Pneumatic isostatic compaction of sintered compacts

DETAILED DESCRIPTION OF THE INVENTION Densified sintered metal compacts are made by the process described hereafter.
Iron particles having particle sizes varying from about 100 microns to about 400 microns in diameter are blended with about 1/2% by weight to about 1 1/4% by weight of a suitable lubricant known to those skilled in the art (e.
g.
, ethylene bisstearateamide sold by the Lonza company under the label ACRAWAX.
TM.
), and uniaxially compacted in a steel die at pressures between about 20 tons per square inch (tsi) and 55 tsi to form an unsintered "green" compact having a density of about 6.
9 g/cc (i.
e.
, 12% porosity) to 7.
35 g/cc (i.
e.
, 5.
7% porosity).
The green compacts can also be made using conventional Cold Isostatic Pressing (CIP) techniques, wherein the compact is made by pressing at about 60,000 psi at room temperature to produce green compacts having a density varying between about 6.
9 g/cc and 7.
0 g/cc.
This compaction may be performed at room temperature, but will preferably be performed at a temperature between about 300.
degree.
F.
and about 500.
degree.
F.
to achieve higher green densities.
When higher temperature compaction is used the iron powder is preferably preheated to about 170.
degree.
F.
-375.
degree.
F.
and the die preheated to about 300.
degree.
F.
-550.
degree.
F.
Best isostatic pressing of the compacts is achieved when the green (i.
e.
, unsintered) compacts have a substantially uniform density throughout and are crack-free at the surface.
Nonuniform green density can result in lower than expected final density and deep surface cracks can result in poor oxide sealing of the surface.
The term "iron" as used herein is intended to include not only pure iron, but also those alloys of iron that are used in the sintered powdered metal industry and include such alloyants as copper, nickel, zinc, tin, molybdenum and manganese, inter alia.
It has also been found to be desirable to add a small amount (i.
e.
, about 0.
4%-0.
8% by weight) phosphorous (i.
e.
, as Fe.
sub.
3 P) to iron particles, to improve yield strength, ultimate tensile strength, magnetic flux density and maximum magnetic permeability,--albeit at some sacrifice to percent elongation at P levels greater than about 0