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The residual stresses on the surfaces of low carbon 12Cr steels used as a nuclear steam turbine blade material have been studied by controlling the flame hardening surface treatments. The temperature cycles on the surfaces of 12Cr steel were controlled precisely as a function of both the surface temperature and cooling rate. The final residual stress state generated by flame hardening was dominated by two opposite competitive contributions; one is tensile stress due to phase transformation and the other is compressive stress due to thermal contraction on cooling. The optimum processing temperatures required for the desirable residual stress and hardness were in the range of 850℃ to 960℃ on the basis of the specification of GE power engineering. It was also observed that the high residual tensile stress generated by flame hardening induced the cracks on the surfaces, especially across the prior austenite grain boundaries, and the material failure virtually, which might limit practical use of the surface engineered parts by flame hardening.
The autoclave corrosion test was performed on the zirconium alloys in various aqueous solutions of LiOH, NaOH, KOH, RbOH, and CsOH, The oxide growth mechanism at metal-oxide interface was evaluated using SEM on the samples which were prepared to have an equal oxide thickness at pre-transition regime and post transition regime. At a low concentration (4.3 mmol), the corrosion rate was gradually decreased as the ionic radius of cation increased. However. at a high concentration (32.5 mmol). the corrosion rate was significantly accelerated in LiOH solution and slightly in NaOH solution. The morphologies and the growth rates of the oxides at metal-oxide interface were dependent on the ionic radius of cation even if the samples have an equal oxide thickness in LiOH, NaOH, and KOH, The grain orientation in Zr matrix also controlled the growth rates and the morphologies of the oxides. Therefore. It is considered that the oxide growth mechanism of Zr alloys can be controlled by the cation incorporation into zirconium oxide.