The influence of remelting processes on the mechanical properties of forged alloys

Abstract

In the past few years, several new melting processes have been industrialized for the production of superalloys, titanium alloys and high-quality steels for use in the turbine-engine industry. These include electron beam, plasma, induction skull and the "triple melt" process of vacuum induction + electroslag remelting + vacuum arc remelting. These developments have all been instituted in response to the major perceived problem of the industry : that of the incidence of random defects in the alloys which cause problems in regards to the predictable lifing of the finished part in service. The direct consequence of this uncertainty is the loss of revenue due to the premature retirement of parts which may have completed only a fraction of their potential service life : a conservatism on the part of design parameters which leads to unnecessary weight in the part : a risk of either service failures or part recalls which interrupt engine performance. The benefits of the process changes in respect of the products' absolute properties and also on the reproducibility and inspectability of those properties have been substantial. It is clear that the processes offer the industry a solution to the present dilemma of how to treat the "rare" defects from the point of view of fracture mechanics design lifing methodology. The use of process controls which guarantee zero defect product is developing into a critical tool for the extension of the range of a given alloy's use. It is very likely to permit substantial gains in both component life and in the allowable stress in rotating parts without the need to introduce new materials carrying new problems of user confidence, production reliability and property database. We conclude that the new processes are at a stage of development where they are ready for wide industrial use in production. They will not add significantly to component cost and they will give a renewed lifespan to the more familiar materials and methods of turbineengine construction.