Effects of the surface temperature and cooling rate on the residual stresses in a flame hardening of 12Cr steel

The residual stress properties of low carbon 12Cr steel, currently used as a nuclear steam turbine blade material, generated by flame hardening have been studied with respect to both the surface temperature and cooling rate. The residual stress state generated by flame hardening was dominated by two opposite competitive contributions, they are, tensile stress due to a phase transformation and compressive stress due to a thermal contraction. As both the surface temperature and cooling rate increase at temperatures above Teqγ, the increment of the tensile stresses was evident. It was, furthermore, found that the cracks were nucleated and propagated across the prior austenite grain boundaries during too high temperature (∼1200 °C) and rapid cooling (∼250 m/s) treatments. This can be explained by a stress relaxation phenomenon, implying a generation of large tensile stress probably with a value close to or higher than the yield stress of the base material. The optimum processing temperatures required for the desirable residual stress and hardness were found in the range of 870–960 °C on the basis of the specification of GE power engineering.