US1915091A - Method of heat treating white iron castings - Google Patents

Description

June 20, 1933.

A. HAYES ET AL METHOD OF HEAT TREATING WHITE IRON CASTINGS Filed Oct. 30, 1930 Decurburized Surface Pearlibe Temper Carbon) Decarburizeol Surface Temper Carbon Ferrite gmnliom Anson Hagen Harry 0. Breaker- @Wza wafiwm;

Patented June 20, 1933 UNITED STATES J'PATENT OFFICE ANSON HAYES, 0F MIDDLETOWN, OHIO, AND HARRY O. BREAKER, OF BUFFALO, NEW YORK, ASSIGNORS TO INDUSTRIAL FURNACE CORPORATION, OF BUFFALO, NEW

YORK, A CORPORATION OF NEW YORK METHOD OF HEAT TREATING WHITE IRON GASTINGS Application filed October 30, 1930. Serial No. 492,329.

This invention relates to the heat treatment of white cast iron, and it has particular reference to a method for producing, in a uniform manner, malleable cast iron of high and predictable quality.

In the iron industry, the terms white cast iron and white pig iron are applied to iron which, in fracture, presents a silvery or white appearance, and which may be, by suitable heat treatment, rendered more ductile and malleable. The product" of the heat treatment, without fusion, of such material, is known as malleable cast iron, and it is characterized by its increasedmalleability and improvement in other physical prop erties over the materialas initially cast.

Commercialmalleable cast iron has heretofore been customarily made by the method which consists of packing a number of white iron castings in a pot, which castings are thencovered with scale, sand, or broken slag, and the pots placed in a coal, oil, or gas fired furnace wherein the heating operationsare conducted; or the castings may be placed uncovered. or unpacked in a mufile furnace and heated therein. After the pots are positioned in the furnace, or the castings placed in the mufile, heat is applied until the temperature reaches a value in the neighborhood of 1600 F., and the heat is then maintained at that value for an appreciable length of time, say fifty-five to sixty-five hours. The application of heat is then discontinued, the furnace is allowed to cool very slowly to about 1000 F., and then the furnace doors are opened and the charge is allowed to cool normally to about room temperature,

The resultant heat treated metal, or malleable cast iron, may have at best an elonga-- tion of about 10% to 25%, a yield point at about 33,000 to 37,000 pounds per square inch, and an ultimate strength of from 50,000 to 57,000 pounds per square inch. In broken section or fracture, there is revealed an outer rim or frame about say one-thirty-second of an inch in depth, of very hard white metal, pearlite, or of soft iron or ferrite, and an inner core of velvety black appearance due to free or temper carbon, which appearance has led to the appellation of black heart malleableiron for the product.

It has been recognized that the effect of the heat treatment of white cast iron is explainable by stating that the iron carbide or cementite, to which the white appearance of the white iron is due, is decomposed by the heat into its chemical constituents, iron and carbon, found subsequently in the malleable iron as ferrite and temper carbon, and that the change in the physical properties of the material is due to the chemical transition effected. Despite the condition that the heat treatment is designed to promulgate a simple chemical reaction, however, it has not been possible heretofore to conduct the operations in such manner as to simplify procedure or to operate with greater efficiency on produce improved satisfactory products on a commercial scale. This is due in part to the circumstance that there-are a number of 'factors which affect the progress of the reaction, and which have not heretofore been evaluated or correlated on a controllable basis. Thus, while it has heretofore been known that the chemical composition of the white iron had an effect on the progress of the desired reaction, it has not been known in the art how to determine the extent of this influence except in a general way. And it has also been accepted that such variable and indeterminate factors as pouring temperature of the white iron, thickness of casting section, etc., have played some role in the conduct of the heat treatment.

Be :ause of the uncertainty of these effects, or in the absence of methods of correlating and controlling them, the manufacture of malleable cast iron is dependent to a large degree upon personal skill, experience, and judgment, and the product of the process has, in the modern arts, but limited applicability. Under favorable conditions of operation, the complete. cycle of heat treatment may be carried out in five to ten days or more, a value which has been arrived at by test and try methods, and those skilled in the art look with considerable suspicion upon proposals for curtailing the time of heating or in otherwise simplifying procedure or increasing the overall efiiciency.

lVe have discovered, however, that it is possible to effect material improvements in operation which lead to the shortening of time of operation, increase in operating efficiency, and enhancement of the valuable properties of the product, and to do so on a practical or commercial scale with predictable results. lVe find that we can make malleable cast iron having properties comparable with the present day product in much less than five days, and that we may also by suitable variation of the details of operation, produce malleable cast iron which is characterized by an increase in tensile strength and bv the elimination of the frame or skin structure. These effects we obtain by a proper correlation and control of certain factors which we have found to be material in the promulgation of the heat treating and graphitizing action.

In setting forth the principles of our invention, we shall deal with white iron cast ings of normal composition, it being understood. however, that other compositions may also be treated according to the methods herein set forth. For the preparation of the white iron casting itself, a suitable melt may beprepared and poured into molds of desired size and shape. The resulting cast, of white iron, contains carbon, silicon, phosphorous. sulfur and manganese. In the making of products such as have heretofore been known as black heart malteable iron, these ingredients will lie between the following values: carbon, 1.75% to. 3.35%; silicon, 1.50% to 0.60% phosphorous less than 0.25%; sulfur up to 0.25%; and manganese less than 0.6 or 0.65% with the factor, manganese minus twice the sulfur, greater than 0.10 and less than 0.20.

Castings within this range of compositions maybe treated in such fashion as to cause a chemical change or decomposition within the casting, whereby free or temper carbon is formed from the cementite or iron carbide in the casting. This iron carbide occurs in two forms, as massive cementi-te and as pearlitic cementite. Since the optimum effectuation of this, graphiti zation or change is dependent upon several factors, such as the chemical composition, rate of solidification of the castings, (which may be evaluated in the foundry as section size), the temperature at which the casting is heated, time ofsuch heating, and the nature of the chemical composition of the gaseous envelope surrounding the casting, these various factors should be known and controlled according to the principles hereinafter illustrated, to ensure the best results in practice.

In the appended drawing, to which reference is hereinafter made:

Fig. 1 is a photomicrograph at a magnification of 80a,- of a completely decarburized casting as made by prior art processes.

Fig. 2 is a photomicrograph at a magnification'of 80a: of normal material as made in a pot furnace and showing partial decarburization.

Fig. 3 is a photomicrograph at a magnification of 8000,. of a product made by our process and showing no decarburization.

In the material represented in Fig. 1, the surface portion of the casting, from the edge to the dark area, is essentially ferrite, and it is devoid of free carbon due to the decarburizing action in the furnace. Below the ferrite rim is a layer of pearlite, while within the pearlite rim is the usualstructure of black heat malleable iron. The specimen reproduced in Fig. 2 typifies the results which have been obtained by carefully conducting prior art processes, and in which the rim is partially decarburized, the small black spots indicating retained carbon. As the core of the casting is approached, these spots become larger and more uniform in size. The material shown in Fig. 3 is illustrative of castings made according to the present invention, wherein the temper carbon in a completely annealed specimen is uniform from the core to the rim, both in particle size and distribution. The matrix is ferrite.

We have found that, for proper control essential to the production of castings of predictable properties, that the gaseous envelope surrounding the castings, during the heat treat-operations, should be of a non-oxidizing f and non-decarburizing nature, and, further,

that the composition of such atmosphere should be maintained substantially constant for any given temperature and pressure condition. In the prior art methods of annealing castings, considerable carbon was removed from the cast itself, during the heat treating operation as shown in Figs. 1 and 2. Such loss is objectionable, as it causes the formation of soft or mushy castings, or, if the loss is restricted to the surfaces layers of the castings, a frame or very hard skin of metal is obtained making machining difficult, and reducing the strength of the casting in proportion to the section. In the long times employed in the prior art processes, these factors were of some significance, but not one which we now recognize, namely, the carbon loss also adds to the uncertainty of determining the time required for graphitization. 190 This, as we believe and may explain, is due to some extent to the circumstance that a critical relationship exists between the chemical composition of the casting, and the time required to produce a given change at a given temperature. As carbon is lost from the casting, during the heat treatment, the time re- .quired to effect the desired change becomes indeterminate, and hence it is a practical impossibility to ascertain what treatment should 139 be given to the metal, in order to obtain the properties sought.

By effecting a proper control and maintenance of the gaseous envelope, we are able not only to prevent frame on the casting, but also to predetermine the treatment required for a given casting,'in order to obtain the desired end product. Castings made by our process, and as illustrated in Fig. 3, show uniform carbon content from the core to the surface; the loss to a depth of about of an inch being not more than 0.40%, while the total carbon loss is of the order of 0.20 7 We have been able, by effecting constancy in the gas envelope, to produce malleable iron containing 2.42% carbon from white iron containing 2.47% carbon, whereas, in the prior art methods with which we are familiar, the total carbon loss was in the neighborhood of 0.80%. We desire to attain therefore, not only an atmosphere which may be, at some given instant, non-oxidizing in character, but we also desire, for best results, to maintain this atmosphere without loss of the carbonin the casting itself. Our preferred atmosphere is maintainedfrom an extraneous source, and not at the expense of the casting.

In order to heat treat castings in such atmosphere, for practical considerations an air tight furnace, to which heat is supplied by transfer of electrical energy is indicated. \Ve have used to advantage a furnace such as shown in the patent to one of us, (Breaker, No. 1,694,964, Dec. 11, 1928), but it will be understood that the principles of the invention may be applied in other types of appa ratus suited to the ends in view.

The atmosphere which we use, and which is nonoxidizing and nondecarburizing, contains preferentially considerable quantities of carbon monoxide. The use of pure carbon monoxide presents some practical difi'culties in comemrcial operation, and we may therefore employ an atmosphere of mixed gases,

containing carbon monoxide, carbon dioxide,

and nitrogen. Lessthan 10 per cent of carbon dioxide is advantageous, and the ratio of carbon dioxide to total oxides of carbon should be less than 0.33. Free oxygen should not be present, quantities greater than 1.5% interfere with best results, while no free oxygen is better. The required atmosphere may be obtained by blowing carbon monoxide into the furnace, or some high carbon chips may be burned in the furnace at the beginning of the heat, and air tight'seals may be used to prevent any change in the atmosphere, once established, after operation begins. Products of combustion are not satisfactory sources of the desired atmosphere, as they may include the elements of water, which should be excluded.

A further consideration with respect to the gas atmosphere is the pressure, or rather, the

variation in pressure, of'the gas during operations. When castings are heated in sealed receptacles, such, for example, as pots, a certain gas pressure may be built up, some gas is eventually forced out, and some air invariably leaks in. When there is gas leakage or variation of this character, fine control of the operations is not attained for a number of successive heats. We also find that the rate of reaction is dependent upon the gas pressure; thus, the reaction will require a different time at live atmospheres than that reuired at normal pressure. To compensate for variations from this effect, we propose to maintain the gas at a substantially constant pressure during operation. The results herein expressed were obtained in a furnace in which a pressure of about one atmosphere -was maintained, and as the gas within the furnace expanded or contracted, provision was made to subtract from or to add to the mass of gas, so that the pressure variations at the pressure given was of the order of four inches of water. Such fine control of the pressure variations is not essential in all cases, but it may be readily effect-ed, and we find it desirable. The gas added to the furnace from time to time was, of course, of the same character as that indicated above.

The temperatures to which the castings are subjected, and the length of time of treatmentat such temperatures, are also impor-- and, for the compositions within the ranges above specified, equilibrium is attained, in plant practice, in times ranging from about seven to twenty-eight hours. In commerc al operations, we have broken down the massive cementite in charges of castings of about a quarter to a half inch in cross-section 1n from eight to fifteen hours at temperatures between 1650" F., and 1750 F. With castings of greater thickness, longer times are required.

Thus, in castings more than one inch in crosssection, the time to effect complete graphitiza'tion may be thirty hours or more.

As described in a copending application by one of us (Hayes Serial No. 209,389, filed July 29, 1927, Patent No. 1,801,742, issued April 21, 1931), the time required for annealing white cast iron depends not only upon the temperature but also on the chemical composition of the white iron treated. Thus it has been shown that in order to completely graphitize free iron carbide in commercial loads of cast-v ings less than one-half inch in section, at a temperature above the critical temperature, such as 17 00 F., iron containing 0.9% silicon and 2.5% carbon must be heated for 11.4 hours. It has also been shown by experiments that with a silicon content of 1.2% the time required for the same size section was 6.6 hours, while an iron with a silicon content of 1.35% required 3.6 hours. There has thus been established a very definite relationship between the silicon content of the white iron and the heating time at an elevated temperature required to completely graphitize the free iron carbide contained in the white iron.

Castings having the same chemical composition but which are materially thicker in section size, require a longer heating period to completely graphitize the free iron carbide. Thus, sections of one and one-half inch of the above compositions were graphitized in 22.8, 13.2 and 7.2 hours respectively.

In plant practice, where it is necessary to treat castings of various sizes and shapes and varying somewhat in their chemical composition we determine the time required for the proper treatment of the most resistant castings, balancing the load to make it as uniform as may be, and employ this factor in laying down the cycle on which the plant is to operate. That is to say, if it be found that the times required for graphitizing the massive cementite lie between say. twenty and twentythree hours, we may employ a hold at the high temperature for twenty-four hours, as such time is economically justified ;it makes the critical periods of the cycle correspond with labor shifts and the like. For We find that by utilizing the other elements of control as herein specified, the graphitizing of the massive cementite may be brought to equilibrium, and no danger will ensue if the precise time is overrun slightly. In general, eighteen to twenty-eight hours will be found about rlght to bring the reaction of decomposition of massive cementite to equilibrium. At this point, there is still a slight amount of free iron carbide which has not been decomposed, and we find it is advantageous to maintaln this residuum.

To effect this retention, we cool the castings as rapidly as possible from the high temperature, as by means of a quenching operation. As the quenching medium, we may use air, water. oil, sand, or the like, but the air quench is sufficient for most' work, and possesses advantages from the cost and practicability,

viewpoints. It is usually desirable to reduce the temperature of the hot castings to a point at which no appreciable further chemical reaction will take place, or, what isofte'n as equally important, no further physical change. The temperature of the material, after quenching, is best around a dark or cherry red heat, and below the critical. Such ture value.

quenching insures the preservation of the nature of the casting as it was at the end of the high temperature treatment, and further treatment may be effected as desired by bringing the castings back to some other tempera- For charges of commercial size, say between ten and twenty-five tons, an air quench of from an hour to an hour and a half will suffice.

After the charge has been cooled or quenched it is heated to a temperature in the neighborhood of the critical temperature. The critical temperature will vary, depending upon the chemical composition of the metal being treated. It is thought however that this temperature lies between 1370 F. and 1420" F. for practical commercial purposes. This second heating at a temperature below the critical temperature range as for instance, between 1270 F. and 1420 F. is for the purpose as we understand, of causing the graphitization of the pearlitic iron carbide and obtaining thereby a malleableized cast iron in which substantially all of the originally combined carbon is now present as free carbon, and substantially all the iron present as free iron or ferrite.

The time required to complete the graphitization of the pearlitic iron carbide depends upon the chemical composition of the metal being treated and the temperature at which that metal 'is heated. Thus, the graphitization (of the pearlitic iron carbide) occurs rapidly in the critical range of temperature, however, the graphitization is less rapid but goes to completion at temperatures removed from or less than the critical temperature. Wehave found that at temperature of 1300 F. to 1350 F., we are able to graphitize the pearlitic iron carbide con stituent of the cast iron in periods of time ranging between twenty and thirty-five hours, although certain compositions may require forty hours for substantially complete graphitization.

At the completion of the heating time at the low temperature, the charge is removed from the furnace and is quenched by any of the suitable methods mentioned hereinabove, as by an air quench.

The heating time at the preferred low temperature required to graphitize the pearlitic iron carbide is, in the case of thick sections, say in the neighborhood of one inch, containing 1.1% silicon and 1.9% carbon, 40.2 hours. WVe have graphitized the pearlitic iron carbide in such sections contained 2.52% carbon, in 25.6 hours, while the same size sections require somewhat less time where the carbon content is increased, being but 16.2 hours for carbon contents of 2.85%. lVhere section sizes are less than that given above the time required to complete this chemical reaction is somewhat less than that given immediately above.

By the use of our method outlined immediately above, we are able to produce a superior grade of malleable iron casting; for instance, a casting containing 0.94% silicon; 0.089% sulfur; 0.33% manganese, and 2.49% carbon, gave a product with a yield point of 37,500 pounds per square inch with an ultimate tensile strength of 54,680 pounds per square inch, and an elongation of 21%. The product was perfectly annealed and showed no frame nor skin structure and possessed an even distribution of carbon from the core to the outside surface of the castin Another sample, of casting placed in a furnace required eleven and one-half hours to heat to 1700- F., was held twenty-six and one-half hours at 1700 F., and quenched, and thereafter heated at 1300 F. for twenty-nine hours showed a yield point of 36,290 pounds per square inch with an ultimate tensile strength of 53,510 pounds per square inch and eighteen and one-half per cent elongation in two inches. The composition of this sample was as follows :1.03-% silicon, 0.105

' sulfur, 0.34% manganese, and 2.60% carbon.

Another bar possessed a yield point of 39,500 with an ultimate tensile strength of 57,140 and an elongation of 27%. This speci- Lnen contained 1.03% silicon, and 2.43% car- These results were obtained in an atmosphere possessing 40.4% nitrogen, oxygen less than 0.1%, 54.4% carbon monoxide, 5.2% carbon dioxide, maintained at substantially atmospheric pressure. No decarburization occurred in the casting surrounded by this particular gaseous envelope. A very good casting was also obtained on heating in an atmosphere containing 68% nitrogen, 0.6% oxygen, 23.8% carbon monoxide, and 7.6% carbon dioxide.

The foregoing data may be regarded as representing a balanced operating condition, wherein the several factors of the chemical composition of the casting, the times and temperatures employed, and the nature of the gaseous envelope are all intercorrelated. As pointed out hereinabove, the composition of I the atmosphere, which is substantially constant for the major portion at least of the times during which the castings are held at elevated temperatures, is. so correlated with the temperatures as to" prevent substantial loss of carbon from the castings. By insuring a constancy in the chemical composition of the casting, the effect of the heat treatment may be accurately predetermined. It will be understood, of course, that in the practical application of the principles of the invention we do not anticipate, in every case, producing a finished product in which the total carbon content is mathematically in accord with the carbon-content of the original white iron. However, making due allowances for losses which will occur in ordinary operation, and

within the range hereinabove indicated, it willbe seen that the furnace atmosphere is maintained at a pressure and within a range of chemical compositions preventing the egress of carbon from the castings. In other words, the furnace atmosphere for the temperature and pressure conditions of operation is in balance with the chemical composition of the casting itself.

From the foregoing, it will be readily seen that the usual muflie or pot furnace practice of malleableizing cast iron has been very materially improved. In the practice of annealing used heretofore, the time of the anneal lasted over all from, one hundred and forty hours to one hundred and sixty hours or more. By the use of our method, this time has been materially shortened and a complete malleablization process performed in seventytwo hours or less. A typical example of our short cycle anneal of about seventy-two hours consists of a heating periodof fifteen hours to 1700 F., twenty-four hours at 1700 F., two hours to cool, and thirty-one hours at 1340 F. for a one inch section casting. With thinner sections and appropriate chemical compositions, heating times may be materially less as shown in the examples given hereinabove. lVe find that we obtain consistent and satisfactory resultson a commercial op eration, and that various other compositions and various other specific cycles may be employed to give marketable products on a practical scale.

What is claimed is 1. In the heat treatment of white iron castings including the steps of heating the casting above the critical range to eflect the decomposition of massive cementite thereinand heating the casting at a lower temperature to effect an improvement in the physical properties thereof, said heat treatment being conducted at predetermined temperatures for predetermined lengths of time, the method of controlling the reactions and maintaining the constancy of the carbon value of the castings comprising surroundin the castings during such heat treatment with a non-oxidizing and non-decarburizing atmosphere containing substantially no free oxygen and no elements of water, and maintaining said atmosphere at a predetermined pressure, said atmospherecontaining carbon dioxide and carbon monoxide in the proportions of less than ten per cent carbon dioxide and more than ten per cent carbon monoxide, the ratio of carbon dioxide to total oxides of carbon being less than one third.

2. In the heat treatment of white iron castings by subjecting the castings to predeter- -mined temperatures for predetermined periods of time to effect desired chemical and physical changes in the casting, the steps of maintaining the ultimate chemical composition of the casting substantially constant by con ducting the heating operations in a nonoxidizing and non-decarburizing atmosphere free from water vapor and excess free oxy en, said atmosphere further containing car on dioxide in an amount less than about ten per cent and carbon monoxide in such quantity that the ratio of carbon dioxide to total oxides of carbon is less than one third, maintaining the composition of said atmosphere as defined from a source extraneous of the castings, and further maintaining said atmosphere at a substantially uniform pressure.

- ANSON HAYES.

HARRY O. BREAKER.

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