| Die steel is the cornerstone of modern industrial development,which has high production efficiency and low consumption rate.It is an indispensable material in mass production.In recent years,the use of die steel has increased year by year,involving a wide range of industries（aviation,medical equipment,automobiles,building materials,etc.）,and the service environment is harsh.Therefore,the requirements for the performance and life of die steel have been improved.In this paper,the near-service performance and damage failure mechanism of new cold work die steel HYC1MOD,hot work die steel H418 and cold and hot work die steel U2 have been studied in detail.The wear and three-point bending fatigue properties and failure mechanism of HYC1MOD steel have been investigated.The thermal fatigue properties and failure mechanism of H418 and U2 steels have been studied.The wear resistance of the new cold working die steel HYC1MOD has been studied by friction and wear experiments.The wear mechanism has been discussed by combining the evolution analysis of weight loss and wear surface morphology.The results show that the microstructure of HYC1MOD steel is composed of martensite and carbides,including M23C6,M7C3,and M6C carbides.The wear map of the new cold work dies steel HYC1MOD is established.According to the wear weight loss,the wear area can be divided into three parts:mild wear,moderate wear,and severe wear.The wear mechanism includes abrasive wear,adhesive wear,and oxidation wear.With the prolonged wear time,oxidation wear gradually dominates the wear mechnism.The size of the M7C3type varies differently.The large-size long strip carbides（≥3μm）are easy to crack and peel off in wear,which can be the third body abrasive to aggravate the wear degree.The large-size spherical carbides（≥3μm）will form a spherical crown after wear,thus enhancing the wear performance.The high cycle fatigue properties of HYC1MOD steel have been studied by a three-point bending fatigue test,and the fracture damage mechanism is discussed by fracture morphology analysis.The results show that the fatigue strength of HYC1MOD steel is461.25 MPa.The fatigue fracture mode is brittle fracture,and the fatigue crack mainly originates from the large size M7C3 carbide rich in Cr element,and the crack source presents a typical’fisheye’shape.Considering the stress field intensity factor of carbide size,the relationship between carbide size and fatigue life is established,and the fatigue life is effectively predicted.The thermal fatigue properties of H418 steel and U2 steel have been studied by thermal fatigue experiments,and the thermal fatigue damage mechanism is explored by combining the crack propagation morphology and hardness measurement analysis.When H418 steel is thermally cycled at 880-550°C,the volume fraction of carbide precipitation increases and the size is enlarged with the increase of the number of thermal cycles.The steel exhibits obvious softening after 3000 cycles.When the U2 steel is thermally cycled at 780-450°C,the volume fraction of carbide precipitation increases and the size is also coarsened as increasing the number of thermal cycles.The softening occurs at 500 cycles.When H418 steel is subjected to a thermal cycle at 880-25°C,carbides precipitate along prior austenite grain boundaries and martensite laths.As the number of cycle increases,the are fraction of carbides increases and the prior austenite grains grow rapidly.The hardness increases firstly and then decreases.When U2 steel is thermally cycled at 780-25°C,with the increase in the number of cycles,the carbides are coarsened.The area proportion of carbide increases firstly and then decreases,resulted from the dissolution of large carbides.When the thermal cycle is 500,U2 steel has been severely softened,and its hardness changes little with the further increase of the thermal cycle.The thermal stress generated by thermal fatigue acts on the surface of the steel so that it preferentially produces cracks on the surface.The fatigue crack propagation mode of H418 and U2 steels is mainly dominatd by transgranular propagation.However,the fatigue crack propagates along the prior austenite grain boundary at the initial stage of or tip of the crack in H418 steel. |
