The Effect of Improved Powders
Recent developments designed to produce, from powder, engineering components that are not merely as good as those currently in production by alternative routes, but to produce better products, have been made possible by improved atomisation techniques that produce clean powders of highly alloyed metals, such as high-speed steels, and complex precipitation hardenable alloys based on nickel and/or cobalt with chromium, now commonly called superalloys.
These powders are isostatically pressed in evacuated metal cylinders and subsequently hot-forged, still in the container, to give a dense billet, which is then processed to the final shape by conventional means.
Critical Powder Production Parameters
The process depends on the use of inert gas, usually argon, to atomise the metal which has been pre-alloyed by vacuum melting. Increasingly centrifugal atomised powders are favoured - e.g. those made by the Rotating Electrode Process (REP).
Advantages of Using Improves Powders
The advantages of material produced are:
• Much better uniformity of chemical composition (lack of segregation)
• Freedom from ingot defects
• Fine and uniform distribution of secondary phases (carbides and other hardening precipitates). The last results from the fact that each particle of powder is virtually quenched from the liquid state.
Production of High Speed Steels
Similar advantages apply to high speed steels and these are now being produced from powder by two different routes.
1. Irregular, water atomised powders are cold compacted either in rigid dies or isostatically, and then sintered in vacuum to give individual pieces of near finished shape and size
2. In a process analogous to that used for superalloys, gas atomised (spherical) powder is HIPed and extruded to produce wrought material such as bar. A number of processes for the production of near-net-shapes have been developed and shown considerable promise. Many of these are pseudo-isostatic hot pressing, the pressure being applied by a hydraulic press to a solid medium in which the powder or powder compact is embedded, the medium being such that, at the operating temperature, it transmits pressure in all directions as if it were a fluid. One such process is the Ceracon process. Another is the so-called fluid die process in which powder is contained in a die made of a metal that is so soft at the compacting temperature that it behaves substantially as a fluid. Superalloy shapes have been made in this way using copper/nickel alloys as the die material.