Study On Electropulsing Induced Microstructural Refinement And Strengthening Mechanisms And Reverse Transformation In Carbon Steels

Steel products are the worlds’ widely used materials. But the energy crisis andenvironmental issues raise the urgent requirements for lowering the energy andresource consumption and shortening the process for steel producing. In order to meetthe demands for weight reduction and longer service life, the steel should possessbetter combination of strength and toughness, and other outstanding applicationproperties. Among the strengthening mechanism, grain refinement is considered asone of the most efficient methods for improving the strength without sacrificing otherapplication properties. Material scientists and engineers have made a great number ofefforts on developing grain refinement methods and studying relevant refiningmechanisms. Development so far, ultragrain refinement has become one of theresearch hotspots in the field of metallic materials. Beginning with the study onelectroplasticity, electropulsing technology has been more and more widely used tomodify the microstructures of metallic materials. As an instantaneous high energyinput processing technique, electropulsing directly inputs the high-energy to lattice ofthe metallic materials, resulting in the microstructure change in very short time. Oneof the advantages of this method is energy conservation. However, few efforts havebeen made to refine or ultra-refine the microstructures of carbon steel by usingelectropulsing. The microstructure transformation is extremely departure from theequilibrium condition during electropulsing treatment. But the underlyingmechanisms on the microstructure transformation and refinement duringelectropulsing treatment are still wanting. Consequently, it is of great significance toinvestigate the refining efficiency of electropulsing on microstructure of carbon steel,and relevant reverse transformation and refining mechanisms, which is beneficial for the practical application of this high efficiency grain refinement and energy-savingprocess.In this paper, the refinement potential of electropulsing treatment on austenite grain,martensite and ferrite/pearlite structures in carbon steel with different compositionsand starting microstructures. The mechanical properties of the steel afterelectropulsing refinement were tested. The reverse martensite to austenitetransformation mechanism during electropulsing treatment was investigated bycomparing with conventional heat treatment. Combining the experimental results withtheoretical analysis, the microstructural refinement mechanisms in carbon steel duringelectropulsing treatment were analysised.Based on the experimental results and analyses in current study, the main researchconclusions are as follows:(1) The prior austenite grains were refined from150μm to20μm in plain lowcarbon steel with the starting ferrite/pearlite structures after electropulsingaustenitization treatment, resulting in the refinement of martensite structureafter quenching by water. And the tensile strength of plain low carbon steelwas improved from1220MPa to1400MPa without the decrease ofelongation. The prior austenite grains in plain medium carbon steel wererefined from150μm to20μm by electropulsing, leading to the mean sizereduction of martensitic laths from414nm to179nm. The hardness wasincreased from49HRC to56.3HRC. And the tensile strength was increasedfrom1616MPa to2000MPa with11.5%fracture elongation, resulting inbetter strength-elongation balance. With the increase of peak temperature,austenite grains were coarsened, causing the decreasing of hardness,strength and elongation. The austenite grains in40Cr steel with the startingtempered microstructure were ultrarefined after electropulsing treatment.The austenite grain growth rate was increased as the pulse current densityincreased. The reverse austenite in metastable austenitic manganese steelwas ten times smaller than the starting austenite after electropulsingtreatment. The tensile strength, elongation and work-hardening rate ofmetastable austenitic manganese steel were improved due to the austenitegrain refinement.(2) Ultrafine grained ferrite in plain low carbon steel was produced byelectropulsing induced rapid recrystallization of cold-rolled lath martensite. The equiaxed ferrite grains with the mean size of1μm were obtained afterelectropulsing treatment. The tensile strength of ultra-refined ferrite samplewas enhanced from530MPa to941MPa with15%uniform and21%fracture elongations. With the increase of peak temperature, therecrystallized microstructure was coarsened, leading to the drop of tensilestrength to648MPa with32%elongation. The results indicate that theelectropulsing induced rapid recrystallization of cold-rolled martensite is aneffective method to produce ultrafine grained steel with good combinationof strength and ductility.(3) The refining effect of electropulsing on austenite grains in carbon steel isattributed to the coupling of the thermal and athermal effects of high densitypulse current. The Joule heat effect made the temperature of the sample riseto above Ac3point by the rate of3.8×104K/s, ensuring the thermodynamicdriving force for austenite transformation. The diffusion of carbon atomswas accelerated by electropulsing due to the athermal effects of high densityof pulse current, therefore, the austenite formation procedure can be finishedin less than one second, in which the austenite formation is nearlyimpossible to be accomplished during conventional heat treatment. Due tothe athermal effect of electropulsing, the thermodynamic energy barrier forthe formation of neonatal γ phase nucleus in α matrix is decreased. Thenucleation rate of austenite during electropulsing austenitization is severalten times higher than that during conventional austenitization. Owing to theshort discharging duration and the high cooling rate induced by quenching,the austenite grains could not grow up easily. Consequently, theultra-grained austenite can be obtained during electropulsing treatment.(4) Comparing the differences of reverse transformation from α′phase to γ phasein metastable austenitic manganese steel under electropulsing andconventional heat treatment conditions, it is discovered that the reverseα′â†'γ transformation is dominated by displacive manner. The martensite andretained austenite underwent decomposition reaction during conventionalaustenitizing treatment, resulting in the formation of ferrite and pearlitestructures. The reverse equiaxed austenite nucleates preferentially at thecenter region of the temper martensite clusters. The whole transformationprocedure of martensite phase during convention heat treatment can beexpressed as α′â†'α+Fe3Câ†'γ diffusional controlled phase transition. The martensite and retained austenite were not decomposied duringelectropulsing treatment. The plate α-phase and retained austenite werecarbon-supersaturated before the starting of α′â†'γ reverse transformationupon electropulsing heating. The α′â†'γ transformation resulted in theformation of plate austenite with high density dislocations. And the α′â†'γtransformation was accompanied with the surface effect. After theaccomplishing of the reverse transformation, the plate austenite underwentrecrystallization, leading to the formation of refined equiaxed austenitegrains. The refinement mechanism of the austenite grains duringelectropulsing treatment by using martensite as the starting microstructure isthe recrystallization of displacive reverse austenite.Results of current research demonstrate the microstructure in carbon steel can berefined or even ultrarefined by electropulsing, resulting in the significant increase intensile strength combined with good ductility. The refining mechanism ofelectropulsing on carbon steel by using the decomposition products of austenite as thestarting microstructures is the acceleration of austenite nucleation rate induced byelectropulsing. While the refining mechanism of electropulsing on carbon steel withstarting martensite structure is the recrystallization of displacive reverse austenite.Ultrafine grained ferrite can be obtained by combining conventional cold rolling oflath martensite with electropulsing induced recrystallization.