Effects Of Coupled Electric-thermal-strain Fields On Microstructure And Mechanical Properties Of 42CrMo/T250 Steels

By virtue of the characteristics of excellent mechanical properties,good form-ability and relatively lower production cost,the advanced high strength steels?AHSS?have been widely applied in automobile industry,military project and aerospace field.Throughout the development history of AHSS from first generation to third generation,to develop super steel with refined matrix and metastable phase based on the structure regulation of“Multi-phase&scale”is gradually favored.Now,in the industrial background of lightweight,intelligent manufacturing,how to develop high performance steels more effectively and efficiently with low energy consumption has become a research hotspot in the field of materials processing.Since the discovery of electroplastic effect,the high-energy and instantaneous electropulsing treatment has successfully attracted the attention of material scholars.In recent years,with the in-depth exploration of the non-equilibrium phase transformation mechanism,the evolution rule of microstructure and the corresponding associated phenomenon under the effect of coupled physical fields,the concept of electrostrengthening has been paid more and more attention,and the industrial application of electropulsing treatment to AHSS has also been realized to a certain extent.Additionally,based on the effect of electron wind and electromigration on rapid transformation and recrystallization,the application of electropulsing in strengthening and toughening of AHSS can fully satisfy the aim and scope of AHSS development.However,the previous studies are mainly focused on the grain refinement effect and the broken of“Trade off/Taking turns”relationship between strength and ductility induced by pulsed current.Unfortunately,some substantial mechanisms about this point have rarely been mentioned.Thereupon,the reveal of the effect of pulsed current on substructure evolution and strengthening mechanism of AHSS is of much importance to further improve the non-equilibrium theory and to demonstrate a new approach to develop AHSS with better properties.In this work,high-energy and instantaneous electropulsing treatment is applied in two steels with totally different strengthening technology?42CrMo steel and T250 steel?.Besides,combined with conventional heat treatment,the detailed effect of pulsed current on microstructure-property relationship is also quantitatively analyzed.It's expected to figure out the difference of affect mechanisms between conventional heat treatment and electro-pulsing treatment.It's interesting to find that:1)The potential barrier of austenitic transformation got reduced by the transient huge energy input.In this way,the nucleation rate of austenite increased significantly.Moreover,the austenitic growth is also restricted under the effect of extremely short period and the following water-cooling.Therefore,the grain size of electropulsing treated 42CrMo steel and T250 steel are reduced by 56.3%and 74.6%,respectively.2)The mechanisms of enhanced stability of retained austenite by electropulsing are proposed:i)The heterogeneously distributed alloying element in untreated steels is responsible for the chemical driven force for the formation of retained austenite after electropulsing treatment;ii)The interface energy between martensite and austenite was increased by the effect of grain refinement and a large amount of crystal defects induced by pulsed current,as a result,the martensitic growth will stop earlier;iii)The volume expansion of martensitic transformation will be inhibited by the thermal compressive stress.3)The morphology and distribution of substructure and the second phase can be remarkably effected by the special physical effect or distribution of physical field during electropulsing.The stacking faults which are hardly exist in steels with high fault energy are formed under the effect of thermal compressive stress;The stacking faults in 42CrMo steel will promote the formation of lamellar carbides during tempering process;The compressive stress distributed between the alloying element dilution area and enriched area is the main reason for the formation of twinning structure;The existence of defects induced by the impact of electron wind can accelerate the active wetting of grain boundaries by the second phase,and if the defects at interface are annihilated,the second phase will passively infiltrating other interfaces;Finally,due to the coincidence of at least two physical fields,some particular phenomenon such as dislocations parallel to current direction in 42CrMo steel,Ni₃?Ti,Al?co-clusters with the uniform length direction parallel to current direction in T250 steel,can be clearly observed.4)The dislocation motion at slip plane is preferentially activated by the pulse current,which will result in some texture in electropulsing treated steels.For example,the{112}//ED texture in electropulsing treated T250 steel at solid-solution state,the{111}//ED texture of retained austenite in T250 steel at TS+EPA state and the{110}//ED texture with local misorientation in T250 steel at EPS+EPA state.5)Multiple phase structure can be derived from electropulsing treated steels.In contrast to the single sorbite in conventional tempered 42CrMo steel,tempered martensite combined with sorbite and retained austenite are found in 42CrMo steel at EPQ+EPT state;Compared with the?-Ni₃?Ti,Mo?in conventional aged T250 steel,the T250 steel at TS+EPA state is composed of Ni₃?Ti,Al?co-clusters,Ni_2.67Ti_1.33.33 phases and NiTi compounds.6)The strength and ductility are both modified by electro-pulsing treatment,and the results of quantitative analysis of strengthening mechanisms indicate that the contribution of grain boundary and dislocation strengthening mechanism to yield strength can be increased by electropulsing assisted quenching and solid-solution treatment;On the other hand,the value of precipitation strengthening is also enhanced by electropulsing assisted tempering or aging treatment.Moreover,through the compatible properties and coordinated deformation of the multiple phase structure,the mechanical properties of steels can also be improved.In summary,the comprehensive properties of the optimized 42CrMo steel and T250steel by electropulsing treatment are 22.82%and 117.26%higher than that of conventional treated steels.Consequently,the electropulsing treatment can be altered as a new strengthening and toughening strategy for AHSS.Meanwhile,the conspicuous substructural change and deformation behavior during electropulsing are also clarified.To sum up,our work offers substantial experimental basis and technical reference for the development of new AHSS.