Effects Of Electropulsing On The Microstructure, Mechanical Properties And Thermal Fatigue Behaviors Of Die Steel Heat Affected Zone

Since the quality and profit of products together with the researching albility of new ones depend on the dies, the technical technical level of production for the dies has become an impotant standard for rating the country's industrialization and innovation ability. The dies are called as"black gold"because of their complicated antrum structure, long-timing of manufacture and processing, and high cost. The hot working dies serve generally in extremely high temperature condition, and the die materials experience multi-cyclic heating and cooling process. The surface of die antrum suffers thermal fatigue easily due to the alternate stress caused by repeated heating and cooling. The thermal fatigue responds to 60%~70% of failure for hot working dies statistically, so the loss caused by thermal fatigue is very large. Therefore, the improvement for thermal fatigue behavior of hot working die possesses important economic significance.The mechanisms for fatigue damnification of metals are very complicated, and the researchers dedicate the research on the fatigue and invalidation behavior all long time. The initiation and propagation of fatigue cracks could be delayed and prevented for long-timing service by improving the microstructure and stress state and enhancing the mechanical properties. From the viewpoint of materials bionics, the living bodies are opening systems with dissipative structure and their fagtigue or damnification could be recoved for long service life by the reorganization with the exchanging of substance and energy from extermal environment. Though engineering materials are not living bodies, they could be regarded as opening system with exchanging energy from external condition when they suffer load and damnification. The biomimetic treatments through inputting adequate external energy not only keep normal structure but also recover fatigued microstructure and reorganize damnification position for controlling material's invalidation and prolonging service lifetime of parts before the fatigued disfigurement didn't assemble and produce microcrack. The purpose of present study focuses on transient electropulsing treatment on the die materials HHD, H13 and 3Cr2W8V steels for improving die material's thermal fatigue performance and prolonging service lifetime in course of thermal fatigue from viewpoint of broad sense of processing bionics. A series of transient dynamic process with incidental phenomenon and regional microstructure evolution are discussed in the course of electropulsing stimulation on the thermal fatigued specimens of die materials. The structural morphology, microstructrue and mechanical properties of heate affected zones of thermal fatigued specimens after transient electropulsing stimulation are further studied for analyzing the behaviors and recovering mechanism for thermal fatigue of die materials. The correlative structural model is constructed using ANSYS softeware and the distributing rules of electric current, temperatue and thermal stress fields are studied through simulating operation for serving theoretical foundation and evidence to analyze practical experimental results.In course of transient electropulsing discharging, due to the existence of prefabricated notch of thermal fatigued specimen, the current channel is locally blocked and electric current would surpass the specimen notch, which induces the current bypassing and concentrating effect. Due to local increase of current density, the Joule heating effect is induced ahead of the specimen notch. The discharging process of electropulsing is very transient with range of millionseconds and the region ahead of specimen notch is heated to austenizing temperature range with high heating rate. However, the region distant from the notch tip is in the state of low temperature due to the unconspicuous Joule heating effect with low current density. Instantly at the end of electropulisng discharging, the region ahead of specimen notch tip with high temperature is cooled by the surrounding region with low temperature rapidly and transformed into martensite microstructure. In the transient process of express heating and rapid cooling of electropulsing stimulation, the region ahead of thermal fatigued specimen notch tip transformed into white-bright heat affected zone that is different from orginal material with distinct characterestics of phrase transformation; grain refinement; high dislocation density and dispersive nanosized carbide particles.The mechanical properties of materials are controlled by their microstructrue. The measure of microhardness at room temperature indicated that the microhardness values of heat affected zones are higher than that of original materials after electropulsing stimulation. In the whole of thermal fatigue process, the microhardness values of heat affected zones are still higher than that of the surrounding matrix. The results of tensile test at room temperature showed that, although tensile prolongation rate possessed a decreased character, the tensile stress exhibited a remarkable increasing trait. Especially in the different stage of thermal fatigue, although tensile strength of electrostimulated specimen HAZ decreased slightly, it was still higher than that of matrix and the HAZ tensile prolongation rate increased gradually with increase of thermal cycles, so the HAZ exhibited higher strength and ductibility in the process of thermal fatigue. The stress measure from X-ray diffraction analysis showed that the residual stress on the surface of fatigued specimen was in the tensile state in the process of thermal fatigue. However, the residual stress of electrostimulated specimen HAZ changed from tensile state into compressive one. What's more, prolonging the discharging time and elevating current density of electropulisng stimulation were both favorable for enlarging the residual compressive stress of electrostimulated specimen HAZ at room temperature. The electropulsing stimuation changed high-temperature wearing morphology of HAZ from adhesive state to furrow one and prolonging discharging duration was favorable for improving the resistance against high-temperature wearing. Otherwise, the electropulsing stimulation with adequate parameter could make HAZ possessing higher impacting toughness.The result of thermal fatigue test showed the ideal operating time of electropulsing stimulation on HHD steel was at 500 thermal fatigue cycle, which could make specimen in possession of higher resistance against thermal fatigue. Prolonging the discharging time and elevating current density of electropulisng stimulation were all favorable for delaying the initiation and preventing the propagation of thermal fatigue crack, so the electropulsing stimulation could recover thermal fatigue damnification and enchance service lifetime of die. The improvement of strengthening with high ductility and anti-oxidizing properties through the inducement of grain refinement, high dislocation density, large amount of nanosized carbide particles separation and residual compressive stress after electropulsing stimulation, especially the matching of strength and ductility of HAZ in the process of thermal fatigue, were the main cause of enhancing the resistance against the initiation and propagation of thermal fatigue crack.The ANSYS simulation results proved the current surpassing and concentrating effect and the Joule heating concentrating release existing in the course of electropulsing stimulation. Comparing with the practical experimental results, the longest discharing duration of electropulsing stimulation was proved as 240 ms through the ANSYS simulation. The ANSYS simulation results proved that prolonging discharing duration and elevation current density of electropulsing stimulation were all favorable for enlarging the acreage and compressive stress of heat affected zone on the surface of electrostimualted specimen.