Improvement of age-hardening process of a nickel-base superalloy, IN738LC, by induction aging

Abstract

Influences of induction heating on the age-hardening process of a Nickel-base cast superalloy, IN738LC, was investigated. In this study cast specimens were undergone a solution treatment process in an argon atmosphere controlled furnace at 1125°C for 2 hours. Then, they were quenched in oil to room temperature to obtain supersaturated solid solution. These samples then subjected to two types of aging with equal heating rate, 30°C/Sec. One was induction aging and the other salt bath aging. Effects of these types of aging on the structure were analyzed and compared with normal aging having an average heating rate of 400°C/hr. The age-hardening behavior and microstructural characteristics were studied by hardness testing, scanning electron microscopy (SEM), electron image analyzing, X-ray diffractometery (XRD) and transmission electron microscopy (TEM) with replica method. According to the results obtained by these experiments, although the rate of heating of the specimens in induction and salt bath aging were equal, the diffusion-control process of nucleation and growth of γ′ precipitates in induction aging were considerably accelerated. Furthermore, desirable characteristics of γ′ precipitates were achieved in induction aging at lower time and temperature in comparison with times and temperatures of other types of aging. Improvement of microstructural characterization obtained in induction aging was related to the existence of an external electromagnetic force produced by induction heating. This electromagnetic force raised the effective driving force necessary for age-hardening process and intensified the nucleation and growth of γ′ precipitates remarkably.