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.

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 atomized powders are favoured - e.g. those made by the REP process already referred to.

The advantages of material produced are:

• 1. much better uniformity of chemical composition (lack of segregation);
• 2. freedom from ingot defects; and
• 3. 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.

Similar advantages apply to high speed steels and these are now being produced from powder by two different routes.

• (1) irregular, water atomized 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; and
• (2) in a process analogous to that described for superalloys, gas atomized (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 already described.
• 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.