Influence of Carbon Addition on Structure and Thermal Properties of Cast Superinvar Alloys

Carbon containing superinvar alloys based on Fe-Ni-Co-matrix has been obtained by casting. It is shown that carbon addition increases coefficient of thermal expansion (CTE) up to 3·10-6К-1, which is unacceptable and can be reduced after a heat treatment using annealing, quenching and quenching with subsequent tempering. Moreover, in order to obtain lower values of CTE it is better to concentrate carbon into spherical graphite phase than into γ-phase (γ-solid solution). Minimal thermal expansion of alloys is achieved after annealing at 800 °C during 3 hours. That mode of heat treatment ensures stable structural components (graphite and depleted in carbon γ-phase) which later stipulates minimal values of CTE. The features of structure formation and behavior of the CTE have been discussed with the main attention paid to the role of eutectic transformation during the crystallization process.