Research On Microstructure Evolution Behavior Of Die-casting Die Steel Based On Thermo-mechanical Loading

Thermal fatigue cracking is the most important failure form of hot die casting molds due to the harsh service environment of molds.Mechanical and thermal stress fluctuations initiate fine cracks that grow larger and finally lead to the failure of hot die casting molds.However,due to the limitations of experimental conditions,most of the researches focuse on the influence of thermal stress on the microstructure of materials during thermal fatigue,but the impact of mechanical stress is rarely involved.Because it is difficult to distinguish the effect of mechanical strain on the microstructure when temperature and strain fields vary simultaneously.This work mainly focuses on the uniaxial loading strain-controlled tests at a constant temperature,and systematically studies the microstructure evolution of hot die casting materials during isothermal fatigue by means of optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),transmission electron microscopy(TEM)and other detection methods.The cyclic softening and damage mechanism of materials during isothermal fatigue are investigated,and the phenomenon of strain-induced carbide precipitation and coarsening in hot die casting materials is firstly revealed,which can provide theoretical basis for the optimization of alloy composition and heat treatment process and also provide theoretical guidance for material failure analysis and mold design.The phase transformation characteristics of 4Cr5Mo2NiV steel were measured by a dilatometer,and the heat treatment process was optimized by considering the grain size,retained austenite and toughness ratio.The results indicate that the optimal heat treatment process is quenching at 1030 ℃,tempering at 600～610℃ for 2 h twice.Comparing with 4Cr5Mo2V steel,the hardenability of 4Cr5Mo2NiV steel is greatly improved,and it still has excellent thermal stability,which meets the requirements of current large-section die casting moldsAn exploratory test was firstly conducted on 4Cr5MoSi/1(H13)which is widely used in the manufacture of die casting molds based on a single-axis mechanical strain control mode at a constant temperature.The results show that mechanical strains cause the deformation of material,which accelerates the softening of the material and also has a great impact on the amount and size of the carbides in the materialThen,the isothermal fatigue properties of 4Cr5Mo2NiV steel were studied.The results show that the isothermal fatigue life of 4Cr5Mo2NiV decreases with the increase of the mechanical strain amplitude.When tested at 600℃ with a cycle time of 40 s per cycle.The isothermal fatigue life of the material decreased from 633 cycles tested by the mechanical strain amplitude of 0.5%to 169 cycles tested by the mechanical strain amplitude of 1.1%.Mechanical strain promotes the deformation and damage of the material,the main crack initiates in the direction perpendicular to the loading direction,and the maximum crack is usually in the middle of the gage length of each sample.Finally,the evolution of martensite,dislocation configuration and carbides in 4Cr5Mo2NiV steel during mechanical loading and static isothermal process were comparatively studied by means of OM,SEM,XRD and TEM.The results indicate that when tested for 4 h under the mechanical strain of-0.7%～+0.7%,the dislocation density of the sample decreased from 21.38×1015 m-2 in tempering state to 8.14×1015m-2.The dislocation configuration had transformed from the high-density entangled state in the tempered state into the reticular distribution.The number of carbides per unit area is 24.7/μm2 in the sample tested for 4 h at the static isothermal temperature,however,the value has reached up to 77.1/μm2 when tested under the mechanical strain of-0.7%-+0.7%for 4 h.The number of carbides per unit area of short rod-shaped,spherical and ellipsoidal carbides increased from 9.5/μm2,7.0/μm2,1.5/μm2 at static test to 28.3/μm2,38.8/μm2,4.4/μm2.The cubic of the average equivalent radius of the carbides during isothermal fatigue is not linear with the test time,and the coarsening rate of the carbides is much larger than that during the isothermal aging process.The recovery of martensite,the movement of dislocations,and the precipitation and coarsening of carbides are the main causes of cyclic softening and cumulative damage of materials.