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Experimental Investigation of the Effect of Almen Intensity and Coverage on Microstructure and Fatigue Behavior of High Carbon Steel Subjected to Severe Shot Peening

Maleki, Erfan | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: English
  3. Document No: 48344 (58)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein
  7. Abstract:
  8. In most cases, material failures occur on their surfaces, such as fatigue fracture, wear and friction, fretting fatigue and corrosion, so surface modification is one of the most effective techniques to improve the strength of materials. Correction of near surface grain size to nanoscale is one of the effective ways to improve the strength, fatigue life, toughness and etc. With increasing evidences of novel properties in nanostructured materials, it is reasonable to propose to achieve surface modification by the generation of nanostructured surface layer so that the overall properties and behaviors of the material are significantly improved.By means of surface severe plastic deformation, the size of the coarse grains in the surface layer of a conventional bulk material can be reduced down to the nanometer scale while its overall composition and/or phases keep unchanged. Among severe plastic deformation methods that result in surface nanocrystallization, shot peening has proved to be a promising technique. Recently the ability of severe shot peening to make nanostructure surfaces according to the depth of nano layer, size of crystal and microstructure features has been shown. Also the shot peening method is more economical than other methods. A lot of papers confirm the capability of shot peening in comparison with other methods such as torsion straining, cold-rolling,equal-channel angular pressing to make nano-structure surface and development of mechanical properties.The aim of this project is investigation of the grain size changing by changing the shot peening parameters including Almen intensity and coverage. Also in this project the effect of grain size on fatigue life of a high carbon steel is studied. In this research after preparation of experimental specimens, at first the effect of Almen intensity in size of shot peened surface grains is investigated; then to survey the shot peening coverage, other specimens with different coverage percentage and Almen intensity will be shot peened. According to the obtained results, the optimum value of Almen intensity and coverage percentage to make the surface microstructure are acquired. Afterward the effects of reduction of surface grains size on fatigue life in specimens that they are close to microstructure conditions are investigated.In recent several years the investigations on microstructural evolution and mechanical properties of strain-induced surface nanostructured layer have been carried out. Bagherifard et al. have investigated ability of shot peening for nanocrystallization of the material's surface. They verified Surface nanocrystallization and affirmed through different experimental procedures.They also performed Rotating bending fatigue tests to evaluate the effect of this high energy shot peening and the nanocrystallized layer on fatigue life. Unal and Varol, also performed severe shot peening with different Almen intensities on commercially pure titanium and then investigated specimens surfaces according to Almen intensity (plastic deformation rate). Also they studied mechanical properties such as hardness and elastic modulus by performing nano-indentation tests
  9. Keywords:
  10. Shot Peening ; Microstructure ; Fatigue Life ; Nanocrystallization ; Severe Shot-peening ; High Carbon Steel

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