Font Size: a A A

Study On The Fatigue Performance Influencing Factors And Life Prediction Of Micro-shot Peened EA4T Axle Steel

Posted on:2022-07-12Degree:MasterType:Thesis
Country:ChinaCandidate:S D ZhuFull Text:PDF
GTID:2492306740959739Subject:Traffic and Transportation Engineering
Abstract/Summary:PDF Full Text Request
The micro-shot peening treatment can significantly improve the fatigue strength of the metallic material and ensure high quality surface finish.It is an excellent surface treatment process with broad application prospects.However,the researches on the effects of the fatigue performance and failure behavior of micro-shot peened materials are still insufficient.Therefore,it is necessary to carry out the research on the optimization of processes selection,fatigue strength influencing factors evaluation,and the fatigue life prediction under variable amplitude loads for micro-shot peening process.In this thesis,the EA4T axle steel was used as the experimental material.Firstly,9kinds of micro-shot peening processes with different shot peening strength or coverage and two kinds of post-polishing treatments were carried out,and the effects of the surface treatments on the surface properties and fatigue strength of the peened specimens were studied.Secondly,two kinds of tempering processes,550 ℃ for 4 hours and 250 ℃ for 2hours,were used to change the surface properties of micro-shot peened EA4T axle steel.The effects of the tempering processes on the surface properties and fatigue strength of peened specimens were tested and analyzed,and the fatigue strength influencing factors of micro-shot peened EA4T axle steel specimens were evaluated.Finally,the effects of the variable amplitude loads on the micro-shot peened specimens were studied.The relaxation processes of surface residual stress and half width of X-ray diffraction of peened specimens under four kinds of constant amplitude load spectrums and four kinds of variable amplitude load spectrums were followed and analyzed.Meanwhile the fatigue strength differences between shot peened and unpeened specimens under variable amplitude loads were studied.The main conclusions are as follows:(1)With the increase of micro-shot peening strength or coverage,the shot peening effects on the surface properties of EA4T axle steel increased gradually,however,the differences among different peening processes were small.Micro-shot peening processes showed a significant fatigue strength improvement of unpeened specimens,but the differences of fatigue behavior among different peening processes were small.Moreover,two kinds of post-polishing processes used in this study could effectively reduce the surface roughness of the peened specimens,but it also removed a certain depth of the surface layer with residual stress and work hardening,which induced the fatigue limits of polished specimens decrease.Both the micro-shot peening process and the post-polishing processes didn’t change the fatigue fracture mechanism of the unpeened specimens,and all the fatigue cracks initiated from the surface.(2)There was a work hardened layer with compressive residual stress on the surface of the micro-shot peened specimen,and the surface grain size was significantly refined.The tempering treatments could change the characteristics of surface residual stress and the surface microstructure strengthened layer(a surface layer with grain refinement and work hardening)of micro-shot peened specimens.The higher the tempering temperature was,the larger the residual stress relaxation and surface hardness changed.Meanwhile the tempering treatments of 550 ℃ for 4 hours could significantly increase the surface grain size.Both tempering processes of 550°C for 4 hours and 250°C for 2 hours would decrease the fatigue limit of the micro-shot peened specimens.In addition,the fatigue strength influencing factors of micro-shot peened EA4T axle steel were quantitatively evaluated,and the results showed that the residual stress and the surface microstructure strengthened layer increased the fatigue strength by about 16% and 23.8% respectively,while the surface roughness reduced the fatigue strength by about 10%.(3)Under the constant amplitude loads,the surface residual stress and half width of X-ray diffraction of peened specimens showed a linear relationship with the logarithm of the loading cycles after 10 cycles.The residual stress and half width of X-ray diffraction of peened specimens sharply relaxed during the overloading,and after that the relaxation rate decreased.The overloading made the fatigue life of unpeened specimens slightly prolonged,and greatly reduced that of the peened specimens,but the fatigue life of the peened specimen was still higher than that of unpeened specimen.A fatigue life prediction method of micro-shot peened specimen based on fatigue loads,residual stress relaxation and half width of X-ray diffraction relaxation was established,and the life prediction errors were within the acceptable range of the engineering.
Keywords/Search Tags:Micro-shot peening, EA4T axle steel, Residual stress strengthening, Roughness, Microstructure strengthening, Variable amplitude load
PDF Full Text Request
Related items