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(11) | EP 0 097 027 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Densification of selected areas of powder metal parts |
| (57) A method of preparing a powder metal body such as a gear having adjacent portions
of different densities. In the case of a gear, the gear teeth and the root adjacent
the gear teeth are provided with the greater density to give the teeth greater impact
and wear resistance. The first step comprises forming a powder metal compact by pressing
powder metal in a mold (10) with at least one multiple depth punch (11, 14) to produce
a compact having a portion of greater length relative to the length of another portion
of the compact. The next step is sintering the compact followed by repressing with
a punch that applies forces only to the portion of the sintered compact having the
greater length. |
[0002] In producing powder metal parts, metal powder is placed in the cavity of a mold and is pressed between upper and lower punches. The faces of the punches and the contour of the mold are configured to impart the desired shape to the finished parts. This practice is generally designed to provide as uniform a density as possible throughout the entire part. If a higher part density is desired, higher compacting pressures are used in molding the part. In an alternate method, a pressed and sintered part is placed in a similarly shaped mold and repressed. Higher compacting and/or repressing pressures are limited, however, by factors such as available press tonnage capacity, excessive tooling stresses, and the quality of the molded part.
[0003] For some applications, it is desirable to have a powder metal part with a higher density in one portion than in another portion to obtain different mechanical properties in the different portions of the part. According to this invention, to obtain a higher density in certain portion of powder metal parts, compacting punches are modified to allow a greater depth of powder fill in one portion relative to another portion. After compacting and sintering, the part is repressed in a similarly shaped mold. The punches, however, are designed to repress only the thicker or over- filled portion to achieve further localized densification. The repressed part may be heat treated or resintered followed by heat treatment.
[0004] It is an advantage according to this invention that after sintering, repressing is restricted to only those portions of the part requiring further densification. Thus the available press tonnage is applied only to those portions of the part.
[0005] This invention has particular application to the manufacture of powder metal gears, although it certainly has many other potential uses. Since prior powder metallurgy methods for gear production are economically attractive, powder metal gears are extensively employed. In the case of a gear, the portions requiring further densification are the teeth and the area of the gear body directly below the root diameter. Gear teeth are subject to severe static and impact loading as well as severe wear.
[0006] An additional benefit is provided, according to this invention, in the case of ferrous-base parts containing carbon which require heat treatment in their entirety or require case hardening. When conventional powder metal parts are made by molding and sintering, they are character- ised by an internal interconnected porous structure. During a heat treating operation, normally conducted in a controlled atmosphere with a prescribed carbon potential, the atmosphere enters the pores of the part and may substantially increase its carbon content throughout. In a similar manner, a carbo-nitriding or carburizing atmosphere designed to impart only a hardened case also enters the part via the interconnected pores. In both cases, this may be deleterious to the mechanical properties of the part. If the interconnected pores are minimised or eliminated by increasing the density of a part (or a portion of the part), the active gas cannot effectively reach the interior of the part.
[0007] Some textbook references relating to conventional powder metallurgy techniques included: Lenel, F.V., Powder Metallurgy - Principles and Applications', Metal Powder Industries Federation, Princeton, NJ (1980), Chapter 17, pp. 433-444. Goetzel, C.G., Treatise on Powder Metallurgy, Interscience Publishers, Inc., New York (1949), Vol. 1, pp. 656-660. Hoganas Iron Powder Handbook, A.B. Boktryck, Sweden (1958), "Sizing and Coining" (Hugh G. Taylor), Vol. 1, Sect. D, Chapter 40, pp. 1-26.
[0008] This invention applies to the art of powder metallurgy. There are two basic steps in the powder metallurgy process to which this invention applies; namely, pressing the metal powder together in a mold at high pressures followed by heating to a temperature near but below the melting point of the metal (i.e., sintering). The powder metal may be derived by using any of the known processes including atcmizing, milling, shotting, reducing oxide powders, electrolytic deposition, or condensation from metal vapors, for example. Typically the powders are pressed in a cold metal die. Pressures generally vary from five to fifty tons per square inch. Sintering is performed in a reducing atomos- phere or in a reducing atmospheric pressure in a vacuum furnace to avoid oxidation of the metal. During sintering, the individual particles are metallurgically bonded together by means of solid state diffusion. According to this invention, a powder metal shape is formed in a mold with at least one multiple depth punch. Thus, the shape will have a portion with a greater length i.e., the linear dimension in the direction of pressing, than the length in an adjacent portion. Preferably, both the top and bottom punches used for initial compaction are multiple depth punches. Practically speaking, the ratio of greater length in the linear dimension to lesser length in the linear dimension after initial compaction is greater than 1:1 and less than 1.5:1. Most preferably, this ratio is greater than 1:1 and less than 1.2:1.
[0009] The powder metal shape is sintered, but not necessarily as fully i.e., to as high a temperature and for as long a time, as it might have been according to prior practices. Specific examples of sintering times and temperatures for particular metals are given in the following examples. The sintering must be sufficient to cause bonding between abutting powder particles. The bonding should be sufficient to maintain the integrity of the particle bond upon repressing whereby the shape can be plastically deformed.
[0010] The next step according to this invention, is repressing in a mold with a punch that applies compacting forces to those portions of the sintered shape which initially had a greater length. Thus, if the punches are simply flat they will bear upon the raised portions of the sintered shape applying pressure first to those portions. The portions of greater length typically have a lower density prior to pressing than the other portions but upon repressing the density of the raised portions becomes greater. The repressing mold must, of course, confine the side walls of the shape and thus will have a shape substantially identical with the mold used to form the powder shape. If a central bore is required, a mandrel will be required in both the powder compaction and the repressing molds. An optional additional step may comprise additional sintering of the repressed shape or merely reheating followed by a quenching.
[0011] This invention has particular applicability to the manufacture of small gears wherein the gear teeth are provided extra density.
[0012] The invention will be more particularly described with reference to the accompanying drawings, wherein:
Figures 1A and 1B illustrate a compacting mold assembly illustrating multiple depth punches;
Figure 2 illustrates a powder shape prior to repressing; and
Figure 3 illustrates the shape of Figure 2 after repressing.
[0013] Referring now to Figure 1A there is illustrated a mold 10 having a lower punch 11 and a mandrel 12 which slides within a bore in the lower punch. The walls of the mold are generated by a line substantially parallel to the axis of mandrel 12 and the bore in the lower punch. The walls of the mold may define the outer edges of gear teeth, for example. In the particular embodiment shown in Figure lA, the top face of the lower punch is stepped to provide that the peripheral area is lower than the central area adjacent the mandrel. In the case of the mold for a spur gear, the area under the gear teeth is thus lower than the hub portion adjacent the mandrel. Note that if the mold is filled evenly to the top with powder metal, the length of the portion of the powder metal at the periphery of the mold is greater than the length of the powder metal in the central portion of the mold.
[0014] Referring now to Figure 1B, an upper punch 14 has a bore to receive mandrel 12 and has a lower face that is stepped so that the peripheral area is higher than the central area. When the metal powder is compacted the resulting part has the cross section shown in Figure 2. In the case of a gear, the body portion length is less than the tooth portion length. Interestingly, it is a fact that the density of the longer peripheral tooth portion after compacting is less than the density of the shorter body portion.
[0015] It should be noted that the punch face does not have to be ground at a right angle to its center line. Thus a taper toward or away from the center line may be made which will give a variation of density in the repressed shape.
[0016] The top and bottom punches may comprise multiple telescoping punches independently positionable to different depths within the mold.
[0017] It is emphasized that the principle of selected densification may be applied to any powder metal part and a gear is only an example.
[0018] After the powder metal part shown in Figure 2 is sintered, it is repressed using flat-faced punches. Initially, at least, the flat faces of the punches only contact the longer peripheral areas of the gear. The extra length of the teeth is now compressed to substantially the same length as the gear body producing a part as shown in Figure 3.
EXAMPLE I
[0019] Ferrous-based powder metal parts were densified at specific locations to much higher densities as follows:
[0020] A blend of low carbon atomized steel powder (Hoeganaes Grade A1000) and 0.9 percent carbon in the form of graphite powder (Southwest Grade 1651) was used. Also included was 0.5 percent of a wax (Acrawax) and 0.25 percent stearic acid powders as die lubricants. The upper and lower punches were ground to the root diameter. When a gear was pressed to a nominal density of 6.8 g/cm , the body of the gear was a nominal 0.500 inch (1.27 cm) long and the teeth were a nominal 0.553 inch (1.41 cm) long.
[0021] The gears were sintered at 2050°F (1121oC) for 30 minutes in a.neutral endogas atmosphere. After sintering they were sized to various lengths shown in Table 1 using flat-faced punches. They were then heat treated for 30 minutes at 1535oF (835°C) in an endogas atmosphere, 0.7 percent carbon potential, and oil quenched. The results are set forth in the following table:
EXAMPLE II
[0022] The identical powder fill as used in Example I was molded between ground top and bottom punches to a nominal density of 6.4 g/cm3 so that the body of the gear was 0.500 inch (1.27 cm) long and the teeth were 0.580 inch (1.47 cm) long. The gears were sintered as in Example I and then sized with flat-faced punches to lengths shown in Table II. After heating for 30 minutes at 1535°F (835°C) in an endogas atmosphere, 0.7 percent carbon potential, and oil quenching, the following data were obtained.
EXAMPLE III
[0023] This example illustrates that the decrease of interconnected pores brought about by selective densification according to this invention inhibits the diffusion of carbon from a gaseous carburizing atmosphere.
[0024] An atomized low carbon steel powder, Hoeganaes AlOOO (without any added carbon (graphite) powder) was pressed into gears at a nominal 6.4 g/cm density. The length of the body was a nominal 0.500 inch (1.27 cm) and the length of the teeth were a nominal 0.578 inch (1.47 cm).
[0025] The gears were sintered in a non-carbonaceous atmosphere, dissociated ammonia, for 30 minutes at 2050oF (1121oC).
[0026] They were then sized to different tooth lengths between flat-faced punches as shown in Table III. The gears were heated for 30 minutes at 1500°F (816°C) in an endogas atmosphere having a carbon potential of 0.7 percent and oil quenched. By way of comparison, a one-inch diameter by one-inch long SAE 1020 cold rolled bar was also heat treated.
[0027] Density measurements were made as shown in Table III for the various length gears. In addition, metallographic cross sections, designed to show the depth of carbon pick-up from the heat treating atmosphere were prepared. The bottom half length, or the half length closest to the bottom punch during the molding and repressing operation, were used since they provided the highest range of densities. The densities and the measured depth of carbon diffusion are shown in Table III.
[0028] The unsized tooth of 0.578 inch length showed carburization throughout as seen in Figure 4. At a density of 7.03,g/cm3 (0.510 inch tooth length), the depth of carburization was irregular and was estimated as 0.56 mm. As the density increased, the depth of carburization decreased. At 7.37 g/cm3, the depth was measured as 0.17 mm. With the 100 percent dense SAE 1020 bar stock, the depth of carburization is about 0.10 mm.
1. A method of preparing an integral powder metal body having portions of different
densities, characterised in comprising the steps of:
(a) pressing metal powder in a mold (10) with at least one multiple depth punch (11,14) to produce a powder metal compact having portions of greater length relative to the length of other portions of said compact;
(b) sintering said powder metal compact;
(c) repressing said sintered compact in a mold with a punch that applies forces principally to said portions of said sintered compact of greater length;
(d) recovering a powder metal body wherein the portions initially having a greater length have a greater density than the portions initially having a lesser length.
2. A method according to Claim 1, characterised in that both top and bottom punches
(11,14) used for initially pressing metal powder are multiple depth punches.
3. A method according to Claim 1 or 2, characterised in that the mold (10) for initially
pressing metal powder has side walls generated by a line substantially parallel to
the pressing axis.
4. A method according to Claim 1, 2 or 3, characterised in that the mold (10) for
initially pressing metal powder includes a mandrel (12) for producing an opening through
the body.
5. A method according to any one of Claims 1 to 4, characterised in that the top and/or
bottom punches (11,14) for initially pressing metal powder are telescoping punches
independently positionable to different depths within themold (10).
6. A method according to any one of the preceding Claims, characterised in that the
mold for repressing said sintered compact has substantially the same side wall configuration
as the mold (10) for initially pressing metal powder.
7. A method according to any one of the preceding Claims, character-ised in that the
portions to have a greater final density have a lower density after initially pressing
metal powder.
8. A method according to any one of the preceding Claims, characterised in comprising
sintering at a temperature and for a time to metallurgically bond the powder particles
into a unitary compact which is plastically deformable upon repressing.
9. A method according to any one of the preceding Claims, characterised in including
resintering said powder metal body after said repressing.
10. A method according to any one of the preceding Claims, characterised in that the
ratio of the length of said portions of said compact having a greater length to the
length of said other portions of said compact after initially pressing metal powder
is greater than 1:1 and less than 1.5:1.
ll. A method according to any one of the preceding Claims, characterised in that the
ratio of the length of said portions of said compact having a greater length to the
length of said other portions of said compact after initially pressing metal powder
is greater than 1.1:1 and less than 1.2:1.
12. A method according to any one of the preceding Claims, characterised in that said
powder metal comprises a blend of low carbon atomized steel powder and a small amount
of carbon in the form of graphite powder.
13. A method according to any on of the preceding Claims, characterised in including
reheating and quenching said powder metal body after said repressing.
14. A method according to any one of the preceding Claims, characterised in that said
powder metal body is a gear.
15. A powder metal body having at least two portions with different densities manufactured
in accordance with the method of any one of the preceding Claims.