Steel Processed by Water Jet Cavitation with Ultrasonic Irradiation
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Alloy steels for machine structural is used for industrial equipment. However, the operating environment is often harsh, requiring the development and application of various surface treatments. The authors focus on the use of water jet cavitation (WJC) with ultrasonication. WJC technology enables the generation of high pressure during cavitation collapse near the surface when a water jet ejected from a nozzle impacts a metal surface. This pressure causes a slight deformation in the impacted surface region and introduces a compressive residual stress due to the elastic constraints of the underlying and surrounding metal. If ultrasonic irradiation is applied to WJC bubbles with diameters of several hundred microns, the bubbles are subjected to alternating high and low sound pressures, which leads to a high-pressure and high-temperature reaction field. This technique is referred to as multifunction cavitation (MFC). The authors have reported several characteristics of specimens processed by MFC to date. For example, titanium oxide powder, which is widely used as a photocatalyst, can be fabricated in the form of nanoparticles by MFC. In addition, the indium tin oxide (ITO) film deposited on the transparent electrode mounted on liquid crystal displays can be melted by MFC treatment, leading to the release of indium, which has a low melting point. In present study, the compressive residual stress and corrosion resistance of Cr-Mo and Ni-Cr-Mo steels were improved by MFC treatment. Moreover, the authors compared conventional WJC technology to MFC technology. MFC was found to lead to higher compression residual stresses and higher corrosion resistances compared to conventional WJC. The corrosion resistance was revealed by the formation of an oxide film through selective oxidation and the concomitant reduction of surface defects. The oxide coating formed by a reaction between the dissolved oxygen in water with Cr on the metal surface during processing.