US1871544A - Cast iron article and method of manufacturing thereof - Google Patents

Description

Aug. 16, .1932. R. H. M cARROLL ET AL 1,871,544

CAST IRON A RTICLE AND METHOD OF MANUFACTURING THEREOF Filed Dec.; 26. 1930 2 Sheets-Sheet 1 lfusse/lHMFCarroll 'bdusla Vvznerlwlm Aug. 16, 1932. 1,871,544

CAST IRON ARTICLE AND METHOD OF MANUFACTURING THEREOF R. H. MQCARROLL ET AL Filed Dec. 26. 1930 2 Sheets-Sheet 2 Patented Aug. 16, 1932 UNITED ,STATES, PATENT, oFr ce RUSSELL H. MCCARROLL- AND GOSTA VENNERHOLM, OF DEARBORN, MICHIGAN, AS-

SIGNORS TO FORD MOTOR OF DELAWARE COMPANY, OF DEARBORN, MICHIGAN, A CORPORATION CAST IRON ARTICLE AND METHOD OF MANUFACTURING THEREOF Application filed December 26, 1930, Serial No. 504,836.

This invention relates to cast iron articles and to methods of manufacture thereof, and has for its principal object to provide a new and improved article made primarily of iron cast to a predetermined shape but which is so processed that the final product has its characteristics improved to such an extent over the ordinary cast iron article that the article so produced possesses in a large measure the desirable high physical properties of steel, while at the same time possessing the desir able attributes of cast iron with respect to low production costs. 7

Another object is to provide a new and improvedzniethod for the production of cast ironarticles by the use of which the physical properties of such articles are vastly 1mproved.

A furtherpbject is to provide an article of iron cast to a predetermined shape, capable of being annealed, heat treated and tempered in a manner analogous to the treatment of steel.

Another object is to provide a new and iniproved process for the treatment of cast iron which will produce not only annealing, hardening and tempering of the matrix, but will also result in a uniform distribution of the free carbon in small rounded globules or nodules, the presence of large graphitic flakes or fissures in the final product is avoided and an article of markedly uniform, homogeneous structure is secured.

The above and other objects will appear more fully from the following more detailed description which sets forth theprinciples underlying the present invention by reference to a satisfactory commercial reduction to practice thereof, the example described,

however, having been selected for the pur pose of the present application for illustrative purposes and not as setting forth the limits of said invention.

In view of the fact that the manufacturing cost of cast iron is so much lower than that of steel, and as cast iron is essentially a steel whose continuity is broken by free graphite, much time,- study, and effort has been given by metallurgists to devise satisactory, commercially practical, methods for present invention.

treating cast iron in a manner analogous to cast iron, but no one, at least as far as we are aware, has heretofore succeeded in obtaining the desired results. Such'methods for treating cast iron in order to harden it as have heretofore been tried, either have not been practical from a commercial standpoint, or it has been foundthat uniformity of results were not obtainable.

The present invention will best be understood by reference to the drawings wherein:

.Fig. 1 illustrates a typical cross-sectiomof the ordinary sand cast grey iron; Fig. 2, a similar section of malleableize iron;

Fig. 3, a. similar section of an iron illus trating the result of the first step of the process of the present invention;

Fig. 4, the result of the second step of the present process; I a

Fig. 5, a cross-section of the final product of the present invention; and

Fig. 6,'a somewhat diagrammatic View of articles manufactured in accordancewith the The cast iron used commercially may be. said to be of the two follow' g principal 'types, namely,

cooling silicon. V

On" the other ha d, white castironabove referred to'is' iron in which the carbon is as above stated, and the presence of rpresen't'in'the combined state, that is, in the 'form of iron carbide and as a solid solution of iron and iron carbide. Iron of this type .d very unstable, and is readily 'deoom ed into iron and carbon, Two factors wil pro mote th s. decomposition, viz, slowrateof accompanying grey cast iron and malleable-. ized iron, the-latter being a heat treated a slow rate of cooling both}.

- cooling resu ting in chilled iron or by the presence of much manganese or sulphur.

The chill cast iron is commercially used for articles in which hard wear resisting surfaces are desired. This type of iron is extremely hard and very brittle. Due to the extreme hardness and frangihility of the chill cast iron and the difficulty of machining it by the ordinary machine shop methods, its usefulness is very limited. Malleableized iron is white iron produced by low carbon and medium high manganese and that has been subjected to a special heat treatment, termed malleableizing. Malleableizing consists in heating .the iron to a high temperature over a long period of time (for example, from to hours and then allowing it to cool slowly). The result is a perfect decom ositionof the iron carbide into iron and car on. The carbon, known as temper carbon, is found to have segregated into small rounded particles imbedded in a soft matrix of substantially pure iron. Malleableized iron is therefore very soft and rather ductile with relatively low strength. Itsusefulness is therefore limited 1 on this account, as well as on account of the slowness of its processing.

Figs. 1 and 2. of the drawings illustrate respectively the characteristic physical structure of ordmary grey cast iron and malleableized iron. By referring to Fig. 1 it will be noted that grey cast iron. contains, interspersed through it, the relatively lar e particles of graphite indicate by the re erence character 10 which particles are commonly referred to as graphitic flakes or fissures. The matrix-of this iron is mainly pearlitic with free ferrite.

. The presence of the graphitic flakes or fissures in grey cast iron 0 viously is one o the factors that contributes to the .relative brittleness of cast iron. While heat treatment, such as is-used in the hardening of steel,

may be resorted to harden the matrix of greycast iron such heat treatment does not appreciably alter the size or distribution of graphite particles, and, as a hardemng of the matrix results in a corresponding increase in its brittleness, no practical advantage, at least insofar as increased strength is concerned, has been apparent in hardened cast iron over ordinary y iron. Therefore heat treatment of cast iron primarily to harden it has not heretofore been commercially practiced although heat treatment has been resorted to for replacing the well known aging process.

As hereinbefore pointed out, the rate of.

ran e in which decomposition may ta eplace.

As iron. having a carbon content of 4.2% is of eutectic composition, iron of such compo sition would have the minimum interval between liquid and solid, and therefore the smallest latent heat range and the fastest solidification. The amount of carbon in euof the desired carbon content, and thereby to secure the fast solidification that characterizes the normal eutectic iron of 4.2% carbon ratio. We have found as a result of a long period of investigation and study, that the physical properties of cast iron can be greatly improved and that an iron can be successfully roduced upon a commercial basis that may e said to be between steel and cast iron, possessin the desirable qualities of the former with the respect to high strength, homogenity, of cross-section, hardening, and temperingi qualities, while at the same time possessing the desirable attribute of cast iron with respect to low production costs. We control the decomposition of the combined carbon and limit the size, shape and distribution of the particles of free carbon, by producin it in aform similar to temper carbon. e aim, primarily, by the process of'the present invention to produce a matrix of approximately eutectoid composition, although we have found in practice that it is not essential to secure an eutectoid matrix.

The present invention has been successfully practiced upon a commercial scale for the f production of cast iron having a carbon ratio not exceeding 3.7% to which we have found the addition of silicon of from 2.5% to 2.9% will produce the desired lowering of the eutectic to bring such eutectic approximately into alignment with the carbon abscissa of the iron-carbon diagram for iron of the desired carbon content. The molten iron is poured into metal molds, preferably of cast iron, to insure a rapid coohng of the. molten metal. As soon as the metal has solidified sufliciently to retain its desired sli'a e, the cast article is released from the mol Due to the quick cooling and to the fact that the latent heat period has been reduced to-a minimum, the product which results from the casting and quick cooling step is found to have.

a structure such as illustrated in Fig.3. It

will be noted by referrin to this figure that there is an absence of the large ,graphitic flakes 10 such as present in the ordinary cast iron illustrated in Fig. I.

After the casting and cooling of the article, it is subjected to a temperature above the critical range in ordertendecompose the combined carbon. The nature of this heat treatment, i. e., the temperature and duration can, of course, be varied. After the heating period, the article is allowed to cool slowly. It is, however, essential that the temperature during heating be above-the critical range.

In practice it has been found that a temperattire-between 1600 and 17 00 F., maintamed for a period of from one to three hours, has given highly satisfactory results. The product that results from the heattreatment ust described is illustrated in Fig. 4 of the drawings, from which it will be seen that the free carbon or graphitic particles have been collected or grouped into small, rounded particles l3 quite analogous to grouping of carbon particles of the malleableized iron illustrated in Fig. 2. i

The above heat treatment is essentially an annealing process, the hardness is drawn from the metal and a soft casting having a Brinnell preferably under 228 is secured. The degree of annealing and the Brinnell hardness can be controlled by the temperature and duration of this heat treatment. After the castings are annealed as above described, they can readily be machined and rough ground to produce an'article of the desired dimensions and configuration.

When the articles have been machined to the desired shape and size, they are then subjected to a further heat treatment at a high temperature above the critical range for producing a matrix inthe form of a solid solution. Due to the fact that the free carbon has been segregated as shown in Fig. 4 into relatively small nuclei, a much larger proportion of the carbon of each nucleus is caused to penetrate into the matrix than would otherwise be possible if the carbon particles were of the larger size such as 1 shown in Figs. 1 and 2. We have found in practice that satisfactory results are obtained y heating the iron to approximately 1550 to 1600 F. maintained for a period of approximately ten minutes. Variations of these figures may, of course, be resorted to in accordance with the chemical composition of the iron, and cross-section of the article.

After subjecting the articles to the last described heat treatment, they are cooled quickly, as for example, by quenching, preferably in an oil bath or water, or any other suitable quenching bath, maintained at-approximately atmospheric temperature. In some cases it may be desirable to resort to air cooling.

If desired, the article may also be temperdrawn after hardening, or subjected to any other treatment to eliminate strains. The resultant product is as illustrated in Fig. 5 from which it will be noted that the free carbon is uniformly distributed throughout the matrix in small rounded nodules. There is an entire absence of large graphitic flakes and fissures. Although the structure illustrated in Fig. 5 is essentially one in which the matrix is characteristically martensitictroostitic, the matrix preferably is not above the martensitic range nor below the sorbitic range. I

In Fig. 6 of the drawings We have 1llustrated various articles. that have been produced by the processes herein described. The reference character 14 indicates a valve tappet or push rod such as employed for internal combustion engines. The reference character 15 indicates a cam shaft. The articles illustrated in Fig. 6 are shown merely by way of example and not as limiting the scope of the invention.

The invention is particularly well adapted for high, speed production methods because of the quick cooling of the castings and the use of metal molds. .In practice the molten metal is cast into the molds which are mounted on an intermittently rotatable table, and after a lapse of time suflicient to permit the metal to solidify, during which time the table revolves through a certain angle of rotation, the molds are opened automatically and the castings are preferably dropped onto a conveyor and passed through an annealing furnace. The above procedure has therefore resulted in a marked reduction in labor production costs over the ordinary sand casting procedure as well as in securing a product highly superior to the ordinary sand cast lIOIl.

While we have for the purpose of this application referred to the manufacture of iron of not exceeding 3.7% carbon and 2.5% to 2.9% silicon, it will be understood that such ratios are given merely by way of example, and, as we have practiced the invention satisfactorily with irons having silicon contents not below 1.75% and carbon not below 2.0%, that the principles of the invention are not limited to use with iron of any specific carbon ratio, as long as the carbon and silicon contents fall within the limits above set forth and the iron is therefore what is termed in the art as high carbon, high silicon iron. Likewise, the specific temperatures and heat periods set forth are merely illustrative examples that have proven successful in practice. Obviously, many changes, variations and modifications may be resorted to according to the chemical composition of the iron, the requirements for any particular article, or the service to which it is to be put.

We claim:

1. An article of manufacture comprising 'cast iron containing not below 1.75 silicon,

and having a matrix not above the martensitic range nor below the sorbitic range and having the free carbon content in the form of temper carbon. h

2. An article of manufacture comprising cast iron containing not below 1.75 silicon,

and having a matrix not above the martensiticj range nor belowthe sorbitic range and free from graphitic flakes or fissures.

3. An article of manufacture comprising cast iron containing not belowv 1.75% silicon and not below 2% carbon, having .a matrix not above the martensitic range nor below the 'sorbitic range, and having the free 'car-, bon content in the form of tem er carbon and free from graphitic flakes or ssures.

4. The process of manufacturing cast iron which consists in preparing a batch of molten metal having a carbon-content greater than 2% and a silicon content. greater than 1.7 5% while maintaining a higher carbon than silicon content, casting the metal in a metal mold to produce an article, allowing said article to cool, then subjecting it to a short cycle anneal of from 1600 to 1700 F. to produce a machinable product having a hardness below, but in the region of, 228 Brinnell, and final-1y subjecting to a reheating above the temperature of the critical followed b quenching to secure a matrix not above t e marten'sitic 1 nor below the sorbitic range.

In testimony whereof we affix tures hereto.

RUSSELL H. MGCARROLL. GOSTA VENNERHOLM.

our signa-

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