CA1078226A - Method of forging powdered metallurgy preforms - Google Patents

Abstract

METHOD OF FORCING POWDER METALLURGY PREFORMS

ABSTRACT OF THE DISCLOSURE
A method of making forged powdered iron base metal articles of high Rc hardness without need for further machining, surface treatment nor heat treatment after quenching comprising forming a pre-alloyed metal powder, compressing said powder into a briquette, sintering said briquette into a preform, heating said preform to a temperature above its austenizing tempera-ture, forging said preform while above its austenizing temperature into a forged article, permitting the article to cool to approximately its austenizing temperature, and promptly quenching said forged article in a quenching bath.

Description

~078226 BACKGROUND OF THE INVENTION
Relatively small forged iron base metal articles can be produced from wrought stock or powdered metal, the latter being a relatively new technique of compacting metal powders.
To produce such an article of high Rc hardness, generally in the order of Rc 60, which means substantially high martensitic content, coupled with good surface finish and dimensional stability requires subjecting the wrought article after forging to at least the additional steps of cooling, machining, and rehardening. Each subsequent step is not only expensive but introduces different metal conditions whi~h must be accounted for in the next following process step.
The forg~ng of powdered metal preforms has consequently come into acceptance as a metal working process because of its chief advantage in eliminating the machining operations necessary with wrought material. In such process pre-alloyed metal powders are admixed with graphite, and lubricant compacted into briquettes, sintered, and forged. ~Yhile desirable, *he full benefits of this relatively new procedure have not been fully realized.
For example, in the manufacture of powdered iron base metal articles having hardness at the working surface of Rc 60 or thereabouts it has hereto-fore always been thought necessary to either a) use a carbon content of 0.2-0.3 percent by weight and then carburize and subsequently harden the article, or b) use a carbon content of 0.5-0.7 percent and then austenize the article and qucnch it. Whilc there arc diffcrcnt spccific ways to accomplisll eacll qP

` 10~8226 processing method (a) and (b), these are well known in the art and it is sùfficient for purposes of illustrating our invention to explain that while each achieves the surface hardness and surface finish desired, each such method causes undesired dimensional changes in the article itself due to the heat effect.
SUMMARY OF THE INVENTION
The present invention overcomes the above stated problems heretofore associated with powder metallurgy forging and permits the one-step forging of articles having the desired density, hardness, and closer dimensional tolerances, aswel~ as articles which are free from surface and internal defects. In addition, our inventive process is capable of producing an article of substan-tially uniform high Rc hardness throughout, rather than mere surface hardness as obtained with carburizing.
Briefly stated, the present invention comprises a method of making a through-hardened, scale-free forged powdered metal article having a density of at least about 99.5% of theoretical density and a surface hardness of at least Rc 40 directly from forging and quenching comprising: compressing an iron alloy powder into a briquettei sintering the briquette into a sintered preform; adjust-ing the temperature of the heated preform to above its austeni~ing temperature, wherein the preform is in an austeniticstate; forging the heated preform while it is in the austeniticstate into a forged article; quenching the forged article substantially immediately after the forging step, and while the preform is still in an austeniticstate; and removing the quenched forged article from the quench-ing bath and stress relieving the article.
DESCRIPTION OF THE DRAWINGS~
The drawing shows a flow diagram illustratinq the main procedural steps employed in carrying out the method of the present invention.

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i 1078ZZ6 DLT.~ILED DESCRIPTION
Referring to the drawing, there is shown a flow diagram of the method, generally designated 10, in which the initial step is preparation of the pre-allowed metal powder 12. The formulation of the powder will vary wide-ly dependent upon the characteristics desired in the final article. Iron, of course is the major constituent with varying amounts of such metals as manganese, molybdenum, and nickel being added. Carbon, in the form of graphite, is added '~ - 2a -.

¦¦ prior to briqucttin~. In actu:ll opcllltion~ lt ls pr~crrccl to llr~t ~lloy the lron and o~her metals in the usual manner and to atomizc thc alloy to produce an lron alloy powder. The iron alloy powder is then ad~ixed with the graphite and with a lubricant. Such lubricants are conventionally wa~y or fatty materials which will be burned off in the sintering furnace as hereinafter described. The methods used to alloy and to atomize are those conventional to this art and form no part of the instant invention.
- TypicaIly, the iron alloy powder can have the follo~ing composition:
% by Weight Manganese 0.25 - 0.5 :
Molybdenum 0.25 - 0.5 Nickel 0.25 - 2.25 Carbon 0.2 - 0.9 Iron Remainder The powder is then formed into a briquette 14 or green compact by pressure compression in a die. This briquetting is used to control densifica-tion, shape, and general dimensional tolerances. Commonly, briquetting can accomplish densi$ication up to about 85~ of theoretical. Here, again, the type o~ compaction that is used can be varied; such as uniaxial or isostatic.
A$ter compression, the briquette or preform is sintered 16. This i8 accomplished by placing the preform into a sintering furnace. These conventionally contain a preheating zone for lubricant burn-off, a hot zone for sintering, and cooling zone. Normally, sintering of ferrous materials is carried out at a temperature of about 2050 to 2100 F. Sintering is carried out in a protective atmosphere, i.e., one free of oxygen, in order to prevent internal oxidation and formation of oxidative scale on the surface of the preform. The furnace atmosphere in which the preforms are sintered contains the carbon potential to maintain the desired combined carbon level of the material. Any of the protective atmospheres conventionally used for this purpose are suitable. Surface enrichment to increase the carbon content and thus provide a carburized condition on the surface of the preform can also be provided. It has been found in practice that such a step not only helps maintain hardness but also eliminates general cracks resulting from the subsequcnt qucnching step.
, 10 78ZZ6 1~78226 An essential feature of the present invention i8 the heating of the preform 18 after sintering to a temperature above its austenizing temperature prior to forging. In conventional sintering furnaces the preform is cooled to ambient temperature. Consequently, the preform must then be subsequently reheated above its austenizing temperature. The particular temperature above austenizing that is chosen is varied depending mainly on the shape and cross-sectional thickness of the preform. Thus, for example, a preform having an austenizing temperature of about 1550 F. can be ad~usted to a preforging temperature of about 1600 to 1900F.

Immediately after being brought to the proper temperature, the preform is subjected to the forging operation 20 in a protective atmos-phere wherein a single blow (one-step forging) of approximately 60 to 80 tons per square inch is used to shape the preform into the desired finished article.
The forged article is then permitted to attain a uniform temperature throughout approximating its austenizing temperature and prom-ptly quenched in a conventional quenching bath 22. This also is essential in the present process.
It is preferred to use a conventional quenching oil bath although other quenching baths, such as water containing ethylene glycol, can be used.

It has been found t.hat the careful regulation of the temp erature prior to forging and quenching eliminates the need for any further heat treatment or working of the article to attain the desired tolerances and hardness throughout the article. Moreover, by operating in a protec-tive atmosphere during sintering and forging, formation of any oxidative scale is avoided. The article 26 is in a finished condition after removal 24 from the quenching bath.
With respect to hardness, the instant process obtains hard-ness that are 40 - 62 Rc or higher. It will be evident that, if a lower ~ _ 4 _ ~ 1078ZZ6 . ~ . . .

hardness is desired, the finished article can be further heat treated.
Similarly, the product can be selective or surface hardened.
The invention will be further described in connectlon with the following examples which are set forth for purposes of illustration only.

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An lron 8110y powder was prcpared havlng tlle.followlng formula:
-% by WeiFht Mangnnese 0.25 Molybdenum 0.5 Nickel 1.8 Carbon .6 Iron , 96.85 . ' '100.00 The iron, manganese, molybdenum and nickel were alloyed in the usual manner and the alloy atomized to produce iron alloy powder. This powder admixed with graphite to supply the carbon and with a lubricant (Acrawax--a hard, white, synthetic wax having a melting point of 94-97C.), was placed into the die cavity of a standard briquetting press where pressure was 15 applied and the powder formed into a briquette (green compact) of sufficient strength to permit further handling.
The briquette was then sintered in a sintering furnace at approx-imately 2050 F. for about 30 minutes in a protective atmosphere. The furnace was of the available type containing a zone for lubricant burn-off, a hot 20 zone for sintering, and a cooling zone. The sintered preform was at a temperature of about 200F. when removed from the sintering furnace. The sintered preform was then heated to a temperature of approximately 1650 F., in a protective atmosphere in an electric heating core. This temperature was above the austenizing temperature (1550F.) of the material.
The heated preform was then promptly transferred to a forging press-and formed by a stroke of approximately 60 tons per square inch pressure into a forged stator clutch race having a smooth O.D. and a splined I.D. After forging, the forged article was allowed to stabilize and cool to its austenizing temperature of about 1550F. and promptly quenched in a conven-30 tlonal oil bath.
The article was free of surface oxidative scale and was tested and found to have a density of 7.82 grams/cc (min.) or a density 99.6% that of theoretlc~l; 7.87 grams/cc being the accepted theoretical maximum density.
Most important, the product had a hardness, throu~hout the part of Rc 59-62.
Equally lmportantly, the artlclc had thc desircd shapc and dimcnsions, thlns -.

.~ 1 1078Z26 elimlna~ing th~ ncc~ Eor furt11er proccssinl-, to ~l~tai11 thc dcslrc(l har(lness of the product.
EX~1PLE 2 An lron alloy powder is prepared in Ex~mple l but having the -following composition:
70 by Weight Nickel 0.5 Manganese 0.3 Molybdenum 0.5 Carbon 0.65 Iron Remainder ' 100.0 ' :
The graphite and lubricant are added as in Example l and the mixture deposited in the die cavity of the die set of a conventional bri-quetting press and compressed into a briquette. This briquette, then sintered, is heated, forged, quenched and stress relieved as described in Example l. The i'orged article thus produced was also found upon inspection to possess an excellent surface quality free from oxidation scale and other sur$ace imperfections and immediately salable without further treatment. Its hardness is $ound by test to extend entirely through the article, with similar properties extending in all directions.

EXA~LE 3 A nickel-content iron alloy powder is prepared as in Example l having the ~ollowing formula:
by Weight Nickel 2.0 Manganese 0.25 Molybdenum 0.3 Carbon 0.5 Iron Remainder 100.0 The processing steps of Example l are followed to form a finished forged article. The forged article thus produced is also found upon inspec-¦~ tio o possess ~n exccllent sorP;ce s~m11ty Pree Prom oxidation sc~1c and oth~

¦¦ surrace imperfcctions so as to bc immcdiatcly salablc witholl~ furtllcr surface ¦ treatme-lt. The h~rdnc~s of the nrticle, as in Ex~mple 1 and 2 a~ovc, is found by test to extend entirely throu~h thc article, ~vit~ similnr properties extending in all directions, and in this instancc was Rc 55-57.
Experience in carrying out the process of the present invention has shown that such an excellent surface quality is obtained that the product can be sold and shipped a`s it was for~ed, without the usual machining to remove 20 to 30 thousandths'of an inch in order to eliminate the surface deterioration which has hitherto occurred. Furthermore, the obtaining, by this method, of the ultimate product immediately after forging, causes the tolerances required when, as previously, the dimensions of the article changed when the artlcle was carburized after forging. Thus, the quenching immed-lately after forging, according to the present method, eliminates the extra tolerance spread acquired by variations in dimensions occurring during the subsequent reheating required by previous procedures, such as carburization.
The high hardnesses achieved by the forging and quenching method of the present invention are ideally suited for mechanical parts which are required to sustain heavy running loads during operation, especially where the hardnesses are so high that the material cannot be machined but can only be ground, namely above 35 Rockwell "C". Anti-friction bearing races, for example, will wear rapidly and will not survive unless they have hardnesses in the neighborhood of 57 or 58 Rockwell "C".
While the invention has been described in connection with a preferred embodiment, it is not intended to limit the invention to the particular form set forth, but, on the contrary, it is intended to cover such alternatives, modii'ications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of making a through-hardened scale-free forged powdered metal article having a density of at least about 99.5% of theoretical , a surface hardness of at least RC 40, and an as forged dimension requiring no further machining other than surface grinding, all directly from quenching comprising:
compressing an iron-base alloy powder containing at least about 0.2% by weight carbon into a briquette;
sintering said briquette into a sintered preform;
adjusting the temperature of said preform to such degree as to put said preform in an austenetic condition;
forging said heated preform in one step while at said austenetic condition into a forged article;
quenching said forged article in an oil quenching bath substan-tially immediately after said forging step while remaining in said austenetic condition; and removing said quenched forged article from the quenching bath, and stress relieving said forged article to produce a forged powdered metal article having a surface hardness of at least Rc 40 and a surface finish and dimension requiring no further machining other than surface grinding.

2, The method of claim 1, wherein the powdered alloy composition comprises, for each 100% by weight, 0.25 to 0.5% manganese, 0.25 to 0.5%
molybdenum , 0.25 to 2.25% nickel, 0.2 to 0.9% carbon and the remainder iron.

3. The method of claim 2 wherein the sintering of said briquette is accomplished in a gaseous atmosphere of high carbon potential and provides a carburizing effect on the surface of the preform.

4. The method of claim 3 wherein the powdered alloy composition comprises 0.5 to 0.9% carbon.

5. The method of claim 1, wherein the adjustment of the preform temperature prior to forging is carried out immediately after sintering to conserve heat energy by utilizing the residual heat of the sintered pre-form toward reaching an austenizing temperature range which assures the preform being in an austenetic condition and also by eliminating the nec-essity of expending heat in subsequent carburization to attain the desired hardness.

6. The method of claim 1, wherein following said forging step and prior to said quenching step the temperature of the forged article is allowed to become substantially stabilized.