We developed a new nozzle in order to greatly increase the temperature and pressure of
multifunction cavitation (MFC), so allowing ultra-high temperature and pressure
cavitation (UTPC) to take place, which produces a new metal surface. Low-alloy steel is
based on carbon steel, but with a low percentage of alloying elements, in many cases less
than 1%. Although its price is slightly higher than that of carbon steel, it offers improved
resistance to specific types of corrosion. However, its corrosion resistance is still less
than that of stainless steel. In conventional heat treatment processes for metals, heat
treatment after cold working produces quite different results to cold working after heat
treatment. In contrast, MFC has the capacity to allow microscale or nanoscale forging, in
which the material is simultaneously worked and heat treated. In this study, modification
of the surface of low-alloy Cr-Mo steel (SCM435) was carried out using UTPC. The
process was found to be effective for improving the residual stress and conferring high
strength and corrosion resistance. It is considered that an oxide film was formed by
reaction of dissolved oxygen in the water with Cr on the steel surface. In this type of steel,
the microstructure consists of a lamellar ferrite phase and a pearlite phase consisting of a
mixture of both ferrite and cementite. The UTPC treatment causes the pearlite to partially
change to granular cementite at the surface and to spherical cementite beneath the surface.
No cracks or voids are observed beneath the surface and a ductile region is formed. The
Charpy impact energy of UTPC has the highest value of 101 J, because the ductile layers
were formed by UTPC processing. The proportion of ductile and brittle in UTPC
processing is higher than the other specimens. Stress relaxation behavior of various
processed materials at a temperature of 500℃ was investigated. The compressive residual
stress of more than -100 MPa is retained after annealing both the WJC and UTPC
processing specimens for 5 hours. After stress relaxation testing, cracks owing to thermal
stress do not occur at the grain boundary in the UTPC material having a tenacious tough
layer inside.
1 INTRODUC