US2352408A - Method of producing ferrous castings having desired physical properties - Google Patents

Description

June 27, 1944. REECE ET AL 2,352,408

METHOD OF PRODUCING FERROUS CASTINGS HAVING DESIRED PHYSICAL PROPERTIES v Filed July 3, 1941 5 Sheets-Sheet 1 HERBEPTA INVENTORS .EEEcE AND BY OLIVER 05MALLEK M v M June 27. 1944. H. A. REECE ET AL 2,352,408

METHOD OF PRODUCING FERROUS CASTINGS HAVING DESIRED PHYSICAL PROPERTIES Filed July 5, 1941 3 Sheets-Sheet 2 AUSTENITE AUSTENITE MARTENSITE GRAPHTE pEARuTE INVENTORS HERBERT A HERE AND OZIVEB SMAZZEY j A rronms.

June 27, 1944. REECE ET AL 2,352,408

METHOD OF PRODUCING FERROUS CASTINGS HAVING DESIRED PHYSICAL PROPERTIES Filed July 3, 1941 I 5 Sheets-Sheet 3 PEARLITE GRAPH \TE SORBO'PEARLITE FERR'TE PEARLITE 1' N VFN T0123 HERBERTA.REECE AND BY OLIVER SMALLEY 'ATTORNEYS Patented June 27, 1944 METHOD OF PRODUCING FERROUS ST- INGS HAVING DESIRED PHYSICAL P OP- ERTIES Herbert A. Reece, Cleveland Heights, Ohio, and Oliver Smalley, Pittsburgln Pa., assignors to Meehanite Metal Corporation, a corporation of Tennessee Application July 3, 1941, Serial No. 400,900

- Claims. (Cl. 148-3) Our invention relates to the art of making ferrous castings.

In the production of foundry castings it is a desideratum to obtain castings having a microstructure best suited to the type of service in which the castings are to be utilized. The metallographic structure of cast iron is an important factor in the ability of the casting to meet the particular requirements of the use to which the casting is to be put. There have been many approaches to the problem of obtaining the desired metallographic structure in cast iron, some of which have been successful to a limited degree but all of which have certain inherent disadvantages, such as cost of the process, the inadequate control of operations, the lack of uniformity in results and inherent limitations upon the scope of the processes.

For example, iron castings which are to be used for Diesel cylinders, brake-drums, crankshafts, liners, cams, gears and other machine parts subjected to rolling or sliding contact, require great wear-resisting properties. To produce castings for such use requires a foundry practice which lends itself to the control of the microstructure of cast iron. One of the attempts to produce cast iron purporting to have physical properties suitable for wear resisting service is shown in United States Patent No. 2,200,765 issued to Bartholemew et a1. It is stated in that patent that the production of an iron casting having an austenitic structure is of value for wear resisting uses as the casting may be "run in to effect a cold-working operation" of the material and a resultant hard and wearresstant surface.

However, the process suggested by the Bartholemew patent requires the expensive and time-taking operations of allowing the castings to cool below the carbide critical range, of reheating them above room temperature, and of quenching the heated castings in a hot bath. Inasmuch as the castings are raised in temperature from that of room temperature, the heat treatment processes of Bartholemew and others involves special heat treatment furnaces, incurs the risk of cracking the casting, and is costly in operation. Moreover, due to several factors the structural stages through which the iron passes upon cooling cannot be completely and precisely retraced by merely heating the iron again as suggested by Bartholemew. The present invention is based upon an entirely different principle from the process suggested by the said Bartholemew patent, obviates the disadvantages and difilculties of that process and other heat treatment processes, and produces results not heretofore obtainable. Moreover, the present invention teaches the control of the microstructure of cast iron in a manner and to an extent not completed by said Bartholemew patent or other disclosures on heat treatment. The references herein to the Bartholemew patent or to the prior art in general are made only for the purpose of demonstrating the novel nature and distinct character of the invention herein described.

It is an object of our invention to provide an economical and accurate method of'producing castings having desired improved and controlled physical properties.

Another object is the provision of a method for controlling the structure of iron castings.

Another object is the provision for retaining a. desired austenitic structure in iron castings.

Another object is the provision for obtaining a sorbitic-austenitic or a troostitic-sorbitiic structure in an iron casting which would otherwise have a predominantly pearlitic structure under the processes in standard practice.

Another object is the provision for retaining varying amounts of austenitic or semi-austenitic structure in iron castings in accordance with predetermined requirements.

Another object is the provision for obtaining an austo-troostitic or a troostitic-sorbitic or a sorbo-pearlitic structure in an iron casting which would otherwise have a pearlitic or a ferritic or a ferro-pearlitic structure without this invention and under the processes utilized in standard practice.

Another object is the provision for obtaining asorbitic-troostitic or an all pearlitic structure in an iron casting which otherwise would exhibit primary or secondary ferrite when produced under the processes in standard practice.

Another object is the provision for inhibiting the formation of free ferrite which may otherwise form under the processes utilized in standard practice.

Another object is the provision for eliminating free ferrite of poor wearing property from an iron casting. Y

Another object is the provision for inhibiting or arresting the normal structural development of cast iron as would take place under the processes in standard practice.

Another object is the provision for obtaining cast iron having an ultimate structure which would otherwise be a transitional structure of' the iron produced under the processes in standard practice.

Another object is the provision for fixing the structural condition of cast iron at a desired stage of development. I

Another object is the provision for imparting wear resistance and toughness to an iron casting.

A still further objectis the provision for obtaining cast iron which possesses superior wear resisting properties and which is at the same time machinable.

Other objects and a fuller understanding of our invention will become apparent from the description herein taken in conjunction with the accompanying drawings, in which:

Figure 1 is a diagram illustrating a typical time-temperature cooling curve of a ferrous casting in a mold.

Figure 2 is a photomicrograph showing a sample of metal in a casting made according to our invention.

Figure 3 is a photomicrograph showing another sample of metal in a casting made according to our invention.

Figure 4 is a photomicrograph showing a sample of metal in a casting left in the mold until a modified transformation of the metal was completed.

Figure 5 is a photomicrograph showing a sample of metal of modified composition, as found in a casting shaken out at one temperature, and

Figure 6 is a photomicrograph showing a sample of metal 01' the same modified composition, as found in a casting shaken out at a lower temperature. The photomicrograph of Figures 2 to 6, inclusive, furnished with this application were made from magnifications of 1000 X, which were reduced in size one to two and a. half, to give a final magnification of 400 X.

The iron casting upon which the diagram of Figure l is based and which is here given by way of example, was made of a cupola charge in the following proportions:

Per cent Pig iron 10 Silvery 4.4 Spiegel l Bought and return scrap 49.6 Steel 35 to which Nickel 1.5

Chromium .3

Molybdenum .6

has been added to the molten metal in the ladle The molten metal of this composition was poured into Diesel engine cylinder liners of intricate design. The removal of the resultant casting from the mold, the timing of the removal and the cooling of the removed casting is the subject matter of this disclosure.

In the diagram of Figure 1 the ordinate represents the degrees Fahrenheit temperature of the metal in the casting. The abscissa represents the time in minutes during which the casting remains in the mold. The line of the descending curve A to U represents the normal cooling curve in standard practice of the described iron casting within the mold, the full extension of the curve to the right being upon the basis of the casting remaining in the mold until the casting is at or near room temperature. This line AU may be referred to as the normal cooling curve as it shows in its extent the relative value in foundry practice prior to our invention and under standard practice.

The following points and ranges marked along the descending curve denote stages in the microstructural development in the particular iron casting used as an example:

Point A indicates the temperature of the metal of the casting at the time that the mold was filled with liquid metal, that is, the temperature immediately after pouring. In the example of iron here given point A is about 2550 F.

Point B indicates the temperature at the start of incipient solidification. In the example here given point B is about 2280" F.

Point C indicates the main temperature where incipient solidification ends.

C to D indicates the temperature of main solidification and is the temperature range in which takes place the main evolution of the latent heat of solidification. The temperature remains substantially at the same temperature degree until point D is reached. In the example here given points C to D are about 2110 F., the temperature at D being just below 2110 F.

In compositions containing phosphorus point F would indicate the solidification temperature of steadite, which consists of phosphide of iron and a separated solution of FeaP in iron. This substance occurs by volume about ten times that of the phosphorus content by weight and its solidification temperature approximates 1740 to 1760 F. Steadite in gray iron is composed largely of a binary cellular eutectic of iron and iron phosphide. It may therefore be said that point F is the temperature of solidification of the phosphide eutectic in ferrous castings containing phosphorus. In the diagram of Figure 1, point F is indicated to be located at about 1752 F.

Following the temperature range C--D, there is a series 01' changes in which the eutectic carbides break up into graphite and austenite. At the higher temperatures preceding point D the metal containing less than 4.3% carbon is a solid solution of cementite in high temperature or gamma iron and is called austenlte plus a liquid magma of carbon dissolved in iron. Metal containing more than 4.3% carbon is a network of cementite in a magma of carbon dissolved in iron. Depending on the composition of the metal the total structure will consist of one of the three combinations (1) austenite and eutectic austermite-cementite alloy; (2) eutectic austenitecementite alloy 0r ledeburite; or (3) cementite alloy, respectively, according to whether the alloy had less, just the right amount, or more, carbon than the eutectoid proportion.

J, on the curve AU indicates the general location of the temperature range in which occurs the eutectoid crystallization of the solid solution of cementite (FeaC) in iron (the said solid solution being called austenite) to form the final eutectoid of cementite and ferrite called pearlite. J may be referred to as 'the carbide critical range and is the temperature of the range where the carbide as pearlite or sorbite is deposited from the solid solution of cementite in iron (austenite) to form the'final eutectoid of cementite and ferrite called pearlite. understood that the eutectoid temperature is not a precise or exact temperature but is a range which may spread more or less than 100F. The location and breadth of this range varies with the composition ofthe metal and addition of certain alloys such as nickel, silicon, manganese or the like causes a shift in the location of this range.

According to the composition of the metal under consideration, the metal in the temperature ranges below and following the eutectoid point J may be comprised of ferrite and pearlite, all pearlite, or cementite, or pearlite with free cementite. There may thus be found in the metal inthe temperature ranges following the eutectoid temperature J (in combinations and proportions dependent upon the composition of the metal), the constituents of cementite: cementite in conjunction with pearlite; or all sorbite or pearlite; or pearlite in conjunction with patches of free ferrite. The proportion of carbon in the pearlitic form of metal is approximately .80% to .89%.

The carbon in cast iron also appears in free form as well as in the combined form (such as combined in austenite, cementite, or pearlite). Under certain conditions and in varying ironcarbon proportions and under the influence of diflerent graphitizing agents, some carbon will become dissociated from the iron and is precipitated out as graphite.

The principal transition in the microstructure of the 'metal during the downward cooling curve following the temperature CD is the progressive change of austenite to cementite or carbide and pearlite, or to pearlite, or to pearlite and ferrite. There are many other modified and transitional stages in the microstructure of the metal during the downward temperature curve dependent upon the character of the charge of materials melted and the chemical composition of the metal under consideration. There are many well known names (such as austenite, martensite,

troostite, sorbite, and pearlite or combinations of these) which have been given to these modified forms of structure .between austenite and pearlite and to mixtures containing them.

Inasmuch as the physical properties of the ulti mately produced iron casting and its ability to meet particular service requirements depend upon the internal microstructure of the cast iron, the selection and control of the ultimate structural form is most important. in foundry practice. In the processes of standard practicev and without our invention, the casting is left in the mold until the casting is cooled appreciably below the critical range of temperature denoted as being in the general location of J and until the casting has cooled to substantially room temperature. Below this critical temperature range the metal has substantially completed the transitional structural development preceding and within'the eutectoid temperature range and becomes converted to the ultimate forms of pearlite, or pearlite and free ferrite, or pearlite and free carbide, depending upon the nature and composition of the cast iron and upon the design and form of the casting under consideration. The conversion to the said ultimate-form of free ferrite is particularly dependent upon the time that the cast- It is to be ing remains in, and is subject to the influence of, the mold.

When castings are poured the temperature of the mold or the molding sand and c'oresandim- 'mediately adjacent the casting may remain above the temperature of iron, particularly the sand of the core, and may retain the heat longer than the iron alone would. This retained heat may cause a breakdown of the metal structure through the austenitic and sorbo-pearlitic stages to the point where free ferrite is formed. The presence of such free ferrite provides poor wearing properties and reduces the physical strength characteristic of the casting. It is desirable that the breakdown into ferrite in the casting or in any portion of the casting be prevented and this may be done by an arresting or inhibiting of the normal and abnormal structural changes otherwise occurring.

Some of the transitional forms of iron and particularly austenite, troostite and sorbite, and

as to what occurs in the running in or cold working" operation it is known that the retention of some of the transitional forms of the iron such as austenite, troostite and sorbite is highly desirable for wear resisting service.

In order to meet the problem of machining the casting, the castings should not be made' too hard and tough. To obtain castings of required hardness and toughness and yet machinable a balance is reached by selecting and obtaining the transitional stage of structural form best meeting all requirements, as for example, cast iron containing in proper proportions austenite and troostite, or troostite-sorbite or sorbite and fine lamellar pearlite or fine lamellar pearlite free from any patches of ferrite.

In the practice of our invention the iron casting is shaken out or removed from the mold at any early stage and before the normal structural development of the metal has been completed. The time at which the casting is shaken out after pouring depends upon the predetermined microstructure desired for a particular use, that is, the time depends upon the transitional stage or structural development which it is desired to secure in the ultimate casting. In all cases the casting is shaken out of the mold while still hot and the shaking out is at least commenced, and preferably completed, before the casting has passed or cooled appreciably below the eutectoid temperature J.

In some cases it is most suitable for an intended purpose to shake the casting out of the mold while at a high temperature on the normal cooling curve and as close to point D as possible. In other cases, it is most suitable for an intended purpose to shake the casting out of the mold at or in the neighborhood of an intermediate temperature G. For other results, it is preferable to shake the casting out of the mold within the critical range but before the eutectoid solidification has been completed. In other cases, it is most suitable for an intended purpose to shake the casting out oi the mold at a temperature just above and preceding the carbide critical temperature J. In other instances other temperatures and time periods along the cooling curve, such as for example, at point D, at point E, or at point H, may be best suited for the separation of the casting from the mold to obtain the predetermined microstructure desired in the metal.

The operation of shaking the casting from the mold is not instantaneous and a lapse of some time takes place between the beginning and the completion of the shaking out operation, depending on the size and character of the casting. The time required for the removal of the casting from the mold is represented by a short length or section of the normal cooling curve between the commencement and the completion of the shaking out operation.

After removal of the casting it is cooled by exposure to the air at a much more rapid rate than it would have normally cooled in the mold.

In other words, a new cooling curve is established for the casting after removal and this new cooling curve is entirely independent of the insulating and heat retaining influence of the mold. The new cooling rate being relatively rapid tends to secure the structure of the metal in the transitional stage at which the casting was removed from the mold and to arrest further structural development in the metal.

The rate of the new cooling curve after separation of the casting from the mold may be adjusted or regulated. In some cases we expose the removed casting to atmospheric air to cool it and the new cooling curve is thus set at one rate. After removal of the casting from the mold the new and independent cooling curve for the casting may be regulated in a number of ways. To retard the new cooling rate (but of course not retarded to the slow rate of the normal cooling curve of the casting in the mold), the casting may be sheltered by suitable means. To accelerate the new cooling rate, an air blast may be blown upon the casting. When still a more rapid cooling rate is desired we subject the casting to a refrigerated atmosphere or to a spray of water or other cooling liquid. By such means a line regulation of the new cooling curve may be obtained, the new cooling curve being entirely distinct and independent of the normal cooling curve of a casting remaining in the mold as in standard practice. The more rapid the rate of the new cooling curve, that is, the more it departs from the normal cooling curve of standard practice, the more of the early transitional forms of the metal are retained and the later transitional forms inhibited. To obtain just the desired mixture or proportion of early forms and later forms, the rate of the new cooling curve is adjusted.

The arresting or inhibiting of the formation of either primary or secondary free ferrite may be brought about by this new cooling curve adjusted to requirements after the separation of casting and mold. By the use of this controlled cooling the entire casting or portion thereof as would otherwise be converted into ferrite may be retained in any desired structural form, as for example, austenite, martensite, sorbite or line lamellar pearlite. Such structural forms provide r y running in quality so as to assure superior wear resistance and provides for "cold working somewhat similar to that found in heat-treated carbon steels, alloy steels and iron. Castings thus produced have superior and improved wear resistance properties over those produced by the processes in standard practice.

In the practice of our invention castings are shaken out of the mold and exposed to atmospheric air in one form or another, such as still air or an air blast. This operation is performed in the light of the discovery and vteachings here set forth to predeterminately obtain the desired results in the service of the casting.

Examples of the practice of our invention are illustrated in Figure 1 in which is demonstrated sample cases of removal of the castings and the subsequent cooling in air. In the first sample case, point K indicates the time and the temperature oi. the example casting when stripping or shaking out was commenced and point L indicates the time and temperature when the separation of the casting and mold was completed. The line K-L, M indicates the new cooling curve or cooling range of the casting after separation from the mold. The photomicrograph of Figure 2 illustrates the ultimate structural form of the metal in our sample casting after being shaken out and cooled according to the line K-L, M, that is, when shaken out about 20 minutes after pouring of the casting and then cooled in still air. The resultant structure is shown as being composed of austenite, martensite, sorbite and graphite.

Upon the swinging of the new cooling curve from point M along the broken line to point V by the use of an air blast on the casting or other suitable means, then more austenite is retained. Upon the swinging of the new cooling curve from point M along the broken line to point Z by the use of sheltered cooling then less austenite is retained.

In a second sample case, point N indicates another time and temperature when the breaking of the mold was commenced and 0 indicates the point where stripping was completed. The line N-O, P indicates the new cooling curve of the casting exposed to the air. The photomicrograph of Figure 3 illustrates the ultimate structural form of the metal in our sample casting after being shaken out and cooled according to the line N0, P, that is, when shaken out about one hour after pouring of the casting and then cooled in still air. The resultant structure ,is shown as containing austenite, martensite,

sorbite, sorbo-pearlite, pearlite and graphite in proportions differing from the 20 minute shakeout of line K-L, M, and particularly in that a. lesser amount of austenite and martensite is present. The new cooling curve N-O, P may be varied by swinging point P to point X and the resulting structure is modified to some extent.

We also show the result of shaking the casting out of the mold at the commencement of the eutectoid temperature J, he stripping of the mold being completed soon after the eutectoid temperature. In this instanc.-, R, in Figure 1. represents the approximate temperature when the breaking of the mold was commenced and 8 represents the approximate temperature when stripping was completed. The line R-S, T represents the new cooling curve of the removed casting. The photomicrograph of Figure 4 shows the earlitic structure resulting from leaving the example casting in the mold 2 hours and live I minutes before stripping. The casting in this instance has a microstructure resulting from leaving in the mold until the decomposition or transformation of the austenite has been completed. The microstructure shown does not exhibit free patches of ferrite which might otherwithdrawing the castings from the molds at selecterl temperatures and within certain time limits prior to the passing. of the critical stages and by cooling the casting or a portion of the cast- -ing out of the mold at proper cooling rates to retain the desired type of structure. By this control the resulting hardness of the casting may be increased as required, the retention of austenite, martensite, troostite, sorbite and pearlite, and the desired proportions thereof, greatly eifecting the physical properties of the casting and its use in service. The method here disclosed eliminates the use of subsequent heat treatment, gives better control of results, and provides a means of obtaining results not heretofore obtainable.

Variations and modifications are of course suggested by the present disclosure. For example. the casting might be shaken out at point E to produce one structural composition, shaken out at point G to produce another structural composition, or shaken out at point B to produce still another structural composition. The design, size and weight of the casting are variable factors which are to be taken into consideration in timing the shaking out operation to obtain the desired microstructure in the ultimate metal obtained. Likewise, the character of the molten metal may be varied to fill requirements. For example, the following charge may be used for the casting here discussed:

The metal of this modified composition was poured into molds in the usual manner and after an interval of time decided on by the type of casting, the section of casting poured and the composition of the metal, the castings were shaken out of the mold. Figure is a photomicrograph of a casting containing metal of this modified composition when shaken out at a temperature above the critical range and cooled at an independent and controlled rate. By way of comparison, Figure 6, being a photomicrograph of a casting of this same modified composition after being left in the mold until after the critical range had been completely passed and decomposition of some of the metal into free ferrite had taken place. The patches of free ferrite resulting from leaving the casting in the mold until the break-down into ferrite had commenced is apparent in the photomicrograph of Figure 6. v

Another sample composition which may well be used in making castings for carrying out our invention, is made of a charge of:

Per cent Pig iron 25 Cast iron scrap 50 Steel 25 and having a chemical analysis of:

. Per cent Silicon 1.6 Manganese .9 Phosphorus v .2 Total carbon 3.25

Although alloys were shown as added to the that the charge need not include the alloys. The eiiect of different alloys in shifting the critical range of the metal in the casting is well known. Our invention is practiced with good results regardless of the efiect of these alloys upon the critical range although the shifts in the critical range are taken into account in determining the timing of the shake-out of the casting at required temperature.

However, the governing principles of our invention remain the same in the modifications of the specific values of time and temperature given in our present disclosure, such as in variations in the chemical content of the metal or in changes of other factors.

The disclosure herein includes the processes described in the appended claims and suggested in the accompanying drawings, which processes are incorporated in, and made a part of, this patent specification. I

Although we have described our invention with a certain degree of particularity, it-is understood that the present disclosure has been made only by way of example and that numerous changes in the details of the process, modifications in the steps undertaken, variations in the materials" used, and diiferent values of time and temperature, may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

We claim as our invention:

1. The improved method of producing iron castings formed in the ultimate shape of use in service and possessed of desired physical wear resisting properties resulting from securement of austenite in the iron at room temperature, comprising the steps of pouring into a foundry mold having a cavity of said ultimate shape molten iron having a total carbon-silicon content of from about 4.06% to about 4.85%, of which percentages about 1.10% to about 1.60% comprises silicon, to inherently produce thereby iron having a volume shrinkage characteristic on cooling of 2% or more and having a microstructure progressively changing from austenite to pearlite during normal cooling of the casting in the mold; separating the mold and the formed casting after solidification but before the casting has cooled in the mold to below the eutectoid critical range; and immediately cooling the separated casting in atmosphere through the eutectoid critical range to approximate room. temperature at a controlled rate greatly accelerated from the normal cooling rate of such a casting remaining in the mold to thereby arrest the normal change of'austenite to pearlite for securing austenite in the said ultimate casting at room temperature.

2. The improved method of obtaining the tough wear resisting quality of austenite in an iron casting at temperatures of service for the casting, comprising the steps of: providing molten iron having a total carbon-silicon content of from about 4.06% to about 4.85%, about onefourth to one-third of said carbon-silicon content being comprised of silicon; pouring said molten iron into a foundry mold having a cavity approximating the shape of the casting for use in service; shaking the casting out of the mold after main solidification of the said iron but while still at an elevated temperature produced solely by the latent heat of the iron and prior to the cooling of the casting in the mold through the carbide critical range; immediately cooling the casting in atmosphere to accelerate the cooling of the casting through the carbide critical range to inhibit the decomposition of austenite as oc-' curring in such a casting slowly cooled in the mold through the carbide critical range; and controlling the rate of said accelerated cooling to obtain the desired degree of retention of austenite in the casting at temperatures below the carbide critical range in which the casting is used in service.

3. The improved method of producing iron castings formed in the ultimate shape of use in service and possessed of desired physical properties resulting from securement of austenite in the iron at room temperature, comprising the steps of: Pouringinto a foundry mold having a cavity of said ultimate shape molten iron having chemical proportions in substantially the neighborhood of the following values: 1.46% silicon, .85% manganese, 3.14% total carbon, 1.27% nickel, .42% chromium, 155% molybdenum, to inherently produce thereby iron having a volume shrinkage characteristic on cooling in the neighborhood of 2% or more and having a microstructure progressively changing from austenite to pearlite during normal cooling of the casting in the mold; separating the mold and the formed casting after solidification but before the casting has cooled in the mold to below the eutectoid critical range; and immediately cooling the sepa-- rated casting in atmosphere through the eutectoid critical range to approximate room temperature at a controlled rate greatly accelerated from the normal cooling rate of such a casting remaining in the mold to thereby arrest the normal change of austenite to pearlite for securing austenite in the said ultimate casting at room temperature.

4. The improved method of producing iron castings formed in the ultimate shape of use in service and possessed of desired physical properties resulting from securement of austenite in the iron at room temperature, comprising the steps of: pouring into a foundry mold having a cavity of said ultimate shape molten iron having chemical proportions in substantially the neighborhood of the following values; 1.10% silicon, 1.05% manganese, 2.96% total carbon, .12% phosphorus, .078% sulphur, to inherently produce therebyiron having a yolume shrinkage characteristic on cooling in the neighborhood oi 2% or more and having a microstructure progressively changing from austenite to pearlite during normal cooling of the casting in the mold; separating the mold and the formed casting after solidification but before the casting has cooled in the mold to below the eutectoid critical range; and immediately cooling the separated casting in atmosphere through the eutectoid critical range to approximate room temperature at a controlled rate greatly accelerated from the normal cooling rate of such a casting remaining in the mold to thereby arrest the normal change of austenite to pearlite for securing austenite in the said ultimate casting at room temperature.

5. The improved method of producing iron castings formed in the ultimate shape of use in service and possessed of desired physical properties resulting from securement of austenite in the iron at room temperature, comprising the steps of: pouring into a foundry mold having a cavity of said ultimate shape molten iron having chemical proportions in substantially the neighborhood oi the following values; 1.6% silicon,

.9% manganese, 3.25% total carbon, and 3% phosphorus, to inherently produce thereby iron having a volume shrinkage characteristic on cooling in the neighborhood of 2% or more and having a microstructure progressively changing from austenite to pearlite during normal cooling of the casting in the mold; separating the mold and the formed casting after solidification but before the casting has cooled in the mold to below the eutectoid critical range; and immediately cooling the separated casting in atmosphere through the eutectoid critical range to approximate room temperature at a controlled rate greatly accelerated from the normal cooling rate of such a casting remaining in the mold to thereby arrest the normal change of austenite to pearlite for securing austenite in the said ultimate casting at room temperature.

HERBERT A. REECE. OLIVER SMALLEY.

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