US2186329A - Making gray cast iron - Google Patents

Description

Patented Jan. 9, 1940- of Delaware MAKING GRAY CAST IRON Fred C. T. Daniels, Bridgeville, Pa., assignor to Mackintosh-Hemphill Company, a corporation No Drawing. Application April s, 1938, Serial No. 200,892 1 5 Claims. (01. 75-130) This invention relates to a method of producing cast iron of particularly high quality in a manner socontrolled as to assure a uniformly high quality of the iron cast from different furnace charges.

a In making iron castings'it has been observed that from time to time the product has been of particularly high quality. For example, in the casting of iron metal-working rolls, there have occasionally been produced rolls which gave satisfactory service through unusually long periods of use. Since this occurred from time to time in usual casting practice, using simple formulae in the composition of the cast iron temperature, and the period of heating in which rather than special alloys, it has been assumed that the particularly desirable results were occa- 'sioned by some slight variation in composition or casting practice. It was discovered, however, that the occasional castings of particularly high quality resulted from the casting of heats from -'ticular composition of the metal.

'I have had in mind the, great desirability of attaining with uniformity the particularly high V quality of iron castings which had previously .."generally to state the. relationship,- I prepareas-i' the major portion of the total metal, a charge containingarelatively,lowsumof carbon and sill- H I I V I con, and in which the proportion of carbon is; bon"relativ'elyjhigh in 'carbcn'by meltingjina '60 been occasionally and accidentally attained, and which in metal-working rolls has occasionally resulted in the production of rolls giving in rolling mills a useful service approximating an increase of 50% over the service usually afforded by'rolls of the same composition.

In accordance with the method of myinvention, and taking as exemplary a simple analysis I ganese, and carbon, I melt this metal in a bathtype furnace, which may be either a reverberatory furnace, open-hearth furnace, or electric furnace, desirably raising the charge to' a temperature substantially above the temperature at which it is to be teemed.- For example, we maytake the normal teeming temperature for a melt of this "type as about 25 50-F., which is below" the temv Y I desirable. qualities, and that "I may do so in a controlled manner, by [making appropriate" addi peraturef range used in forming pearlitic'; iron. I limit somewhat rigidly the carbon'an'dfsilicon content of the metal. Taking a standard formula manganese .24%, my'furnace charge contains an appreciably lower percentage of silicon, and, at

least when the charge is finished, contains an.

appreciablylower percentage of carbon. :More

reduced in the furnace. The 'quantitive decrease in carbon efiected by the melting operation varies from 0.20% to 0.50%, (for example a reduction from 2;90%' carbon to from. 2.70% carbon to 2.40% carbon) with particular furnaces and furnace operators, and the atmospheric conditions under which the melting operation is conducted. Thus the percentage of carbon being reduced, a

tendency toward fine grain formation exists -unless the proportion of silicon in the charge be unduly high.

In accordance with my preferred practice, the charge in the bathfurnace is brought to a temperature at least as high as its normal teeming it is raised to its maximum temperature may be considered to be a washing treatment for the metal. This treatment greatly decreases the oxides in the metal, apparently by reduction with carbon and evolution of carbon combined with oxygen in gaseous state. It may be noted as a general principle that the greater the dilution of carbon and silicon in the major, bath furnace charge,the higher will be the state of purity to which the charge is brought. Since the molten metal contains toward the end of the treatment metalloids and carbon in high dilution, the metal is thoroughly cleaned and refined, with elimination of any undesirable inherent'qualities which may be possessed by the scrap or pig iron includedirr the furnace charge. Also primary graphitic carbon in the metal, which is normally I present in improperly distributed and improperly sized particles, is thoroughly resolved. By the term resolved I means that the carbon is brought into a condition ,of solution, combination, Y

or extreme dispersion. Were the bath, as so prepared, to be teemed without alteration, it would be a very hard white castv iron, containing too high a percentage of its carbon in combined form,

and because of its extreme hardness and brittle- Y tion to the molten charge of. the bath 1 furnace. In 1 so i'doing' I employ, a ferrous metal high; in

which usually-means a "high' content of; silicon.

I carbon," may prepare a; ferrous solution} of carcupola a mixture of steel scrap and pig iron or simply steel scrap, and making adequate addition of silicon or other graphitizing agent. Also direct blast furnace iron and mixer metal may be considered to be the full equivalent of pig iron as a source of the carbon-additive ferrous solution of carbon. However produced, I have found that the ferrous solutionmay desirably contain as much as from 3.5% to 4.5% carbon, and as much as from 2% to 2.5% silicon. This metal I mix at a temperature sufiiciently high for teeming, with the charge low in carbon and silicon, to .lmpregnate that charge, and make up the carbon deficiency therein.

Upon addition of the carbon-additive charge to the major charge, the mixture is thoroughly stirred, and is quickly drawn off and teemed. The addition of as little as 2.5% of a ferrous solution of high carbon content is of advantage. I. have found it desirable to add the ferrous carbon-additive solution in a quantity equal to about 10% of the total charge, as that approximate percentage of the iron high in carbon and silicon gives the desired results economically. 25% of the ferrous solution in the total charge, my process decreases greatly in economy. Within reasonable limits, the properties of the total metal are thus controllable by proportioning the carbon-additive charge to the bath furnace charge.

While it is most convenient to add the carbon-supplying ferrous solution to the major charge, or bath, in the furnace, the charges may alternatively be mixed in the ladle in preparation for teeming without altering the effect on the total metal,

It will be appreciated that my method involves the uniform mixing of a carbon solution through the body of the total charge, giving upon solidification of the total charge a very fine dispersion of carbon throughout the body of the metal. When the combined charge is teemed into molds, the carbon added to the total charge by the carbon-additive charge, which upon solidification is finely and uniformly dispersed in graphitic state throughout the body of the metal, does not interfere with the formation of a fine grain structure as would a high initial silicon and carbon content in the metal of the bath. In this connection it should be understood that in my process I add-carbon in a condition to promote a fine and uniformgraphite dispersion in the casting, and that I do not seek merely to promote precipitation of carbon'alreadypresent. My mode of making up in the total charge a carbon deficiency in the major portion of the-charge is not to be confused with the usual metallurgical adjustments, such as the addition of-ferro-silicon or ferro-manganese before teeming, which metallurgical adjustments may or may not be made in conducting my process.

It should be emphasized that the tendency of the metal to form a fine grain structure, attendant upon the fwashing treatment which has been described is not effectively inhibited by the addition of the carbon-additive charge of carbon in iron solution. I thus obtain a cast iron of exceptionally cleanquality and fine structure, which cast iron is in complementary manner tough and machinable to an order appropriate to the use Above Example No. 1

This example may be considered to be a desirable adaptation of my method. to the casting of metal-working rolls of standard analysis. It comprises the following compositions for the major, low carbon, charge and for the minor, carbon-additive, charge:

Percent by Percent Percent Percent weight C Si Mn Low carbon metal. 2. 55 .60 22 Ferrous carbon solution 20 4.30 .82 .37 Final analysis 100 2.90 .65 24 It Will be noted that in'this example the final analysis is identical with the analysis given above as standard for metal-working rolls of chilled iron.

Example No. 2

This example may be considered to be a desirable adaptation of my method to the preparation sand-cast iron metal-working rolls. It comprises the following compositions of the charges:

l lrgc Weight Percent Percent weight C S1 Mn Low carbon metal 80 2. 40 l. ()0 .60 Ferrous carbon solution" 20 4.00 75 Final analysi 100 2. 72 65 Example N0. 3

.This example may be considered to be a desirable adaptation of my method to the preparation of iron for making large section sand castings. It comprises the following compositions of the charges:

gg Percent Percent Percent weight Mn Low carbon metal 75 3.00 l 40 .00 Ferrous carbon solution. 25 4. 00 2 10 1. 00 Final analysis.. 3. 25 l 65 .70

Example No. 4

This example may be consideredto be a desirable adaptation of myrnethod to the preparation of iron for casting chilled-iron rolls. It comprises thefollowing compositions of the charges:

Example N0. 5

This example may be considered to be a desirable adaptation of my method to the preparation Percent Percent Percent Percent by Si M weight n Low carbon metal 90 2. 78 60 .22 Ferrous carbon solution. l0 4. 00 l. 10 i 42 Final analysis 100 2. 90 65 24 t\ In each of the examples the temperature of the major charge in the bath furnace was brought to above 2600 F., and the charge of carbonadditive iron, molten at about 2300" F. was added to the bath furnace charge at approximately the highest temperature of the latter. It will be noted that each example contains manganese in the formulae of both charges, although manganese has not been-discussed as a factor in conducting my method. This is for the reason that it has but a relatively moderate graphitizing eflect on the ferrous metal. Whereas a normal percentage inclusion of manganese has but little graphitizing effect, other elements, such as nickel and titanium, are more active in promoting graphitization unless balanced by chromium or its equivalent. Silicon has been considered herein as the typical 'graphitizing agent, but the gist of the desired conditions in the major, bath furnace charge is that the low proportion of carbon and/or the negative graphitizing balance is such as to minimize the presence of graphitic carbon. in that charge, and thus to make a hard melt (i. e. a melt of white cast iron). This is effected most simply by depending primarily upon a low percentage of carbon in the metal of the bath furnace charge. A negative graphitizing balance may be established in the major charge by including a relatively high percentage of chromium, so apportioned to the graphiti ng agent or agents supplied by the impregnating ferrous solution that graphite precipitation is not largely prevented in the molds.

It will readily be understood that my-method results in producing high quality iron castings intentionally and uniformly, and thus gives a predictable result which has been hitherto obtained only accidentally and at long intervals by following known methods for the preparation of casting irons. The graphite is controlled quantitatively, and a fine grain structure is maintained, without the hardness and brittleness usually attendant upon fine grain structure in iron castings.

Microphotographs of iron castings made in accordance with my method'show a more uniform and finer dispersion of carbon than may be expected in standard practice; and also show pearlite more finely and uniformly developed than may be expected in standard practice. They also show cementite patches lessmassive than those commonly found. These three characteristics are all indicative of cast iron of particularly high quality. It is to be emphasized that in casting iron in accordance with my method uniform results from heat to heat may be obtained and high quality castings uniformly produced.

Theoretically, but not conclusively, I attribute my uniformly good results to the following considerations. In the standard process for making cast iron, the physical properties of the metal, and most particularly the size, shape, and distribution ofthe graphite crystals are not wholly a function of the immediate composition, or cooling rate of the casting. Much of the physical character of the metal is inherited from the metals melted to compose the casting charge, the

good or bad properties of the components of the charge tending to persist throughout the melting and casting of the charge, and the transformations attendant upon cooling to a molded product. Thus, if the raw materials of a charge contain large, platy, graphite crystals, there is a tendency for a like structure to persist in the castings made from them.

In my process I melt the major part of the required weight of metal as a ferrous solution of carbon and silicon which is relatively dilute with respect to their ultimately desired concentration in the metal. This effects complete dissolution of the graphite of the raw materials, with attendant destruction of the initial size and shape of the graphite crystals. The bath of metal is thus in such condition that its graphite-forming tendencies may be exactly controlled by the addition of carbon and metalloids in ferrous solution of predetermined concentration. By practicing my process, the raw materials of the iron may therefore be selected wholly on the basis of their composition, without attempting to make the difficult and frequently deceptive determination of the physical structure of the raw materials.

I claim as my invention:

1. The herein described method of preparing gray cast iron which comprises making a major bath furnace charge of ferrous metal having the approximate composition of white cast iron, bringing the bath to its teeming temperature, and by mixing with the major bath charge of white cast iron when ready for teeming a minor charge of molten ferrous metal having a positive graphitizing balance and containing a proportion of carbon higher than that ultimately desired in the cast iron making a total charge of gray cast iron having a carbon content which upon solidification of the charge is largely in graphitic state and which is proportionally greater than in the bath charge of white cast iron.

2. The herein described method of preparin gray cast iron which comprises making a bath furnace charge of ferrous metal having the approximate composition of white cast iron with from approximately 2.40% to3.00% carbon in a state of resolutionin a quantity of the bath approximately equal to from 75% to 95% the desired total weight of cast iron, bringing the bath to its teeming temperature, and mixing with the major bath charge of white cast iron when ready for teeming a minor charge of molten ferrous metal with from approximately 3.50% to 4.50% carbon and with silicon adequate to give a substantial positive graphitizing balance in a quantity of the minor charge equal to from 5% to the desired total weight of cast iron to make a total charge of gray cast iron having a carbon content which upon solidification of the charge is largely in graphitic state and which is quantitatively greater than in the bath charge of white cast iron.

3. The herein described method of preparing gray cast iron of a standard analysis suitable for rolls which comprises making a major bath furnace charge of white cast iron having about 2.55% carbon, bringing the bath to its teeming temperature, and by mixing with the major bath charge of white cast iron when ready for teeming a minor charge of molten ferrous metal having about 4.30% carbon and having a positive graphitizing balance in a quantity approximating 20% the total desired weightof cast iron making a tent oi. about 2.90% which upon solidification of the cast iron is largely in the graphitic state.

4. The herein described method of preparing gray cast iron of an analysis suitable for large section sand castings which comprises making a major bath furnace charge of white cast iron having about 3.00% carbon, bringing the bath to its teeming temperature, and by mixing with the major bath charge of white cast iron when ready for teeming a minor charge of molten ferrous metal having about 4.00% carbon and having a positive graphitizing balance in a quantity approximating 25% the total desired weight of cast iron making a total body of gray cast iron having a carbon content of about 3.25% which upon solidification of the cast iron is largely in graphitic state.

.5. The herein described method of preparing gray cast iron 01' an analysis suitable for chilled rolls which comprises making a major bath furnace charge of white cast iron having from about 2.78% to 2.84% carbon, bringing the bath to its teeming temperature, and by mixing with the major bath charge of white cast iron when ready for teeming a minor charge of ferrous metal having from about 2.90% to 4.00% carbon and having a positive graphitizing balance in a quantity of from about 5% to 10% the total desired weight of cast iron making a total body of gray cast iron having a carbon content of about 2.90% which upon solidification of the cast iron is largely in the graphitic state.

FRED C. T. DANIELS.