US3811878A - Production of powder metallurgical parts by preform and forge process utilizing sucrose as a binder - Google Patents

Abstract

Metal particles are intimately mixed with a sufficient amount of sucrose to effect the desired degree of deoxidation and/or carburization. The mixture is poured into a mold and is then processed by (a) baking at low temperature to form a green compact with sufficient handling strength for further sintering and/or hot working or (b) heating to above sintering temperature to form a stronger compact similarly useful for hot forging.

Description

United States Patent [191 Chao et al.

[ May 21, 1974 1 PRODUCTION OF POWDER METALLURGICAL PARTS BY PREFORM AND FORGE PROCESS UTILIZING SUCROSE AS A BINDER [75] Inventors: Hung-Chi Chao, Monroeville;

Robert R. Judd, Murrysville; Roger L. Rueckl, Murrysville; Charles K. Russell, Murrysville, all of Pa.

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

221 Filed: Dec. 6, 1972 211 App1.No.:312,461

[52] U.S. Cl ..75/21l, 75/200, 75/20l,75/203, 75/204, 75/226, 264/111, 156/336 [51 1 Int. Cl. B22f 1/00, B22f 3/00 [58] Field of Search 264/111; 75/200, 201, 203, 75/204, 211, 226; 156/336 [56] References Cited UNITED STATES PATENTS 2.158.845 5/1939 Ayer 156/336 2,176,302 10/1939 Romp 75/204 2,509,838 5/1950 Oswald 2,279,003 4/1942 Matush 3,006,859 10/1961 Allemann et a1. 252/30l.l

FOREIGN PATENTS OR APPLICATIONS 951,681 3/1964 Great Britain 264/63 Primary ExaminerLe1and A. Sebastian Assistant Examiner-B. H. Hunt Attorney, Agent, or FirmArthur J. Greif [5 7] ABSTRACT Metal particles are intimately mixed with a sufficient amount of sucrose to effect the desired degree of deoxidation and/or carburization. The mixture is poured into a mold and is then processed by (a) baking at low temperature to form a green compact with sufficient handling strength for further sintering and/or hot working or (b) heating to above sintering temperature to form a stronger compact similarly useful for hot forging.

7 Claims, 2 Drawing Figures L OOSE- PACK PROCESS AS- ATOM/ZED POWDER INT/MATELY MIX WITH .SUCROSE POUR MIXTURE IN TO MOL D FORM PREFORM BY SINTER/NG IN PROTECT] VE ATMOSPHERE FORM PREFORM DY BAKING INA/R PATENTEUHAY 2 1 197-1 38 1 1.878

saw 1 0r 2 CONVENTIONAL PROCESS AS- A TOM/ZED POWDER ANNEAL IN REDUCING ATMOSPHERE WITH CONCURRENT PRODUCTION OF CAKE GRIND CAKE TO ACHIEVE METAL POWDER M/X WITH DIE LUBRICANT FORM PREFORM UNDER HGIH PRESSURE .SINTER IN PROTECTIVE ATMOSPHERE FORGE .SINTERED COOL T0 PRE'FORM //v ROOM TEMP. CLOSED 0/5 REHEAT FOR FORGING' FORGE l/V CLOSED 0/5 FIG. I.

ATENTEDMAY 2 1 m4 SHEET 2 UF 2 LOOSE- PACK PROCESS AS- ATOM/ZED POWDER //V T/MATELY M/X WITH SUOROSE POUR MIXTURE INTO MOLD FORM PREFORM BY S/NTERl/VO IN PROTECTIVE ATMOSPHERE FORM PREFORM BY BAKING INA/R AT 350-500F.

FORGE .Sl/VTEREO PREFORM IN CLOSED O/E COOL r0 HEAT FOR HEAT r0 .SINTER ROOM TEMP. FORE/N6 a 70 ROOM 7544p.

05/1541 FOR 50005 nv 55/1547 FOR FORG/NG 040550 p FORG/NG 50005 IN FORGE nv. CLOSED 0/5 01.0550 0/5 PRODUCTION OF POWDER METALLURGICAL PARTS BY PREFORM AND FORGE PROCESS UTILIZING SUCROSE AS A BINDER This invention is related to the production of powder metal preforms and is particularly related to a process in which such preforms may be made from economical, as-atomized metal powders.

There are a number of different methods by which metal powders (particulate metals) useful in the production of powder metal compacts, have been produced. These methods include, for example, electrolytic processes, ore reduction processes and gas and water atomization processes. The latter process has recently come to the forefront, especially in the production of ferrous metal powders, since the process is generally more economical and produces particles of a shape and density which provice a powder compact with enhanced physical properties. US. Pat. No. 3,325,277 is illustrative of a water atomization process which is being commercially employed. In order to produce a powder useful for further compacting, the asatomized powder must first be annealed in a reducing atmosphere to soften the powders and reduce the oxide surface thereof. As a result of this annealing procedure, the particles tend to agglomerate and form a cake-like structure, thereby necessitating an additional grinding stage to break-up the cake and finally achieve the desired particle shape and size distributions required for further compacting. In the conventional processes, these powders are then compacted under pressure and then heated to elevated temperature to form the desired powder metal part or, in a more recent development, are similarly compacted under pressure and then heated to elevated temperature to produce a preform, which is then employed for production of the final part.

It is therefore an object of this invention to provide a process by which high quality powder metal preforms can be produced from economical particulate metals such as as-atomized metal powders.

Another object of this invention is to eliminate the limitations imposed by practically sized compacting presses, in the production of powder metal preforms.

Still another object of this invention is to provide a process which enables the use of relatively inexpensive and expendable molds in the production of powder metal preforms.

These and other objects and advantages of the invention will be more apparent from the following description and appended claims when taken in conjunction with:

FlG. I which is a flow diagram of the conventional process for the production of powder metal preforms, and

FIG. 2 which is a flow diagram of the basic embodiments of this invention for the production of powder metal preforms.

It has now been found that as-atomized powder can be directly employed, if the powder is initially admixed with sucrose, which serves to (a) reduce the oxidized surface of the powder, (b) act as a carburizing agent to achieve the desired carbon content in the powder metal preform, and in a further embodiment, (c) act as a binder when heated to low temperatures, serving to provide a green preform which may be handled and transported for further processing. It may be seen in comparing FIGS. 1 and 2, that utilization of sucrose in combination with the outlined procedures permits the elimination of both the annealing and grinding steps of the conventional process. Additionally, a number of further benefits are achieved by following the procedures of this invention. Referring to FIG. 1, it may be seen that in the conventional process, the preform is produced by admixing the annealed and ground powders with a lubricant, and then compacting under high pressures, generally in excess of 30 tons per square inch. Utilizing such a procedure, it is necessary that fully processed (annealed and ground) powder exhibiting a considerable degree of irregularity of particle shape be employed to insure adequate strength for handling after pressing. The resulting green preform is then sintered under a protecting atmosphere at temperatures of about 2,000 F. In some commercial procedures the pressing and sintering are accomplished simultaneously. This procedure has not received significant commercial utilization, because of the severe limitations imposed by the necessity of providing die materials which exhibit very high strength at rather elevated.

temperatures.

In contrast with these conventional procedures (i.e., embodiment (I) of the instant invention) the blended mixture of powder metal and sucrose is poured into a ceramic or metal mold, preferably vibrated to a bulk density substantially in excess of apparent density, and then heated at 1,2002,400 F in a protective atmosphere to effect annealing and sintering in one step. For purposes of this invention, the term sintering is directed to the joining together of metal particles/by the application of heat in the absence of substantial ex- I ternal pressures, i.e. pressures in excess of l tsi. In view of this sinteringof the as-atomized powder in combination with sucrose, the carbon reducible oxides (e.g. various forms of iron oxide as well as the oxides of nickel, copper, molybdenum, etc.) of the powder are reduced and the metal powders softened in a manner analagous to that achieved in the annealing step of the conventional process. Since this sintering produces a preform with good green strength, the grinding and pressure compaction procedures of the conventional process are clearly unnecessary. Thus, additional economies are realized through the elimination of the rather expensive high-pressure press. Of equal importance, the attendant size limitations of the preforms made by conventional process are eliminated. In the conventional process, the pressed preforms are limited (at least in a practical sense), by the size of available presses, to the production of relatively small preforms, generally less than 10 pounds. In contrast, significantly larger preforms, ranging up to several hundred pounds, may be sintered by the instant process and then forged to the desired part. Finally, the lower density of the sintered only preform, permits better metal flow during forging, resulting in both significant reductions of the energy required for forging and in better die filling characteristics.

In the second embodiment (II) of this invention, the' as-atomized metal powder-sucrose mixture is poured into a mold and baked at a temperature (generally 350500 F) sufficient to soften the sucrose and thereby form a cohesive green preform. The relatively low-temperatures which may be employed in this baking procedure, allows the use of a variety of inexpensive, expendable mold materials such as various plastics or rubbers or even paper; the only requirement being that the mold material be capable of withstanding the rather low baking temperature. Therefore, while ceramic or metal molds may be utilized. the full economic benefits of this embodiment will be realized by utilizing such inexpensive, expendable molds. Ceramic molds present a further problem in that it is often difficult to remove the preform without the necessity of special precautions being taken. After the cohesive baked preform is discharged from the mold, it may be processed by either of two alternative routes, dependent primarily on equipment availability and the size of the preform. In the first of these routes, the preform is heated in a protective atmosphere and forged in a manner similar to the conventional preform and forge process. In the second route, the baked preform is sintered (heating for at least minutes at temperature, preferably l,800-2,200 F) in a protective atmosphere and then forged directly, making use of the sensible heat of sintering; or cooled and then reheated for forging at a later time.

In experiments leading to the instant invention, a variety of potential carburizing materials were evaluated for their effectiveness in providing a suitable binder. in the tests reported below, all the bonding agents were essentially of the same particle size, i.e., minus 200 mesh. The metal powder was all minus 6 mesh and had the following screen analysis:

Mesh Size Percent Retained 80 I8. l I00 2.0 I40 4.3 200 I 3.2 230 4.0 325 [06 pan 47.8

The metal powder-binder combinations were blended and poured into the preform mold, which was mechanically vibrated to achieve a bulk density substantially in excess of apparent density.

4 strength to be easily removed frornthe fiioldand handled.

Further tests were conducted to determine if more uniform coating of the metal particles could be achieved by use of solutions of sucrose in water. Surprisingly, no improvement in distribution was achieved. More importantly, it was determined that any substantial percentage of moisture was, in fact, detrimental. Thus, at low levels of about 1 to 5 percent moisture, the metal powder-sucrose mixture would not flow properly even when vibrated, thereby resulting in an incompletely filled mold. At higher water levels, the mixture did effectively fill the mold. However, this necessitated an extra step of preliminary drying with the attendant requirement for the taking of rather impractical precautions. Thus, drying had to be achieved at a temperature below 212 P, so that the packed powder was not disturbed by the water boiling-off. Drying therefore became a lengthy and time consuming process, primarily because of the small exposed surface area of the powders in the mold. Even with such preliminary drying, it was found that the baked preforms did not achieve the same high density as those made with essentially dry mixtures. It is therefore preferable that the metal powder-sucrose mixture be essentially dry, i.e. less than 0.5

percent moisture.

In general, the features of the instant invention are applicable to metal powders or particles from virtually any source. However, a few instances do exist in which one or the other of the two embodiments may be ruled out as inapplicable to the desired objective. For purposes of understanding the applicability of these embodiments, source powders may be divided into two categories: (a) relatively pure metal powders with carbon reducible oxygen contents below about 200 ppm (e.g., inert gas atomized powder, electrolytic powders, rotating electrode powders) and (b) metal powders or particles with carbon reducible oxygen contents sub stantially in excess of 200 ppm (e.g., as-atomized pow- TABLE 1 Weight Baking conditions of binder Binder type (percent) Temp. (F) Time (min.) Results Dextrose 2.5 400 Stuck to mold. no strength, could not be handled.

5.0 400 60 Do. 2.5 550 60 Do. 5.0 550 60 Do. 5.0 400 90 Do. 5.0 550 90 Do. Lactose 2.5 400 60 No bond. remained powder.

5.0 400 60 Slight bond, however, could not be handled. 2.5 400 90 No bond, remained powder. 5.0 400 90 Slight bond, however, could not be handled. 2.5 550 90 No bond. remained powder. 5.0 550 90 Slight bond. however. could not be handled. Maltose 2.5 400 60 No bond.

5.0 400 so Developed some bond, but softened after cooling. some sticking to mold. 2.5 400 90 Very slight bond. could not be handled. 5.0 400 90 Developed some bond, however. softened on cooling.

stuck badly to the mold. 5.0 550 90 Binder ran to bottom of mold. very severe sticking to mold. Potato starch 5.0 400 90 No bond 5.0 550 90 Do. Methyl cellulose..... 5.0 550 90 Do. 5.0 550 90 D0. Sucrose 2.5 400 60 Excellent bond. no sticking. adequate strength for all handling. 5.0 400 60 Do.

It may be seen from the above, that irrespective of der, mill scale). For purposes of this invention, carbon binder concentration and baking temperature, only sucrose provided a baked, green preform which did not stick to the mold and which exhibited sufficient reducible oxygen is meant to include those metal oxides which are capable of being reduced by carbon at temperatures below about 2,400 F. As stated hereinabove, the admixture of the metal powder with sucrose serves to reduce the oxidized surface of the powder, act as a carburizing agent and in embodiment (ll), act as a binder when baked at low temperatures. Thus, if pure metal powders of category (a) are employed, and there is no requirement for the carburization thereof, only the baked preform route, i.e., embodiment (II) would be applicable. In this instance, the carbon would then be removed as a result of heating in a controlled atmosphere during sintering and/or prior to forging. Similarly, there are instances when it is only desirable to increase the carbon content by as little as 0.04 percent. If relatively pure powders are employed (no attendant oxygen reduction), the amount of sucrose added in such a case, will be insufficient to act as an effective binder in the production of a baked preform, i.e. route II, and only the sinter preform embodiment would be applicable. However, for the production of most powder metallurgical parts, it is generally desirable to effect significantly greater increases in the carbon content of the starting powders (e.g. 0.2 percent). Thus, in many cases, even when pure iron powders are employed, the required amount of sucrose will be sufficient to permit the utilization of both embodiments of this invention.

Although applicable to pure metal particles, the instant procedures are of particularly notable advantage when employing metal particles of category (b), i.e. those with carbon reducible oxygen contents substantially in excess of 200 ppm. If the latter type particles crose, when employed in a relatively pure state, preferably less than 2 peicent ash content, exhibits an exceedingly high and uniform reactivity, approaching that of the better natural graphites.

The ferrous metal powder-sucrose combination is intimately mixed, i.e., by blending, to achieve a uniform distribution; poured into the mold; vibrated to increase density and then baked at temperatures in excess of about 350 F, to glue the particles together and achieve sufficient green strength for further processing. At least about 1.5 wt. percent sucrose is required to achieve a baked preform with sufficient handling strength. Typically, 'water atomized ferrous powders (with carbon reducible oxygen contents of 1,000 to 20,000 ppm) require the addition. of from about 2 to 10 percent sucrose. For economic reasons, the baking is generally accomplished in air; in which case temperatures in excess of about 500 F are undesirable due to excessive carbon oxidation. Obviously, no such temperature limitation is imposed, if the baking is accomplished in a non-oxidizing atmosphere.

The method above was employed for the production of a differential gear and test bars, from a modified TABLE II.COMPOSITIO N OF MODIFIED 4600 GRADE STEEL EVALUATED-PERCENT BY WEIGHT C Mn P S Si Cu Ni Cr Mo Al N Total 0 are employed it is desirable to know the oxide content (i.e. hydrogen loss) of the particles, since it is first necessary that the sucrose reduce the oxides before it can effectively combine with the iron powder. Thus, the amount of sucrose which is added is dependent on both the hydrogen loss of the particles and the desired carbon content of the final part. With a knowledge of the hydrogen loss of the particles, it would of course be possible to calculate the stoichiometric amount of sucrose required to achieve such a desired final carbon content. However, it is preferable that the required amount be determined empirically, since it has been found that the efficiency of carburization is, to a large extent, affected by the characteristics (e.g. grain size, shape) of the powders employed.

In the recarburizing of iron powders, it is already known in the art that even when sufficient amounts of carburizing agent are employed, that the mechanical properties of the final product are strongly dependent on the reactivity of the carburizing agent. Thus, lampblacks, carbon blacks and synthetic graphites exhibit poor reactivities and are generally considered unsuitable as carburizing agents for the production of powder metal parts with optimum mechanical properties. Even the natural graphites vary considerably in the reactivity they exhibit. Surprisingly, it hasbeen found that suempirically determined that 3.2 wt. percent sucrose was required for this particular powder. The blend of powder metal and sucrose were poured into a mold, vibrated to increase density and baked in air at 400 F for about 40 minutes. After cooling, the baked preform was removed from the mold and sintered in a hydrogen atmosphere at 2,050 F for 30 minutes. The baked and sintered preform was cooled and shipped to another facility for further processing, which comprised heating the preform inductively (in an atmosphere of 5 percent H percent N to various temperatures within the range of 1,200 to l,700 F. The heated preforrns were then immediately forged at about 60 tons/in and then air cooled. The resultant mechanical properties of the so forged test bars are shown in Table III. Noteworthy, is the relatively high ductility and good notch toughness achieved, especially in view of the significant costreductions realized using the instant process. The differential gears were then further evaluated in the drift-pin test. In this test, a tapered, hardened steel pin is pressed into the bore of the gear until failure occurs. If the gear sustains a load of 20,000 pounds without failure, it is considered satisfactory. Shown in Table IV are the results obtained under a variety of forging conditions. Even the gears forged at comparatively low temperature and pressure, passed the test.

TABLE lII.-ROOM-TEMPERATURE ME(HANICAL PROPERTIES OF PREFORMED AND FORGED TEST BARS MADE FROM MODIFIED 4600 GRADE STEEL Yield strength Fracture 10.2% Tensile Elongation Reduction Average Energy Lateral appearance offset) strength i l i nch of area hardness absorber? expansiom (percent C di i (ks tksi) (Percent) (percent) n) (ft-lb) (mils) shear) Test bars forged from 6-mesh powder As-forged' 86.1 94.2 21 4 89 30 39 I Heat-treated 90.4 I 09.0 I 2 26 97 32 35 I00 Test bars forged from -80-mesh powder As-forged' 79.3 91.9 27 52 92 30 32 I00 Heat-treated 89.4 I 09.0 I8 57 95 38 43 100 Cha V-notch test results with standard size specimens. A 'PX A.

' Test bars were stress-relieved for one hour at I000F before testing. 2 Test bars were austenitized for one hour at I600F, oil-quenched and then tempered for one hour at 800F.

TABLE IV.RESULTS OF DRIFFPIN TEST ON MODIFIED 4600 GRADE STEEL GEARS FORGED FROM PREFORMS MADE BY LOOSE-PACK PROCESS Increase Forging Maximum Pin Energy to in bore Gear temperature load (I000 displacement failure (I000 diameter designation (F) pounds) linches) inch-pounds) (percent) a I660 36.5 1.65 3i.2 22 b.. I660 39.8 3.00 76.2 43 0... I660 40.9 2.82 76.2 38 d... I545 45.3 2.65 72.6 36 e... 1575 2L0 l.80 23.8 24 f.... I510 33.5 I94 30.7 25 g... I565 25.6 I66 25.2 22 h... I555 22.9 1.90 28.5 25 I650 24.] 2.60 43.9 34 j I343 26.5 1.36 21.8 18

Note: All gears were forged with a 4 to I die-lubricant-water mixture except for gear j, for which an 8 to I mixture was used. The gears were stress-relieved for one hour at I000F before testing.

heating the filled mold to a temperature of at least about 350 F, but substantially below that at which said metal particles will sinter, said heating being conducted for a time at least sufficient to soften said sucrose to form a baked preform with sufficient strength for handling and further processing.

2. The method of claim 1, wherein said metal particles are ferrous base metal powders with a carbon reducible oxygen content substantially in excess of 200 ppm and said sucrose is present in an amount sufficient to reduce said oxygen and increase the carbon content by a value greater than 0.2 percent, during the carburization of said ferrous particles.

3. The method of claim 2, wherein said heating is accomplished in air at a temperature below about 500 F.

4. The method of claim 3, wherein said blended mixture is essentially dry and contains from about 2.0 to 10.0 percent sucrose.

5. The method of claim 4, wherein the particles in said mold are packed to a bulk density substantially in excess of apparent density.

6. The method of claim 5, wherein said mold is composed of an inexpensive, expendable material with sufficient refractoriness to withstand said heating temperature.

7. The method of claim 6, wherein said baked preform is cooled and removed from said expendable mold.

Claims (6)

1. 2. The method of claim 1, wherein said metal particles are ferrous base metal powders with a carbon reducible oxygen content substantially in excess of 200 ppm and said sucrose is present in an amount sufficient to reduce said oxygen and increase the carbon content by a value greater than 0.2 percent, during the carburization of said ferrous particles.
2. 3. The method of claim 2, wherein said heatIng is accomplished in air at a temperature below about 500* F.
3. 4. The method of claim 3, wherein said blended mixture is essentially dry and contains from about 2.0 to 10.0 percent sucrose.
4. 5. The method of claim 4, wherein the particles in said mold are packed to a bulk density substantially in excess of apparent density.
5. 6. The method of claim 5, wherein said mold is composed of an inexpensive, expendable material with sufficient refractoriness to withstand said heating temperature.
6. 7. The method of claim 6, wherein said baked preform is cooled and removed from said expendable mold.

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