US2706681A - Alloy for addition to iron or steel - Google Patents

Description

United States Patent ALLOY FOR ADDITION TO IRON OR STEEL Daniel Leonard Edlund, Bethel, Pa., assignor to Vanadium Corporation of America, New York, N. Y., a corporation of Delaware No Drawing. Application August 20, 1952, Serial No. 305,509

4 Claims. (Cl. 75167) This invention relates to a uniform, homogeneous alloy 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.

This application is a continuation-in-part of my copending application Serial No. 183,663 filed September 7, 1950, now abandoned.

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 influence of the process details are observed in the qualities of the prod uct. Alloying is employed to alter the properties of some cast irons; also some iron castings both alloyed and not alloyed are subjected to thermal treatments to produce desired results in castings for specific uses. The 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 as-cast 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 constructional 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. Phosphorous occasionally strengthens iron but in large amounts renders it quite brittle.

2,706,681 Patented Apr. 19, 1955 Certain elements usually considered 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 addition agents and below the temperatures at which cast iron is tapped and poured, causes extreme difficulty in controlling the overall recovery of magnesium from raw material to the finished cast iron product.

The boiling point of magnesium is about 2030 R, which results in very high losses 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 difiiculty 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 increases 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 compositions previously mentioned, does not satisfactorily overcome these difliculties. 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 magnesiumcontaining cast iron of predictable and reproducible properties, that by a process involving the manufacture and use of certain uniform, homogeneous alloys of magnesium, silicon and lead, an unexpected solution to these problems is presented. The alloys may also contain iron or manganese or both iron and manganese. Primarily, my invention consists in a process whereby magnesium may be converted to a form satisfactory for the treatment of cast iron with greater overall recovery of magnesium from the raw materials to the finished iron casting.

More particularly, my discovery consists in a certain critical combination in the proportions of magnesium,

silicon and lead in a uniform, homogeneous alloy, which alloy may contain iron or manganese or both, the proportions of the ingredients being such as to substantially increase the recovery of magnesium in the manufacture of the addition agent as well as when this agent is added to molten cast iron. This combination of effects provides a significant cost saving. More significant, however, is the fact that when using alloys within the limits of my invention, the desired improvements in the finished cast iron are obtainable more readily and with much greater regularity. \Vhile cast iron in which substantially all of the carbon is in the form of spheroids has been produced frequently, it has been difiicult to obtain structurally perfect castings under production conditions with a variety of designs, melting equipment, raw materials and other foundry conditions. A much closer approach to perfection can be attained in the use of alloys herein described.

The alloys which have produced the advantages described are uniform and homogeneous and contain about 5 to 30% magnesium, about to 40% silicon, and about to 70% lead. Contrary to the experience of prior practice, it has been found that in the presence of this amount of lead in these complex alloys, the maximum magnesium content may be somewhat higher than customarily employed without danger of severe spattering or explosive violence; presumably the vapor pressure of magnesium is beneficially affected by association with the lead. Iron or manganese or both may also be present in these alloys. Similarly, while contrary to expectations, based upon their relative ease of combination with oxygen, nitrogen, etc., the silicon, and the manganese when present, in these alloys, have the effect of restraining the loss of magnesium on addition to molten iron, thereby reducing the amount of magnesium that need be added to effect a given magnesium recovery in the solidified iron. Iron or manganese or both in the presence of the minimum percentage of lead above noted, appear to behave similarly to the lead in restraining the violence of the reaction. The limits, however, upon iron and manganese appear to be critical. The manganese should not exceed about 12% in view of the fact that for the magnesium addition customarily practiced the amount of manganese introduced would tend too greatly toward stabilizing pearlite in the cast iron matrix and render it difiicult to secure the maximum ductility that results from maximum ferrite in the cast iron matrix and which is often desirable. Higher manganese also creates difficulty in securing readily a homogeneous product during the manufacture of this addition alloy. Too much iron will have a similar effect during manufacture. Where the alloy does not contain any manganese, the iron can be up to but should not exceed about 40%. If the alloy contains manganese, the sum of the iron and manganese should not exceed about 40%. If the alloy does not contain iron, the manganese can be up to but should not exceed about 12%. Customary impurities such as sulphur and phosphorus, and minor elements such as carbon, chromium, vanadium, titanium and molybdenum, can be present in an amount not exceeding a total of about 4%. 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 alloys, aluminum is a detrimental element if the amount exceeds about 1.5%, but it is preferably kept below 0.6%.

As previously stated, the alloys of this invention are uniform and homogeneous in composition, rather than being a mere mechanical mixture such as may be obtained by pressing the ingredients into briquettes, or as contrasted with compositions in which only some but not all of the ingredients are actually alloyed. It is a relatively simple matter to alloy magnesium and lead but such alloys, not containing silicon, are not adequate for the purposes of this invention, as will be pointed out hereinafter. It is more difficult to produce a uniform, homogeneous alloy of lead, magnesium and silicon, but such alloys can be made by taking suitable precautions, particularly by using relatively high melting temperatures, as for instance temperatures above about 1900 F., and in some instances temperatures approximating the minimum values customarily employed in the casting of gray iron. The following are examples of procedures by which uniform, homogeneous alloys according to my invention can be made.

EXAMPLE A.Mg-Si-Pb ALLOY Charge: Grams Magnesium metal 4,000 97% silicon metal (MW-H0 M.) 3,400 Lead 12,600

Total 20,000

All the magnesium metal was charged at the bottom of a plumbago crucible, the 97% Si metal A+10 M.) was placed over the Mg metal, and finally covered with part of the lead. The charge was melted down in a gas furnace, the balance of the lead was added, the melt was heated to about 1900 F., the alloy stirred well and poured into a cast iron ingot mold. The alloy analyzed:

Alloy N0. Mg Si Pb A 23. 28 15. 35 Balance.

EXAMPLE B.Mg-Si-Pb-Fe ALLOY Charge: Grams Lead-magnesium alloy (60 Pb-40 Mg) 1,000 97% silicon metal (MW-+10 M.) 610 Lead 200 Steel punchings Total 2,000

The silicon metal, lead and steel punchings were melted down in a high frequency furnace using a magnesia crucible. When the bath was molten, the lead-magnesium alloy was added, the bath was stirred well, and the molten alloy poured at about 2400 F. into a cast iron ingot mold. The solid alloy analyzed:

punchings and lead. This charge was melted in a high frequency furnace and then heated to about 2200 F., the molten bath was stirred well, and poured into a cast iron ingot mold. A very uniform, homogeneous ingot was obtained. The final alloy analyzed:

Alloy No. Mg Si Pb Fe O c. 18. 13 28. 72 40. 06 Balance.

EXAMPLE D.Mg-Si-Pb-Mn ALLOY Charge; Grams Silicon-magnesium alloy (60 Si-40 Mg) 627 Lead-magnesium alloy (60 Pb-40 Mg) 250 Lead 800 Manganese-silicon alloy (62 Mn-21 Si) 323 Total 2,000

The above charge was melted down in a high frequency furnace using a magnesia crucible. The molten alloy was stirred well, and tapped at about 2000 F. into a cast iron 1ngot mold. The alloy analyzed:

Alloy No. Mg Pb EXAMPLE E.-Mg-Si-Pb-Mn ALLOY Charge: Grams Magnesium metal 400 97% silicon metal 10 M.) 400 Manganese metal (10 M.) 200 ead 1,000

Total 2,000

Pb I Mn Each of these alloys A to E is a uniform, homogeneous alloy as shown by examination of its fracture. Each of the fractures was uniform and showed only a single constituent, there being no indication in the fracture of any of the original charge constituents.

My uniform, homogeneous alloys are a distinct improvement over compositions known prior to my invention, and particularly in comparison with those compositions disclosed in Becket Patent 1,622,078. I have made compositions corresponding to Examples I, II and III of the Becket patent, and have used them in the treatment of molten cast iron and have found that they are not nearly as effective as my uniform, homogeneous alloys, and have also found that they are distinctly less desirable for other reasons hereinafter referred to.

The following Example F corresponds to Example I of the Becket patent.

EXAMPLE F An alloy containing:

Per cent Lead 92 Magnesium 8 was made by melting down 11,000 grams of lead in a plumbago crucible heated in a gas furnace to a temperature of about 1000 F., and thereafter adding metallic magnesium in amount sufiicient to produce the alloy containing 92% lead and 8% magnesium. The magnesium dissolved readily in the bath. The molten bath was well stirred, and then poured into a cast iron mold. The product of this example is hereinafter referred to as composition F. The fracture of this binary alloy showed a uniform, homogeneous product. Upon breaking the alloy, the fracture became masked with a black, powdery product after /2 to 1 minute exposure in air. When the alloy was crushed to 1" by down, the crushed product ignited spontaneously in air after about 3 to 5 minutes, glowed intensely, and burned to a yellow powder. The product, due to its instability, was entirely unsuited as an addition alloy.

The following Example G corresponds to Example II of the Becket patent, and the composition resulting therefrom is referred to hereinafter as composition G.

EXAMPLE G parts by weight of magnesium shavings were mixed with 56 parts by weight of lead shot and 34 parts by weight of a manganesersilicon alloy containing about 45% silicon and 55% manganese, and the mixture was pressed into briquettes.

As is obvious from the method of making the briquettes and also as was shown by examination of their fracture, the briquettes were merely heterogeneous mechanical mixtures of the individual components of the original mix.

The following Example H corresponds to Example III of the Becket: patent, the composition resulting therefrom being designated as composition H.

6 EXAMPLE H A mixture consisting of:

Grams Magnesium metal 2,550 Lead 8,400

was melted in a plumbago crucible heated in a gas furnace. This charge was heated to 1600 F., stirred well, and then 2,025 grams of a manganese-silicon alloy (minus mesh) containing about 43% Si, 53% Mn and 2%Fe was stirred into the molten bath and finally tapped into a cast iron mold. Even after long soaking in the furnace, the finely crushed manganese-silicon alloy was not dissolved in the molten magnesium-lead bath. The fracture of this cast showed a non-homogeneous com position in which the fine particles of the manganese-silicon alloy were entrapped in the magnesium-lead alloy. Furthermore, the cast product after standing in air for 24 hours disintegrated into a fine powder.

Tests were made to show the effectiveness of alloys B and C (according to the present invention) and compositions F, G and H (according to the Becket patent) when added to molten cast iron baths. The cast iron had the composition:

T. C. Si P The different addition agents containing magnesium were added to the molten cast iron having a temperature of about 2700 F., the bath was held /2 to 2 minutes, inoculated with 78% ferrosilicon, held to 1 minute, and then cast into standard arbitration test bars (1.2" dia. x 18" span). In heat No. 1, since composition F contained no silicon, 0.27% silicon was added to the melt down charge to bring the final silicon content of the iron to the same level as the other heats. The five sets of bars were then subjected to bend tests. The results of these tests as well as other pertinent data are given in the following table:

Table Percent Si Cast Iron Mg Added by Breakmg Deflection, Heat No posmon f Fe-Si f Inch Added Percent Inoculation bs.

F 0 55 0. 38 2, 127 0. 27 G 0 55 0. 38 1, 770 0. 24 H 0 55 0. 38 2, 000 0. 25 B 0 45 0. 38 3, 495 0. 33 C 0 78 0. 38 3, 270 0. 28

The table shows that cast iron, treated with the uniform, homogeneous alloys (alloys B and C) of the present invention were greatly superior to those produced either by alloy F made according to the Becket patent and containing 92% lead and 8% magnesium but no silicon, or by the heterogeneous briquette G made according to the Becket patent, or by the non-homogeneous composition H made according to the Becket patent.

The amount of my uniform, homogeneous alloy to be 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 which the addition is made, the sulphur content 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 /2 times the sulphur content of the iron. This is not an absolute amount but varies acording of the precise composition of the alloy 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 leave 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 sufficiently 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. Other than taking suitable precautions to protect against lead fumes which may be released during treatment of the 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 to 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.

In improving the mechanical properties of the cast iron the alloys covered by this invention regulate the microstructure of the gray iron so as to produce nodular graphite in substantial amount, usually to the extent of complete conversion of the carbon to this form.

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 sulhpur 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 hitherto 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. 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-lead 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 The amount 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, that is, attainable with the final chemical composition of the iron, 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 for the chemical composition also results in maximum ductility of the iron composition.

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 sulphur content decreased flfiJlTl about 0.030% to about 0.020% by the use of these a oys.

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 uniform, homogeneous alloy for addition to iron or steel, said alloy consisting essentially of magnesium, silicon and lead except for customary impurities and minor elements in an amount not exceeding a total of about 4% and except for nickel in an amount up to about 2%, the alloy containing about 5 to 30% magnesium, about to 40% silicon, and about to 70% lead, said alloy resulting from solidification of a completely liquid solution and upon fracture showing only a single constituent.

2. A uniform, homogeneous alloy for addition to iron or steel, said alloy consisting essentially of magnesium, silicon, lead and iron except for customary impurities and minor elements in an amount not exceeding a total of about 4% and except for nickel in an amount up to about 2%, the alloy containing about 5 to magnesium, about 15 to silicon, about 20 to lead, and not over about 40% iron, said alloy resulting from solidification of a completely liquid solution and upon fracture showing only a single constituent.

3. A uniform, homogeneous alloy for addition to iron or steel, said alloy consisting essentially of magnesium, silicon, lead and manganese except for customary impurities and minor elements in an amount not exceeding a total of about 4% and except for nickel in an amount up to about 2%, the alloy containing about 5 to 30% magnesium, about 15 to 40% silicon, about 20 to 70% lead, and not over about 12% manganese, said alloy resulting from solidification of a completely liquid solution and upon fracture showing only a single constituent.

4. A uniform, homogeneous alloy for addition to iron or steel, said alloy consisting essentially of magnesium,

silicon, lead, iron and manganese except for customary impurities and minor elements in an amount not exceeding a total of about 4% and except for nickel in an amount up to about 2%, the alloy containing about 5 to 30% magnesium, about 15 to 40% silicon, about 20 to 70% lead, and not over about 12% manganese, the sum of the iron and manganese not exceeding about 40%, said alloy resulting from solidification of a completely liquid solution and upon fracture showing only a single constituent.

References Cited in the file of this patent UNITED STATES PATENTS 1,622,078 Becket Mar. 22, 1927 2,485,760 Millis et al. M Oct. 25, 1949

Claims (1)

1. 3. A UNIFORM HOMOGENEOUS ALLOY FOR ADDITION TO IRON OR STEEL, SAID ALLOY CONSISTING ESSENTIALLY OF MAGNESIUM, SILICON, LEAD AND MANGANESES EXCEPT FOR CUSTOMARY IMPURITIES AND MINOR ELEMENTS IN AN AMOUNT NOT EXCEEDING A TOTAL OF ABOUT 4% AND EXCEPT FOR NICKLE IN AN AMOUNT UP TO ABOUT 2%, THE ALLOY CONTAINING ABOUT 5 TO 30% MAGNESIUM, ABOUT 15 TO 40% SILICON, ABOUT 20 TO 70% LEAD, AND NOT OVER ABOUT 12% MANGANESE, SAID ALLOY RESULTING FROM SOLIDIFICATION OF A COMPLETELY LIQUID SOLUTION AND UPON FRACTURE SHOWING ONLY A SINGLE CONSITUENT.