US2870004A - Method of producing nodular cast iron - Google Patents

Description

United States Patent METHOD OF PRODUCING NODULAR CAST IRON James W. Estes, New Brunswick, and Grant E. Spengler, Plainfield, N. J., assignors to Air Reduction Company, IYIICCETPOIHtGd, New York, N. Y., a corporation of New N 0 Drawing. Application February 7, 1955 Serial No. 486,714

5 Claims. 01. 75-51 This invention relates to the desulfurization, upgrading and nodulizing of cast iron and particularly concerns means therefor which include provision for the addition of elemental magnesium metal to molten cast iron.

It has previously been found that an upgraded or nodular gray cast iron having properties superior to conventional gray cast iron can be produced by treating the molten iron so that, when cast, the graphite will be compacted rather than in flake form. One such treatment involves the introduction of magnesium into a molten iron bath of such composition that it would, if cast normally in the absence of any treatment, produce conventional gray cast iron in the as cast condition. Before such magnesium treatment will be effective to produce partial compacting (upgrading) or complete compacting (nodulizing so that substantially only spheroidal graphite exists in the cast iron), it is usually necessary to eliminate sulfur in some manner. It is highly desirable that the entire process be done so that there is a small time loss, a small heat loss, and a small cost for treating agents.

Magnesium is a well-known nodul'izing-impelling or compacting agent. However, the problems involved in adding the metal itself to molten cast iron have resulted in statements in issued patents that it is impossible to add magnesium metal per se to molten cast iron without a very violent reaction and complete loss of the magnesium. The problems encountered in adding metallic magnesium to molten cast iron are due principally to the rapid conversion of magnesium to gas at the temperatures involved. It compares .to dropping Water on boiling grease. Another factor is the reactivity of magnesium with oxygen which results in pyrotechnic display.

The primary object of the instant invention is to provide an improved, rapid, economical injection treatment process for the upgrading and nodulizing of cast iron and an improved treating agent for use in sucha process.

Another object of the instant invention is to provide an improved method for introducing elemental magnesium The accomplishment of the foregoing objects others, along with the features and advantages of the invention, will be apparent from the following description.

Generally speaking, the present invention comprises forming a molten quantity of cast iron, injecting beneath the surface of theiron a relatively dense stream of an inert gas, calcium carbide, and magnesium into the iron and then promptly casting the iron. Finely-divided calcium carbide and magnesium powder are injected and cooperatively affect the iron so that compacting of the graphite results when the iron is cast. The magnesium must be of such a size thatit will pass through a 20 mesh Tyler standard sieve or screen and most' will be retained on a mesh screen. The magnesium powder must not exceed 15% of the mixture of calcium carbide and magnesium and preferably about 1-6 pounds" of magnesium per ton of iron are injected. The carbide must be of such size that it will pass a 20 mesh screen and most (90%) will be retained on a 200 mesh screen. The density of the injection stream should be from about /1-2 cubic feet of gas per'vpound of treating agent. The gas, of course, must bemaintained at a pressure suflicient to overcome the static head of the molten iron and thus prevent entrance of the metal into the passage through which the mixture'is injected. The rate of feeding can vary from about 5 to about 30 pounds of mixture per ton of iron perminute depending on the sizeof the ladle or quantity of iron-and the type of injection apparatus. The entire injection should be performed so that not more than about -10 minutes are required for the injection. The casting should be made promptly, that is, begin casting within about 10 minutes after injection.

' Since desulfurization is required in most instances,-it is decidedly preferred to inject finely-divided calcium carbide by meansof an injection tube in order to reduce the sulfur level to at least 0.03%, more preferably appreciably less than 0.03% or about-0.01%, and then to inject the carbide-magnesium mixture immediately through the same injection tube. In most instances, it is also preferred to inoculate the treated iron with ferrosilicon (75% silicon) or a similar inoculating agent after treating.

The amount of magnesium used will vary within the above preferred range or the operable range of /2 vto 12 poundsper ton of iron depending on several circumstances.

More magnesium is used if it is not desired to desulfurize mesh but greater than 200 mesh. The apparent density of the magnesium powder preferably'is about 0.7 gram per milliliter. Only slight tapping of the graduated vessel to eliminate voids was eifccted. V.

' The iron which is benefited by the instant invention is an iron initially having a gray cast iron composition, that is, .an iron which, if cast without treatment, would produce a cast iron having a tensile strength and the flake graphite, typical of gray cast iron. The molten iron should be less than 2750 F. at the time of injection. It is to be noted that the product resulting from the instant invention will normally contain retained magnesium in an amount which by itself would be insuflicient to produce the type of product obtained. The fact that less retained magnesium is necessary to produce a given structure is due to the action of the carbide. The apparatus for injecting the instant treating agent can be, for example, the screw feeder device shown in the Hulme Patent No. 2,577,764; the batch feeder as disclosed in the Clenny-Rowen U. S. application Serial. Number 430.864: or the fluidizer injectiondevice shown in the Muller. Fisher and Hulme U. 8'. application Serial Number 324.998 (all assigned to-the assignee of the instant application). All'of these apparatus are so built as to provide .a solid gas stream having a density of about /1 m2 gas-to-solid ratio (cubic feet of gas per pound of mixture). The preferred inert gas which is used to inject the instant treating agents and calcium carbide is dry nitrogen. Other gases which can be used are helium, argon, and carbon dioxide. The term, inert gas, herein meansa gas which does not react with magnesium or iron under the described conditions.

The instant treating a ent is prepared by intimately mixin by any conventional means between 10 to 17 parts of finely-divided calcium carbide with l to 2 parts magnesium owder so that the ma nesium constitutes less than The calcium carbide is a commercial grade calcium carbide, quite free of impurities, as made in electric arc furnaces from calcium oxide and hi h quality coke. As above mentioned, the calcium carbide is sized so that all of it pas es through a mesh screen and no more t an about of it passes through a 100 mesh screen (standard Tyler screen). The magnesium powder is in the form of very small irregular chip-like particles. Under the microscope, these particles have irregular edges which are frequently rolled-up. These magnesium particles must be sized, as above mentioned, so that they pass a 20 mesh screen and 90% are retained on a 100 mesh screen. A typical screen analysis of the magnesium was 100% minus 20; 86.7% minus 48; 42% minus 65; and'6.8% minus 100. The calcium carbide particles are also graduated as to size although more particles are finer than with the magnesium. The quantities of the components of the instant treating agent which are used per ton of gray cast iron range between 10 and 80 pounds for calcium carbide and between /2 to 12 pounds for magnesium. In any event, the magnesium must not exceed 15% of the total.

In the initial tests made with small particles of magnesiurn, it became apparent that preventing plugging of the injection tube was a major consideration independent of hazards as to the amount of magnesium or the efii ciency of the injection. With the proportioning and particle sizes herein taught, plugging of various sized injection tubes is substantially avoid ed. Thus, the instant features resolve both the problem connected with the feeding of magnesium particles through a red-hot carbon injection tube to beneath the surface of molten cast iron and the hazardous problem of violence involved when a quantity of magnesium contacts molten cast iron which is above the vaporization temperature of magnesium. This latter problem primarily involves a change of state hazard, due to a solid being suddenly converted to a gas occupying many times the volume. There is the further hazard of magnesium oxidizing at the surface or flaring. The present invention minimizes this flaring problem since the magnesium is dispersed beneath the surface, is effectively dissolved into the iron '4 and is partially shielded at the surface by the slag formed by the carbide. It is to be appreciated also that the instant mixture contributes to safety in storage and handling. Segregation of the magnesium during feeding is also minimized due to the interlocking of the rough edges of the magnesium and carbide.

To illustrate in detail the application of the process provided by the instant invention, a preferred embodiment th reof will be described. A molten cast iron bath was established which, if then cast, would result in gray cast iron containing flake graphite. This bath was injected with 16 pounds of carbide through a carbon injection tube in order to reduce the original sulfur percentage of 0.082 and further benefit the iron. Next an intimate and homogeneous mixture of magnesium and calcium carbide was injected immediately through the same tube. Dual hoppers can be used or fast recharging can be done so that not more than one minute is required between injections. The mixture was proportioned 36 pounds of carbide per ton of iron to 4 pounds of magnesium per ton of iron so that the magnesium constituted about 10%. Dry nitrogen was used as the carrier gas. The injection was at the rate of 28.5 pounds of mixture per ton of iron per minute. The gas to solid ratio was 0.418 cubic feet per pound. The carbon injection tube was immersed more than /2 the depth of the basic-lined vessel. A depth of at least 6 inches is essential. The temperature of the iron was about 2700 F. Ferrosilicon (75% silicon) was added immediately after injection in an amount equivalent to one percent per ton. The iron was cast within 3 minutes after completion of the injection. A fully nodular cast iron was obtained. This iron had an ultimate tensile strength of 62,250 p. s. i., a Brinnell hardness number of 143, an elongation of 18.5% in two inches, and a chill 0f of an inch. The foregoing test is test number 882-l in the following tables which give comparable data for other tests as well as chemical analyses.

Some cast irons contain small amounts of elements which are sometimes referred to as subversives or inter fering elements. The elements lead, arsenic, tin, and

antimony are some of the elements believed to be subversive. When such irons are encountered as can be determined by results which are inconsistent with those shown as generally obtained with treating with calcium carbide and magnesium, it is preferred to incorporate a small amount of rare earths oxides (REO) in the carbidemagnesium mixture. Apart fromthe foregoing consideration, it is preferred to use REO when the compacted graphite as herein described is desired and small quantities of magnesium are used since rare earths eifect compacting. Between one and two pounds of REO per ton of iron are preferred. The REO is finely-divided, being at least 20.by 0 mesh-size. The REO contains about 50% cerium oxide, 20% lanthanum oxide, and the remainder is carbide which is added is somewhat in excess of 10 pounds of carbide per pound of sulfur to be removed so that the sulfur level of less than 0.03% is rapidly obtained and the carbide can affect the iron in a beneficial way and make it more receptive to the effect of magnesium. The following table shows the proportioning, otherconditions, and results when carbide treatment is effected first.

Table l lnjleggij rrrolfgix Chemical Analysis Properties Temp Test No. F.

Percent Base Final Chill GaC: REQ Mg 2l91 T. 0. Si Mn 3 S P GE U. T.S. B. H. N. Elong. P8!

815-1 2,080 0.75 3.58 2.68 .22 .013 .000 .03 4.47 09,200 103 18.510 07-1 1.0 2,050 0.75 3.59 2.20 .08 .088 .003 .043 4.34 69,800 103 7.82 g 32 879-1 }2.0 2,700 1.00 3.50 2.57 .12 .082 .003 .028 4.43 03,200 140 18.02: s '4 880-1 2.0 2,700 1.00 3.00 2.34 .12 .107 .022 4.45 04,200 140 17.01.- 30 881-1. $3 1.19 2,700 1.00 3.50 2.57 .14 .100 .027 4.43 04,000 105 19.0 32 382- 2,700 1.00 3.00 2.43 .14 .082 -.028 4.48 02,250 i 143 18.57: '20 883-1 1.33. 266 2,700 1.00 3.55 2.32 .30 .113 .033 34.33. "81,300 180 11.52 24 884-1 2.0 2,700 1.00 3.55 2.40 .33 .103 .030 4.38 07,750 102. 0.5 32 003-1 1.0 2,700 1.00 3.57 2.44 .37 .000 .034 4.38 66,850 15 17.5.5 .10

The data under the heading Chemical Analysis ref Flakes were eliminated in substantiallyall of the tests fers to the final percent analysis as to'silicon, manganese and phosphorus. Base (initial) and final sulfur percent are both shown. The other data include pounds of the materials injected per ton of iron and properties such as ultimate tensile strength (U. T. S.) and elongations (Elong) as percentage in twoinches. The top quantity of carbide in each line is that usedfor initial desulfurizing .andbenefiting. The second, or bottom, quantity is the amount of carbide incorporated into the compacting mixture. The abbreviations T. C. and C. B. respectively refer to total carbon and carbon equivalent. The temperature is that of the iron at thebeginning of the treatment. The term, percent late FeSi, refers to the addition of ferrosilicon (75% silicon) as a percentage of the iron being treated. The ferrosilicon was added by reladling, although it can be injected. When no value is given, ferrosiliconwas not added.

herein described. It was quite apparent that thewormy graphite was a strength-contributing factor. Examples of irons having compacted graphite as a mixture of nodules and wormy formations are.tests No. 884-1 about 60% nodules (supra); No. 871-1 about nodules (infra); and No. 872-1 about 50% nodules (infra). In general, the higher the percentage of nodules, the higher the tensile strength. Sometimes it may be desirable to treat with mixtures 0 magnesium and calcium carbide without immediate prior carbide treatment. This can be done either when low sulfur iron is available or when, with higher sulfur iron, there are advantages in using only one mixture comprised .of magnesium and carbide. The following Table II shows the proportioning, conditions, and results when magnesium and carbide were injected together.

Table II Injection Mix Chemical Analysis Properties (Lbs/Ton) Test No. Temp., F. Percent Base Final Chill 0.02 Mg 7 Se T. C Si Mn S S P O. E U. T. S B. H. N Elong. 142

25 3. 3 2, 740 0. 6O 3. 61 2. 34 42 077 015 11 4. 42 51, 300 167 2. 0 12 36 4. 0 2. 595 3. 61- 2. 08 43 .083 008 .11 4. 34 59, 300 285 Or 118 36 4. 0 2, 595 0. 3. 61 2. 48. 43 083 009 11 4. 47 78, 200 176 7. 5x 10 44 6. O 2, 715 0. 3, 15 2. 57 42 O88 017' .11 4. 04 62, 300 211 2. 0 12 34 6. 0 2, 650 0. 60 3. 45 2. 42 51 087 013 11 4. 29 81, 400 174 9. Oz 20 44 6. 0 2, 550 0. 75 3. 18 2. 66 44 082' 012 11 4. 10 79, 500 187 8. OJ: 16 44 6. 0 2, 500 3. 42 2. 21 21 023 006 O3 4. 14 70, 350 203 3. 51' 118 44 6. 0 2, 500 0. 60 4 3. 42 2. 58 21 023 006 03 4. 28 64, 400 149 21. 89: 14 18 2. 0- 2, 700 1. 40 3. 41 2. 77 0.13 .006- .066 .024 4. 34 56, 250 143 8.7 p 18 36 4. 0 2, 700 1. 00 3. 40 2. 55 0. 14 022 003 027 4. 26 63, 850 149 21. 02: 32 36 4.0 2, 700 1. 00 3.37 2. 62 0. 16 .013 004 03 4.24 V 66, 200 163 8. 5

, In test No. 883-1, the magnesium was 65- by 100 mesh. This magnesium powder passed the first screen and was retained on the second screen. This test shows the eifectiveness of the carbide in cooperation with very small particles of magnesium. I

The chill figure in the above and following tableswas "obtained by casting into a sand mold against a graphite 'block asa base. The wedge formed was 3% inches high, of an inch wide at the top and of an inch at the bottom. The tabulated figures are the depth (height) of the chill from the bottom of the wedge contacting the block in 32nds of an inch. I The irons in the above andfollowing tables which have an "xafter the elongation values were fully nodular, that is, the graphite was present only as nodules. The

.other irons were upgraded, that .is, not flaky but having compacted graphite in the. form -ofnodules and wormalike formatiqns;(thick, short strands).

Asindicated by Table II, it is apparent that it is pos- .sible to desulfurize and compact a relatively high base sulfur iron by using. only a'mixture of magnesium and carbide For instance, test No. 781-2 shows such a result and gave a fully nodular iron. It is to be noted that compacted graphite is obtained with temperatures as low as 2500 F. Tests No. 781-2 and 791-3 show this result and the making of fully nodular iron. Tests No. 783-3 and 858-1 respectively gave irons having 14% and of the graphite in the formof nodules, the remainder of the graphite being wormy graphite.

The following table III indicates the proportioning, other conditions, and results when using magnesium, RED and calciumcarbide-without immediate; prior dej sulfurizing sincetheinitial-sulfur level was lowlgin all tests as can be easily, and preferably is, efiected by carlbi e reatmen Y Tabfe. 'lIl Injection Mix Chemical Analysis Properties (Lbs/Ton) Tergp,

Test Percent Base Final Chill use, REO Mg Late '1. O. Si Mn S S P C. E U. T. S. B. H. N Elong. fiz

FeSi

An upgraded iron was also obtained from a low sulfur hypoeutectic iron by injecting 15 pounds of carbide per ton of iron and 1% pounds of magnesium per ton, with a 0.5% late ferrosilicon inoculation. The magnesium constituted about 7.5% of the mixture. The tensile strength was almost doubled although no nodules were made. This test indicates the preferred lower quantity andnecessary proportion formagnesiurn, that is, about one pound per ton of cast iron and about 5% of the mixture. a

It is to be noted that the instant invention is very useful for obtaining a nodular iron having a low silicon content, that is, less than 2.5% silicon, because the only silicon added is in the inoculation. Such an iron gives a higher impact strength than many nodular irons having-a greater percentage of silicon and can be used to replace brass in some applications.

By reference to the tests set forth in the above tables, it can be seen that many types of cast irons can be produced, ranging from fully nodular to upgraded irons having' tensile strengths of about 50,000 p. s. i. noted that these tests were made with cast irons which are at or near the eutectic point, 4.3% carbon equivalent (C. E.) which is the carbon percentage plus of the sum of the percentages for silicon and phosphorus. The preferred C. E. range is 4.0 to 4.6 although the instant invention is useful in treating irons within the range of 3.8 to 5. It is to be understood that basic-lined or acid-lined vessels or fore-hearth type structures with either type of lining can be used.

All mesh sizes herein specified are standard Tyler screen sizes. All percentages are weight percentages.

It is to be understood that the instant invention comprehends injecting a mixture of magnesium powder, fine- It is to be ly-divided calcium carbide and a small amount of other finely-divided substances such as rare earth substances,

magnesium oxide or graphite. However, the carbide should always constitute more than 80% of the mixture ;and the magnesium must not exceed 15% of the total.

In a test identical to No. 783-3, except for the omission of the late ferrosilicon addition, the simultaneous injection "of carbide, magnesium, and graphite (about 0.3% graphite in relation to the iron) was effected with good 7 results.

The aforementioned quantities of magnesium and other agents, the manner of introducing, and the size of the p'articles 'p'rovide for adding magnesium without exces'sive violence and without excessive magnesium flare "at the surface "of the cast iron and yet provide an improved, rapid and economical treatment. It is to be noted that the granular-like slag similar to the usual slag fro'mcarbide alone, is believed to be helpful in minimizing magnesium flare.

It is to be appreciated that the instant invention provides-an improved practical method for making upgraded lowered below that of alloy addition treatment, and the treated metal and foundry ladles are free of any drossy slags or scum. Lower temperatures of the iron at the time of treating do not reduce the eflectiveness of the carbide-magnesium treatment. It is also to be appreciated that the instant invention substantially eliminates or minimizes the violence and flaring sometimes encountered in adding elemental, pure magnesium and, equally important, substantially avoids plugging in the immersed injection tube.

As persons skilled in the art will understand, modifications and variations of and in the present invention as above described in detail can be resorted to without departing from the scope of the invention as defined in thefappended claims.

We claim:

1. A method for producing cast iron containing compacted graphite which comprises establishing a bath of molten cast iron composition which if cast Without treatment would contain flake graphite; continuously introducing a stream of an inert gas and a mixture of finely divided solid calcium carbide and elemental magnesium particles into said bath beneath the surface thereof; said magnesium particles constituting about 515% of the weight of said mixture; said carbide and magnesium particles being of a fineness suiilcient to pass a 20-rnesh screen and incapable of passing a ZOO-mesh screen; the ratio of said gas to said solid mixture being about A to 2 cubic feet of gas per pound of mixture; and promptly casting the resulting treated metal to obtain a casting containing a substantial amount of compacted graphite, said mixture effecting a product improvement which is significantly greater than the sum of the improvement that would be obtained by treatment with the same amount'of said calcium carbide alone plus the improvement that would be obtained by treatment with the same amount of said elemental magnesium alone.

2. The method according to claim 1 in which said stream contains a small amount of a finely divided rare earth substance, said amount constituting about one to two pounds per ton of said molten iron.

3. The method according to claim 1 in which a stream of inert gas and finely divided calcium carbide particles are injected into said iron immediately prior to said step of injecting said calcium carbide and said magnesium, and continuing the injection of said stream of inert gas and calcium carbide particles until the sulfur level is below 0.03%.

4. A composition for treating molten cast iron at normal founding temperature to produce a cast iron containing compacted graphite which comprises a mixture of finely divided calcium carbide particles and elemental magnesium particles; said magnesium particles constituting between 5% to 15% of said agent; said calcium carbide and magnesium particles being of a fineness sufficient to pass through a ZO-mesh screen and being of a size which is substantially entirely retained on a 200- mesh screen; and :saidcalcium carbide-and elemental magnesium -:particles being thoroughly mixed,

5. The composition a's' defined in 'c laim' 4 and being further characterized in that a small amount of finely divided oxides of rare earths is thoroughly mixed therein, said amount constituting about one to two pounds per ton of said molten iron.

. 5 References Cited in the file of this patent UNITED STATES PATENTS 2,488,512 Morrogh Nov. 15, 1949 2,527,498 Jordan Oct. 24, 1950 2,529,346 Millis et a]. Nov. 7, 1950 2,530,368 Jordan NOV. 21, 1950 10 Offenhauer Jan. 16, 1951 Jordan Mar. 6, 1951 Mills et a1 Aug. 14, 1951 Ziiferer Dec. 11, 1951 Hulme Dec. 11, 1951 Crome Dec. 16, 1952 Hignett Sept. 15, 1953 Morrogh Dec. 1, 1953 Morrogh May 29, 1956 FOREIGN PATENTS France Nov. 3, 1950

Claims (1)

1. A METHOD FOR PRODUCING CAST IRON CONTAINING COMPACTED GRAPHITE WHICH COMPRISES ESTABLISHING A BATH OF MOLTEN CAST IRON COMPOSITION WHICH IF CAST WITHOUT TREATMENT WOULD CONTAIN FLAKE GRAPHITE; CONTINUOUSLY INTRODUCING A STREAM OF AN INERT GAS AND A MIXTURE OF FINELY DIVIDED SOLID CALCIUM CARBIDE AND ELEMENTAL MAGNESIUM PARTICLES INTO SAID BATH BENEATH THE SURFACE THEREOF; SAID MAGNESIUM PARTICLES CONSTITUTING ABOUT 5-15% OF THE WEIGHT OF SAID MIXTURE; SAID CARBIDE AND MAGNESIUM PARTICLES BEING OF A FINENESS SUFFICIENT TO PASS A 20-MESH SCREEN AND INCAPABLE OF PASSING A 200-MESH SCREEN; THE RATIO OF SAID GAS TO SAID SOLID MIXTURE BEING ABOUT 1/4 TO 2 CUBIC FEET OF GAS PER POUND OF MIXTURE; AND PROMPTLY CASTING THE RESULTING TREATED METAL TO OBTAIN A CASTING CONTAINING A SUBSTANTIAL AMOUNT OF COMPACTED GRAPHITE, SAID MIXTURE EFFECTING A PRODUCT IMPROVEMENT WHICH IS SIGNIFICANTLY GREATER THAN THE SUM OF THE IMPROVEMENT THAT WOULD BE OBTAINED BY TREATMENT WITH THE SAME AMOUNT OF SAID CALCIUM CARBIDE ALONE PLUS THE IMPROVEMENT THAT WOULD BE OBTAINED BY TREATMENT WITH THE SAME AMOUNT OF SAID ELEMENTAL MAGNESIUM ALONE.