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

Abstract

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.

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

“…Cracks, which are likely to occur because of high-temperature corrosion, have been reported to form on surfaces subjected to compressive residual stress [ 3 ]. We therefore investigated the residual stress on each of the processed surfaces at each time ( Figure 5 ).…”

“…We speculate that this compressive residual stress suppresses stress corrosion cracking and voids that are likely to occur inside by delaying the time to change to tensile stress caused by the temperature difference between the surface and inside during heating. In fact, the compressive residual stress has been reported [ 3 ] to prevent voids, which easily occur inside the specimens. However, the effect of this compressive residual stress on the surface is not clear; this issue should be addressed in detail in future studies.…”

“…WJP and MFC were performed in the same way as in the previous study [ 3 ]. In addition to the aforementioned processing techniques, a swirl flow nozzle (SFN) [ 19 , 20 ] was installed to increase the number and size of cavitation bubbles at the tip of the WJ nozzle.…”

“…The high-temperature corrosion cycle tests were also performed during the coating tests. T he coating test was performed in the same way as in the previous study [ 3 ]. This test was performed 10 times.…”

“…Residual stress was performed in the same way as in the previous study [ 3 ]. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS, JSM-7100F, JEOL, Ltd.) was used to observe the surface.…”