Effects Of Electropulsing On The Microstructure And Mechanical Properties Of The High Strength Steels Along With The Numerical Simulation Analysis

Under the treatment of electropulsing, the energy of electricity, heat and stress can beinputted into the metallic materials instantaneously. With the coupling effect of theelectricity-heat-stress multi-field, a series of instantaneous, non-equilibrium and dynamicprocesses and its associated phenomena can be brought about. In-depth study of thesenon-equilibrium processes, associated phenomena, evolution laws and the mechanism couldfurther reveal the essence of electro-strengthening on the metals, exploring and developingthe new material strengthening methods, and being significant theoretical and practical valueof greatly improving the mechanical properties of the metallic materials. The high-strengthsteels are widely used in the modern industrial in a large amount as the important basicmaterials. To dig their performance potential is the hotspot and eternal theme for the materialresearches for a long time.In this paper, the effects of transient high-energy electropulsing onthe microstructure and mechanical properties of high-strength steels were studied.Meanwhile, the numerical simulation was employed to analyse the current densitydistribution, temperature and stress field in the materials during the electropulsing treatment.The effects of electropulsing and the physical fields caused on the crystallographicorientation selection of the new nucleating phase and the decomposition of the differentphase pre-existing in the matrix were investigated as well.After the electropulsing treatment with the finest parameters, the cold-rolled22MnB5boron steel exhibites the excellent mechanical properties, reaching the tensile strength of2GPa with good ductility. Moreover, the hardening capacity of the electropulsing hardenedboron steel is also better than that of the steel hardened by the hot-stamping process. Thehigh strength and toughness of the parts could improve the safety performance of the carwhen suffering a collision accident. Due to the thermal and athermal effects of the pulse current, the austenite nucleation rate was improved greatly. For the extremely shortelectropulsing duration with the rapid cooling effect of the water-cooled copper electrodes,the size of the newly formed austenite grains could not be enlarged easily. Therefore,thesmall lath-martensite microstructure was obtained in the boron steel during the subsequentquenching process, which greatly improves the mechanical properties of the hardened boronsteel. The electropulsing cyclic treatment can cause the refinement effect in the boron steel.However, this refinement trend is limited. That is to say that the electropulsing cyclictreatment could not refine the grains infinitely. Moreover, under the same experimentalconditions, for the effect of the grain refinement, the cyclic electropulsing treatment isinferior to the electropulsing induced recrystallization in the cold-rolled boron steel.Numerical simulation results for the specimen of cyclic electropulsing treatment showed that,during the electropulsing treatment, for the cross-sectional area of the current channel at thearc-transition region decreases abruptly, some current will choose the closest distance to passthrough, resulting in the significantly increased current density at this location. Thetemperature field numerical simulation results indicates that in the middle part of thespecimen the average temperature is the highest, while the average temperature at the ends isthe lowest. At the arc-transition area between the middle and ends of the specimen, thetemperature distribution is gradiental, and with the extension of the electropulsing duration,the slope of this gradient is increasing. The results of the stress field numerical simulationshowed that the stress concentration at the arc-transition region is most obvious, while in themiddle and two ends of the specimen the stress value is small. In the same way, theelectropulsing duration is longer, the the stress gradient slope is bigger, which is resultingfrom the temperature gradient obviously.The local austenite nanocrystalline structure formed at the transition region between themiddle and ends of the specimen from the cold-rolled boron steel when subjected to thetransent electropulsong in the high current density and the conditios of rapid cooling. Thisaustenite nanocrystalline structure is pure in the single phase, and no other phases orimpurities coexisting. Numerical simulation results showed that in the location where thenanocrystalline structure was found, the current density, temperature and stress values are alllarger relatively. On the formation mechanism of the local austenite nanocrystalline structure,we believe that the rapid temperature rising and the lowered thermodynamic barrier are thetwo major reasons. Meanwhile, a high concentration of alloying elements into thenanocrystalline structure and the rapid cooling provide the thermodynamic and kineticconditions for keeping it to the room temperature. Judging from the numerical simulation results, the instant coupling of electricity, hot and stress exsit in this arc-transition region.Therefore, as to the formation of local austenitic nanocrystalline structure, the mechanism ofhigh temperature recrystallization has the certain rationality. After the analysis of thereported orientational nanocrystalline, we believe that the kinetics process is critical to theformation of nanocrystals. Through the numerical simulation of the current densitydistribution, temperature and stress field in the system of the matrix with the different phasecoexisting, it was provided data for studying the impact of kinetics on the process of thenanocrystals formation. From the numerical simulation results of the physical fields, it canbe concluded that, when there is a different phase with the high conductivity in the matrix,the current density distribution, temperature and stress field could be beneficial to itsgrowing up in the shape of columnar along the current direction and detriment of itsdecomposition. However, when there is a different phase with the low conductivity in thematrix, the physical fields could be beneficial to its decomposition instead of growing up.Electropulsing tempering treatment of the quenched high-carbon steel saw bladeresulted in that the mechanical properties of it was increased, the teeth has the highesthardness which is bigger than that at the back saw blade, and the hardness is the lowest at thetooth root. The high hardness in the tooth could improve the cutting performance, while thelow hardness in the back could increase the toughness of the saw blade. Meanwhile, theplastic area at the tooth root could prevent the initiation and propagation of cracks thereeffectively. Thus, the strength and toughness of the saw blade had been significantlyimproved. Simulation results show that due to the detour effect of the pulse current, thevalue of current density at the tooth root is the highest, resulting in the largest temperaturerising and consequently the biggest thermal stress there. The high current density,temperature and the stress at the tooth root could make electroplasticity there carry out moreeffectively, improving the effect of crack healing and thereby increasing the strength andtoughness of the saw blade. When the12.9-grade high-strength bolt in the state of quenchedand tempered subjected to the low-temperature electropulsing, the mechanical propertiescould be improved, having the high tensile strength and ductility simultaneously. Numericalsimulation results of the bolt subjecting to the electropulsing showed that due to the detoureffect of the pulse current, at the thread root the value of the current density is the highest,and that at the thread tip is the lowest, while inside the bolt the current density value issomehow between them uniformly. Thus, the highest temperature occurs at the thread root,and at the thread tip the temperature is the lowest. Such temperature distribution couldimprove the toughness of the bolt as well as the strength at the thread at the same time. Meanwhile, the stress value at the thread tip is larger relatively, and that is lower at thethread root, while inside the bolt the stress value is the lowest, which is in favor of thefatigue damage resistance of the thread and the whole bolt as well.