Reduction of Thermal Expansion of Ferritic/martensitic Heat Resistant Steels -Alloying Effects on Thermal Expansion of <i>α</i>-Fe Phase-

Alloying effects on the thermal expansion of the α-Fe phase in connection with the magnetic states were investigated with a final goal to design ferritic/martensitic heat resistant steels with a reduced thermal expansion coefficient at high temperatures. The thermal expansion coefficient decreases with increasing the content of the alloying elements M (M = Co, Cr and V) regardless of the types of the elements in the ferromagnetic state well below the curie temperature in each alloy. The thermal expansion coefficient increases with temperature more significantly in the paramagnetic states compared to the ferromagnetic states. The temperature dependence of the thermal expansion coefficient in the paramagnetic states is not much influenced by the type of alloying element. As a result, the lower the Tc is, the higher the thermal expansion coefficient tends to become in the paramagnetic states at high temperatures. Based on the results obtained, a way is proposed to design heat resistant ferritic steels with reduced thermal expansion coefficients; to add both Cr and Co as major alloying elements such that the Tc and the A3 temperatures are placed above their service temperature.