Casting processes are reviewed from the point of view of the type of defects they produce and their consequential properties of the castings they produce, particularly resistance to fracture, and therefore, their reliability in service. The ingot casting of steels is criticized for unnecessary degradation of the steel. The fundamental problems of continuous casting of aluminum alloys and steels are seen to be lying in inattention to the details of the processes. Vacuum casting, particularly vacuum arc remelting, as currently executed, is seen to be fundamentally unreliable for any safety critical purposes, particularly its history of helicopter tragedies resulting from its use in helicopter drive trains.
Part of the book: Casting Processes and Modelling of Metallic Materials
The writer describes his experience of a lifetime of casting metals, and how the casting technique controls the quality of the metal and offers answers to engineering failures. In view of the wide denial of this aspect of process metallurgy, the author takes the opportunity to present a personal view, backed up by additional evidence in a bibliography. It is a concern that the failure of wind turbine bearings continues, on occasions, to defy substantial metallurgical efforts. It is proposed here that there is good reason to identify the casting process as the generator of pervasive defects, which the writer calls bifilms. These defects originate from the casting process during the pouring of the liquid steel. They are simply doubled-over oxide films originating from the surface of the melt. They are inherited by the solidified steel and are resistant to bonding by mechanical working. They, therefore, exist in finished steel components as a substantial population of cracks. These pre-existing cracks are usually the initiators of fatigue failure, as well as other failure modes. Techniques to eliminate bifilm cracks during the casting of steel are now known and require to be implemented to produce steels that will naturally eliminate failure. We shall have, for the first time, steels we can trust.