Effect Of Electropulsing Treatment On Microstructure And Properties Of Low-alloy Steels

With the increasing demands on material performance,it is significant to exploit the potential of material performance.The properties of materials are determined by their structure,and controlling the microstructure of materials is an important means of improving their properties.Electropulsing treatment has a unique"electric-thermal-force-magnetic"effect,which induces phase changes in materials and introduces electrical effects as well as thermal,force,magnetic and other secondary effects,resulting in complex changes in microstructure,macroscopic morphology and precipitation that are difficult to be produced by other heat treatments.It is of great importance to exploit the potential of materials by controlling the parameters of the electropulsing treatment to regulate the microstructure and coordinate various strengthening effects to improve the mechanical properties of materials.In this study,the electropulse-induced the microstructure evolution behaviors of different initial tissues(pearlite+ferrite and martensite),macrostructure evolution,and local phase transformation were investigated.Compared with conventional heat treatment,the effects of electropulsing treatment on the microstructure evolution,macroscopic morphology and final mechanical properties of low-alloy steel were investigated in comparison with conventional heat treatment.The main research results are as follows:(1)The microstructure evolution of 35CrMo steel transformation from pearlite+ferrite to austenite under electropulsing treatment was investigated,and the effects of the four driving forces induced by electropulsing treatment(chemical potential,temperature gradient,stress field and electric field)were analyzed during the phase transformation.The study reveals that the austenitization transformation induced by electropulsing treatment could be completed within 500 ms,and the difference in austenitization rate between ferrite and pearlite is greatly weakened,breaking the perception that the size distribution of ferrite determines the austenitization process in the traditional heat treatment process;among the four driving forces,the electromigration effect generated by electric field is considered to be the main role in accelerating the diffusion of elements and thus changing the phase transformation process.(2)The electropulse-induced microstructure evolution from martensite to austenite in two low-alloy steels with different carbon contents(22Mn B5 and 35CrMo steels)was investigated,and the types of phase transformation and cementite precipitation behavior were analyzed.The results showed that the pulse current could inhibit the decomposition of martensite and carbide precipitation during the heating process,and it was the first time confirmed that martensite transformed to austenite in a diffusionless manner in low-carbon low-alloy steel(22Mn B5 steel);the phase transformation was reversible,i.e.,the acicular austenite growed explosively inside the prior austenite grains without any change in composition and merges to reconstitute grains with the same size,shape and orientation as the prior austenite grains.However,the reversibility of martensite transformation cannot be achieved in the low-alloy steel with high carbon content(35CrMo steel)only by suppressing martensite decomposition,and the growth of acicular austenite was suppressed during the phase transformation.The high dislocation density led to recrystallization and disruption of the original orientation relationship,which leads to failure of the prior austenite grain reconstruction and grain refinement.(3)The mechanical properties of 35CrMo steel treated with different initial tissues were investigated by the electropulsing treatment.The best comprehansive mechanical properties were obtained when the initial microstrucutre was martensite.Its yield strength,ultimate tensile strength and elongation at fracture were 1413 MPa,1478 MPa and 13.6%,respectively.Its yield strength and ultimate tensile strength were 6.7%and 5.8%higher than that of sample with ferrite and pearlite as the initial microstrucutre,respectively,and its yield strength and ultimate tensile strength were43.6%and 23.1%higher than those of the conventional tempered specimens,while the elongation at fracture was basically unchanged.(4)The effect of electropulsing treatment on the macroscopic distribution of the material was investigated.The rapid preparation of lamellar structures was achived.The results show that lamellar structures can be rapidly prepared in homogeneously35CrMo steel by using the difference in current density distribution in various phases during electropusling treatment.The lamellar structures are the result of the difference in elemental distribution,where the C-,Mn-and Cr-rich layers have higher hardness(800 HV)and corrosion resistance,while the Mo-rich layer has a lower hardness(665HV),so that the material shows alternating light and dark lamellar structures after corrosion.(5)The effect of electropulsing treatment on hot-rolled texture was investigated;the relationship between texture evolution and current density was analysed;and the mechanism of texture evolution induced by pulsed current was elucidated.The results showed that electropusling treatment could eliminate hot-rolled texture and generate a new<100>texture in the current direction by increasing the current density,which was a result of the inhibition or promotion of grain growth in each orientation caused by the difference of collision between grains of different orientations and electrons.(6)The correlation between the phase transition region and the current density during the electropulsing treatment was used to realize the local phase transition of the material,so as to prepare the material composed of soft and hard phases with a special structure.The results showed that the surface-strengthened steel plate made of Q235 steel had high wear resistance,high plasticity,and low strength.This means that the steel plate has high wear resistance and good formability.Its wear resistance,elongation and impact toughness were about 7.99×10~6 N·m/g,22.6%,and 196.3 J/cm~2,about 1.22,3.32,and 6.42 times that of the quenched sample.In addition,the integrally strengthened steel plate made of 35CrMo steel maintained 15.93%elongation at the break while its tensile strength reached 1400 MPa,and its tensile curve integral reached 19100 MPa·%.Compared with the traditional tempering sample,its ultimate tensile strength,elongation-to-break,and tensile curve integral were increased by 16.5%,34.7%,and 47.5%respectively.