Effect Of Heat Treatment Processes On Low Cycle Fatigue Behavior Of 42CrMo Steel

The fatigue failure problem of engineering structural components after experiencin g cyclic loads has always been a hot topic of concern for many scholars and research ers,especially the cyclic hardening/softening behavior of structural components after ex periencing cyclic loads,which may directly cause excessive plastic deformation of the structural components,ultimately leading to damage to the entire equipment.This study focuses on the cyclic hardening/softening behavior of 42 Cr Mo steel,which is widely used as engineering structural components,after undergoing cyclic loading.Firstly,four different heat treatment processes(annealing,isothermal annealing,normalizing,and quenching and tempering processes)were designed to obtain different microstructures.Secondly,conventional mechanical properties were obtained through experimental methods for 42 Cr Mo alloy steel samples with four different heat treatment processes.Provide basic mechanical performance parameters for low cycle fatigue testing.Low cycle fatigue tests were conducted again on fatigue specimens of 42 Cr Mo alloy steel using four different heat treatment processes to investigate the effects of different heat treatment processes on low cycle fatigue behavior(including stress/strain response,temperature response,ratcheting behavior response,and other low cycle fatigue behaviors).Then,the microstructure characterization method is used to characterize the structures with different phenomena,revealing the mechanism of the influence of different structures on the cyclic hardening/softening of 42 Cr Mo alloy steel,and providing theoretical guidance for the failure analysis of large 42 Cr Mo alloy steel cross-sectional structural components.The results of physical and chemical inspection and mechanical property testing show that the microstructure of the annealed 42 Cr Mo alloy steel is pearlite ferrite structure,with a Vickers hardness value of 220 HV,a yield strength of 424 MPa,a tensile strength of 802 MPa,and an elongation of 23%.The microstructure of 42 Cr Mo alloy steel produced by isothermal annealing process is pearlite ferrite structure,with a Vickers hardness value of 169 HV,a yield strength of 407 MPa,a tensile strength of 762 MPa,and an elongation of 20%.The microstructure of the quenched and tempered 42 Cr Mo alloy steel is a tempered sorbite structure,with a Vickers hardness value of 342 HV,a yield strength of 1022 MPa,a tensile strength of 1137 MPa,and an elongation of 16%.The normalized 42 Cr Mo alloy steel has a bainite ferrite structure,a Vickers hardness of 368 HV,a yield strength of 812 MPa,a tensile strength of 1212 MPa,and an elongation of 13%.The results of low cycle fatigue tests indicate that under stress controlled loading conditions,the fatigue specimens of 42 Cr Mo alloy steel subjected to four heat treatment processes all exhibit cyclic softening behavior,while the fatigue specimens of 42 Cr Mo alloy steel subjected to annealing and isothermal annealing processes exhibit significant cyclic hardening during the initial stage of cyclic loading.The fatigue specimens of 42 Cr Mo alloy steel annealed and isothermal annealed under medium strain amplitude loading conditions underwent a transition from cyclic hardening to cyclic stability behavior.The fatigue specimens of 42 Cr Mo alloy steel subjected to both normalizing and quenching and tempering processes exhibited cyclic softening behavior.The fatigue specimens of 42 Cr Mo alloy steel annealed under high strain amplitude loading conditions and subjected to isothermal annealing processes exhibited cyclic hardening,cyclic stability,and secondary hardening.The fatigue specimens of42 Cr Mo alloy steel subjected to normalizing processes exhibited severe cyclic softening.The microstructure characterization results show that compared with the cyclic softened ferrite bainite structure and tempered sorbite structure,the number of small angle grain boundaries in the ferrite pearlite structure increases significantly under the loading condition controlled by the strain amplitude,the Taylor factor of the grain increases significantly,and the entanglement and accumulation of dislocations in the ferrite lamella form a relatively stable substructure,which makes it difficult for dislocations to slide,Eventually,the fatigue behavior of cyclic hardening occurred in the ferrite pearlite structure.