Study On Microstructures And Mechanical Properties Of High Carbon And High Manganese Austenitic Steels

Mechanical properties of conventional and N+Cr alloyed high carbon and high manganese austenitic steels,namely Mn12 and Mn12 CrN respectively,were studied,including tensile properties at different strain rates,low cycle fatigue properties at different total strain amplitudes,and wear properties at different applied loads.The effect of N+Cr alloying on the microstructures and mechanical properties were studied.Meanwhile,the influence of pre-hardening treatment on low cycle fatigue properties of high carbon and high manganese austenitic steel were analyzed through comparing the low cycle fatigue properties of water-quenched and pre-hardened high carbon and high manganese austenitic steels.Different deformation modes(i.e.,explosion deformation and cold-rolling deformation)were applied to harden high carbon and high manganese austenitic steel to a similar hardness level.The microstructures in these pre-deformed high carbon and high manganese austenitic steels were observed and tensile properties were tested.Results showed that,the strength and plasticity of the Mn12 CrN steel were superior to those of the Mn12 steel at the same strain rate during tensile deformation.With increasing the strain rate,the changes in strength and plasticity of the Mn12 CrN steel were less sensitive to tensile strain rate compared with the Mn12 steel.The effects of grain refinement and N+Cr alloying on dynamic strain aging and deformation twining behaviors were responsible for this lack of sensitivity to strain rate.During cyclic deformation,the Mn12 CrN steel underwent cyclic hardening,softening and stability at low strain amplitudes.Only cyclic hardening and softening followed by fracture failure were observed at high strain amplitudes.The slip mode of the Mn12 CrN steel transited from planar slip to wavy slip with the increase in total strain amplitude.Meanwhile,fine deformation twins were observed in Mn12 CrN steel when the total strain amplitude reached 0.8%.N+Cr alloying affected the dislocation configurations and twinning behavior.The deformation twins also interacted with dislocations to reduce the local stress concentration.These explained the different cyclic deformation behaviors and prolonged fatigue life in the Mn12 CrN steel.The wear behavior and work hardening characteristics of the Mn12 CrN steel were different from those of the Mn12 steel under different wear loads.The Mn12 CrN steel presented common wear resistance under low wear loads,but strong wear resistance under high wear loads.The worn surface hardness of the Mn12 CrN steel increased slowly under low applied wear loads but increased rapidly and approached the saturation value under high wear loads.The effect of N and Cr combination alloying on initial strength and twinning behavior was responsible for the different hardening characteristics of the Mn12 CrN steel during wear test.Significant differences in fatigue behaviors were found between the pre-hardening(PH,with a cold rolling deformation degree of 40%)and water-quenched(WQ)high carbon and high manganese austenitic steels.Initial hardening followed by cyclic stability behavior happened in the PH high carbon and high manganese austenitic steel under cyclic loading,whereas cyclic softening behavior was barely observed.The fatigue life of the PH high carbon and high manganese austenitic steel was higher than that of the WQ high carbon and high manganese austenitic steel at low strain amplitudes,while a contrary result was obtained at high strain amplitudes.At low strain amplitudes,the deformation twins induced in the PH high carbon and high manganese austenitic steel could enhance the multiplication and slip of dislocations,which actually improved the deformation uniformity.The long-range motion of dislocations was intensified at high strain amplitudes.However,the dislocation motion was also blocked by twin boundaries.As a result,the interactions between dislocations and deformation twins enhanced,finally causing severe dislocation accumulation.These two effects of deformation twins on dislocation motion eventually resulted in different low-cycle fatigue behavior of the PH high carbon and high manganese austenitic steel.Explosion deformed and cold-rolling deformed high carbon and high manganese austenitic steels presented different microstructure characteristics and tensile properties with a similar hardness level.Unlike cold rolling condition,most grains remained their equiaxed morphologies in the exploded sample.The strain concentration was localized at the grain boundaries.Moreover,dislocations and deformation twins were uniformly formed in the high carbon and high manganese austenitic steel under the explosion condition.These microstructure characteristics finally resulted in higher yield strength but lower tensile strength and elongation of the exploded sample.