Microstructure And Performances Control Of 40Cr Steel Based On Electropulsing Treatment

Micro-alloyed high strength steel is one of the most widely used metallic materials in the industrial field,and its strengthening and toughening have always been a key topic in steel research.However,the conventional treatment process is generally characterized by high cost,a long period,and serious pollution,which is unable to fully develop the potential of materials.As an instantaneous high-energy input technology,electropulsing treatment has been proved to be an effective method to modify the microstructure and improve the performances,which is featured with efficient,economical,energy-saving and environment-friendly.In this work,electropulsing was applied to the quenching and tempering processes in 40 Cr steel,and the influences and mechanism of current pulses on the solid phase transition were systematically studied by observing the microstructure,fractures,and micro internal stress evolutions.Compared with the conventional heat treatment,the effect of electropulsing treatment on the mechanical properties and anti-delayed fracture property of 40 Cr steel was analyzed.And the optimum parameters of electropulsing treatment were obtained.(1)Due to the ultra-short holding time at the elevated temperature and the promotion of current pulses to the austenite nucleation,the annealed cold-drawn sample after electropulsing quenching(EQ)can obtain finer martensite than that of conventional quenching(CQ).The optimum parameter of EQ is 480 ms,while the microhardness is ~690 HV and the average grain size of prior austenite is ~14.65 ?m at this time.Compared with CQ,480 ms EQ makes higher dislocation density,and correspondingly,the micro residual stress is also larger,which can be attributed to the extremely non-equilibrium condition of phase transition under electropulsing.Moreover,EQ also induces a dramatic change of grain orientations,which contributes a larger Schmid factor and the <110>-fiber texture distributed along the current direction.480 ms EQ sample after conventional tempering(CT)at 520 ? meets the requirements of 12.9-grade bolts(while the sample subjected to CQ and 520 ? CT only obtains the mechanical properties equal to those of 10.9-grade bolts).(2)The optimum parameter of electropulsing tempering(ET)for 480 ms EQ sample is triple-cyclic ET with the duration of 100 ms each time(3×100 ms ET);at this time,the microhardness is ~654 HV,the ultimate tensile strength is ~2241 MPa,and the elongation-to-failure is ~15.2%.Compared with 250 ? CT,3×100 ms ET leads to less reduction in dislocation density,while causes almost the same amount of decline in micro residual stress.The rapid stress relief during ET can be attributed to the accelerated dislocation slip under the combined effect of Joule heat,electron wind and thermal compressive stress induced by current pulses.Besides,due to the inhibition of current pulses on the formation of a low conductivity phase,no ?-carbides were precipitated in 480 ms EQ sample after 3×100 ms ET.(3)Cyclic EQ with proper parameters can further refine the prior austenite,which is mainly attributed to the increased crystal defects density induced by phase transition,i.e.phase transition hardening.The optimum parameter of cycle EQ is three times EQ treatments with each duration of 440 ms,400 ms and 380 ms in turn,at this moment,the average grain size of prior austenite is ~4.98 ?m and the hardness is ~780 HV.After the subsequent 3×120 ms ET,optimum comprehensive mechanical properties can be obtained: the hardness is ~692 HV,the ultimate tensile strength is ~2336 MPa,and the elongation-tofailure is ~15.1%.(4)Electropulsing treatment is favorable to the removal of hydrogen and can reduce the tendency of hydrogen embrittlement.The delayed fracture duration of the optimum parameters-electropulsing quenched and tempered sample increased by ~0.9 times than that of conventional quenched and tempered sample.The improvement of delayed fracture resistance may be related to the microstructure refinement(finer grains,more dispersed carbides)and the change of internal stress caused by the current pulses.Furthermore,it is suggested that the hardness of 40 Cr bolt steel quenched and tempered by electropulsing should be controlled in the range of 360-390 HV.