Abstract
The microstructure and hardness on and below the surface of Cr-Mo steel (SCM435) treated by water jet peening (WJP) were investigated using scanning electron microscopy and micro Vickers hardness measurements. The change of the surface residual stress caused by the WJP treatment influenced the surface microstructure and surface hardness of the SCM435 steel. Cementite in the pearlite phase tended to protrude as the duration of WJP was increased. Voids were formed in the area 0.5 - 1.0 mm below the surface and also at grain boundaries between ferrite and pearlite grains, whereas no voids were formed in the depth range from 2.0 to 3.0 mm below the surface.
Highlights
Structural machine alloy steels have been used for a wide range of machine parts, mainly automobile parts
To further increase the compressive residual stress generated after water jet peening (WJP) processing, the specimen surface was first subjecting to hard grinding, which imparted tensile residual stress
Tensile residual stress was imparted in the grinding direction of the specimen not subjected to WJP processing, while compressive residual stress was imparted in the vertical direction
Summary
Structural machine alloy steels have been used for a wide range of machine parts, mainly automobile parts. Improvement of the fatigue characteristics of these alloy steels by conventional surface modification used at present is not always satisfactory, because the environment of use for such materials becomes increasingly severe. In the WJP process, high pressure occurs when a cavitation caused by high pressure water jetted from a nozzle collapses on the metal surface. This impact pressure results in slight plastic deformation of the surface layer and generates compressive residual stress by an elastic restraining force from the lower layer portion and the surroundings. The dependence of the microstructure and hardness on the WJP processing time was investigated
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