INFLUENCE OF CARBON AND NICKEL ON LINEAR THERMAL-EXPANSION COEFFICIENT IN LOW THERMAL-EXPANSION FE-NI-C ALLOYS

Taking note of the segregation of Ni in the primary austenite, the influence of C and Ni content was investigated on the linear thermal expansion coefficient (α) of Fe-Ni-C alloys. When C below 2 mass% was added to Fe-(34-40 mass%) Ni alloys above the Invar composition, the α value increased in the temperature range from 373 to 500 K. With more than 2 mass%C additions, the α value increased in the temperature range from 373 to 773 K. On the other hand, with the addition of C up to 3.42 mass%C to Fe-(27-30 mass%) Ni alloys below the Inver composition, the α value decreased in the temperature range from 450 to 750 K because the composition of the primary austenite is reaching the Inver composition. The α depends mainly on the Ni concentration and not on the graphite structure. An effective distribution coefficient of Ni (keNi) in Fe-Ni alloys was smaller than unity, on the contrary, keNi in Fe-Ni-C alloys increased with increasing C content and was larger than unity. It was concluded that such a change in α was caused by the segregation of Ni in the primary austenite.