Effect of MC Type Carbides on Age Hardness and Thermal Expansion of Fe-36 wt%Ni-0.2 wt%C Alloy

Fe-36 wt%Ni alloy, also called "Invar alloy", is known for its low-thermal expansion. Since it has only mediocre strength and accordingly its application is limited, effects of addition of several carbide-forming elements, Ti, V, Zr, Nb, and Ta, on age hardness and thermal expansion of Fe-36 wt%Ni-0.2 wt%C alloy were studied for the purpose of developing a high-strength and low-thermal expansion alloy. All the alloys except Zr-added sample showed maximum hardness when aged at 650 °C. Among these additives, V had the largest age-hardenability with vanadium carbides precipitated abundantly and finely. In addition, vanadium carbides were formed with their crystallographic axes parallel to the matrix, causing about 15% of misfit strain. In the viewpoint of strengthening by carbide precipitation, large solubility in the matrix and structure and lattice constant similar to the austenitic matrix are important, and V is a preferred additive to Fe-36 wt%Ni-0.2 wt%C alloy. On the other hand, the V-added alloy in solution-treated condition showed the largest thermal expansion coefficient near room temperature. This was because it composed of larger amount of C and V in solid solution state but it is expected that its thermal expansion will decrease when these elements are eliminated from the matrix by appropriate aging. © 2013 Springer Science+Business Media New York and ASM International.