The present paper demonstrates the development of a peening technique using recirculating shot accelerated by a water jet that can be used as a risk free treatment in power and chemical plants.Although it is important to increase the reliability of structural materials used in large scale structures, such as power and chemical plants, it is hazardous to treat these structures by general shot peening (SP) techniques because of the risk of sparks and dust explosions. In the present study, an SP technique using a water jet to accelerate shot has been developed, and the conditions under which it is used have been optimised in terms of nozzle geometry, standoff distance and amount of shot. In order to demonstrate the effect of SP, austenitic stainless steel Japan Industries Standards SUS316L was peened and evaluated in terms of its residual stress, fatigue strength and resistance to stress corrosion cracking. IntroductionIn large scale structures, such as power and chemical plants, it is necessary to ensure its long time operation by enhancing the fatigue strength and suppressing the stress corrosion cracking (SCC) in the materials from which the structure is made. Shot peening (SP), which utilises the impact generated by shot, can be applied to enhance the fatigue strength of mechanical components by introducing compressive residual stress on the surface layer. 1-6 In order to carry out this treatment while the plant is in its operating condition, there must be no risk from sparks and dust explosions generated by shot colliding. In the present paper, we describe an SP technique in which the recirculating shots are accelerated by a water jet.It is well known that the residual stress on the surface layer introduced by mechanical processing or heat treatment is one of the major factors that affects the fatigue strength and the resistance to SCC. 7-13 The SCC usually occurs when we have a material under tensile stress, including residual stress, in a corrosive environment. As it is difficult to improve a material's resistance to corrosion and reduce its exposure to the environment during operation, its resistance to tensile stress should be improved. This can be performed by introducing compressive residual stress on the surface for example using peening techniques, such as SP, cavitation peening 14-17 or laser peening. [18][19][20] In general, air blast systems and centrifugal blast systems are used for accelerating shot. The shots hit the surface in high velocity and broke due to colliding. 21,22 There is a possibility that sparks and dust explosions generate in such situation. 23,24 For this reason, general SP treatment is always exposed to risk of sparks and dust explosions. In order to avoid the risk of sparks and dust explosions caused by solid body collisions, it is essential to use a dust collector and a separator. As those systems make the SP apparatus larger, it becomes more difficult to treat some structures. Therefore, an alternative technique for accelerating shot to replace air blast systems or centrifugal ...

Introducing compressive residual stress by a cavitating jet into the sub-surface of components used in nuclear power plants can mitigate stress corrosion cracking in these components. Although applying the jet is an effective method for this purpose, it should be used without causing damage to the surface from water jet droplets arising from high-pressure injection of the water jet. Thus, in introducing compressive residual stress, the injection pressure needs to be optimized. In this paper, in order to determine the optimum injection pressure, the residual stress of stainless steel treated by a jet at various injection pressures was measured using an X-ray diffraction method. The injection pressure of the jet was varied from 5 MPa to 300 MPa, and the diameter of the nozzle throat of the jet was varied from 0.35 mm to 2.0 mm. The variation of residual stress with depth was measured by alternating X-ray diffraction measurements with electropolishing. It was revealed that a cavitating jet at an injection pressure of 10 MPa with a nozzle diameter of 2.0 mm can introduce higher compressive residual stress to deeper into stainless steel compared with a jet at 300 MPa with a nozzle diameter of 0.35 mm when the downstream pressure of the nozzle was constant.

In this study, steel rollers and gears were treated by the cavitation peening with processing times of 1 min and 5 min, and they were fatigue-tested using a roller testing machine and a gear testing machine. The surface hardness and the compressive residual stress of the rollers and gears were increased by the cavitation peening, while, the surface roughness of the cavitation-peened rollers and gears was almost the same as that of the non-peened ones. As a result of the fatigue tests, the fatigue strength of the test specimens were improved by the increase in surface hardness due to the cavitation peening, and those without the increase in surface hardness were not increased so much. The fatigue strength of the cavitation-peened test specimens was similar to that of the shot-peened ones obtained by the previous fatigue tests. This study revealed that the cavitation peening is expected to put to practical use in the improvement in fatigue strength of steel gears.

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