US2191936A - Manufacture of porous iron articles - Google Patents

Description

Feb. 27, 1940. F. v. I ENEL 2,191,936

MANUFACTURE OF POROUS IRON ARTICLES Filed March l0, 1938 2 SheeiSfSheet 1 0. CHANGE mLeNe-m m Vwoo oF nu mcd Pelz mcvl ATTORNEY Feb. 27, 1.940. F. v. LENEI. 2,191,936

MANUFACTURE OF POROUS IRON ARTICLES Filed March l0, 1938 2 Sheets-Sheet 2 O cnuee m Leuer m \/|ooo oF An 'men Pea men FRn-z Lema.

@iii www f am ATTORNEY Patented Feb. 27, 1940 UNITED STATES MANUFACTURE OF POItOUS IRON ARTICLES Fritz V. Lenel, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware yApplication March 10, 1938, Serial No. 195,053

2 Claims.

This invention relates to a method of fabricating ferrous articles from iron powder.

The primary object of the invention is to provide a method of fabricating an article from iron powder and carbon so that the size of the article after sintering thereof is predetermined, the article being formed in an undersized condition so that upon sintering thereof the increase in size will cause the article to be substantially the size desired.

In carrying out the above object it is a further object of the invention to accurately control the percentage of carbon utilized, the briquetting pressure and the sintering temperature in order to obtain the results desired.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

Fig. 1 is a graphic representation of the change in size of briquettes made from sponge iron and carbon in varying percentages at increasing sintering temperatures, all the briquettes beingcompressed at a pressure of 30,000 lbs. per sq. in.

Fig. 2 is a chart similar to Fig. 1 with the eX- ception that the briquettes were compressed at a pressure of 45,000 lbs. per sq. in.

Fig. 3 is a chart similar to Fig. 1 in which the briquettes Were compressed at a pressure of 60,000 lbs. per sq. in., and

Fig. 4 is a chart similar to Fig. 1 in which the briquettes were compressed at a pressure of 75,000 lbs. per sq. in.

Comminuted sponge iron, such as is obtained by the reduction of iron ore or by reduction of the oxides of iron is now being used in industry in the fabrication of ferrous articles. In proceeding to make such articles, the sponge iron powder is uniformly lled into a die cavity, having an internal configuration corresponding to the desired shape of the article and is then briquetted under a high pressure to form the iron powder into a self-sustaining mass having the shape desired. The briquette so formed is then sintered at a suitable high temperature, under nonoxidizing conditions, for such a time as to cause the particles of iron to sinter together and thereby form a porous ferrous article having a high degree of mechanical strength and of the desired configuration.

It has been found desirable to include carbon in the preliminary mixture of iron powder so that the nal article, subsequent to the sintering i il step is carburized, to some extent, and also lncludes some free carbon, mechanically held within the pores of the article. The free carbon adjacent the outer surface of the article aids in lubricating the surface when the article is utilized under any condition of frictional contact with another surface. The carbon which is absorbed by the iron increases the hardness thereof, thereby increasing the wear resistance of the article. Carbon in the initial mixture is further desired as it aids in the formation of the briquette, acting -as a lubricant in the briquetting die.

I have found that when iron-carbon mixtures are briquetted and sintered that the final size of the sintered article does not correspond to the size of the briquetted article. The present invention is particularly concerned with this phase in the manufacture of iron articles from powdered iron and solves the problems of manufacturing ferrous bodies which include carbon in appreciable percentages.

The inclusion of carbon in excessive quantities tends to lower the mechanical strength of the nished article. 'I'his is explained by the fact that the free carbon, which is mechanically held within the pores of the iron, reduces the area of contact between the individual particles or grains of iron and thereby reduces the strength of the bond between the adjacent particles after the sintering thereof. It has been proved by test that carbon below 7% is most desirable from a mechanical strength standpoint, although articles having a carbon content in the order of 12% have suilicient mechanical strength for many industrial and commercial uses.

The fabrication of ferrous articles from powdered iron is most desirable from the aspect of production and economy, since in many instances it is possible to briquette irregularly shaped articles having a plurality of apertures therein and then sinter the same to forni a rigid ferrous article of substantially the size desired. Many articles, that require a great number of finishing operations when made by the usual casting methods, may be fabricated by the present method with sufcient accuracy to completely eliminate such machine work. When a very high degree of accuracy is desired a final machining operation may be utilized but such a machining operation does not require any appreciable time or labor since the body made from powdered metal is so very close to the size required. In other words articles formed by briquetting and subsequently sintering powdered actors 'that controls the to produced cr stated one o the factors controlling the change the oriquette during sintering 'i'.hereci.

found that the hriquettirig pressure the sintering temperature are likewise of great i partance in controlling the change sise of the hriquette during sintering thereof.

-Referring to the accompanying drawings it will be observed that articles having under 3% carbon in the initial rnix always shrine'. upon sintering thereof regardless of the sintering temperature or briquetting pressure. 'it is also manifest that under certain conditions articles having 3% and over of carbon, in the initial mixture, can he caused to increase in size, or swell, during the sinterne` thereof by control of the pressure and temperatures. Thus a briquette having 3% carbon which has been briquetted between 30,000 and 40,000 lbs. per sq. in. will increase in size when sintered at a temperature of 1750o F. Articles having over 6% carbon therein and briquetted at pressures of 60,000 lbs. per sq. in. and over will increase in size when heated to temperatures approximating 1850 F. Likewise, articles that include r1'1/f77 carbon in the initial :nix and briquetted at pressures of 30,000 lbs. per sq. in. and over will increase in size when heated to a temperature of not over 1950 E. Similarly ar-v ticles including 11% or over of carbon and briquetted at 30,000 lbs. per sq. in. and above will increase in size when sintered at temperatures up to 2050D F.

From the foregoing it is manifest that by the proper choice of sintering temperature and briquetting pressure that articles may be formed including 3% or more of carbon which will increase in size a desired amount during sintering thereof. Therefore, it is possible to briquette the article to a predetermined smaller size in all drections than the size desired, so that upon sintering thereof the article will increase in size to substantially the size desired. A specific example, for illustrative purposes only, may be noted in the manufacture of a hollow cylindrical article having a closed end such as a piston. Such an article may be formed from sponge iron including carbon, in for example, 6 parts of carbon to 94 parts of iron. These ingredients are thoroughly mixed and then briquetted in a suitable die under a pressure of approximately 60,000 lbs. per sq. in. The briquette is then sintered under non-oxidizing, or reducing conditions for at least onehalf an hour at a temperature approximating 1850 F. During this heating period it has been found that between 1 and 2% of the carbon is absorbed by the iron. The article according to my discovery will increase in size during the sintering step. Thus, when making ferrous articles having 6% carbon, the forming die should be of the shape desired but of slightly reduced dimenollows: c. heated with iron to high temperatur suon a sintering temperature, there partial lcaticn ci' the carbon clue to combination ci the carbon with the oxygen combined with the C surface oxides on the iron, such gases cartion monoxide and carbon dioxide. These escape from the hriquette after the internal pressure of the gases is in excess of atmospheric pressures. This internal pressure causes a slight increase or" size over the initial size thereof. When carbon in percentages of under 3% is utilized there is sumeient absorption thereof to substantially prevent such an evolution of gas. When using higher percentages of carbon at briquetting pressure the porosity of the briquette is sufliciently great to allow easy escape of the gases without the build-up or" internal pressure.

will be noted from the curves shown in the accompanying drawings that briquettes having 3% carbon therein do not follow any predetermined course in their action but are irregular and unpredictable. The nature of briquettes having 3% carbon therein cannot be explained other than it appears that 3% carbon is a critical percentage.

The points used in plotting the curves as shown in the accompanying drawings were obtained by actual tests in which small pellets or slugs approximately an inch long were briquetted with the required amount of carbon and under the pressures designated. These slugs were accurately measured as to the green length thereof and were then sintered at the temperatures indicated, cooled and remeasured. The increase or decrease in length was noted and these figures yield the basis for curves of the chart. In each case three slugs were utilized and an average of the dimensional change taken as the figure for the curve. It is interesting to observe that there was very little variance between the dimensional changes of the three check samples i. e. the thousandths of an inch increase or decrease was quite uniform in all cases, where samples were tested under similar conditions.

While sponge iron as herein defined is preferred in the fabrication of porous articles due to its low market price and to its purity' and resultant constant reactant qualities, it is to be understood that substantially similar dimensional changes occur under like conditions when comminuted iron, electrolytic iron, iron from iron carbonyl, or any other form of iron powder is used.

While the embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

lower 230 1. The method of fabricating articles from sponge iron including carbon in percentages of y 3% comprising the steps of: forming a briquette of the desired shape out of intimately mixed nely divided sponge iron and carbon under a pressure of between 30,000 to 45,000 lbs. per sq. in. and of a predetermined size which is slightly less in all directions than the size ultimately desired, and then sintering the briquette so formed in a nonoxidizing atmosphere at a temperature of about 1750 F. for a. time suicient to cause the particles of iron to sinter together and to absorb at least v a portion of the carbon and simultaneously cause the sintered briquette to increase in size to substantially the size desired.

2. In the manufacture of a hollow cylindrical porous ferrous body, the steps of cold molding sponge iron with graphite in percentages of 3% under pressures greater than 30,000 and less than 45,000 lbs. per sq. in. to form a cylindrical blank of the shape desired but of predetermined dimensions which are less in all directions than the dimensions ultimately desired and then heating the blanks so formed at a. temperature of about 1750 F. for causing the particles of iron to sinter together and to absorb at least a portion of the graphite and simultaneously cause the sintered body to increase in size to substantially the size desired.

FRITZ V. LENEL.

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