Optimizing The Microstructure And Mechanical Properties Of The Cold-Rolled Austenitic Steel By Electropulsing Treatment

When pulsed electric current passes through the metal,recrystallization,phase transformation,crack-healing and amorphous crystallization usually occur in the material due to the Joule heat and electronic wind force effects.As a result,some phenomena i.e.electroplasticity,grain refining and damage-healing have also be observed.Thus,the application of electropulsing treatment(EPT)on the recovery and improvement of mechanical properties of engineering materials has become a hot research topic.Since the 316L austenitic stainless steel(316LSS)is widely used in industry due to its excellent mechanical properties,machining performance and corrosion resistance,in this work,by appling the EPT on the 316LSS,the electroplasticity,crack-healing,corrosion resistance and cryogenic tensile properties were systematically studied.And the recrystallization and crack-healing mechanisms via the combined thermal and althermal effects by EPT were also discussed.The microstructure and mechanical properties evolutions of the cold-rolled 316LSS plate after EPT were studied.By varying the EPT discharging voltages,the micstructure evolution of the material were systematically characterized via SEM,EBSD,and TEM technologies.After EPT,the microhardness and tensile properties were then measured.It was found that with the increase of pulse voltage,the material hardened first and then softened,the hardening was mainly due to the decrease of the movable dislocation density in the lamellar grains,and the softening was mainly because of the growth of recrystallized grains.The microstructure and mechanical properties evolutions of the cold-rolled 316LSS plate after EPT along different directions were studied.When appling the EPT along the RD and TD of the 316LSS plate sample,the electroplasticity sensitivities of each sample were conculiated based on the changes on the mechanical properties.The EPT induced athermal and thermal effects on each sample with different current direction were different.It was found that the lamellar grain in RD sample has a larger length-wide axial ratio,which leads to its stronger Joule heat(thermal effect)and electronic wind action(athermal effect).By comparing the microstructure and mechanical properties of 316LSS treated by EPT and the tranditional annealing treatment,it was found that EPT can be used as instantaneous heat treatment and has better corrosion resistance than traditional heat treatment.The EPT induced recrystallization mechanism was different to the annealing method.In addition,the EPT inhibited the precipitation of the second phase with lower conductivity.And the reduction of the second phase increased the difficulty of pitting nucleation.So that the samples after the EPT had better corrosion resistance than that were treated by the traditional annealing.The cryogenic(77 K)tensile deformation behaviors of a cold-rolled 316LSS plate after EPT was studied.The phenomena of EPT induced hardening and softening in the cryogenic temperature were analysised.The deformation mechanism of the steel during cryogenic tension after EPT was revealed.It was found that in the process of tension at room temperature(293 K),the deformation mechanism was dominated by TWIP effect and supplemented by TRIP effect.However,in the process of tension at cryogenic temperature(77 K),the deformation mechanism is completely dominated by TRIP effect.In addition,after EPT treatment,the materials show the phenomenon of cryogenic electrohardening and cryogenic electrosoftening.The EPT induced crack-healing behaviors of two kinds of cracks in a twinning induced plasticisty austenitic steel(TWIP)were studied.The microstructural changes of penetrating large cracks and rolled microcracks after EPT treatment were characterized,and the electrohealing mechanisms of these two types of cracks were analyzed and discussed in comparison.It was found that EPT treatment had obvious healing effect on both large penetrating cracks in compact tensile(CT)specimen and microcracks in rolled plate.Under the action of the thermal compressive stress at the crack tip and the crack side,the healing mechanism of the large penetrating cracks is mainly due to the healing of crack tip,the passivation hole and hardening area also play the role of inhibiting the crack growth,and the healing mechanism of the internal microcracks is the result of the joint healing of crack tip and crack side.