Mold is the foundation and core equipment of the modern manufacturing industry.The level of the mold industry has been one of the main standards for measuring the manufacturing level of various countries.H13 hot work die steel is widely used in the manufacture of light alloy die casting dies,aluminum alloy extrusion dies and hot forging dies,but it is locally damaged due to impact and wear during service,and it is prone to thermal fatigue during the process of chilling shock crack and fail.With the rapid development of modern manufacturing,people are increasingly demanding to increase production efficiency,reduce production costs,save resources and improve product quality.Deep cryogenic treatment is a treatment method that can improve the overall performance of the material,but its effect mechanism on H13 hot work die steel is still unclear.In this paper,using the optical microscope,XRD,SEM,TEM,Rockwell hardness tester,impact tester,resistivity tester and internal friction tester and other test methods to study the effect of deep cryogenic treatment on the structure and properties of H13 hot work die steel,Revealing the transformation of the microstructure and the evolution of the second phase of the experimental steel during tempering after deep cryogenic treatment.The following main conclusions are obtained:1.The hardness of the tested steel increased by more than 1.5HRC after deep cryogenic treatment at different times and reached the highest hardness when the deep cryogenic treatment time was 24h.It has little effect on the hardness when prolonging the deep cryogenic treatment time.The hardness of the tempered steel after deep cryogenic treatment is 3.5HRC higher than that of the directly tempered steel after quenching,but the impact toughness decreased.2.The electrical resistivity of the experimental steel after deep cryogenic treatment is lower than that of the quenched experimental steel.The internal friction peak strength of the experimental steel SKK after cryogenic treatment is higher than the internal friction peak strength of the quenched experimental steel SKK.The internal friction peak strength of the experimental steel after deep cryogenic treatment is tempered.It is higher than the peak strength of SKK internal friction after tempering of the experimental steel without deep cryogenic treatment.The results of resistivity and internal friction analysis show that deep cryogenic treatment promotes the migration of interstitial atoms and produces a strong interaction with dislocations.3.After long time tempering,the hardness of the deep cryogenic treatment tested steel is increased by 2HRC-4HRC compared to the without deep cryogenic treatment tested steel,and the decreasing trend of the hardness of the deep cryogenic treatment steel decreased significantly with the extension of the tempering time after tempering for a certain period.According to the calculation of tempering kinetics,the maximum temperature of cryogenically treated experimental steel tempered to 35 HRC in 2 h is733℃,and the maximum temperature of quenched experimental steel tempered to 35HRC in 2h is 696℃.The activation energy of tempering transformation of cryogenically treated experimental steel is greater than the activation energy of tempering transformation of quenched experimental steel,indicating that deep cryogenic treatment is beneficial to improve the thermal stability of experimental steel.4.The microstructure study before and after deep cryogenic treatment shows that deep cryogenic treatment is beneficial to promote the transformation of retained austenite into martensite.More and evenly distributed M23C6 carbides were precipitated from the matrix during tempering after deep cryogenic treatment,which resulted in an improvement in the hardness and thermal stability of the tested steel. |