Structural machine alloy steels have been used for a wide range of machine parts, mainly automobile parts. However, 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. To solve this problem, the development of surface treatments and their practical application are required. As one method, the authors had a particular focus on the use of water jet peening (WJP) [1] [2] [3] . 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. When the compression force is converted to compression deformation, the deformation returns to its original state after cavitation collapse; however, if a small amount of plastic deformation occurs, then compressive residual stress is generated after cavitation collapse. Shot peening is generally used to apply compressive residual stress to metallic materials, which can result in macrostrain by compression. However, the collision of shot causes microstrain, such as dislocation and lattice defects, to increase, so that hydrogen can easily enter the metal, which tends to cause hydrogen embrittlement [4] . In contrast, WJP treatment can reduce microstrain for each crystal grain introduced by machining or heat treatment while applying macro distortion [5] . Much research on WJP treatment has been related to the sustainment of compressive residual stress and improvement of the resistance to stress corrosion cracking; however, there are few detailed reports on the internal structure in the depth direction from the surface layer of the specimen after WJP treatment.

In the present study, the microstructure and micro Vickers hardness at and below the surface of WJP-treated Cr-Mo steel (SCM435) were investigated using scanning electron microscopy (SEM) and micro Vickers hardness measurements. In addition, the dependence of the microstructure and hardness on the WJP processing time was also investigated.

The material used for these tests was Cr-Mo steel (SCM435), a structural machine steel, the chemical composition of which is shown in Table 1.

Round bar (rod) specimens were heated at 860˚C as a solution treatment, followed by quenching. Tempering was performed at 600˚C. The specimens were subsequently cut into rectangular specimens with dimensions of 100 mm × 100 mm × 3 mm. Figure 1 shows a schematic diagram of the equipment used for