US2555014A - Composition for addition to cast iron or steel - Google Patents

Description

Patentecl May 29, 1951 COMPOSITION FOR ADDITION TO CAST IRON OR STEEL Jerome Strauss, New York, N. Y., assignor to Vanadium Corporation of America, New York, N. Y., a corporation of Delaware No Drawing. Application September 7, 1950, Serial No. 183,661

8 Claims.

This invention relates to a novel composition of matter for the control and improvement of the physical properties of cast iron and for other uses, such as the treatment of steel in the molten state.

Cast iron is a generic term which includes gray irons, white cast irons, chilled cast irons and malleable irons. The properties of these metals depend in part upon chemical factors (principally the percentages of carbon and silicon) and in part upon physical factors, both as influenced by conditions related to the manufacturing methods. This is so because cast iron is essentially the product of a process and the infiuence of the process details are observed in the qualities of the product. Alloying is employed to alter the properties of some cast irons; also some iron castings both alloyed and not alloyed are subiected to thermal treatments to produce desired results in casting for specific uses. Th physical structure (i. e. the nature and distribution of the micro constituents), and consequently the properties of the metal are influenced not only by these several factors but also by adjusting the maximum temperature attained by the molten iron and the cooling rate both during and after solidification.

Cast iron is sometimes considered unsatisfactory as a material of construction because of its low strength and lack of ductility when compared to steel and certain other engineering alloys. In such a comparison cast iron is usually described as being inherently brittle, especially in the ascast condition; this brittleness can be reduced only to a very limited extent by heat-treatment, except for long and costly treatments applied to irons in the malleable iron composition range.

The cast iron which is most used commercially for construction applications, however, is gray iron, but this term actually covers a Wide range of compositions with corresponding widely varying properties. The composition of gray iron is usually confined within the limits 0.50 to 2.75% silicon and 2.70 to 3.60% total carbon. Within this range of composition the tensile strength may vary from 15,000 to 60,000 pounds per square inch and occasionally higher.

While a part of the carbon present in gray cast iron may be in the combined form as iron carbide, the greater amount is present in elemental form as graphite. The relative amounts of free carbon and combined carbon, as well as the shape, size and distribution of the particles are dependent upon the remaining chemical composition of the iron and the influences referred to above, namely,

maximum temperature in the liquid state, rate of cooling during and after solidification, and types of heat treatment, if any, applied to a solidified casting.

Elements incidental to the production of cast iron may add or detract from its properties, such as strength, toughness and ductility. Sulphur, for example, is usually kept as low as possible because while it increases the strength of the iron to some degree, it very markedly reduces ductility. Control of sulphur depends, however, on raw materials and process and often it cannot be held to very low values because only high-sulphur raw materials are available, the cost of electric furnace refining may not be permissible and chemical treatment may not be adequate or sufficiently uniform. Phosphorus occasionally strengthens iron but in large amounts renders it quite brittle.

Certain elements usually considers as alloying agents have been added to control the structure and properties of cast iron. In some cases the addition of such alloys results in one property being improved to the detriment of another, for example, strength may be improved but the toughness of the iron be reduced unless some adequate form of heat-treatment is provided to retain the toughness. In other cases improvement in strength is accompanied by reduction in the machinability of the iron; in some of these cases heat-treatment, such as annealing, may restore the machinability.

It is known that magnesium added to iron which would otherwise cast gray or nearly so contributes to this iron high strength and some ductility. The ductility has in some cases been found to be increased by an annealing treatment with relatively little sacrifice in this superior strength.

However, serious difficulties have beset attempts to make this magnesium practice one that would provide with assurance successive reproduction in continuous manufacturing operations of iron articles of high strength or high strength and good ductility. The boiling point of magnesium being considerably below both the melting temperatures of those metals with which it is preferably combined for making additional agents and below the temperatures at which cast iron is tapped and poured, causes extreme diniculty in controlling the overall recovery of magnesium from raw material to the finished cast iron product.

The boiling point of magnesium is about 2030 E, which results in very highlosses due to Vaporization of the magnesium when it is added to molten copper and molten nickel to produce such addition agents as 20% magnesium and 80% copper, and 20% magnesium and 80% nickel. Further difficulty is presented when it is attempted to introduce iron into these compositions. Iron is a very desirable carrier metal when magnesium is to be introduced into iron alloys, such as gray cast iron or steel. The introduction of iron into the copper or nickel base alloys mentioned above, however, only increase the already high losses of magnesium during manufacture.

Since the temperature of cast iron flowing from the cupola is normally in the range 2300 to 2800 F., it is obvious that additional losses will be encountered through vaporization when magnesium alloys are added to iron. When pure metallic magnesium is added to cast iron at such pouring temperatures, its volatilization can be explosive in character. Magnesium continues to vaporize as long as the iron remains molten and the time element combined with the initial violent reaction results in variable losses and properties that vary beyond the limits of satisfactory commercial control. The use of alloy addition agents, such as the 20% magnesium and 80% copper, and 20% magnesium and 80% nickel composition previously mentioned, does not satisfactorily overcome these difilculties. For example, if the cast iron to which such alloys are added is on the high side of the above temperature range, a violent reaction will occur with high losses of magnesium through volatilization as well as loss of metal and danger to the operators through spattering of the molten cast iron. If, on the other hand, the temperature of the iron is low, incomplete solution may occur, resulting in segregation and in variation in both structure and properties throughout the product.

I have discovered in the course of extended experiments directed toward the development of a commercially satisfactory practice for producing magnesium-containing cast iron of predictable and reproducible properties that certain combinations, preferably in alloy form, of magnesium, silicon, copper and iron afford an unexpected means of simplification of these problems. Primarily, my discovery consists in a certain critical combination in the proportions of magnesium, silicon, copper and iron, which substantially increases the recovery of magnesium in manufacture of the addition agent as well as when this agent is added to molten cast iron. This combination of effects results in a substantial cost saving. More significant, however, is the fact that, when using alloys of compositions within the limits of my invention, the desired improvements in the finished cast iron are obtainable more readily and with much greater regularity. While cast iron in which substantially all of the carbon is in the form of spheroids has been produced frequently, it has been difficult to obtain structurally perfect casting under production conditions with a variety of designs, melting equipment, raw materials and other foundry conditions. An infinitely closer approach to perfection can be attained in the use of the alloy herein described. The mechanism of the behavior of this magnesium-silicon-copper-iron compositions within the critical ranges hereinafter set forth has not been fully explained but the results have been obtained with such regularity both in production of the alloy and. in its use with gray cast iron over a substantial amount of production and a large number 01. 1

'4 ferent foundries that a distinct advance in the art through its use has been established.

The ranges of composition which have produced the advantages described are 5 to 25% magnesium, 20 to 45% silicon, 3 to 20% copper, the balance substantially all but not less than 20% or more than 60% iron, except for customary impurities and minor elements, such as carbon, manganese, sulphur, phosphorus, etc., which generally do not exceed a total of 5%. Nickel is not included among these minor elements; it is present today in some pig irons and in most iron and steel scrap and hence may be present in these alloys up to about 2% and is not detrimental to the functioning of the alloys. In my compositions, aluminum is a detrimental element. It should be kept below 1% and preferably below 0.6%. Within the ranges of composition already cited, I prefer to restrict the relationship of the several elements so that the ratio of silicon to magnesium is not less than about 2:1 and not greater than about 6: 1 and preferably not greater than 4.5: 1, and the ratio of magnesium to copper is not less than 1:2 and not greater than 4:1 and preferably not greater than 2.5: 1. Following are three examples of compositions within these critical ranges and possessing the preferred ratio of elements which were manufactured and used successfully.

Iron Bal. Bal. Bill.

The definite advantages obtained with an iron content between 20% and 60% are twofold: the iron increases the specific gravity of the alloy, aiding it to penetrate the molten metal, and it raises the melting point, thus slowing down the rate of solution in the molten cast iron, effecting more uniform distribution throughout the mass. In addition to melting too rapidly in the absence of iron, the cost of production becomes excessive if the iron is held below about 20%. If the iron content is permitted to exceed 60%, then the magnesium losses during manufacture of the alloy become excessive and very costly, and the rate of solution of such alloys in molten cast iron is too slow and their effectiveness is greatly impaired.

By use of alloys having compositions within the described limits, superior results have been achieved in respect to formation of nodules of graphite in cast iron and reduction of shrinkage of cast iron, over and above what had been previously attained by the best known and most widely used alloy, namely, nickel and 20% magnesium, together with the development of maximum amounts of tensile strength.

In addition, quite unexpectedly, it has been discovered that the slag forming on iron following the addition of magnesium-containing agents is much more fluid when using the alloys of this invention instead of previously known magnesium alloys, thus avoiding slag entrapment in the finished castings.

The amount of alloy added to molten cast iron to be cast into gray iron castings varies with the nature of the iron (as influenced by the raw materials used and the conditions of processing during melting), the maximum temperature attained by the molten iron, the temperature at of the iron and perhaps other factors. In general, the addition approximates 0.12% to 0.20% of contained magnesium plus an amount of contained magnesium equal to 1 times the sulphur content of the iron. This is not an absolute amount but varies according to the precise composition of the ailoy being used as well as the various factors influencing the character of the molten cast iron, as pointed out above, and the method of adding the alloy to the molten iron. It is, however, a good starting point in establishing the practice in a specific foundry while on a particular melting practice so that a limited amount of experimentation varying the addition upward and downward readily establishes the optimum addition required. In general, the amount of alloy added should be such as to have in the solidified cast iron a total magnesium content of 0.04 to 0.10%, preferably 0.05 to 0.08%.

Another advantage possessed by these alloy addition agents is that of increased recovery of magnesium during both the manufacturing process and during use in the treatment of gray iron. Alloys Within these ranges of composition have the peculiar characteristic of being dissolved sufliciently easily, in cast iron that they may be effectively used at the lowest customary pouring temperatures which characterize modern commercial foundry practice. On the other hand, solution. is sufficiently slow that the alloy and its components will be uniformly distributed throughout the melt and the magnesium will not be lost so rapidly as to prevent it from exerting its influence throughout the entire mass of cast iron. No additional care and no modifications of prior practice are required, the addition of the alloy to iron that would otherwise cast gray, and of low moderate strength, being the sole requisite. The reason or reasons why these alloys melt at such critical rates so as to be usable at full effectiveness over such a wide range of casting temperatures, but still melt slowly enough so that all ingredients, and particularly the magnesium are distributed uniformly throughout the cast iron have not been clarified but the observations have been sufficiently numerous to be conclusive.

When alloys within composition ranges of the present invention are added to cast iron, in-

creases in strength are secured ranging from about 50% to well over 100%. In improving the mechanical properties to such high degrees the alloys covered by this invention regulate the microstructure of the gray iron so as to produce nodular graphite in the required amount, usually to the extent of complete conversion of the carbon to this form.

The following is an example of the procedure used in practicing the present invention.

EXAMPLE 1 Five hundred pounds of cast iron were poured into a ladle containing twenty pounds of alloy having the following composition:

Per cent Magnesium 5.67 Silicon 32.82 Copper 5.80 Iron Balance The composition of the cast iron before and after treatment with the alloy is shown in Table 'I.

TABLE I Composition of treated and untreated iron 3 5 Manufacture 'r.o. Mn er s P Mg Cu 1 Untreated. 3.77 .69 1.65 .11 .19 None .11 2 Treated... 3.10 .67 3.36 .01 .16 .075 .38

) Benefit to the iron by treatment with the alloy may be seen in Table II; where the hardness is shown to have been greatly increased and tensile strength has been more than tripled.

In a second example, samples were taken of the untreated iron, of the iron after treatment with an alloy containing 5.88% magnesium, 34.05% silicon and 5.56% copper, balance iron, and of the treated iron after annealing. Compositions of these three samples are given in Table III.

TABLE III Sample Manufacture 'I. 0. Mn Si S P Mg Cu 3 Untreated 3.79 .78 2.43 .142 0.20 None .09 4 Treated.-." 3.20 .74 4.15 .011 0.19 .002 .35 5 Treated and 3.18 .74 4. 22 .010 0.19 .094 .35

Annealed.

In this case the iron was of low strength, due possibly to its high silicon and carbon contents,

but after treatment strength was more than doubled and the hardness was greatly increased as may be seen in Table IV.

TABLE IV Tensile Brinell Sample Manufacture Strength, Hardness p. s. i. No.

provement in machinability without reducing the strength.

EXAMPLE 3 iron and the cast iron treated with the same addition alloy as was used in Example 2 are given in Table V.

TABLE V Sam- Manufacpi 6 mm '1 0. Mn S1 S 6 Untreated". 3.30 .65 1.61 .12 .12 None .09 7 Treated 2.93 .60 3.18 .14

The compositions of another untreated cast In this case the treated and untreated coupons were tested in the as-cast condition.

TABLE VI Tensile Brinell Sample Manufacture Strength, Hardness p. s. i. No.

6 Untreated 33, 600 190 7 Treated 95, 750 283 While the advantages of my invention are obtainable with alloys within the composition range already given, namely, magnesium 5 to 25%, silicon 20 to 45%, copper 3 to 20%, iron 20 to 60%, I prefer to use alloys with compositions Within the following narrower limits in order to develop the best properties with most economic advantages.

Per cent Magnesium '7 to 14 Silicon 25 to 38 Copper 5 to 12 Iron 35 to 55 EXAMPLE 4 An alloy containing 13.52 magnesium,

33.80% silicon, 9.87% copper, balance iron, was used in the treatment of electric furnace iron. The analysis of the treated electric furnace cast iron was:

Properties of the electric furnace cast iron as cast and after annealing were as follows:

TABLE VII Yield Tensile Elong., R. A., Brinell Izod Point, Strength, Per Per Hard- Impact, p. s. i. p. s. i. Cent Cent ness Ft.-Lbs.

As-Oast. 67, 500 87, 200 1. 5 269 39 Annealed. 61, 500 85, 400 3. 0 2. 229 183 These properties are far superior to those obtained from ordinary electric furnace cast iron.

EXAMPLE 5 The same alloy as given in Example 4 was used to treat cupola cast iron, which gave the following analysis after treatment:

Per cent Carbon 3.50 Silicon 2.60 Manganese .15 Phosphorus .025 Sulphur .015 Magnesium .050 Iron Balance The properties obtained in the as-cast and annealed states were as follows:

TABLE VIII Yield Tensile Elongn, R. A., Brinell Izod Point, Strength, Per Per Hard- Impact, p. s. i. p. s. i. Cent Cent ness Ft.-Lbs.

As-Cast 52, 800 74, 700 ll. 0 8. 8 187 50 Annealed. 48, 900 66, 200 17. 2 15.0 163 260 Methods of sampling and analysis currently in use show after the addition of the alloys of this invention that the carbon content and the sulphur content of the untreated irons have been reduced by the treatment. The reduction in the sulphur content is no doubt due to the combination of magnesium with at least part of the sulphur and its removal as a slag or a slag forming constituent. The reason for the change in the carbon content is not so obvious and it may be that the change is apparent rather than real due to the difference in the form and the distribution of the carbon and creating a necessity for different methods of sampling and analysis than those commonly in use. Completely satisfactory procedures have not yet been devised or discovered.

It has heretofore been an absolute requirement in the production of cast iron having all or nearly all of its carbon in the form of spheroids that there be added following the addition of the magnesium alloy a second addition rich in silicon, such as ferrosilicon, the amount in most cases being substantial. In the use of the alloys of this invention such final addition of ferrosilicon is not a necessity and may be dispensed with in many cases. The amount of silicon represented by the addition of the alloys according to my invention is very much less than that which would be added according to prior art methods subsequent to the incorporation of the magnesium alloy. Even in those instances where a final addition of silicon-rich alloy is made, the total silicon content represented by this addition plus the silicon content added by means of the magnesium-silicon-copper-iron alloy is distinctly less than the silicon added as a late ferrosilicon addition according to prior art methods. When the late ferrosilicon addition is actually employed in conjunction with the alloys of this invention, the maximum amount of ferrite in the microstructure may be attained with a minimum silicon addition, leaving not more than a very small amount of pearlitic structure in the matrix and through this means advantage in respect of machinability and reduction in the wear on tools used for machining will result. Production of an iron with minimum pearlite also results in maximum ductility of the iron composition.

The behavior in use of alloys with and without the prescribed amounts of copper have been compared in manufacture and use. Copper in these limited amounts aids in the manufacture of the alloys, adds to the stability and melting characteristics, and very significantly improves the regularity of the production, as well as improving the properties of the cast iron after treatment. At the same time the copper content is not so high that normal remelting of returns (heads, gates, etc.) with new pig iron, other iron scrap, etc., causes an undesired building up of the copper content of the product which might adversely affect some castings.

While fairly satisfactory results can be obtained in some instances in the treatment of cast iron with compositions of matter within the ranges previously specified, not in the form of alloys but in the form of mechanical mixtures, much better results are obtained by the use of these compositions in alloy form.

Alloys of this invention have also been added to molten steel for lowering the sulphur content of the steel. For example, steels have had their 9 sulphur content decreased from about .030 to about 020% by the use of these alloys.

The invention is not limited to the preferred embodiment but may be otherwise embodied or practiced within the scope of the following claims.

I claim:

1. A composition of matter for addition to cast iron or steel, said composition comprising about to 25% magnesium, to 45% silicon and about 3 to 20% copper, the balance being substantially all iron, the iron being in an amount between about 20 and 60%.

2. A composition of matter for addition to cast iron or steel, said composition comprising about 7 to 14% magnesium, about to 38% silicon and about 5 to 12% copper, the balance being substantially all iron, the iron being in an amount between about and 55%.

3. An alloy for addition to cast iron or steel, said alloy comprising about 5 to 25% magnesium, 20 to silicon and about 3 to 20% copper, the balance being substantially all iron, the iron being in an amount between about 20 and 60%.

4. An alloy for addition to cast iron or steel, said alloy comprising about 7 to 14% magnesium, about 25 to 38% silicon and about 5 to 12% copper, the balance being substantially all iron, the iron being in an amount between about 35 and 5. An alloy for addition to cast iron or steel, said alloy comprising about 5 to 25% magnesium,

about 20 to 45% silicon and about 3 to 20% copper, the ratio of silicon to magnesium being between about 2:1 and 6:1, the balance being substantially all iron, the iron being in an amount between about 20 and 6. An alloy for addition to cast iron or steel, said alloy comprising about 5 to 25% magnesium, about 20 to 45% silicon, and about 3 to 20% copper, the ratio of magnesium to copper being between about 1:2 and 4:1, the balance being substantially all iron, the iron being in an amount between about 20 and 60%.

7. An alloy for addition to cast iron or steel, said alloy comprising about 5 to 25% magnesium, 20 to 45% silicon and 3 to 20% copper, the ratio of silicon to magnesium being between about 2:1 and 6:1, the ratio of magnesium to copper being between about 1:2 and 4:1, the balance being substantially all iron, the iron being in an amount between about 20 and 60%.

8. An alloy for addition to cast iron or steel, said allo comprising about '7 to 14% magnesium, about 25 to 38% silicon, and about 5 to 12% copper, the ratio of silicon to magnesium being between about 2:1 and 45:1, the ratio of magnesium to copper being between about 1:2 and 2.5:1, the balance being substantially all iron, the iron being in an amount between about 20 and 60%.

JEROME STRAUSS.

No references cited.

Claims (1)

1. A COMPOSITION OF MATTER FOR ADDITION TO CAST IRON OR STEEL, SAID COMPOSITION COMPRISING ABOUT 5 TO 25% MAGNESIUM, 20 TO 45% SILICON AND ABOUT 3 TO 20% COPPER, THE BALANCE BEING SUBSTANTIALLY ALL IRON, THE IRON BEING IN AN AMOUNT BETWEEN ABOUT 20 AND 60%.