Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Patent
1993-12-10
1996-10-22
Mai, Ngoclan
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
75246, 75355, 419 11, 419 38, C22C 3300
Patent
active
055678904
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an iron-based powder which after powder compacting and sintering gives dimensionally stable products, i.e. products inherently exhibiting similar dimensional changes, also in the event of local density variations.
A major advantage of powder-metallurgical processes over conventional techniques is that components of varying complexity can be sintered into final shape immediately after powder compacting, and they therefore require but a relatively limited aftertreatment as compared with e.g. a conventional steel blank. Also in the development of new powder-metallurgical materials, it is an aim to ensure that the dimensional change is small during sintering, since it has been found difficult in practice to maintain the dimensional stability if the dimensional change is considerable. This is especially important in the case of high-strength materials which are difficult to adjust to correct measurement after sintering. Therefore, it is vital that the dimensional change is minimal and as independent as possible of variations in the process parameters sintering time, sintering temperature, carbon content and distribution of alloying substances. In the development of high-strength diffusion-alloyed materials during the 1970s, the primary objective precisely was to make the dimensional change as independent as possible of these process variables.
By the diffusion-alloying technique, the alloying substances Ni, Cu and Mo have become uniformly distributed in the material and the contents of these substances can be so selected that variations in the other process parameters time, temperature and C-content have but a small effect on the dimensional change. On the other hand, it has been found that the dimensional change is not constant for different density levels in these materials. In the compaction of powder mixtures, the density may in fact vary considerably within the compacted component and in particular if the geometrical shape is complex. For example, density differences about 0.4 g/cm.sup.3 are not at all unusual in practice. This, in turn, may give rise to different dimensional changes locally during sintering, thus making the material "warp", which may mean that it will have to be rejected.
One object of the present invention is to provide a dimensionally stable sintered product. The expression "dimensionally stable" as used in this context means that the product undergoes a similar dimensional change despite inherent density differences. Thus, it is possible according to the invention to produce a product which, although exhibiting inherent density differences, has a variation in the dimensional change of at most about 0.07%, preferably at most about 0.05% at a minimum density of about 6.7 g/cm.sup.3, especially in the density range of 6.8-7.2 g/cm.sup.3. The dimensional change during the sintering process need however not be zero, since the pressing tools can be adjusted in size already at the design stage so as to obtain the correct shape after sintering.
Another object of the invention is to produce an iron-powder-based material which after compacting and sintering yields a dimensionally stable product having high strength. For instance, it is possible with the iron-powder-based material according to the invention to produce sintered products having a tensile strength above about 450 MPa, especially between 500 and 1000 MPa, and preferably between 550 and 950 MPa, without the sintered product being subjected to subsequent heat treatment.
Yet another object of the invention is to produce a powder which by a simple and inexpensive low-temperature sintering process yields a product having the properties specified above.
The invention embraces also such powders as after compacting and sintering exhibit not only good dimensional stability and high strength but also high fatigue strength. In these powders, the nickel content is comparatively high and preferably is in the range of 2-4.5% by weight.
According to the invention, these objects can be achieved by a powder compositio
REFERENCES:
patent: 4128420 (1978-12-01), Esper et al.
patent: 4561893 (1985-12-01), Takajo
patent: 4702772 (1987-10-01), Engstrom et al.
patent: 4954171 (1990-09-01), Takajo et al.
Second International Search Report for the present application (PCT/SE92/00399) (Dated Sep. 22, 1992).
International Preliminary Examination Report for the present application (PCT/SE 92/-0399).
First Search Report for the present application ("International-type Search Report" dated Jan. 21, 1992.
Johansson Bjorn
Lindberg Caroline
Hoganas AB
Mai Ngoclan
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