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Heat-resisting alloys of nickel (the Nimonics) and iron (chromium-nickel steels) are finding increasing application for the construction of dies used for the hot forming of metals and metal powders. Although these alloys are strong and tough at temperatures in the range of 1350 to 1750 F, they are liable to suffer from surface erosion, particularly in extrusion, for example, where the rate of flow of the deforming metal is considerable. This surface erosion is caused mainly by two effects: (1) partial welding of the deforming metal, followed by tearing away of portions of the surface of the die material, and (2) the formation of relatively low melting point eutectics (from constituents of the deforming metal and the die material) which are continuously squeezed out of the reaction zone due to the high pressure. Such effects can be reduced by preforming a suitable adherent oxide film on the dies (for example, chromium oxide on Nimonic alloys) and by the use of die lubricants such as graphite, but these methods are not infallible, and the films formed are very thin and readily abraded. It was considered that better protection would be afforded by an adherent coating of suitable alloys, high melting point eutectics, or intermetallic compounds, formed with, and on the surface of, the die material. Preferably such a coating should be formed by a relatively simple technique, such as diffusion of a mechanically applied coating into the surface of the base metal.
Anglo American Corporation of South Africa, Johannesburg,