Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Patent
1990-02-22
1992-04-28
Lechert, Jr., Stephen J.
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
419 25, 419 56, 419 57, 419 54, B22F 900
Patent
active
051084929
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to corrosion-resistant sintered alloy steels which are made by powder metallurgy and also to a method for making such steels.
PRIOR ART
In recent years, the manufacture of sintered parts by powder metallurgy has been remarkably developed and the field of application of the sintered parts has now been extending. In particular, automobile parts, electronic and electric parts and office parts become more complicated in shape and their manufacturing technique is now undergoing a change from the machine work to powder metallurgy.
However, sintered alloys produced by the powder metallurgy are disadvantageous in that voids or pores are present in the alloy and give an adverse influence on corrosion resistance and mechanical characteristics. To avoid this, the sintered alloy should have a density as high as possible with a density ratio of not less than 92% being required.
For the manufacture of sintered parts by powder metallurgy, if known press molding is used wherein starting powders have a size as large as several tens to 150 micrometers, the density ratio attained by the molding and sintering is in the range of 80 to 90%. Thus, a satisfactory high density could not be obtained. Since the starting material is in the form of a coarse powder or grain, the space between particles is great and voids having a size of not less than 50 micrometers are present. The voids are not reduced nor disappear during the sintering, but remain in the structure of the resultant sintered product. The presence of the void leads to considerable deterioration of the corrosion resistance.
In order to improve the corrosion resistance, sintered alloys of high density have been developed wherein other alloy elements are added to stainless steel powder to cause a liquid phase to appear.
For instance, as described in Japanese Laid-Open Patent Application No. 58-213859, there is known a sintering material in which Co or B is added to and dispersed in a matrix so that during sintering, a liquid phase containing Co or B is produced to fill the voids or pores therewith. In Japanese Laid-Open Patent Application No. 61-253349, there is proposed a sintered stainless steel of high density wherein P is added so as to cause a liquid phase to appear.
However, if Co metal which is an expensive powder is added, the product cost rises with a sacrifice of economy which is one of merits involved in powder metallurgy.
When P is added, the liquid phase portion occluding P as a solid solution is left, after cooling, as a brittle phase, resulting in deterioration of mechanical characteristics.
Accordingly, the high densification technique wherein alloy elements are added and high density is attained by the liquid phase sintering has to be avoided. Further, in order to reduce residual pores, which directly influence the corrosion resistance, to an extent as low as possible, there is known a method wherein sintering materials are subjected to re-compression or re-sintering, or also to hot forging or hot isostatic treatment. This presents the problem that the procedure becomes complicated with the need of a specific type of device and complicated working operations.
Since stainless steels contains Cr which is a hard-to-reduce element, the dew point should be at a level not higher than -50.degree. C. for sintering in a reductive atmosphere. This is difficult from the industrial viewpoint and the common practice is to sinter in vacuum as is well known in the art. When the sintering is effected in vacuum, the Cr element having a high vapor pressure evaporates from the surface which is exposed in vacuum. This will lower the Cr concentration at the surface of sintered product, thus leading to considerable deterioration of the corrosion resistance at the surface. This has been experimentally confirmed by us. From this, it is assumed that the sintered product of high density obtained by the known vacuum sintering is a sintered alloy whose corrosion resistance is poor.
DISCLOSURE OF THE INVENTION
An object of the inve
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Kiyota Yoshisato
Matsushita Masakazu
Ohta Junichi
Ohtsubo Hiroshi
Sakurada Ichio
Kawasaki Steel Corporation
Lechert Jr. Stephen J.
Miller Austin R.
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