has become the most widely used and most representative hot work die steel.However,after conventional heat treatment,the martensite lath in the microstructure of H13 steel is wider,the grain size is larger,and the carbide is easy to coarsen,which is easy to cause thermal stress concentration and accelerate the initiation and propagation of thermal fatigue cracks,and reduce the service life of the mold.Therefore,how to further improve the performance of H13 steel and improve the service life of the mold to maximize resource conservation and reduce carbon emissions has become an urgent problem to be solved in the mold industry.In this study,QPQT new heat treatment process(Quenching + Precipitation +Critical Quenching + Tempering)was used to refine austenite grains and martensite laths of H13 steel.The effects of QPQT new heat treatment process on the microstructure,mechanical properties and thermal fatigue properties of the tested steel were studied by means of metallographic microscope,scanning electron microscope,transmission electron microscope,Rockwell hardness tester,impact tester and thermal fatigue tester.Compared with conventional heat treatment,the initiation and propagation mechanism of thermal fatigue cracks in the tested steel after QPQT new treatment was revealed.The main conclusions are as follows:1.Compared with conventional quenching and tempering treatment,the QPQ heat treatment process is conducive to the precipitation of smaller and more uniform carbides during tempering.After the QPQ heat treatment,the austenite grains are refined by about 60 % on the basis of conventional quenching and tempering treatment,and the average width of martensite laths is refined by about 51 %.2.The QPQT treatment reduces the size of carbides in the test steel and makes the distribution more uniform.Due to the double refinement of austenite grains and martensite laths,the hardness of the test steel after QPQT heat treatment is similar to that of the conventional quenching and tempering treatment,but the impact toughness is increased by 34.8 %.The test steel has good strength and plasticity by carbide preprecipitation at 730 ℃ and critical quenching at 930 ℃.3.After long time tempering,the hardness of the tested steel decreases with the increase of tempering time and tempering temperature,and the carbide size increases with the increase of tempering time and tempering temperature.However,the hardness decreases rate and carbide coarsening rate of the sample after QPQ treatment during tempering are lower than those of conventional quenching treatment.Based on the Hollomon-Jaffe relationship,the diffusion activation energy of the tested steel after QPQ treatment is greater than that of the conventional quenching heat treatment,indicating that the QPQT treatment significantly improves the thermal stability of the tested steel.4.The average crack length,crack density and surface hardness of the tested steel after QPQT treatment are lower than those of the conventional quenching and tempering treatment,indicating that the new QPQT treatment can effectively improve the thermal fatigue performance of the tested steel.The mechanism is due to the precipitation of uniform and fine carbides and grain refinement,which passivates the crack tip and delays the initiation and propagation of cracks.