US1980578A - Art of treating cast iron castings - Google Patents

Description

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lE STATE was PATENT QFFIE ART OF TREATING CAST IRON CASTINGS N0 Drawing. Application February 10, 1922,

,Serial No. 535,652.

17 Claims.

This invention relates to the treating of cast iron.

Cast iron is an alloy of iron with various other elements, such as carbon, silicon, sulphur, manganese, phosphorus, etc. The composition and structure, however, vary depending upon the conditions during casting and also upon the mixtures employed.

White cast iron, whether produced by casting a white iron mixture'or by chilling during casting, has a low tensile strength, and its elastic limit is close to and practically coincides with its ultimate breaking strength. Moreover, since the carbon is almost wholly in the combined form and in solution, it is so hard, that it cannot be machined. It is, therefore, useful only where its bulk can be suflicient to withstand the stresses and strains or where a hard chilled surface is, desired.

Grey iron is produced by permitting sufliciently slow cooling of the molten metal to cause precipitation of the carbon in the form of graphite; the latter, however, forms veins or plates which, while they may result in a comparatively soft casting, nevertheless reduce its strength considerably; in fact, its tensile strength is comparatively small and its elastic limit is close to its ultimate breaking strength. In the making of small grey iron castings, the cooling is frequently so sudden that the casting produced is in part chilled and, therefore, brittle so that it cannot withstand strains. Moreover, parts thereof may be so hard in spots, on account of the presence of combined carbon which has not completely precipitated from the solution, as to render its machining difiicult. Grey iron castings, especially where small, require in most cases practically the exclusive use of high grade pig iron, which renders them rather costly.

Scrap iron is available and can be readily cast, but such castings are usually found hard when the proportion of scrap iron in the mixture is high or even medium. Whenscrap iron is exclusively employed, the casting, especially if small, is generally so hard that it cannot be machined at all and moreover has, in such a case, a low tensile strength. Accordingly, scrap iron cannot be economically employed by itself and the proportion of scrap iron in a mixture must be kept low, resulting in high cost.

Malleable cast iron is obtained by subjecting white iron to a decarbonizing action. There is thereby formed, however, only a thin skin of decarbonized metal, while the core remains in Renewed December 15,

practically its original brittle condition. While, therefore, malleable cast iron hasa higher tensile strength than white or grey iron, it cannot be machined as this destroys the protecting skin giving its strength. Even where thetreatment is extended for a long period so as to penetrate the casting, it nevertheless has practically no elasticity or toughness, which renders it unfit where the casting is subjected to. bending or torsion stresses or strains. Moreover since malleable cast iron is soft, it cannot be used with success where toughness and hardness are required. The production of malleable cast iron moreover requires a treatment of a considerable period, extending over days, which not only requires a treating plant of large capacity, but the casting produced will be frequently blistered. Steel castings are not only expensive but difficult to cast, especially when small, on account of the difiiculty of attaining a sufliciently high temperature to obtain a perfect casting. Such castings are moreover not only difiicult to machine, but are frequently hard so as to prevent their use where a bearing surface is required.

Cast iron has, however, the favorable characteristic of a low melting point, enabling it to be readily cast to small and intricate shapes and it is, therefore, in every way desirable. At best, however, it has a low tensilev strength and has practically no elasticity or toughness. It moreover has a low magnetic permeability, rendering its'use for electrical machinery limited, especially where low weight is a requisite. The result is, therefore, that cast iron castings, especially where they are to be machined, or used to provide a wearing surface, must be originally cast large and then machined down to size, which obviously is not economical from a manufacturing standpoint. To produce soft castings, or those having the desired characteristics, requires not only expert foundrymen, but even then the resultant product is'usually so uncertain that the loss on account of this uncertainty is comparatively great which, of course, must be considered in the cost of any good castings. It is to be noted, however, that it is not difl'icult at all to provide a hard casting, but rather the reverse, so that nearly all of the foundrymens troubles are in avoiding hard spots and hard castings. The result is, therefore, that small castings, especially those -that must be machined, are made of moreex'ensive alloys, such as brass, aluminum, etc. I

One of the objects of this ntion, therefore, is to develop a cast iron casting which, while rebreak into a number of pieces. strength of such a casting is, therefore, very low,

taining all of the advantageous features of normal grey iron, obviates its disadvantages and has additional distinctive advantageous characteristics.

Another object of this invention is to develop a casting which has high tensile and compressive strength, high elasticity and toughness, is close grained and has high magnetic permeability.

Another object of this invention is to provide a process of heat treatment whereby the physical characteristics of the casting are improved.

Another object of this invention is to provide a process of heat treatment whereby hardness in whole or in part of cast iron castings is obviated, so that unusual care need not be taken in avoiding such hardness, whereby the final characteristics of castings are really independent of the condition of casting and of the skill of the foundryman.

Another object of this invention is to provide a process and develop a casting, whereby scrap iron may be utilized to produce a high grade castinghaying all of the desirable characteristics.

Another object of this invention is to develop a cast iron casting which is close grained, but the softness or hardness of which is susceptible of control to meet requirements.

Further objects will appear from the detailed description and a number of examples illustrating different embodiments of this invention.

Taking as a first example, a chilled casting having a weight of about two ounces and a section of by inch, which is cast with a chill by being cast in contact with a sheet of iron,

of such thickness as to form a smooth surface, but still prevent adherence of the metal thereto; it will be found that such a casting is so hard that it cannot be touched with a file, and is so brittle that if it is dropped on the floor, it will The tensile and it is really unfit for commercial use.

The analysis of a representative casting showed .0% of uncombined carbon, 3.45% combined carbon, 2.31% silicon, 040% sulphur, ,.75% manganese, .73% phosphorus, which is the chemical composition of an ordinary grey ironmixture,

but on account of the chill has produced a white iron casting. The micro-structure of this sam-,

ple shows that the iron is entirely in the martensite condition; that is? to say, it is a distinct entity as a result of fcementite being in complete solid solution in beta iron.

This casting is then packed in a menstruum of cast iron borings, containing about 1% of cop- I per or copper alloy (such as bronze) turnings or of the menstruum, as distinguished from iron used as a cutting tool, such as a hatchet, or as a shavings, which are distributed throughout the mass. Cast iron borings are used for the iron part oxide or iron ores'. In practice the castings are packed in suitable receptacles, such as cast iron boxes, so that they are entirely surrounded by the menstruum. These boxes are then placed in aa period of about ten hours.

, copper alloy turnings or shavings.

It is found that the heat treated casting is highly elastic, tough and has a high tensile strength. It can readily be bent at an angle of 45 and can be twisted a quarter of a turn in one inch. It is, however, so soft that it can be readily machined and provides a very eflicient bearing surface. For instance, when used as a piston ring it will not score the cylinder, but will take a high polish, and is so tough that it can be bent, twisted and stretched considerably without breakage. At the same time, it is highly elastic as it has considerable spring before taking a permanent set. The tensile strength is over 50,000 pounds per square inch. The magnetic permeability has been increased over 50% above that of normal cast iron.

The chemical composition of the heat treated casting shows as follows: Uncombined carbon 3.51%, combined carbon .06%, silicon 2.24%, sulphur .05l%, manganese .75%, phosphorus .75%. The micro-structure shows finely grained temper carbon in a matrix of ferrite, with a small proportion of cementite in a transition stage of partial solution. The result seems to be an almost complete precipitation of carbon with a slight resolution of the same. Practically it. would indicate an arrested anneal with some slight cementation caused thereby or absorbed from the heat treating menstruum or packing. It will, however, be noted that the treatmenthas extended through the casting, as the structure is uniform throughout and is not limited to the skin effect usually found in malleableized castings.

Another example illustrating another embodiment of this invention involves the treatment of a white iron casting of one pound weight and of varying cross section, such as an ordinary hatchet, which is cast using the same mixture and the same procedure as in the production of white iron castings intended to be malleableized. The fracture of the original casting shows the original white iron structure, being brittle and extremely hard. The treatment of this casting is'performed as previously described, except that the temperature (furnace temperature) is raised over a period of two to three hours to from 1800 to 1900" F. and held at that temperature for a soaking period of about one hour, it being found that better results are obtained with a white iron 'mixture and a heaviencasting if the final temperature is incerased. The casting is, however, similarly packed using a similar menstruum of cast iron borings with-copper or The charge is moreover allowed to cool down in the furnace in the same manner as previously described, that is, over a period of about ten hours.

When the treatment is at the temperature arid period just described, it is found that the casting is sufficiently hard and tough to enable it to be striking tool, such as a hammer. It can be 'she rpened and will form a keen edge, and as the it can be subjected to the twisting strains, to

which hatchets and like tools are ordinarily subjected, without danger of breakage. If the temperature is maintained for a longer period,

3.58% graphite, .os% combined carbon, 2.11%

than as described, the casting will become softer; accordingly by intelligent control of the soaking period, a casting of the desired hardness or softness can be obtained.

This process can be used to advantage in the production of dies (such as cutting dies, forming dies, drop forge dies) drill bushings,- and other machine parts required to be hardened. The casting is in this case made of white iron in the same manner as just previously described, using the usual white iron mixture and producing the usual white iron casting intended for malleableizing. As an example, a white iron casting of ten pounds weight having a diameter of about three inches, is subjected to the heat treatment just previously described. The soaking can, however, be continued if desired, until the required softness has been obtained. The casting is then soft and canbe readily machined to the desired form. The article can then be hardened and tempered, using the same process of tempering as is usedwith tool steel. It is found-that a polished surface of this casing has the appearance closely similar to that of tool steel, while it will produce the same characteristic spark (exploded spark) as is found in tool steel. The casting is exceedingly close and uniformly grained and will stand wear, shocks and strains and give service closely approaching that of tool steel. Its advantages of .cheapness, its adaptation of being cast close to its final shape, and the ease with which it can be machinedare, of course, obvious.

Another example illustrates the application of this process to the treatment of grey cast iron A small casting having a cross section of 3 2 by 1 inch is cast in a sand mold and has a weight of about 8 ounces. The usual grey iron mixture is used and its chemical composition is as follows: Graphite 3.14%, combined carbon .52%, silicon 2.08%,' sulphur 043%, manganese-.98%, phosphorus .62%, this being the usual analysis of a good quality sand cast grey iron. The microstructure shows large flakes of graphite in a matrix of perlite and ferrite. In other words, it shows the ordinary micro-constituents of grey iron. On account of the thinness of the casting, however, it has hard spots.

These castings are packed in amenstruum of cast iron borings and copper or copperalloy turnin'gs', as previously described, and heat treated as previously described, except that the temperature is increased from normal to between 1600 and 1700 F. over a period of two to three hours, and the furnace is then kept at that temperature for a period of about one hour, as it is found that the temperature required for the proper heat treatment of grey iron castings need not be carried as high as for chilled or white iron castings. The charge is moreover allowed to cool down in the furnace in the same manner as previously described, that is, over a period of about ten hours.

It is found that this treatment has improved the grey iron considerably; not only has it absolutely eliminated all hard spots, but it is tougher and more readily machined than before treatment. Although its tensile strength has not been increased to the same extent as in the heat treated white iron castings, its elasticity and toughness have been considerably increased and it can be subjected to-considerably greater bending and twisting strains, althoughnot as much as can heat treated white iron castings.

The chemical analysis after treatment shows silicon, 073% sulphur, .95% manganese and .060%' phosphorus. The micro-structure shows partial transition of the graphitic carbon into the amorphous form or temper carbon in a matrix of ferrite, but also partial transformation of the cementite to finely grained temper carbon. Moreover the areas of the temper carbon are somewhat extended, thereby indicating either a conversion of the graphitic carbon into temper carbon, or an arrested anneal with some slight cementation caused thereby or absorbed ,from the heat treating menstruum or packing. At any rate, while the improved characteristic features are not as distinct as in the heat treated white iron casting; nevertheless the desirable characteristics have been considerably enhanced. The magnetic permeability has also been considerably increased over that of normal grey iron.

Another example shows the application of this process to the treatment of castings made wholly from scrap iron. The casting under consideration is a small motor frame having varying being melted in a crucible and the metal being poured into a sand mold. The casting is so hard that it cannot be machined at all and its magnetic permeability is low.

The heat treatment of this casting is carried out as previously described in the treatment of the small chilled casting, the same menstruum or packing being used, and the treatment being extended over a rising period from two to three hours to 1700 F., and the furnace temperature being then maintained at 1700 F. The casting is then cooled in its packing and in the furnace for the same period of about ten hours.

It is found that thisjtreatment has radically changed the scrap iron casting. Notonly has the iron become softened but all hard spots are eliminated so that it may be readily machined. The metal is close grained, and theheat treatment has extended uniformly so as to produce a uniform grained casting. It is also found that the magnetic permeability has been increased by over 50% as compared to that of ordinary grey iron. The tensile and breaking strength have. moreover been increased, and this is also true of the toughness and elasticity.

The advantages of enabling scrap iron to be noted that this approximates the intermediate critical point of iron (890 0.). Although in the treatment of grey iron and small chilled iron castings improved results have been obtained with the temperature as low as 1300 F.,

the best results are obtained .when the final temperature at which the "charge issoaked is 1700' F. Similarly, while it has been found that improved results can be obtained in the treatment of castings utilizing a whiteiron mixture of as much weight as one pound, with the final soaking temperature as low as 1600 F., the best results are obtained when this final soaking temperature is between 1800 and l900 F. In other words, it seems that the best results can be obtained when the final or soaking temcal point of iron.

While the best results are obtained when a menstruum or packing of grey iron borings, hav-.

ing a small percentage of copper or copper alloy turnings, is used, it has been found that the excellent results can be obtained if copper or copper alloy is used alone as packing, especially in the heat treatment of a casting made from a white iron mixture, as previously described. A copper or copper alloy packing is, however, more expensive and not as economical to utilize as one using a small proportion of copper. It is found in this connection that after treatment, there will frequently be deposited on the castings thin platings of copper, which is significant with the temperature employed at the intermediate critical point, where there is a thermoelectric charge. It is probable that the copper or copper alloy, especially when used in connection with the iron borings or turnings, acts as a catalyzer.

It has also been found that improved results can be obtained when a packing or menstruum of only grey iron borings is used. In such a case,

however, it is advisable to somewhat extend the soaking period. Better results are, however, obtained when copper or copper alloy turnings are used in connection with cast iron borings. It is found advisable to renew the cast iron bor ings periodically and 'in' practice, they should be renewed after having served for ten heats. It is also of advantage to use foreign cast iron borings, such' as borings the metal of which was not cast from the same cupola furnace as the casting to be treated. While improved results are obtained by using a packing of sand, especially when, copper turnings are distributed throughout the same, it is found that in such a case, the soaking period should be somewhat extended. However, such superior results are ob--.

' tained when using a menstruum of high. heat conductivity and more particularly grey iron borings with copper or copper alloyturnings.

It will be noted thatin accordance .with invention, the heat treating" isc'arrieu onfor'a comparatively short period, ,as distinguished from the long periodvof treatmentiwhich is employed in'the usual malleableizing process, as the treatment in accordance'with this invention is a matter of hours (the actual heating and soaking extending over only a few hours) while the malleableizing treatment extends over a period of days. Accordingly the treatment, as well white iron mixture and scrap iron castings show close grained structures, which are entirely different from the structure of malleable cast iron.

This'invention distinguishes from the simple annealing of grey iron, which has been heretofore attempted, mainly for the purpose of relieving the strains set up during casting, for in such annealing processes, the temperature is necessarily kept low and much below the intermediate critical point of iron. Attempts to carry the temperature higher have invariably resulted in a weakening of the iron in that it has caused complete precipitation of the combined carbon with the result that the final casting was considerably weaker than the original casting. In accordance with this invention, however, the treatment is such that weakening is not only avoided but the structure is actually strengthened, and its elasticity as well as its toughness and permeability have actually been increased. It is found that a grey iron casting heat treated in accordance with this invention as heretofore described can be bent through a considerable angle and can be twisted considerably. The physical characteristics of grey iron are, therefore, radically improved by the employment of the process embodying this invention.

As applied to castings employing wholly or mainly scrap iron as a casting metal, the improvements of the physical characteristics are marked for not only has the casting comparatively high tensile strength and elasticity, but it has been rendered soft so that it can be readily machined; in addition, its permeability has been enormously increased. The structure is close grained. In fact it closely resembles tool steel in its structure and spark. i

In accordance with this process, therefore, it is entirely practical to utilize castings employing wholly or mainly scrap iron as the casting metal to produce high grade castings, thereby enabling scrap iron to be used for purposes heretofore thought impossible.

While it is advantageous to allow the'casting to cool slowly for an extended period, while enveloped in its menstruum, it is not necessary that the cooling be down to normal or room temperature. After the packed casting has cooled down to ,about 700 F. it can then be removed from its enveloping menstruum and allowed to then cool further at random. Good results have been obtained by allowing cooling from the soaking temperature (1700-1900 F.) in three hours, while the casting is enveloped in its menstruum,

and then removing the castings, allowing it thereafter to cool at random.

- As previously described, in carrying out this process the fuel used is preferably an oil used in the usual oil burner, while the castings are packed and enclosed, although the enclosure is porous permitting entry to some extent of the products of combustion to the casting. It has been found that the use of an oil burner, which produces only a slight oxidizing atmosphere, is

superior to the employment of artificial gas,

a comparatively short period as distinguished from the long period of treatment which is employed in the usual malleableizing processm In the specific embodiments described in the specification, the heating period is from three to four hours; however, the heating period might be extended somewhat without detriment. In order, however, to be clearly distinguished from the malleableizing process, it is to be understood that the expression a comparatively short period contemplates the heating period not in excess of eight hours. The temperature range has been specified in the specification from 1600 to 1900 F. depending upon the character of the cast iron under treatment; this range of temperature is approximate the intermediate critical point of cast iron.

While certain theories have been advanced, for the purpose of explaining the scientific principles of the process embodying this invention; it is to be understood that they are not advanced as the only true or correct theories, or as even necessary for a disclosure of the process; accordingly it is to be understood that this invention is not to be limited to any theory or theories. It is further obvious that various changes may be made in details without departing from the spirit of this invention; it is, therefore, to be understood that this invention is not to be limited to the specific details described.

Having thus described the invention, what is claimed is:

1. In the art of heat treating white cast iron, the process comprising, heating the casting, While packed with a suitable menstruum, at a temperature and for a period sufficient to partially convert the cementite throughout the casting to temper carbon in a matrix of ferrite throughout the casting.

2. In the art of heat treating cast iron, the process comprising, heating the casting while, packed with a menstruum of cast iron, for a comparatively short period, approximate the intermediate critical point, in order to readjust the carbon content.

3. In the art of heat treating cast iron, the process comprising, heating the casting while packed with a menstruum of copper or copper alloy, for a comparatively short period, approximate the intermediate critical point, in order to readjust the carbon content.

4. In the art of heat treating cast iron, the process comprising, heating the casting while packed with a menstruum of cast iron and copper or copper alloy to a temperature approximate the intermediate critical point.

5. In the art of heat treating cast iron, the process comprising, heating the casting, while packed with a menstruum of cast iron and copper or copper alloy, by the action of an oil flame, in order to adjust the carbon content substantially as and for the purpose set forth.

6. In the art of heat treating cast iron, the process comprising, heating the casting while packed with a menstruum of cast iron and copper or copper alloy, for a comparatively short period, approximate the intermediate critical point.

7. In the art of heat treating cast iron, the process comprising, heating the casting while packed with a menstruum of unoxidized cast iron particles to a temperature approximate the intermediate critical point.

8. In the art of heat treating cast iron, the process comprising, heating the casting, while packed with a suitable menstruum, to-wit, granular cast iron, at a temperature, to-wit, 1600-' 1900" F. and for a period, to-wit, 2 to 4 hours, sufficient to cause an arrested anneal with cementation by absorption from the packing.

9. In the art of heat treating cast iron, the process comprising, heating the casting, while packed with a suitable menstruum, to-wit, granular cast iron, for a comparatively short period,

to-Wit, 2 to 4 hours, approximate the intermediate crtitcal point, substantially as and for the purpose set forth.

10. In the art of heat treating cast iron, the process comprising, heating the casting, while packed with a suitable menstruum, to-wit, granular cast iron, for a comparatively short period, to-wit, 2 to 4 hours, approximate the intermediate critical point, and allowing the casting to gradually cool over a period of 3 to 10 hours while so packed.

11. In the art of treating cast iron, the process comprising, heating the casting, while packed with a suitable menstruum, to-wit, granular cast iron, in a substantially non-oxidizing atmosphere, to a temperature approximate the intermediate critical point for a comparatively short period, to-wit,'2 to 4 hours, substantially as and for the purpose set forth.

12. In the art of making iron castings, employing wholly or mainly scrap iron as the casting metal, the process of heat treatment comprising, heating the casting, while packed with a suitable menstruum, to-wit, granular cast iron to a temperature approximate the intermediate critical point for a comparatively short period, to-wit, 2 to 4 hours, substantially as and for the purpose set forth.

13. In the art of heat treating cast iron castings, the process comprising, heating the casting while packed with a suitable menstruum, towit, granular cast iron, to approximate the intermediate critical point for a short period, to;- wit, 1 hour, maintaining that temperature for a short period, to-wit, 2 to 3 hours, and permitting the casting to cool in the packing for a comparatively short period, to-wit 3 to 10 hours.

14. In the art of heat treating grey iron castings, the process comprising, heating the casting while packed with a suitable menstruum, to-wit, granular cast iron, at a temperature of about 1600 to 1'700 F. for a comparatively short period, to-wit, 2 to 4 hours, and permitting the casting to cool while in the menstruum for a comparatively short period, to-wit, about 10 hours.

15. In the art of heat treating white iron castings, the process comprising, heating the casting while packed with a suitable menstruum, to-wit, granular cast iron, at a temperature of about 1800 to 1900 F. for a comparatively short period, to-wit, 2 to 4 hours, and permitting the casting to cool while in the menstruum for a comparatively short period, to-wit, about 10 hours.

16. In the art of treating castings made wholly or partially of scrap iron, the process comprising, heating the casting while packed with a suitable menstruum, to-wit, granular cast iron, at a temperature of about 1700" F. for a comparatively short period, to-wit, 2 to 4 hours, and permitting the casting to cool while in the menstruum for a comparatively short period, to-wit, about 10 hours.

1'7. A heat treated grey iron casting, the micro-structure of which shows throughout the casting the conversion of graphitic carbon to temper carbon in a matrix of ferrite with partial transformation of cementite to finely grained temper carbon and in which the combined carbon of hard spots has been reduced to a fraction of one per cent of the casting.

JOHN W. FORD.