An attempt has been made to develop new alloys with high thermal fatigue resistance and interpret the thermal fatigue behavior based on the relation between plastic strain range and related elongation in tension test at elevated temperatures. The thermal stresses that lead to thermal fatigue failures are generally induced by rapid change in temperature due to start and stop of plants, so that the total number of cycles involved is relatively small. Resistance to thermal fatigue depends on the material properties such as a coefficient of thermal expansion, heat conductivity and resistance to the strains incidental to temperature change. At present work, three types of modified NCF625 alloys were candidated to control the material properties by changing the contents of elements such as aluminum, titanium and iron, and were cast as ingots for the test. Thermal fatigue behaviors of examined alloys were evaluated at elevated temperatures between 773K and 1073K, and between 773K and 1073K. As a result, it was derived that the modified NCF625 alloy containing 1% titanium, 1% aluminum, and 1% iron showed the highest thermal fatigue strength. It was found, form the thermal fatigue tests, that this alloy had thermal fatigue resistance about two times that of NCF625. The alloy thus selected was gas atomized to powder and PTA welded on a carbon steel plate, and it was confirmed that there was no problem found on the metallurgical structure and hardness distribution.
