Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Patent
1997-02-06
1999-03-09
Jenkins, Daniel J.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C22C 3302
Patent
active
058813560
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for the case-hardening of higher-molybdenum alloy sintered steels.
Prior Art
It is known to produce shaped bodies of any desired geometry from metallic powders, for example, from steel powders, by means of sintering. During this process, the metallic powders are heat-treated below a specific temperature which is just below the melting point of the sintered materials. During the heat treatment of sintered steels, regions having different lattice structures are known to form in which appear so-called .alpha.-iron, .gamma.-iron or a mixed structure comprised of .alpha.-iron and .gamma.-iron. Conventional sintered steels, which usually comprise carbon, sinter in the region of the .gamma.-iron. Here, the sintering process proceeds 10.sup.2 to 10.sup.3 times slower than in the .alpha.-iron region at the same temperature. If, for example, steel powders having an elevated molybdenum content are sintered, the sintering occurs at sintering temperatures of approximately 1250.degree. C. in the region of the .alpha.-iron. Since an entirely carbon-pure sintering must take place, a drawback of this process is that, during a subsequently required case-hardening, a carbon pick-up in the edge regions of the shaped sintered body is possible only with difficulty and, as a result, a brittle carbide network develops.
ADVANTAGES OF THE INVENTION
But, in contrast, the method according to the invention having the features listed in claim 1, offers the advantage that higher-molybdenum-alloy sintered steels can be case-hardened without resulting in the development of a brittle carbide network. Since, after sintering, the sintered steels are subjected to a heat treatment in the presence of carbon at temperatures at which a minimum portion of .gamma.-iron is present in the sintered steel, it is possible in an advantageous manner to create such a lattice structure, in particular, in the edge regions of the sintered steel, which lattice structure is subsequently suited for the pick-up of carbon. The additional heat treatment is carried out in one working step immediately following the sintering, with the protective gas atmosphere or the vacuum present during the sintering being replaced by a carbon-emitting agent, that is, carbon-containing atmosphere.
In the method according to the invention, cooling takes place from the higher sintering temperature down to the temperature range of the two-phase region in which a minimum portion of .gamma.-iron is present. It is therefore necessary for the case-hardening of sintered steels that a minimum portion of .gamma.-iron be present. If heating up starting from room temperature would take place, the supply of carbon during the heating up would have to be blocked because, also below the two-phase region, only .alpha.-iron is present (see FIG. 2) and the harmful iron carbide would develop in this region. Consequently, a long heating phase would be required which ultimately represents idle processing time. To avoid this idle time, cooling takes place from the sintering temperature down to the temperature of the two-phase region (.alpha.-iron and .gamma.-iron). Thus, the thermal energy supplied during sintering is utilized at the same time.
If the heat treatment is carried out preferably at a temperature of 1120.degree. C., approximately 40% of the material volume of the higher-molybdenum-alloy sintered steel is comprised in the necessary lattice structure region of the .gamma.-iron if the molybdenum content is 3.5 wt %. This favors the initial pick-up of carbon.
A preferred embodiment of the invention provides that, apart from the additional heat treatment after sintering, a case-hardening is subsequently conducted at the usual case-hardening temperatures of 840.degree. to 950.degree. C. This accomplishes that by means of the additional heat treatment carried out between sintering and case-hardening, an activation of the higher-molybdenum-alloy sintered steel occurs so that the incorporation of carbon becomes possible without resulting in the development of a
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Schelb Bernhard
Schneider Rudolf
BT-Magnettechnologie GmbH
Jenkins Daniel J.
Spencer George H.
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