Development of energy intensive multifunction cavitation technology and its application to the surface modification of the Ni-based columnar crystal superalloy CM186LC.

The present work demonstrates a technique for the hot forging of metal surfaces in water at 1000 °C or higher, termed energy-intensive multifunctional cavitation (EI-MFC). In this process, the energy of cavitation bubbles is maximized, following which these bubbles collide with the metal surface. This technique will be employed to improve the surface structure of CM186LC/DS, a Ni-based columnar crystalline superalloy used to manufacture the rotor blades of jet engines and gas turbines that are exposed to high-temperature oxidizing environments, with the aim of improving creep strength. EI-MFC processing induces compressive residual stress in the metal that prevents the occurrence of surface cracks and also increases surface hardness, improves corrosion resistance, and increases the coefficient of friction. The latter effect can enhance the adhesion of thermal barrier coatings applied to Ni-based superalloys by thermal spraying. The technology demonstrated herein can be applied to present-day jet engine and gas turbine components and also to the production of hydrogen combustion turbines operating at 1700 °C with higher combustion efficiency than the current 1500 °C class gas turbines. In addition, the high processing energy obtained using the EI-MFC technique has the potential to flatten rough surfaces resulting from the stacking pitches of various metals manufactured using three-dimensional printers, and so improve surface strength.

Sonoluminescence from ultra-high temperature and pressure cavitation produced by a narrow water jet.

This work developed a small-scale processing apparatus for ultra-high temperature and ultra-high-pressure cavitation (UTPC) incorporating a small diameter (0.1 mm) water jet nozzle. This instrumentation comprised a swirl flow nozzle (SFN) installed on the water jet nozzle so as to obtain UTPC from a multifunction cavitation (MFC) setup. Multi-bubble sonoluminescence (MBSL) assessments using two types of photon counting heads were employed to assess UTPC, MFC, ultrasonic cavitation (UC), water jet cavitation (WJC) and SFN-WJC. The SL intensity was found to increase in the order of SFN-WJC, WJC, UC, MFC to UTPC. Because UTPC produced the most intense emissions, this process evidently attained the highest processing temperature. Assuming a UC bubble temperature of 4000 K, the temperatures associated with UTPC, MFC and WJC were determined to be 5400-5900, 5300 and 3200-3300 K, respectively. The energy density of a single bubble during UTPC was calculated using the Rayleigh-Plesset and Planck equations for an initial bubble radius of 100 µm together with photon measurements from many bubbles and employing Planck's law. The highest SL intensity of UPTC is thought to exist due to the high energy density of UTPC. This research demonstrates that it is possible to increase the energy density of cavitation bubbles within a small reaction area.

Rapid Nitriding of Titanium Alloy with Fine Grains at Room Temperature.

Multifunctional surfaces are required to design safe engineering products for human lives. Heating in a nitrogen atmosphere (nitriding) improves the tribological properties but reduces the strength of titanium (Ti) alloys owing to grain coarsening. A rapid nitriding method for Ti alloys forms the nitrided layer on the surface of a Ti alloy by bombarding with commercially pure Ti fine particles with a nitrided phase at room temperature within a short period. Furthermore, fine grains of Ti alloy are formed in the nitrided layer because of the impact of the Ti particles. These results reveal that this room-temperature method resolves the trade-off between the rapid formation of a nitrided layer and the suppression of grain coarsening for Ti alloys.

High-temperature corrosion behavior of high-temperature and high-pressure cavitation processed Cr-Mo steel surface.

In this paper, long-term high-temperature corrosion at 500 °C and high-temperature corrosion at the melting temperature of a corrosive ash mixture were examined because the use of high-temperature equipment such as boilers and gas turbines increases year over year. To investigate the optimum cavitation processing conditions for the specimens used in high-temperature corrosion tests, the surface properties of each processed specimen were examined. In specimens processed using multifunction cavitation (MFC), the compressive residual stress was high when the processing time was 10 min and the Cr content on the surface was greater than on the surface of an unprocessed specimen. On the other hands, in specimens subjected to water-jet peening (WJP), the compressive residual stress was high when the processing time was 10 min. In the present study, the processing time was selected to be 10 min and all high-temperature corrosion tests were conducted by the coating method. In the case of long-term high-temperature corrosion at 500 °C, the corrosion loss of the MFC-processed and WJP-processed specimens was small, whereas the corrosion loss of the unprocessed specimen was large.

Thermal Stress Relaxation and High-Temperature Corrosion of Cr-Mo Steel Processed Using Multifunction Cavitation.

This research investigated high-temperature corrosion (500 °C) of Cr-Mo steel processed using water jet peening or multifunction cavitation (MFC), and the suitability of such steel for high-temperature boilers and reaction vessels. High-temperature corrosion was induced using an embedment test and a coating test using sulfide-type K2SO4-Na2SO4 powder. To measure the relaxation of the residual stress due to the decrease in work hardening caused by an increase in specimen temperature and the difference in thermal shrinkage between the surface and interior of the specimen, a thermal cycling test was conducted. For the MFC-processed specimen, the oxide film that formed on the surface suppressed mass loss, prevented crack formation, and reduced the compressive residual stress caused by high-temperature corrosion. MFC-processed Cr-Mo steel is thus suitable for a high-temperature corrosion environment.

Sustainability of compressive residual stress on the processing time of water jet peening using ultrasonic power.

Water jet peening (WJP) is used as a stress improvement method and a countermeasure against stress corrosion cracking (SCC) in the internal structures of reactors in nuclear power plants. However, when residual stress is converted to compressive stress and applied to the specimen surface as a countermeasure against SCC, voids and cracks can easily form inside the specimen because of the increase in the pressure applied to the surface during WJP processing. Recently, multifunction cavitation (MFC), which is WJP using ultrasonic power, has been developed as an alternative to WJP. In MFC-processed low-alloy steel, when the residual stress is converted to compressive stress as an SCC countermeasure, voids and cracks do not form inside the specimen. In this study, to further improve current MFC techniques, the surface modification of low-alloy steel (Cr-Mo steel) was further investigated using 1200 W ultrasonic power. In MFC using 1200 W ultrasonic power, the corrosion resistance, compressive residual stress, and strength of the specimens were improved when the processing time was 10 min; however, decarburization occurred at longer processing times, causing these characteristics to worsen. The decarburization that occurs at high ultrasonic outputs may be caused by an increase in the water temperature and of the heating of the specimen surface. The evaluation of the surfaces of specimens processed for 30 min at ultrasonic powers of up to 1200 W revealed that decarburization does not occur on the specimen surface as long as the power does not exceed 720 W.