US2253502A - Malleable iron - Google Patents

Description

Patented Aug. 26, 1941 UNITED ASTATE s PATENT OFFICE I MALLEABLE IRON Alfred L. Boegehold, Detroit, Mich., assignor to General Motors Corporation, corporation oi.' Delaware Detroit, Mich., a

Application February 23, 1938, Serial No. 191,876

(Cl. 'l5-123) v 12 Claims.

This invention relates to a ferrous alloy and to a process of producing the same. More particularly the invention relates.to irons suitable for making malleable iron castings and the manufacture thereof. .l

In my prior Patent 1,707,753, dated April 2,

carbon during solidication of the iron without retarding the action'of the graphitizing agent in promoting :formation of temper carbon during the subsequent annealing operation when the graphitizing agent is present in excess of an amount equivalent to about 1.3% silicon. This 1929, I- have described andclaimed certain im- ,f

provements in the manufacture of white iron castings for .malleabilizing whereby the time necessary to anneal the same may be materially reduced over the prior art practice. The patent teaches the use of a graphitizing agent, such as silicon, capable of promoting formation of temper f carbon during annealing and an ingredient having the property of obstructing formation of ilake' graphite during solidication of 4the casting.

In my prior patent, the proportion ofthev graphitizing agent used was considerably larger than in the teachings of the prior art. This was made possible by the use of the ingredient having the property of obstructing formation of ake graphite during solidication of the casting.

I have now found a new method. and new alloy for making white iron castings in which the time in ake form during solidification of the casting.

In order to reduce the annealing time it is desirable that the graphitizing agent be present in lsuch amount asl to facilitate the formation of temper carbon during the annealing operation. My invention, therefore, comprises the use of small quantities of tellurium in iron compositions containing graphitizing agents in amounts equivaient in effect to 1.3%, or more, of silicon, which compositions are such that without the addition of tellurium grey iron would be formed upon solidication. In this way thel amount of the graphitizing agent-may be greatly increased and lstill attain white irons that may be readily annealed to form malleable iron.

I have found that tellurium has a powerful eii'ect in preventing the formation of grapliitit;

permits, if desired, the use of compositions of higher carbon contents, with the resulting advantages ci' greaterfluidity of the iron, as well as the use of increased amounts of silicon which greatly reduces the time of annealing and also permits maintaining a higher silicon content in heavier-sections than possible with ordinary iron without appearance of primary graphite. v

The amount of tellurium necessary to stabilize the carbide in accordance with the invention` is relatively small. Additions of tellurium to the molten metal as low as .0001% have an appreciable effect in light sections and, in general, amounts greater than .2% are unnecessary. Larger amounts thanl.2% may be used without detrimental effect on the malleabilized product; however, the higher amounts for most'purposes are not recommended for the reason that the addition of tellurium to a molten ferrous metal bath results in fumes'which are disagreeable and have av toxic eil'ect. Consequently, the lower amounts of 'tellurium are less objectionable on this ground and for this reason are preferred.

For most purposes tellurium within the range .003% to .02% is entirely satisfactory for castingsto be malleabilized having approximately 1.75% silicon.

The form 1n which the tenurium is added to` the iron is immaterial to the invention. MetallicA tellurium may be added to the molten iron coming from the cupola or electric furnace or, if desired, tellurium oxide may be added. 'I'he oxide is easily reducible and the iron reduces the same so that the tellurium alloys with the other constituents of the iron, as in the case of the addition of metallic tellurium. Other alloys or compounds containing tellurium may be usedI as the tellurium addition agent.

The following illustrative examples of practice in accordance with my invention may be given:

Example 1 An alloy containing 3.25% carbon, 2.25% silicon, .40% manganese and the balance iron with small amounts of impurities, such as phosphorus, sulphur, etc., normally present in.. cast irons was cast into a test bar 1/2". x 11/2" X31/s". The structure of the iron in the bar was grey through- Qut. Metallic tellurium was added to a molten iron of the same analysis to the amount of .10% of the alloy and cast into the same sized bar as beiore. The bar was white throughout. Annealing by heating the bar up to 1700 F. for about six hours substantially completely decomposed the iron carbide.

Example 2 An alloy containing 2.75% carbon, 1.75% silicon and the balance iron plus about .40% manganese, .07% phosphorus, and .11% sulphur, was cast into a test bar 1/2" x 11/2" x 31/2". The bar showed a grey structure throughout. The addi- An alloy was made containing 3.38% carbon, 1.91% silicon, and 0.40% manganese and the balance iron plus usual impurities such as phosphorus and sulphur in small amounts. To m01- ten iron of the above analysis the following amounts of tellurium were added: .0001%, .0005%, .001%, .003%, .005%. The several irons were cast into wedge bars, tapering from a point to a 3" square section at the opposite end to determine the minimum amount of tellurium required. Fractures of these bars showed that the smallest quantity of tellurium had a measurable effect as a carbide stabilizing agent and indicated that .003% tellurium is suicient for making white iron castings of compositions having a silicon content as high as 1.75% in castings having a cooling rate in the mold similar to a square by 7" long bar.

In general, as the proportion of silicon is increased or decreased the amountof tellurium that may be necessary to obtain white iron also will be increased or decreased, respectively. The same holds true for an increase or decrease in the carbon content of the iron. In the larger amount present is thought to be somewhat less than the amounts added to the molten metal due to volatilization and with the larger amounts of tellurium addition the quantity present in \.the casting will be proportionately less than with the smaller additions. Qualitative tests show that in each case the solidified white iron casting Acontains tellurium. It has also been shown that upon remelting the iron, the fracture obtained upon recasting is typical of the one to which tellurium had been added which is proof that the tellurium remains in the iron in eiective amounts.

'Ihe amounts of manganese and carbon in the iron may vary within ranges of known whitek irons suitable for annealing. If desired to increase the manganese content over the amount ordinarily used in standard compositions, i. e. about .40%, or to add other elements to the iron which would modify the physical properties of the annealed iron it may be necessary to raise or lower the amount of silicon present depending on whether the addition agent is a carbide stabilizer or a graphitizer. Elements in addition to silicon which may be classed as graphitizersinclude aluminum, barium, calcium, copper, lithium, magnesium, nickel, potassium, sodium, titanium and zirconium, while antimony, boron, chromium, manganese, molybdenum, sulphur, tin, tungsten and vanadium may be classed as carbide stabilizers. For example, if it is desired to obtain higher strength by increasing the manganese content over that of the standard composition, it may be necessary to slightly increase the silicon content to counteract the carbide stabilizing iniiuence of the manganese. On the other hand if it is desired to add an element such as nickel or copper for increasing the tensile strength of the malleable iron it will be necessary to decrease the silicon content because these elements act as graphitizers in the same manner as silicon except that they are not as powerful. These examples, as well as others, are illustrated in the following table with an iron containing 2.75% carbon and show how the silicon should Percent Percent Percent Percent Percent Percent Percent C Te Mn S P oy Regular malleable 2. 75 1. 20 40 11 08 Tellurum only 2. 75 1. 60 003 r .40 11 08 Manganese increased. 2. 75 i. 75 .003 1. 00 11 08 Copper added 2. 75 1. 20 .003 .40 1l .08 1.00 Cu Molybdenum added 2. 75 1. 75 .003 40 1l .08 .50 Mo Copper and manganese added 2. 75 1. 50 003 1. 00 11 08 1.00 Cu sections it may be desirable to increase the tellurium content somewhat.

As an example of the effect of tellurium and silicon on the fractures of test castings two inches in diameter and twelve inches long, attention is directed to the iigure. The curves indicate thevapproximate amounts of the tellurium and silicon necessary to obtain White, mottled and grey irons. In addition to the tellurium and silicon the test specimens contained 2.75% carbon, .40% manganese, .12% sulphur, .07% phosphorus and the balance iron.

The amount of tellurium in the white iron casting prior to annealing is not known definitely. Quantitative tests for tellurium in the small amounts used herein are not accurate. Consequently, the exact amount of tellurium in the casting is not known. In each case the One method for converting a grey iron melted in the cupola, into a white iron suitable for the manufacture of malleable iron, is to add steel. The amount of steel required depends upon the carbon and silicon content of the cupola greyl iron. Even if it requires only 3 or 4% steel addition to convert a grey to a white iron, considerable labor is involved. About 1 to 2 ounces of tellurium per ton will serve substantially the same purpose at a considerably lower cost for labor.

The term white iron casting as used herein means a carbon containing iron free of appreciable amounts of carbon in flake form that characterizes ordinary grey iron castings. I do not intend the term to exclude annealable castings that may contain some nodular or temper carbon but contain substantially no iiake carbon. White iron as used commercially for the manufacture of malleable iron may have a small percentage of the structure containing ake graphite without detriment to the properties of the annealed product.V These small areas are known as mottles and are similar to grey iron in structure.

All white iron castings are not suitable for the manufacture of malleable iron castings. AA white iron casting containing no graphitizing element is unsuitable because the iron carbide will not decompose in a commercially useful period of time into iron and temper carbon upon subsequent annealing. Also, if the carbide stabilizer added to obtain a white iron of high silicon content is of such a nature that it makes the carbide so stable that it will not readily break down during annealing the iron is unsuitable for commercial malleabilizing, For example, chromium added to an iron with high silicon content may increase the tendency of the iron to solidify white but it also renders the carbides so stable theyare very difficult to break down upon subsequent an-v nealing. I have found that carbide stabilizers differ greatly in their relative stabilizing effect during solidification of the iron compared to their stabilizing effect during the annealing operation. Whereas chromium increases somewhat the power of an iron to solidify white, tellurium is exceedingly powerful in this property. On the other. hand chromium is powerful in preventing decomposition of iron carbide at annealing temperature even when the silicon content is high while tellurium has very little carbide stabilizing effect at the annealing temperature when 1.50% or more silicon is contained in the iron. In this respect tellurium is especially advantageous. In the drawing it is shown that an iron with 2.75% carbon and 1.75% silicon solidifles grey in a two inch round casting and is useless for making malleable iron, whereas the addition of .01% tellurium to this iron by its effect on the carbide makes the iron solidify white and yet it will anneal so rapidly because of the 1.75% silicon that the rst stage of annealing is complete iny three hours as compared to ten hours for malleable iron of the usual composition containing about 1.20% silicon.'

Annealed castings in accordance with my invention s how the same microstructure that charfor use Wherever malleable castings may be employed.

My invention is not limited to irons having carbon contents suitable for ordinary malleable castings. For example, it may be desired to cast ingots eight or ten inches in diameter containing as low as 1.5% carbon, which, aftery partial graphitization or annealing may be rolled into bars, or pierced and drawn into tubes, Small additions of tellurium permits the use of higher silicon content Without the appearance of flake graphite in the ingot as cast. This facilitates the annealing orv graphitization and may be useful in modifying the physical properties lof the final product due to the alloying effect of the silicon. v

It. will be understood that various modifications and changes may be made in the embodiment of my invention set forth herein by skilled metallurgists without departing fromy the principle and spirit of my invention and I do not desire to limit the patent granted thereon except as necessitated by the prior art.

I claim: l. An annealable white iron casting substantially free from flake graphite and characterized l by the rapidity with which it can be annealed to form.malleable iron containing, at least 1.50% carbon, atleast about .40% manganese, a graphitizing agent equivalent in effect to approximately .range 2-3.50% and the graphiti'zing agent is present in amount equivalent in effect to approximately 1.3-2.25% of silicon.

3. A white iron casting characterized by the rapidity with which it can be annealed to form malleable iron, composed of the following elements as essential constituents in substantially the proportions stated:

Per cent Carbon l 1;503.50 Silicon LBO-2.25 Manganese .4G-1.00

a small but effective amount of tellurium up to about `.2% sufficient to obstruct formation of flake graphite during solidification of the casting without appreciably retarding the decomposition of iron carbide during annealing, bal- Vance substantially all iron.

' 4. A white iron`casting substantially free of flake graphite characterized by the rapidity with which it can be annealed to form malleable iron containing approximately 15o-3.50% carbon, approximately 1.30-2.25% silicon, approximately .40-1.00% manganese and a small but effective amount of tellurium up to about .2% sufficient to obstruct formation of flake graphite during solidiiication of the casting without appreciably retarding the decomposition of iron carbide during annealing to form malleable iron.

5. A white iron casting substantially free of ake graphite characterized by the rapidity with which it can be annealed to form malleable iron containing approximately LBO-3.50% carbon, a graphitizer equivalent in eiect to approximately 1.30-2.25% silicon, approximately .4G-1.00% manganese and a small but effective amount of tellurium up to about .2% sufficient to obstruct formation of flake graphite duringsolidiflcation of the casting without appreciably retarding the decomposition of iron carbide during the annealing to form malleable iron.

6. An annealable white iron casting substantially free from flake graphite characterized by the rapidity with which it can be annealed to form malleable iron containing, at least 1.50% carbon, a graphitizing agent equivalent in effect to at least 1.3% silicon and a small but effective amount of tellurium up to about .2% sufficient to obstruct formation of ilake graphite during solidication of the casting without appreciably retarding the'decomposition of iron carbide during the annealing to form malleable iron.

7. An annealable white iron casting substantially free from ake graphite and characterized by the rapidity with whichit can be annealed to form malleable iron composed as follows: approximately 2.00-3.50% carbon, approximately LSG-2.25% silicon, approximately .4G-1.00% manganese, a small but effective amount f tellurium up to about .2% suflicient to obstruct formation of flake graphite during solidication of the casting Without lappreciably retarding the decomposition of iron carbide during annealing, and the balance substantially all iron.

8. An annealable white iron casting substantially free from flake graphite and characterized by the rapidity with which it can be annealed to produce malleable. iron containing at least 1.50% carbon, at least 1.3% silicon, and a small but -about ..2% suiilcient to obstruct formation of `ake graphite during solidication of the cast-ing.

10;. As an article of manufacture, a-malleableized cast iron having temper carbon and containing 1.50% to 3.50% carbon, a graphitizing agenty equivalent in effect to 1.30% to 2.25% silicon, and tellurium in small but eiective amount not to exceed .2%.

'11. Process of making a malleable cast ironV product comprising casting a fused mixture containing carbon in excess of 1.5%, a graphitizing agent equivalent in eii'ect to at least 1.3% silicon and iron substantially the remainder, in the presence of a small but effective amount of telluriurn sufficient to substantially obstruct separation of carbon in flake form during solidiflcation, said amount being between .0001% and .2% of the mixture, thereby producing a white cast iron body, and then subjecting said body to an annealing treatment to cause separation of at least a portion of the carbpn as temper carbon.

12. Process of making a malleable cast iron ar-1 ticle comprising casting a mixture containing 1.50%-8.50% carbon, 1.30%-2.25% silicon and not lessthan about 94% iron, in the presence of a small but eiective amount of tellurium, from .0001% to .2% of the mixture, sumcient t obstruct formation of carbon in ilake form during solidification, thereby producing a white cast iron article, and then annealing the article under conditions to cause separation of at least'a portion of the carbon as temper carbon.

ALFRED L. BOEGEHOLD.

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