High density stainless steel product and method for the preparation thereof

Abstract

The invention concerns a method of preparing compacts having a sintered density of above 7.3 g / cm3. This method comprises the steps of subjecting an annealed, water-atomised, essentially carbon free stainless steel powder, which in addition to iron, comprises at least 10% by weight of chromium, not more than 0.4%, preferably not more than 0.3% by weight of oxygen, not more than 0.05%, preferably not more than 0.02% and most preferably not more than 0.015% of carbon, not more than 0.5% by weight of Si and not more than 0.5% of impurities, to HVC compaction with an uniaxial pressure movement with a ram speed of at least 2 m / s, and sintering the green body.

Description

[0001] This invention relates to the general field of powder metallurgy. Particularly the invention is concerned with high-density stainless steel products and a compacting and sintering operation for achieving such products.[0002] Currently used methods for preparing high density products, such as flanges, of stainless steel powders involve compacting the stainless steel powders to densities of between about 6.4 and 6.8 g / cm.sup.3 at compaction pressures of 600-800 MPa. The obtained green body is then sintered at high temperatures, i.e. temperatures between about 1250.degree. C. and 1400.degree. C. for 30 to 120 minutes in order to get densities of about 7.25 g / cm.sup.3. The requirement for the long sintering times at the comparatively high temperatures is of course a problem considering the high energy costs. The necessity for special, high temperature furnaces is another problem.OBJECTS OF THE INVENTION[0003] An object of the invention is to provide a solution to these problems a...

AI Technical Summary

Benefits of technology

[0021] A particular advantage of the invention is that the compacts having near theoretical density may be sintered at low temperatures, such as 1120-1150.degree. C., in conventional furnaces, such as belt furnaces. This is in contrast to conventional compaction methods where it is not possible to obtain such high green densities and where a high sintered density is obtained by high temperature sintering, which causes shrinkage of the compacts. By using the HVC compaction method with no or a very small amount of lubricant included in the powder composition to be compacted, the green density will be essentially identical with the sintered density. This in turn means that very good tolerances are obtained.

[0022] The method according to the invention permits the manufacture of green and sintered compacts having high density, such as densities above 96 or even above 98% of the theoretical density. For stainless steel powders having the usual chemical compositions this corresponds to densities above 7.25, 7.30 and even 7.35 g / cm.sup.3

[0023] The invention as described in the present specification and the appended claims is believed to be of especial importance for large sintered stainless steel PM compacts having a comparatively simple geometry, such as flanges. Other products which may be of interest are gas-tight oxygen probes. The invention is, however, not limited to such products.

[0024] The invention is further illustrated by the following example:

Problems solved by technology

The requirement for the long sintering times at the comparatively high temperatures is of course a problem considering the high energy costs.

The necessity for special, high temperature furnaces is another problem.