Process for preparing a powder mixture and its use

Details Process for preparing a powder mixture and its use Process for preparing a powder mixture and its use

1995-10-31

1997-05-06

Jordan, Charles T.

Powder metallurgy processes

Powder metallurgy processes with heating or sintering

Consolidation of powder prior to sintering

419 11, 419 57, 419 58, 75231, 75246, 75355, 75950, B22F 312, B22F 908, C22C 3812

Patent

active

056280467

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of Invention
The invention is directed to a process for producing compression ready a powder mixture of steel powder and to the use of such a powder mixture for fabricating sintered articles with high toughness and density.
2. Description of the Prior Art
The fabrication of mechanical structural component pans from ferrous materials by way of sintering techniques, as opposed to production by cutting or chip-removing machining (e.g., turning, boring, milling), has the great advantage that the actual shaping can be effected in a single work step practically without waste and is therefore faster and more economical for duplicated or series-produced articles. For example, the articles are pressed to form green compacts on a hydraulic metal powder press in a die using a pressing pressure of 7 t/cm.sup.2, for instance, and are then sintered in a furnace at approximately 1120.degree.-1150.degree. C. (normal sintering) or at approximately 1250.degree.-1280.degree. C. (high-temperature sintering) in order to gain a sufficient static and dynamic strength. Owing to conditions of fabrication, the density of the sintered articles is always lower than that of the corresponding solid work material (theoretical density), since the articles are penetrated by pores. In ferrous materials, the actual density of the sintered articles is normally in the range of roughly 80-92% of the theoretical density depending on the applied pressing pressure and the shape of the article. This inevitably leads to impairment of the mechanical properties, of the article. Due to this sintered articles were previously not used under particularly high mechanical stresses, especially since greater dimensioning to compensate for this disadvantage is generally not acceptable due to the resulting increase in volume and weight. In addition, the pores contained in the sintered article can act as inner notches which in particular can lead to a drastic reduction of the dynamic strength characteristics.
In order to reduce the pore volume of sintered articles, it is known to use ferrous base powder with a higher phosphorous content. This leads to noticeable shrinkage during the sintering process and accordingly to an increase in density. The shrinkage of the sintered article is taken into account in the geometrical form of the press die by means of suitable overdimensioning and can accordingly be compensated to a great extent. However, the addition of phosphorous, which can be effected either by appropriate alloying of the melt used in the powder atomization or by admixture of phosphorous compounds with the ferrous base powder, has the disadvantage that it can only be used to a limited extent to increase density, since higher phosphorous contents tend to produce brittleness in the sintered articles and accordingly further increase susceptibility to notching.
Another method for achieving a higher density, i.e., for reducing the pore volume, is the so-called double sintering technique in which the compacted body, after first being sintered generally at a temperature of approximately 700.degree.-900.degree. C., is subjected to another pressing process and a final finish sintering. This is a very cost-intensive process due to the double pressing and sintering.
A ferrous base powder which ensures a comparatively high impact strength is known from WO 91/19582. The prescribed alloying elements compulsorily contain 0.3-0.7 percent by weight phosphorous and 0.3-3.5 percent by weight molybdenum. The sum total of any other alloying elements which may be present is limited to a maximum of 2 percent by weight. The molybdenum content is preferably 0.5 to 2.5 percent by weight and the phosphorous content is preferably 0.4 to 0.6 percent by weight (added in the form of Fe.sub.3 P in particular). A maximum carbon content of 0.07 percent by weight is recommended. This ferrous base powder is suitable for normal sintering temperatures (below 1450.degree. C.). The test results presented in this reference show that there are optimum quantit

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