Title:
Method of producing powder metal parts using induction sintering
Kind Code:
A1


Abstract:
A method of forming a part from a metallurgical powder includes compressing a metallurgical powder including no more than 0.3 weight percent of lubricant within a die to provide a green compact. The green compact is subsequently sintered by induction heating up to a sintering temperature of the green compact and maintaining the compact at the temperature for a time sufficient to provide a sintered compact. The sintered part may be subsequently hot formed and/or forged to densify and adjust the dimensions of the sintered compact.



Inventors:
Kosco, John C. (St. Marys, PA, US)
Newman, Keith E. (Ridgway, PA, US)
Application Number:
09/887287
Publication Date:
02/27/2003
Filing Date:
06/22/2001
Assignee:
KOSCO JOHN C.
NEWMAN KEITH E.
Primary Class:
International Classes:
B22F3/105; C22C33/02; (IPC1-7): B22F3/24
View Patent Images:



Primary Examiner:
JENKINS, DANIEL J
Attorney, Agent or Firm:
K&L GATES LLP-Pittsburgh (PITTSBURGH, PA, US)
Claims:

What is claimed is:



1. A method of forming a part from a metallurgical powder, the method comprising: compressing a metallurgical powder comprising 0 up to 0.3 weight percent of internal lubricant within a die to provide a green compact; and sintering the green compact by induction heating the green compact to provide a sintered compact.

2. The method of claim 1, wherein the metallurgical powder further comprises a particulate metallic material that may be heated by induction.

3. The method of claim 2, wherein the particulate metallic material is least one of a pure iron powder and an iron-containing powder.

4. The method of claim 2, wherein the metallurgical powder is one of a single powder and a powder blend.

5. The method of claim 1, wherein the metallurgical powder comprises 0 up to 3 weight percent graphite, 0 up to 12 weight percent nickel, 0 up to 3 weight percent molybdenum, 0 up to 10 weight percent copper, 0 up to 2 weight percent manganese, 0 up to 20 weight percent chromium, and iron.

6. The method of claim 1, wherein the metallurgical powder comprises 0 up to 1 weight percent graphite, 0 up to 6 weight percent nickel, 0 up to 2 weight percent molybdenum, 0 up to 3 weight percent copper, 0 up to 1 weight percent manganese, 0 up to 3 weight percent chromium, and iron.

7. The method of claim 1, wherein sintering the green compact comprises induction heating the green compact to a sintering temperature within the range of 1900° F. to 2500° F.

8. The method of claim 1, wherein the metallurgical powder is free of internal lubricant.

9. The method of claim 1, wherein the metallurgical powder comprises at least one internal lubricant selected from ethylene bis-stearamide, zinc stearate, and stearic acid.

10. The method of claim 1, wherein compressing at least a portion of the metallurgical powder comprises compressing at least a portion of the metallurgical powder in at least one of a self-lubricating die and a die including a die wall coated with a material to reduce friction and die wear.

11. The method of claim 1, wherein compressing the metallurgical powder comprises compressing the metallurgical powder at a pressure in the range of 10 up to 70 tsi.

12. The method of claim 5, wherein the green compact has a green strength of at least 1000 psi.

13. The method of claim 5, wherein the green compact has a density of at least 4.0 g/cc.

14. The method of claim 1, wherein induction heating the green compact comprises inducing a current within the green compact to increase the temperature of the green compact to a sintering temperature of the green compact.

15. The method of claim 1, wherein induction heating the green compact comprises placing the green compact within an induction work coil while passing alternating current through the coil to thereby induce a secondary current within the green compact through electromagnetic induction.

16. The method of claim 14, wherein the green compact is maintained at the sintering temperature for a time in the range of 2 up to 600 seconds.

17. The method of claim 14, wherein the green compact comprises 0 up to 3 weight percent graphite, 0 up to 12 weight percent nickel, 0 up to 3 weight percent molybdenum, 0 up to 10 weight percent copper, 0 up to 2 weight percent manganese, 0 up to 20 weight percent chromium, and balance iron and incidental impurities, and further wherein the green compact is maintained at the sintering temperature for at least 20 up to 180 seconds.

18. The method of claim 14, wherein the sintering temperature, measured in Fahrenheit degrees, is 60 up to 95 percent of the melting point of the primary component of the metallurgical powder.

19. The method of claim 1, further comprising: processing the sintered compact by at least one of hot forming and forging the sintered compact.

20. The method of claim 19, wherein hot forming and forging the sintered compact comprises placing the sintered compact in a heated die and applying pressure to the sintered compact.

21. The method of claim 20, wherein the heated die is at a temperature in the range of 200 up to 800° F. (93° C. up to 427° C.).

22. The method of claim 20, wherein 20 up to 90 tsi pressure is applied to the sintered compact.

23. The method of claim 20, wherein about 40 tsi pressure is applied to the sintered compact.

24. The method of claim 20, further comprising prior to hot forming the sintered compact, cooling the sintered compact to a temperature less than the sintering temperature.

25. The method of claim 24, wherein cooling the sintered compact comprises cooling the sintered compact to a temperature in the range of 1400 up to 2100° F. (1371° C.).

26. A method of forming a part from a metallurgical powder, the method comprising: compressing a metallurgical powder comprising predominantly iron and further comprising 0 up to 3 weight percent graphite, 0 up to 12 weight percent nickel, 0 up to 3 weight percent molybdenum, 0 up to 10 weight percent copper, 0 up to 2 weight percent manganese, 0 up to 20 weight percent chromium, and up to 0.3 weight percent internal lubricant, within a die at a pressure in the range of 10 up to 70 tsi to provide a green compact; sintering the green compact by induction heating the green compact to a sintering temperature of the green compact, and maintaining the sintering temperature for a time in the range of 2 up to 600 seconds to provide a sintered compact; cooling the sintered compact to a temperature lower than the sintering temperature; and processing the sintered compact by at least one of hot forming and forging the sintered compact by placing the sintered compact in a heated die and applying a pressure in the range of 20 up to 90 tsi to the sintered compact.

27. The method of claim 26, wherein the green compact has a density of at least 4.0 g/cc.

28. The method of any of claims 1 and 26, wherein the part is selected from a gear, a sprocket, a spur pinion, a helical pinion, a cam lobe, a bushing, a bearing, and a bearing race.

29. A part formed by a method comprising: compressing a metallurgical powder comprising 0 up to 0.3 weight percent of internal lubricant within a die to provide a green compact; and sintering the green compact by induction heating the green compact to provide a sintered compact.

30. The part of claim 29, wherein the method further comprises: processing the sintered compact by at least one of hot forming and forging the sintered compact.

31. A part formed by a method comprising: compressing a metallurgical powder comprising predominantly iron and further comprising 0 up to 3 weight percent graphite, 0 up to 12 weight percent nickel, 0 up to 3 weight percent molybdenum, 0 up to 10 weight percent copper, 0 up to 2 weight percent manganese, 0 up to 20 weight percent chromium, and up to 0.3 weight percent lubricant, within a die at a pressure in the range of 10 up to 70 tsi to provide a green compact; sintering the green compact by induction heating the green compact to a sintering temperature of the green compact and maintaining the sintering temperature for a time in the range of 2 up to 600 seconds to provide a sintered compact; cooling the sintered compact to a temperature lower than the sintering temperature; and processing the sintered compact by at least one of hot forming and forging the sintered compact in a heated die by applying a pressure in the range of 20 up to 90 tsi to the sintered compact.

32. The part of any of claims 29 and 31, wherein the part is selected from a gear, a sprocket, a spur pinion, a helical pinion, a cam lobe, a bushing, a bearing, and a bearing race.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

[0003] The present invention is generally directed to a method of forming parts from metallurgical powders. More particularly, the present invention is directed to a method of forming powder metal parts including the step of sintering a green compact by induction heating. Even more particularly, the present invention is directed to a method of forming powder metal parts including the step of induction sintering a green compact including little or no internal lubricant and, optionally, further including a hot forming or forging step subsequent to the induction sintering step. The method of the present invention obviates the need for sintering a green compact using a conventional sintering furnace, such as a belt, pusher, or batch sintering furnace. The method of the present invention may provide substantial cost and time savings.

DESCRIPTION OF THE INVENTION BACKGROUND

[0004] The following conventional process is used to form parts from a metallurgical powder. One or more metal or metal-containing powders satisfying applicable process constraints and meeting requirements of the desired end product are selected. An internal lubricant, discussed below, is included with the metallurgical powders, and a homogenous blend of the one or more metallurgical powders and the internal lubricant is prepared. As used herein, a “metallurgical powder” refers to a homogenous blend of particulate materials that includes one or more metal or metal-containing powders, whether alloyed or unalloyed powders, and that also may include one or more internal lubricants and other additives as are known in the powder metallurgy art. As used herein, an “internal lubricant” is a lubricant that is present substantially homogenously throughout a metallurgical powder. An internal lubricant may be contrasted with an external lubricant, which is applied either to the external surface of a powder metal compact or to the surface of a molding die in which a powder metal compact is molded.

[0005] All or a portion of the lubricant-containing metallurgical powder blend is placed in a molding die and pressed into a green compact of a desired shape. The green compact is shaped so that a powder metal part having the desired shape and dimensions results after further processing of the green compact. The green compact is then sintered, typically at a temperature in the range of 1900-2400° F. (1066-1316° C.) for iron-base compacts, in an electric or gas-fired belt, pusher, or batch sintering furnace to consolidate the powder particles and increase the density of the compact. The internal lubricant is gassed off during the sintering step. Thus, the sintering step must occur at a temperature above the melting temperature of the internal lubricant.

[0006] It is known to use a hot forming or forging step following a conventional sintering step to increase the density of the compact and to improve mechanical and/or physical properties. To prepare the sintered compact for hot forming or forging, the compact is first reheated to a temperature substantially below the sintering temperature of the compact, typically in the range of about 1400° F. to about 2100° F. (about 760° C. to about 1150° C.). The reheating step may be carried out by induction heating the already sintered compact. This is typically done by passing the compact through the interior of a helically arranged metal induction work coil carrying an alternating current. The alternating current creates a varying magnetic field around the coil, and the magnetic field heats the compact by inducing electrical resistance and hysteresis losses in the compact. Once the compact has been reheated, it may then be hot formed or forged, typically at 40-70 tons/in.2 (tsi), in a heated die that is slightly larger than the die used to form the original green compact. The heated die is at a temperature less than that of the reheated compact, and typically is maintained at, for example, a temperature within the range of 400° F. to 800° F. (204° C. to 427° C.) for iron-base compacts.

[0007] In the case of iron-base compacts, for example, the conventional sintered and hot formed or forged compact typically has a density in the range of 7.6-7.85 g/cc, which is 97-99.8% of theoretical full density. As used herein, an “iron-base” compact is a compact including greater than 50 weight percent iron, whether in elemental or alloyed form.

[0008] In conventional powder metallurgy techniques, internal lubricants are employed as a means of facilitating removal of the green compact from the die and to aid in particle rearrangement during compaction. In the absence of an internal lubricant, the powder metal parts may adhere to the surface of the die, which may result in galling of the die surface after only a very small number of compacts have been formed in the die. Consequently, the die wears quickly and has to be replaced on a frequent basis, resulting in substantial expense. Lubricants, while alleviating the problems of adherence and die damage, are problematic in a number of ways. For example, the cost of the lubricants increases the cost of the finished parts. The lubricants also must be substantially removed from compacts during processing, and the evolution of gaseous lubricant decomposition byproducts from compacts during sintering may cause micro-cracks in the part and/or raise environmental concerns. Thus, eliminating internal lubricants would be of great value.

[0009] As just noted, the internal lubricant must be substantially removed from the powder metal compacts during processing. Failure to remove the internal lubricant reduces the maximum part density that may be achieved and otherwise compromises the properties of the finished part. For example, corrosion resistance of stainless powder metals may be degraded if all internal lubricant is not removed.

[0010] Conventional electric or gas-fired belt sintering of iron-base powder metal compacts typically subjects the compacts to high temperatures for about 20 minutes, allowing the internal lubricant to decompose and escape. Electric and gas-fired sintering furnaces are relatively inefficient heating devices and require large energy expenditures. Thus, the length of the sintering step required to process each compact involves substantial energy consumption and cost, and the sintering step constitutes a significant portion of the overall processing time and expense necessary to produce powder metal parts. In addition, belt, pusher, or batch sintering furnaces are large and expensive machines, requiring significant capital expense and a large floor space in the plant. Thus, in addition to the elimination of internal lubricant, it also would be advantageous to form powder metal parts without the need for conventional belt, pusher, or batch sintering.

[0011] Prior efforts to eliminate the use of internal lubricants in the fabrication of powder metal parts have met with only moderate success. One approach, seen in U.S. Pat. No. 5,682,591 issued to Inculet (“the '591 patent”), teaches the use of an electrostatic process to charge lubricant particles which are then sprayed onto the die surface prior to each molding cycle. It is intended that lubricant on the die surface would prevent adhesion of the compact and inhibit die wear. The entire disclosure of the '591 patent is hereby incorporated herein by reference. Success with the process of the '591 patent, however, has been limited because coverage of the die surface by the lubricant spray can be inconsistent if improperly sprayed. At the high molding pressures required to form green compacts, even one mis-spray can produce die damage. Molding dies could better withstand insufficient die coverage resulting from mis-sprays if lower molding pressures could be used. However, lower pressures result in compacts with lower green strength and lower densities. Green strength refers to the ability of a green compact to maintain its size and shape during handling and storage prior to sintering. Low densities may be raised in subsequent working operations, but a green strength less than about 1,000 lbs./in2 (psi) is unacceptable because the compacts may easily crumble or fracture when handled.

[0012] Accordingly, it would be advantageous to provide a method of forming powder metal parts wherein little or no internal lubricant must be included in the metallurgical powder. It also would be advantageous to avoid the necessity for the use of conventional belt, pusher, or batch sintering techniques in order to reduce time and expense associated with those processes.

SUMMARY OF THE INVENTION

[0013] The present invention addresses the above-described needs by providing a method for producing a part from metallurgical powder including up to 0.3 weight percent of internal lubricant. The method includes the step of compressing all or a portion of the metallurgical powder in a die to provide a green compact. Preferably, the metallurgical powder lacks an internal lubricant. The green compact may be formed using a self-lubricating die or any other device suitable to form a green compact having acceptable green strength.

[0014] In a subsequent step, the green compact is sintered by induction heating to provide a sintered compact. The sintered compact may be further processed by other powder metallurgy techniques, including hot forming or forging. In the case of hot forming or forging, the induction sintered compact may be cooled from the sintering temperature to a temperature suitable for hot forming or forging, and then placed in a heated die and hot formed or forged to densify and adjust the dimensions of the compact.

[0015] The present invention further provides a method of forming a part from an iron-base metallurgical powder that may include, for example, 0 up to 3 weight percent graphite, 0 up to 12 weight percent nickel, 0 up to 3 weight percent molybdenum, 0 up to 10 weight percent copper, 0 up to 2 weight percent manganese, 0 up to 20 weight percent chromium, and up to 0.3 weight percent of internal lubricant. All or a portion of the metallurgical powder is molded in a die at a pressure in the range of about 10 up to about 70 tsi to provide a green compact. In a subsequent step, the green compact is sintered by induction heating the green compact to a sintering temperature and maintaining the temperature for a time in the range of 2 up to 600 seconds. The sintered compact may be hot formed or forged in a subsequent step. For example, the sintered compact may be hot formed or forged, after cooling the sintered compact from the sintering temperature to a suitable hot forming or forging temperature, in a heated die at a pressure in the range of about 20 up to about 90 tsi. Alternatively, if the sintered compact has cooled to a temperature below that which is suitable for hot forming or forging, it may be re-heated to a suitable temperature and then hot formed or forged.

[0016] The present invention also is directed to powder metal materials and parts formed using the method of the present invention. Such parts may include, for example, gears, sprockets, pinions (both spur and helical), ring gears, bushings, cam lobes, races, and bearings.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0017] As discussed above, internal lubricants included in metallurgical powders are conventionally removed by sintering parts molded from the metallurgical powders in a belt, pusher, or batch sintering furnace. It would be advantageous to eliminate the lubricant removal step from the process. The inventors have discovered that inductive heating can be used to successfully sinter compacts of metallurgical powder when the internal lubricant content of the compacts is no greater than about 0.3 weight percent. For comparison, the typical internal lubricant content of conventional metallurgical powders ranges from about 0.5 to about 1.3 weight percent. The sintered compacts may then be densified and dimensionally adjusted in a hot forming or forging die in a subsequent step, if desired. Because inductive heating heats compacts to sintering temperature quickly, inductive sintering will require substantially less time, and also less expense, than conventional sintering such as electric or gas-fired belt, pusher, or batch sintering. Self-lubricating dies, for example, may be used to form the green compacts from metallurgical powders including not more than 0.3 weight percent internal lubricant so as to prevent adhesion to the die surface and unacceptable die wear.

[0018] When conventional powder compaction processes are utilized to form fully dense compacts, the compacts are typically sintered in a conventional electric or gas-fired belt, pusher, or batch furnace. The furnace heats the parts by conventional processes (i.e., conductive, convective, and radiative processes) at a sufficiently slow rate to permit the escape of gaseous byproducts from the decomposition of internal lubricants. However, the inventors have determined that the effective heating of green compacts from metallurgical powders by induction requires that the compact contain no more than 0.3 weight percent of internal lubricant. The inventors have determined that internal lubricant levels greater than about 0.3 weight percent affect the induction sintering process in two distinct manners. First, the excessive internal lubricant inhibits effective thermal and electrical coupling of the individual powder particles under the action of an induction coil. The second factor complicating induction heating of green compacts is the observation that the byproducts of internal lubricant decomposition evolve more rapidly than in conventional sintering. If excessive, these gases are unable to be liberated from deep within the part quickly enough to eliminate the risk of part fracture from large internal stresses generated by the entrapped gases.

[0019] When hot forming or forging is used in the production process, compacts are typically coated with a graphite slurry before being heated in an induction coil to sub-sintering temperatures just prior to being placed in the hot forming or forging die. The carbon in the graphite slurry will diffuse into the compact and undesirably harden the compact's surface unless the induction heating is done very quickly. Thus, the inventors believe that heating a compact including greater than about 0.3 weight percent of internal lubricant for a prolonged period as a means to remove the internal lubricant just prior to hot forming or forging would not be effective.

[0020] The metallurgical powder used in the method of the present invention may include one or multiple metal-containing powders. As used herein, “metal-containing” powders should be interpreted in a broad sense, and include metals alloyed with metals and/or other elements, mixed powders including metals, and pure metal powders (such as pure iron powder). All powder components may include incidental impurities. The metallurgical powder must include a metallic material capable of being heated through the action of alternating electric fields.

[0021] In a preferred form, the metallurgical powder includes one or more of a pure iron powder and an alloyed iron powder. More preferably, the metallurgical powder is composed predominantly of iron in elemental and/or other forms. One possible embodiment of the metallurgical powder has the following composition, which is provided by preparing an appropriate blend of particulate materials: 0 up to about 3 weight percent graphite; 0 up to about 12 weight percent nickel; 0 up to about 3 weight percent molybdenum; 0 up to about 10 weight percent copper; 0 up to about 2 weight percent manganese; 0 up to about 20 weight percent chromium; 0 up to 0.3 weight percent internal lubricant; and iron and incidental impurities. A more preferred form of the metallurgical powder has the following composition: 0 up to about 1 weight percent graphite; 0 up to about 3 weight percent nickel; 0 up to about 2 weight percent molybdenum; 0 up to about 3 weight percent copper; 0 up to about 1 weight percent manganese; 0 up to about 1.5 weight percent chromium; 0 up to 0.3 weight percent internal lubricant; and iron and incidental impurities.

[0022] As disclosed in a co-pending patent application entitled “Method of Producing Powder metal Parts From Metallurgical Powders Including Sponge Iron” naming John C. Kosco as inventor and filed on even date herewith (referred to herein as the “co-pending application”), the entire disclosure of which is hereby incorporated herein by reference, the metallurgical powder may consist of or include sponge iron powder. While sponge irons exhibit only modest compressibility, achieving typical densities of 5.0-6.6g/cc when compressed at 15-50 tsi, they possess outstanding green strength. Sponge iron is a coherent, porous mass of substantially pure iron produced by solid-state reduction of iron oxide and, as its name suggests, is sponge-like in appearance. As described in detail in the incorporated co-pending patent application just mentioned, green compacts having green strengths in excess of 1000 psi were formed using compaction pressures as low as 5-15 tsi, pressures substantially lower than the pressures conventionally used to form green compacts. Mixes of sponge iron powders and atomized powders may be desirable because atomized powders are generally more compressible than sponge iron powders and can be pressed at 30-50 tsi into green compacts having densities of about 6.65-7.25 g/cc. Thus, the addition of sponge iron to a metallurgical powder will permit the molding of green parts having acceptable green strength at lower molding pressures, and this may permit the use of little or no internal lubricant in the metallurgical powder.

[0023] While atomized powders exhibit lower green strengths than sponge irons when pressed to green compacts at the same pressures, compacts of atomized powders typically possess excellent sintered properties. Thus, powder metal structural parts can be made from metallurgical powder including primarily atomized powders, while metallurgical powder having substantial amounts of sponge iron powder are more commonly used for bearings. Normally, the mechanical properties obtained with sponge iron are inferior to atomized iron powders because of the porosity inherent in sponge irons. However, a hot forming or forging step may be used to close up a significant amount of the porosity typical of sponge iron compacts. Therefore, hot formed or forged articles of blends of atomized iron or iron alloys and sponge iron, or of pure sponge iron with elemental additions or of prealloyed sponge iron, can yield properties comparable to sintered compacts of pure atomized powders.

[0024] It is preferred that the metallurgical powder be free of internal lubricant. However, as noted, internal lubricant may be present in amounts up to about 0.3 weight percent without resulting in significant disadvantages. Accordingly, the cost associated with providing internal lubricant may be wholly or substantially avoided. Internal lubricant is typically in the form of a wax or stearamide compound, but any lubricant commonly used in the powder metallurgy art may be used, including, but not limited to, ethylene bis-stearamide, zinc stearate, and stearic acid.

[0025] At least a portion of the metallurgical powder is compacted in a die under pressure sufficient to form a green compact. Preferably, the green compact has green strength of at least 1,000 psi and a density of at least about 6.0 g/cc. In one embodiment, the metallurgical powder is compressed at a pressure in the range of 10 up to 70 tsi to form a green compact. The green compact may be formed in a die employing die wall lubrication as is taught in the '591 patent. In addition, the die may be of a type known in the art employing various die cavity coatings to reduce friction and wear during molding. Lubrication of the die wall eliminates or reduces the need for internal lubricant by reducing adhesion of the compact to the die wall and abrasion between the die wall and the compact. Thus, a green compact having sufficient green strength and little or no internal lubricant is provided.

[0026] Following compaction, the green compact is sintered by induction heating the compact to a suitable sintering temperature and maintaining the compact at the sintering temperature for a time period suitable to consolidate and densify the compact. The sintering temperature typically is, for example, a Fahrenheit temperature of 60-95% of the melting point of the primary component of the compact. Those having ordinary skill in the powder metallurgy art may readily ascertain a suitable sintering temperature and time-at-temperature for a particular green compact.

[0027] The green compact may be induction heated in, for example, the interior space of an induction coil. The design and specific manner of operating the induction coil to sinter green compacts will be dictated by, for example, the shape and composition of the green compact and may be readily ascertained by one having ordinary skill in the art. Although, to the inventors' knowledge, induction sintering is not currently used in the commercial production of powder metal parts, those of ordinary skill in the powder metallurgy art are generally familiar with the use of induction coils to heat powder metal compacts. For example, induction coils are used to heat powder metal compacts to sub-sintering temperatures prior to hot forming or forging the compacts. Such persons of ordinary skill, after becoming aware of the present invention, may readily adapt available induction heating devices for use in the present method.

[0028] Sintering by induction heating is a significant improvement over prior art powder metal sintering methods. For instance, induction sintering eliminates the need for sintering the compacts using an electric or gas-fired belt, pusher, or batch sintering furnace, so energy costs are reduced and a typical twenty-minute sintering time is eliminated. Induction heating may be carried out by inducing a current in the green compact so as to raise the compact to a suitable sintering temperature, typically in the range of 1600-2500° F. (871-1371° C.), for a time at temperature sufficient to produce a sintered compact. Alternating current may be passed through the coil to thereby induce a secondary current through electromagnetic induction within the green compact.

[0029] Preferably, the sintering time using inductive sintering is in the range of about 2 to about 600 seconds time-at-temperature. For example, an induction sintering time of 20 to 180 seconds may be employed to sinter green compacts used to form a pinion gear, wherein the compacts have a one-inch outer diameter, a length of one inch, and are formed from a metallurgical powder including substantially iron powder and, optionally, relatively minor amounts of graphite, nickel, molybdenum, copper, manganese, and chromium, and including sponge iron. It will be understood that the suitable range of sintering times will vary based on part size, geometry, and composition. One of ordinary skill in the art may readily determine an appropriate induction sintering regimen for any particular powder metal compact using available induction heating equipment.

[0030] A more specific example of the application of the method of the present invention is as follows. The inventors have determined that green compacts formed at a pressure between 30 tsi and 40 tsi to create a pinion gear, wherein the compacts have an outer diameter of approximately one inch and a length of approximately one-half inch, and are composed of a metallurgical powder including 0.17 weight percent graphite, 0.5 weight percent manganese sulfide, no internal lubricant, and the balance a water-atomized 0.6 weight percent molybdenum, 1.8 weight percent nickel, pre-alloyed iron powder, may be suitably processed by the method of the present invention by heating the green compacts in an induction coil using a 100 kHz power supply to approximately 1900° F. for about 60 seconds, cooling to approximately 1800° F., and subsequently hot forming at a pressure of approximately 70 tsi. After a subsequent conventional hardening heat treatment, the pinions were compared to identical pinions produced using a conventional powder metal fabrication practice including the following steps: pressing at 30-40 tsi portions of a metallurgical powder including 0.17 weight percent graphite, 0.5 weight percent manganese sulfide, 0.75 weight percent ethylene bis-stearamide internal lubricant, and the balance a water-atomized 0.6 weight percent molybdenum, 1.8 weight percent nickel, pre-alloyed iron powder; sintering the green compacts in a conventional electrically heated sintering furnace for about twenty minutes at a temperature above 2000° F.; cooling the sintered compacts to room temperature; heating the sintered compacts in an induction coil using a 100 kHz power supply to approximately 1800° F. (a sub-sintering temperature) for about 20 seconds; and subsequently hot forming the heated compacts at approximately 70 tsi. As shown in the following table, all measured physical and mechanical properties of the parts made using the process of the present invention met or exceeded those of the parts made by the conventional process. 1

Mold-Belt Furnace
Mold-Induction Sinter-Sinter-Hot Form-Heat
PropertyHot Form-Heat TreatTreat
Density7.837.82
(g/cc)
Crush Strength12,71610,216 (min.)
(pounds)
Effective Case0.0210.018-0.030
Depth (inches)
Apparent59-60 (surface)58-62 (surface)
Hardness30-32 (core)25-40 (core)
(Rockwell C)

[0031] As indicated above, the initial powder metal blend used in the present invention includes little or no internal lubricant. Because the amount of lubricant in the metallurgical powder used in the method of the present invention may be reduced or eliminated, there is little or no insulating film between the individual powder particles. As such, thermal coupling between the powder particles is improved to the point that compacts can be induction heated rapidly, (usually in less than 60 seconds for iron-base compacts of typical sizes) through their entire mass to sintering temperatures in the range of 1600-2500° F. (871-1371° C.) using an induction coil.

[0032] Following induction sintering, the sintered compacts may be further processed using known powder metal processing techniques. For example, the sintered compacts may be cooled immediately after being inductively sintered to a temperature below the sintering temperature, and then hot formed or forged to densify and adjust dimensions of the compacts. Typically, the sintered compacts would be cooled to a temperature in the range of 1400-2100° F., and more typically between 1700-1900° F. for iron-base compacts, before being hot formed or forged. Hot forming and forging are carried out in a heated die. Typical die temperatures are in the range of about 200° F. up to about 800° F. (93° C. up to 427° C.), and preferably are in the range of about 400° F. to about 600° F. (204° C. to about 316° C.). The preferred die temperature range is believed to minimize hot forming and forging tool wear, better control heat loss from the part, and better control final part dimensions after hot forming or forging. Pressures applied to the powder metal part in the hot forming or forging die typically are in the range of about 20 tsi up to about 90 tsi, and preferably the hot forming or forging pressure is about 40 tsi.

[0033] One having ordinary skill may, without undue experimentation, readily determine suitable temperatures and pressures for a hot forming process used in the method of the present invention. As an example, a green compact is molded from an iron-base metallurgical powder optionally including up to 3 weight percent graphite, up to 6 weight percent nickel, up to 3 weight percent molybdenum, up to 10 weight percent copper, up to 2 weight percent manganese, up to 3 weight percent chromium, and with at least a portion of the iron being present as a sponge iron powder. Assuming a sintering temperature of about 2500° F. (1371° C.) for the green compact, the hot forming or forging temperature for the sintered compact may range from a minimum of about 1200° F. (1371° C.) up to about 2100° F. (1149° C.), with the preferred hot forming or forging temperature being about 1800° F. (982° C.).

[0034] The following table illustrates the advantages that may be gained by including some portion of sponge iron in the metallurgical powders used in the method of the present invention. The table lists the green strength (psi) of green compacts formed at various compaction pressures from metallurgical powders including atomized iron powder, sponge iron powder, or a combination of atomized and sponge iron powders, both with and without internal lubricant. Acrawax™ ethylene bis-stearamide (“EBS”) was used as the internal lubricant. Green strength was evaluated by test method MPIF Standard No. 15 (ASTM B312, ISO 3995). 2

Green Strength (psi) of Compact
35 weight
percent
R80 + 65
HP85R80 withweight
with 0.750.75percent
weightweightR80 withHPB5 and
MoldingpercentHP85 nopercentnono
PressureEBSinternalEBSinternalinternal
(tsi)lubricantlubricantlubricantlubricantlubricant
102341,463565
156052,7941,184
206951,1464,380
301,3112,3397,042
401,8153,4349,247
502,15710,894

[0035] R80 is a sponge iron produced by Pyron, Niagara Falls, N.Y. HP85 is an atomized iron alloy powder including 0.85% molybdenum and is available from Hoeganaes, Riverton, N.J. As seen in the table, a suitable green strength (at least 1,000 psi) was not achieved with a metallurgical powder composed of HP85 atomized iron-molybdenum powder and 0.75 weight percent lubricant until 30 tsi of pressure was applied. An acceptable green strength was achieved with HP85 powder lacking internal lubricant at a molding pressure of 20 tsi, producing a green strength of 1,146 psi. This evidences an advantage to not including an internal lubricant in the powder. As seen from the table, major improvements in green strength are possible by including R80 sponge iron powder in the metallurgical powder mix. For example, a powder including R80 sponge iron powder and EBS internal lubricant molded at 20 tsi had green strength in excess of 4000 psi, while R80 powder without internal lubricant had green strength of 1463 psi when molded at only 10 tsi. Thus, the addition of sponge iron permits good green strength at lower molding pressures, and also allows suitable green strengths with lower levels of internal lubricant. Further evidence of the advantages provided by addition of sponge iron is shown in the incorporated co-pending application.

[0036] Green compacts of a blend of sponge iron and atomized iron-molybdenum powder had satisfactory green strength (1,184 psi) at 15 tsi. Green compacts of sponge iron may have a density of as little as 4.0 g/cc, and more typically about 5.1-5.4 g/cc, when molded at low pressures. The addition of an atomized powder in the metallurgical powder should result in increased density relative to green compacts formed from sponge iron only. Green compacts including sponge iron only or including both sponge iron and atomized iron or iron alloy powders may be processed according to the present invention as generally described above, but may be molded with satisfactory green strength at lower pressures relative to compacts including only atomized iron powders. As an example, the inventors molded a green compact at 20 tsi from a metallurgical powder including 97 parts R80 sponge iron powder, 3 parts copper powder, and 0.8 parts powdered graphite. The compact had a green strength of 4400 psi and a density of 5.2 g/cc. The compact was sintered at 2080° F. in a N2-5%H2 environment, cooled to 1800° F., and hot formed at 60 tsi. The hot formed part had density of 7.6 g/cc, RC15 hardness, and 100 ksi tensile strength. Thus, the use of sponge iron yielded a handleable part at low pressures with minimal tool wear.

[0037] The ability to produce powder metal compacts having acceptable green strength at low molding pressures (for example, less than 20 tsi) is a significant improvement over the prior art. This development solves certain compact adhesion and die wear problems encountered in molding compacts with little or no internal lubricant. Substantial or entire elimination of internal lubricant may now be accomplished without significantly compromising the service life of the molding die because the lower usable molding pressures eliminate or significantly reduce the occurrence of wear and scoring of and adherence to the die wall. Available die wall self-lubrication systems also may be used with greater confidence as there would be less chance at the lower molding pressures of ruining the tooling if coverage of the die wall with lubricant is incomplete. Thus, the useful life of the die may be extended, reducing capital costs. Green compacts produced at the lower molding pressures that may be used with the method of the present invention may be handled and may be induction sintered as generally described herein. The capability to use molding pressures as low as 10 tsi also allows for the use of smaller molding presses, which may be run at a faster rate.

[0038] It is to be understood that the present description illustrates those aspects of the invention relevant to a clear understanding of the invention. Certain aspects of the invention that would be apparent to those of ordinary skill in the art and that, therefore, would not facilitate a better understanding of the invention have not been presented in order to simplify the present description. Although the present invention has been described in connection with certain embodiments, those of ordinary skill in the art will, upon considering the foregoing description, recognize that many modifications and variations of the invention may be employed. It is intended that all such variations and modifications of the inventions be covered by the foregoing description and following claims.