Suppression of Mean Thermal Expansion Coefficient of Precipitation Strengthened Austenitic Heat Resistant Steel by Means of Invar Effect

Abstract

In this study, the effect of Ni and Cr composition on mean thermal expansion coefficient (CTE) and γ’-Ni₃ (Al, Ti) strengthening microstructure of austenitic heat resistant steel, Alloy 286, was investigated. The measurements revealed that the CTE of Alloy 286 decreases at wide range of temperature up to 800°C with addition of Ni and reduction of Cr concentration. The magnetization-temperature curve showed that the magnetic transition temperature, which the alloy transforms to ferromagnetic state to paramagnetic state, raise with addition of Ni and reduction of Cr. The CTE of alloys had good relations with magnetic transition temperature and mean valence electron numbers (e/a), suggesting that the magneto-volume (invar) effect and the suppression of lattice expansion strongly appear at high Ni and low Cr concentration. Ni and Cr optimized alloy exhibits γ phase microstructure with fine γ’ particles dispersed within the grain, and the hardness of grain interior is larger than Alloy 286. This alloy has low CTE comparable to conventional ferritic steels, together with higher strength than conventional austenitic steels.