In this study, 30CrMnSiNi2A steel specimens were treated by shot peening technique. In order to evaluate the effects of shot peening process on the surface characteristics, the surface topography and roughness of the treated and untreated specimens were observed and analyzed using scanning electronic microscope (SEM) and surface profile tester. Surface microstructure and residual stress were observed and analyzed using transmission electron microscope (TEM) and X-ray stress apparatus respectively. Results show that the surface roughness of milling specimens could be reduced from Ra 4.45 μm to 2.39 μm using shot peening process. Severe plastic deformation and grain refinement were also induced by the process, and the average grain size of nano-structured regime was about 46 nm. Moreover, a 300 μm compressive residual stress layer with the -766.5 MPa compressive residual stresses was induced. At the same time, the anti-fatigue mechanism of shot peening was discussed.
To analyze the strengthening mechanism of 30CrMnSiNi2A steel welded joint with ultrasonic impact treatment (UIT), the welded joint specimens were full coverage strengthened by the technology. The microstructure of the surface layer in fusion zone of the welded joint with and without UIT was investigated by optical microscopy (OM). The hardness and residual stress distributions along the thickness direction were also measured by micro-hardness tester and X-ray diffraction method respectively. The results show that the microstructure in fusion zone of the untreated 30CrMnSiNi2A steel welded joint were coarse dendrite, and there were many welding defects in this zone. UIT has the ability to achieve more compact microstructure with only small welding defects. The average hardness value of the treated specimens reached 571 HV, increased 14.4% as compared with that of the untreated specimen (499 HV). A residual compressive stress layer with thickness of 850 μm was also obtained from by UIT, and the maximum residual compressive stress was-347 MPa. The grain refinement, work hardening and residual compressive stress in fusion zone introduced by UIT increased its anti-fatigue performance.
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