| Thermal cracking is the most important failure form of die-casting dies,which is often due to the fatigue damage caused by the combination of thermal cycling and mechanical loading cycling,i.e.,thermo-mechanical fatigue(TMF).TMF is the most complicated subject of all fatigue behaviors.It has been due to the difficulty,time-consuming and low success probability of the test,which has led to a lag in TMF research.It has focused on the self-restricted pure thermal fatigue and isothermal low-cycle fatigue behavior research of die-casting die steels at home and abroad.However,these studies do not conform to the actual service conditions of die-casting die steels and do not consider the collective effects of temperature and mechanical loading.Therefore,the study of TMF behavior and damage mechanism based on strain-based control of die-casting die steels was carried out by means of MTS thermomechanical fatigue hydraulic servo testing system,X-ray diffraction,scanning electron microscopy,transmission electron microscopy,ultra deep depth and optical microscope,combined with the carbon replica technique and mechanical property testing in this paper.It provides reliable theoretical support for thermal cracking failure behavior and mechanism of die-casting dies,and also provides a reasonable prediction model for the service life of die-casting dies.First,TMF mechanical behavior at in-phase(IP)and out-of-phase(OP)loading of H13 steel under different mechanical strain amplitudes(0.3%?1.3%)cycling during 200?600 ? and 400?700? were studied based on the mechanical strain control mode of symmetric tension and compression.Stress-strain hysteresis loops exhibit asymmetry due to high temperature strength and Young's modulus constantly changed at the non-isothermal of H13 steel during TMF cycling,i.e.,the specimens bear the compressive average stress under IP-TMF condition and the tension average stress under OP-TMF condition.The stress relaxation results from creep strain was found at the high temperature half cycle during TMF cycling,and a "loop elongation method" for quantitatively distinguishing creep strain from plastic strain was proposed.The stress and strain cycling response curves indicate that TMF damage of H13 steel is mainly characterized by continuous cycle softening.Meanwhile,three stages of TMF cyclic response process of the H13 steel were clarified,i.e.,adaptation stage,softening stage and failure stage.Then,it was found that TMF damage of H13 steel is oxidation-fatigue-creep interaction damage behavior by the analysis of oxidative damage,TMF fracture,crack initiation and propagation of H13 steel.Fatigue cracks start from the surface of specimen with multi-source cracking transgranular initiation.Oxidation is the main reason for inducing cracking initiation at surface.The continuous filling of oxides in cracks can accelerate the initiation and propagation of cracks.The cracks propagated by intergranular and transgranular under IP-TMF condition,while the cracks propagated by transgranular under OP-TMF condition.Moreover,grain boundary trigeminal cracks or wedge-shaped cracks occur in the steel because of creep damage.The main crack length,root width and crack number under IP-TMF and OP-TMF condition increase with the increase of mechanical strain amplitude.At the same mechanical strain amplitude,the main crack length is longer under IP-TMF condition,while the main crack root width and crack number are wider and more under OP-TMF condition,respectively.TMF fractures are severely oxidized.The fatigue source zone,crack propagation zone and fracture zone can be observed in the fracture of OP-TMF.The propagation zone is dominated by fatigue striations with secondary cracks and holes.The fracture zone is dominated by a large number of dimples and holes.The fatigue fracture of IP-TMF is rather ambiguous,and the fracture zone is dominated by quasi-cleavage,tear ridges and dimples.Afterwards,the micro-damage of H13 steel during TMF cycling mainly includes the widening of martensite laths,the recovery of dislocations and the aggregation and coarsening of carbides.The over-tempering effect with synergistic cyclic strain and the interaction of carbides and dislocations under thermo-mechanical coupling are the essential reason for TMF softening damage.During TMF cycling of H13 steel,the tempered strip-like cementite decomposes into alloy carbides under the thermo-mechanical coupling.In the initial stage,acicular Mo-and Cr-rich(Mo/Cr ratio is small)metastable-state M2C-type carbides and small short rod-like Cr-and Fe-rich(Cr/Fe ratio tends to 1)metastable-state M7C3-type carbides were precipitated.With the increase of cycles,metastable-state M2C-type carbides transform into sharp-edged rectangular M7C3-type carbides.After reaching a certain number of cycles,M2C-type carbides transform into a steady-state Mo-and Fe-rich cobblestone-like M2C-type carbides,while M7C3-type carbides transform into a steady-state angular rounded thick rod-like M7C3-type carbides with Cr/Fe ratio greater than 2.The two type carbides are coarsing most seriously.The concept of strain-induced carbides precipitation and coarsening at the service temperature of die-casting die steels was proposed through the comparative analysis of number and size of carbides.Finally,the TMF strain-life curve of H13 steel was established.The intersection point of strain-life curves was appeared.When the mechanical strain amplitude is small,the lifetime of OP-TMF is higher.When the mechanical strain amplitude is large,the lifetime of IP-TMF is higher.It is related to the dominant damage mechanism in the oxidation-creep-fatigue interaction.In addition,a normalized model(0.84 · ??·??in)· Nf 1.03958 2921.66219 for TMF life prediction of die-casting die steels was proposed based on the viewpoint of hysteretic energy damage and TMF data of H13 steel.The model was successfully applied to TMF life prediction for another die-casting die steel named 4Cr5Mo2V.The band of predicted life and test life is basically within the factor of 1.5. |
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