The Influence of Different Melting Methods on High-Temperature Properties of Heat-Resisting steels

Abstract

As a result of the rapid development of vacuum melting process, many super-alloys have been developed. It is evident that the high temperature properties of these super-alloys depend on various melting processes. It is the purpose of this study to point out the high-temperature properties of A-286 and Incoloy T containing some scavenging elements such as Ti and Al. Therefore, in order to obtain a good comparision between an air-melting and various vacuum-melting techniques, it was first necessary to obtain heats of similar chemical compositions. 10kg electrolytic materials were melted in an alumina or magnesia crucible with a laboratory-scale vacuum-induction equipment which had 25KW capacity and about 60kC frequency.
Degassing process during vacuum induction melting was done by two methods of carbon and hydrogen refining.
Attainable oxygen and nitrogen levels in these specimens refined by carbon are lower than those of these specimens refined by hydrogen. 18mm ∅; forged bars made from both air-and vacuum-melted ingots were used for age hardening, short-time tensile test, rotating-bending fatigue test, high-temperature creep-rupture test, electron-microscopy and X-ray examinations.
In terms of the relationship between rupture time and gas content of each specimen, hightemperature rupture properties of Incoloy T were improved by reducing the gas content.
On the other hand, the same result was not obtained in the case of A-286. Those were much the same in age-hardened characteristics, short-time tensile strength and rotating-bending fatigue strength by both melting methods but the ductility after tensile test for the vacuum-melted specimens of A-286 were superior to those for the air-melted. According to the results of long-time creep-rupture tests at 650 and 815°C, the rupture strength in the vacuum-melted Incoloy T specimens was especially superior to those of the air-melted.
The instability break points which were found on stress-rupture design curves at each test temperature depended on the translation from a transcrystalline to an intercrystalline type fracture, and the break points of vacuum-melted Incoloy T appeared after a longer time than the air-melted alloys.
It was believed that the strengthening characteristics of vacuum-melted Incoloy T were caused by precipitations in grain boundaries and matrices.