Melting and Refining Technology of High-temperature Steels and Superalloys a Review of Recent Process Developments

The recent increase in number and capacity of special melting techniques and refi ning processes is related to the continuing growth of high-temperature alloy usage in wrought and cast form and to the fact that the metallurgical and technological potential of conventional melting and casting processes have been largely exhausted. The evol ution of the major melting processes for high-performance materials and the increasing trend toward separation of rnodern melting and refining operations into distinctive indi vidual functions for maximum utilization of each unit are outlined. Use of auxiliary re equipment after air melting is most attractive for producers of specialty steels fining but has also found application for nickel-base alloys. Some of the more sophisticated auxiliary refining technologies are described in the light of their capability of performing multiple functions, like degassing, desulfurization, carbon deoxidation, precise alloying, and decarburization. Metallurgical process characteristics, principal features and shortcomings of various production and pilot melting techniques are discussed and compared with each other on the basis of their technological potential. The present wrought alloy manufacturing processes provide flexibility in the selection of the most suitable combination of specific melting processes for the large variety of alloys to achieve the best material quality most economically. Emphasis is placed on the technology of the most important double vacuum melting process which combines vacuum induction melting and vacuum arc remelting. Electraflux remelting in combination with air melting or vacuum induction melting is increasingly used as an alternative process to vacuum arc remelting for certain alloys. Metallurgical, operational, and quality aspects of the widely used production melting techniques are discussed regarding raw material selection, composition and impurity control, refining capabilities, and control of ingot structures. Additionally, electron beam melting, which is in limited production use today for hightemperature alloys, and the promisin tensive development are considered. 1 plasma arc melting process presently under inn outlook into the future position of the various melting and refining schemes conclude the review.