Low thermal expansion superalloy and manufacturing method thereof

Abstract

A low thermal expansion superalloy is composed of, in mass %, 0.1% or less of C, 0.1-1.0% of Si, 1.0% or less of Mn, 25-32% of Ni, more than 18% but less than 24% of Co, more than 0.25% but 1.0% or less of Al, 0.5-1.5% of Ti, more than 2.1% but less than 3.0% of Nb, 0.001-0.01% of B and 0.0005-0.01% of Mg, with the balance of Fe and unavoidable impurities, while satisfying Mg/S≥1, 52.9≤1.235Ni+Co<55.8%, (Al+Ti+Nb) is 3.5-5.5%, and the F value is 8% or less. In the superalloy, a granular intermetallic compound containing Si, Nb, and Ni alone or in a total amount of 36 mass % or more is precipitated at a grain boundary of an austenite matrix, and an intermetallic compound including a larger concentration of Ni, Al, Ti, and Nb and having 50 nm or smaller of an average diameter is precipitated in the austenite matrix.