US2661283A - Lithium treated cast iron - Google Patents

Description

Dec. 1, 1953 O SMALLEY 2,661,283

LITHIUM TREATED CAST IRON Filed Aug. 24, 1950 RANGE OF TEMPERATURE IN WH/CH PRIMARY AUSTE/V/TE FORMS F/NAL SOLID/FICAT/ON TEMPERATURE Patented Dec. 1, 1953 UNITED OFFICE 2,661,283 LITHIUM TREATED CAST IRON Oliver Sinalley, Larchmont, N. Y., assignor to Meehanite Metal Corporation, a corporation of Tennessee Application August 24, 1950, Serial No. 181,129

2 Claims.

application No. 71,387, filed January 17, 1949, now

issued as Patent No. 2,527,037, dated October 24, 1950.

The gray cast iron to be treated is one which may be either hypo or hyper eutectic, that is, one in which the uncombined carbon appears in saucer or flake form when cast in sand. Such gray cast iron is characterized by being a more or less weak and brittle material having uncertain engineering properties. The poor mechanical properties of common gray cast iron are attributed to the presence of massive flakes of graphite which may occupy 8% to 12% of the volume of the casting and cut the matrix up much in the same way as wood shavings would do to concrete if indiscriminately mixed therein.

Many attempts have been made to improv the properties of gray cast iron by treatment with special metals and I particularly refer to the Patent No. 2,483,511 relating to the use of a small portion of cerium uncombined with sulphur; also to Patent No. 2385,761 relating to the use of .02 to 134% magnesium and Patent No. 2,485,760 relating to the use of 04% to about 3% of magnesium.

It has been discovered that through simple treatment with lithium or lithium bearing alloy either in the spout or in the ladle that the mode of occurrence of the graphite flakes in such iron can be materially affected in a beneficial manner tion to provide a gray cast iron in which at least a substantial part of the graphite exists in noduto improve both the castability and physical lar lorm in the as-cast condition.

3. It is still a further object to provide a gray cast iron having improved physical and mechanical properties even in the presence of relatively low or relatively high carbon content.

l. It is a further object of the present inven- ,tion to produce improved mechanical and phys ical properties by treating an iron that is impure and without having regard to complete removal of sulphur although lithium itself acts to some extent both as a desulp-hurizer and as a graphite stabilizer.

5. A still further object of this invention is to improve the graphite structure to assure mechanical properties and service behavior markedly superior to the same iron not treated with lithium or a lithium-bearing alloy.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings, in which:

Figure 1 is a graph illustrating a cooling curve plotted with temperature and time as ordinates;

Figure 2 is a photomicrograph taken at 250 magnification according to the teaching of this invention;

Figure 3 is a photomicrcgraph of the same iron taken at 2000 magnification and Figure 4 is a photomicrograph taken at 250 magnification of a cast iron of a second test made in accordance with the teaching of this invention.

The processes of the present invention involves in its simplest form only the addition of an appropriate amount of lithium to a molten cast iron of appropriate composition and solution of the lithium in the iron preferably in conjunction with a carrier metal.

It has been found that lithium has a definite whitening effect on gray cast iron. Thus a gray cast iron melt containing lithium in accordance with the invention will usually produce castings having a cementite network structure unless graphitization is employed following the lithium treatment. However, Where special properties such as wear resistance and heat resistance are desired, primary carbide may be present. Thus, white cast irons and alloy white cast irons may be improved by the present invention. In the case of gray cast irons, a graphitizing of lithium containing melt shortly before casting is an important feature of the invention and effectively prevents the formation of the aforementioned carbide network structure. The graphitizing treatment is preferably made shortly after the lithium treatment but may be made simultaneously therewith and shortly before the casting of the cast iron. The lithium containing improved gray cast iron of the invention will generally contain over 2% and less than 4.5% of carbon. Silicon content of preferred gray cast irons in accordance with the invention, will be at least 1% and usually fall within the range of 1.5% to 3.5 but not over 6%. The sulphur content is not of importance in that sulphur contents of .1% can be readily taken care of by standard desulphurization treatment while metal containing .l% or under of sulphur may be treated with lithium satisfactorily. The phosphorus may be any value according to the use to which the casting is to be put in service. If ductility is required without annealing or with heat treatment of any kind, then it is important that the phosphorus content be under .07%. Ductility is not always a requisite of nodular iron castings, but when required or improved impact properties are necessary accompanying higher tensile properties, then it is important that the phosphorus content be under 07%. If the irons are to be used for such purposes that the phosphide eutectic network is desirable, such as in certain bearing castings, then the phosphorus content may reach 1.5%. The gray cast irons may contain all usual amounts of such alloy elements as nickel, copper, molybdenum, chromium, and maganese, etc. The nickel may be present in amounts up to 40%, molybdenum in amounts up to 2%, copper in amounts up to manganese in amounts up to 2% and chromium in amounts up to 2%. So far as is known there has been no information published or any discovery made that lithium will increase the strength of gray cast iron above what is considered normal for gray cast iron, viz., 32,000 p. s. i., nor has any information been published showing that lithium will not only effect a change in the form of graphite from flake form to one predominantly nodular with resultant notable improvement in mechanical properties of the material much higher than what is considered standard to even high-test cast iron and that has a lower Brinell hardness and a superior machinability; nor has it been published that in conjunction with these properties such an iron would have markedly improved fluid life and castability; nor has it been shown previously that lithium in contra-distinction to other nodularizing agents such as cerium and magnesium may be used to effect a nodularization of the graphite in the presence of over 02% and up to .1% sulphur.

Notwithstanding the distinct novelties of the cast irons of the present invention it should be clearly understood that improved graphite structures and nodular graphite in cast iron have been a subject of common discussion in technical literature over the past decade. Also, as is well known, the production of nodular cast iron by heat treatment of a white iron has long formed the basis of a process for the production of malleable cast iron.

To enable a better understanding of the invention and the mechanism of graphite change a MECHANISM OF THE FORMATION GRAPHITE IN CAST IRON In high-carbon cast iron (hyper eutectic) microscopic graphite specks may exist in the molten iron. When this is present, these inclusions of graphite act as nuclei for the growth of massive flakes of graphite as the iron cools down. The so-called flake of graphite is not an atten uated curved string as portrayed by a photomicrograph, but rather has the form of a bent saucer or an irregular lens.

When there is no free graphite present (hypo eutectic), then primary austenite or solid solution of carbon in iron freezes out in the form of dendrites which continue to grow down to eutectic temperature. A typical cooling curve appears as indicated in Figure l of the drawings.

At 2210 F. solidification commences and proceeds until 2100 F., when solidification is complete.

When the iron reaches 2100 F. it does not solidify immediately but takes quite a period of time. All of the primary austenite solidifies in the form of a pinetree leaving eutectic liquid in the interstices. Graphite flakes do not begin to form until the eutectic material between the branches begins to solidify. As soon as the eutectic is completely solid, the form of the flakes is essentially complete, but the flakes continue to grow or stay put according to whether the rate of cooling is reduced or speeded up.

Under certain conditions, irons of particular characteristics possess a predominant form of graphite structure which determines in a general way the mechanical properties. However, it should be recognized that graphite, even in common high-carbon gray cast iron, is not always present in a particular form. Actually, graphite assumes various forms, and these are something as follows:

. Massive Lamellar Random Rosette Intergranular Euteotiform Interdendritic Nodular Spherulitic Rarely is it that any cast iron contains only the one type. More often two or even more types are found together in the same iron.

The basic factors controlling the form of graphite are:

1. Composition 2. Rate of Cooling However, it is rarely that graphite in any form is ever pure carbon. It is nearly always contaminated with some foreign substance, either gaseous or solid. Common impurities are silicates and sulphides, etc. Therefore purity of the iron has a vital influence upon both the form the graphite assumes and upon the physical characteristics of the iron itself.

So far as rate of cooling is concerned, we know that in general the faster the rate of cooling the less opportunity is there for growth and the finer is the form of the graphite, everything else being equal.

We know also that cooling rate is not only an influence of section size of type of mold, but that actually it is also influenced by both the constitution and composition of the iron. it is this latterlong known but little understood-that has confused the subject so long and made rection but away from the actual composition of the alloy considered, and this is particularly true of high carbon-silicon cast irons.

A characteristic of such cast irons is that con-- tinued abstraction of heat and lowering temper-- ature of the molten mass results in a continued decrease in the extent and rapidity of vibration 'of the molecules of the liquid iron, until at 'the solidification temperature the molecules fall into relatively fixed positionswith respect to each other. These "positions might perhaps'better be called centers of oscillation, for although the random to-and-iro motion of molecules characterizing the vapor and liquid states has disappeared, still all motion theniolecules has by no means ceased and continues all through the freezing range of 221W to 2060 F. and even after solidification. It isme'relyfreduced to oscillation about afixed point.

It is not difficult to understand, thereiorel-that if the liquid iron drops below its true ireezing point before crystals become visible and then graphitization is induced during this process of undercooling, an entirely new and different form of graphite may result.

If undercooling is effected'to the extent that there is no return to graphitization, then the iron becomes white, i. e., the critical change for the deposition of graphite is suppressed. However, if we anneal this white iron in the range of 1600 F. for a sufiicient length of time, the graphite is precipitated in nodular form and we have what is known as malleable cast iron.

EFFECT OF NUCLEI Besides the phenomenon of undercooling, there is another factor that influences the form of graphite structure in cast irons-what may be termed nuclei action.

Where submicroscopic silicate slime, oxides or sulphides of nianganese, etc., contaminate the molten metal, it is not possible to effect controlled undercooli. There is,fr exampla'definite evidence that sulphur contamination is a frequent cause of coarse flake graphite. Actually, we do not know suiiicient about the effect of foreign nuclei on the formation and structure of the graphite, and for that matter on the effect "of dissolved gases and chemical reactions taking place in the molten cast iron during the process of solidification.

all solution. and deposition of crystals require time, the rate of fall of temperature will obviously afiect the completeness of carbon deposition and the degree of undercooling or su- 'persaturation obtained. In general, with rapid cooling we promote a condition of supersaturamagnesium, silicon, copper or nickel, etc.

.6 'tion, which may eventually be offset by the deposition of carbon in the form of iron carbide.

"Wherethere is a :superficiency of nuclei present, the tendency'will be to throw graphite out of solution early in the solidifying cycle, thus reducing undercooling.

However, whether cast iron be of the hypo or :liypereutectic variety, whether is is pure or impure, whether it is "subject to a nucleating treatment (graphitization) whether it is cooled slowly or quickly, and whether it is or is not subject to a .nodularising treatment, it must solidify through a wide range of temperature; hence the mechanism of theformation of graphite is always complex, and rarely is it possible to produce an iron casting wherein the graphite assumes only one of the types enumerated above.

The extent to which the other forms of graphite appearis often aggravated by the design of the casting itself, particularly in complex designs thatcreate hot spot conditions and in designs of marked sectional variation.

In this invention, therefore, no claim is made to produce only one particular type of graphite, nor :is it claimed that nodules of graphite are wholly spherulitic, consisting of an aggregation of graphite crystallites radiating from a common center or nucleus. In fact, many so-called spherulites are truly nodules, that is, they are aggregates of graphite assuming more or less spheroidal or spherical form. Nor is this too important, for it has been discovered that so far as the ultimate physical properties and service behavior of the casting are concerned, they exert much the same effect.

The purpose, therefore, of this invention is that improvement in the graphite structure such as to assure mechanical properties and service behavior markedly superior to the same iron not treated with lithium or a lithium bearing alloy.

LITHIUM Lithium metal has a melting point of 367 F. and :a boiling point of 2437 F. Its higher boilingjpoint gives it the advantage over most other nodularizing agents, in that it is safer to introduce into the molten metal-free from explosions and pyrotechnic display and more positive in control.

Lithium may be introduced into the molten cast iron as such but it should be protected from the atmosphere preferably in sealed copper, iron or nickel sheathes as it reacts with the nitrogen in the air.

Lithium alloys can be used to better advantage than metallic lithium. The more convenient methods of producing lithium alloys are:

1. To melt and mix with the component metals.

2. To .eleotro-deposit lithium on or into the other alloying metals or 3. To co-deposit'the lithium with the other alloying constituents.

Lithium alloys readily when added to molten cadmium, aluminum, lead, zinc and bismuth The melting point of the alloy usually rises with the increasing lithium content.

.In the'case of the more reactive elements, such as alkaline earth metals, the method preferred is to electro-deposit the lithium together with the alkaline earth metal from a fused bath con taining lithium and alkaline earth halides. In the case of certain alkaline earth metals, e. g., calcium, there is .but one single step for the production of lithium-calcium alloys. Lithiumcalcium, lithium-barium; lithium-strontium or lithium-magnesium alloys can be handled readi- 1y. Lithium alloyed with sodium or potassium, etc. reacts violently with moisture, oxygen and carbon dioxide of the air.

In actual practice we have used lithium metal contained in copper sheathes, also lithium alloyed with silicon and manganese; also lithium alloyed with calcium and magnesium; also lithium alloyed with magnesium and lithium alloyed with nickel and magnesium.

The following examples are included as typical:

Example 1.Test No. 1

Gray cast iron containing:

Balance substantially iron was melted in electric furnace. .5% lithium metal was added in copper sheathes followed by 4% silicon (as ferro-silicon).

TEST RESULTS Graphite structure quasi flake and nodular. 1

Test No. 2, same original melt 1.00% lithium metal in copper sheathes followed by 50% silicon (as ferro-silicon).

extremely small sections and crevices which magnesium.

The cast iron treated contained:

Balance substantially iron and was melted in a 42" cupola.

' Three ladles were taken. To one was added 1%% of alloy; to another 7 and to another The alloy was added in lump form (approx. 2 inches x 1 /2) directly to the surface of each ladle.

As the alloy was added, a cover was placed over the ladle and when the action ceased, additions of ferro-silicon (90% Si) were made. The metal stirred until action ceased and test bars and castings were poured.

The action was different from that of magnesium bearing alloys. It was quiet and sufficiently slow to control physical changes. The amount of slag produced was exceedingly small and the final metal much more fluid than that normal to ordinary nodular cast iron. The results of the tests are indicated below:

Test No 1 2 3 Amount of Alloy Added 1%% Silicon Added 57 .5% .5

Nodular with Microstructurc from 1 sample type Total Carbon of graphite.

Sil1con Manganese. Sulphur before Sulphur after: Transverse Gcst Defiectiorn Tensile p. s. Brine1l rns'r RESULTS fi f ggi Deflection Brinell Before 23, 000 2, 100 .28 187 After 73, 000 5, 100 .41 1 255 Graphite structure all nodular. See Figures 2 and 3 of the drawing.

Applicant has shown tensile strengths of 51,000

It will be observed that the carbide efiect was not obtained with additions of the alloy under /s%.

We have found also, with these and other tests, that with a base metal sulphur content of .06-.08, at least of this alloy is necessary to promote nodularity. A striking feature of these various tests is that full nodularity is obtained with under 04% residual magnesium and under .015% lithium in the presence of a residual sulphur content as high as 047%. See Figure 4 of the drawing.

Two specific examples have been set forth to illustrate the invention and its advantages, but the invention is broader in scope than those illustrated, and the following description of the process is intended to set forth the invention in fuller detail.

This invention is a process of producing an improved gray cast iron wherein the gray cast iron is nodular and quasi-flake. The first step comprises melting an iron which gives a gray cast iron containing 2%-4.5% carbon; 1%6% silicon; .1%2% manganese; .005%-.15% sulphur; .005 %-1% phosphorus. To this melt, before casting, is added an amount of lithium or lithium bearing alloy which will result in a gray cast iron containing not less than .005% nor more than .l% of lithium. Thereafter an alkaline earth metal may be added in an amount such that not less than .005% nor more than .2% remains in the final cast iron. In the usual casting, the melt is graphitized after the addition of the lithium treatment with a graphitizer selected from the group consisting of ferro-silicon, silicon, manganese, zirconium, calcium silicon, nickel silicon, aluminum calcium silicon, or similar graphitizing agent.

Other lithium bearing alloys given below have been selected from the experiments carried out in the course of the researches leading to this invention.

The following sets forth the approximate composition of some lithium containing alloys which can be employed as addition agents for the purpose of introducing lithium into the molten bath in the amounts required by the invention:

Per cent Mn Alloy N0.

Other combinations are possible and the above gives a general idea. The alloy may be introduced in either lump or granulated form.

It may be introduced by placing on top of the ladle as explained previously or a mechanical mixing device may be used either in the stream as the metal flows from the furnace or in the ladle direct.

In summary, I have discovered that lithium will increase the strength of gray cast iron above what is considered normal for gray cast iron. I have discovered that lithium will not only eifect a change in the form of graphite from flake form to one predominantly nodular with resulting notable improvement in mechanical properties of the material much higher than what is considered standard to even high-test cast iron. I have discovered that lithium can produce a lower Brinell hardness and a superior machinability. Also, I have found that lithium produces a markedly improved fluid life and castability. And further, I have discovered that lithium, in contra-distinction to other nodularizing agents such as cerium and magnesium, may be used to eifect a nodularization of the graphite in the presence of over .02% and up to .1% sulphur.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. An article of manufacture comprising, a cast iron casting containing lithium and up to .08% sulphur which if untreated with said lithium would have resulted in a gray cast iron having a flake graphite structure, said casting being characterized by a substantially nodularspherulitic form of graphite containing a nodularizing agent consisting of lithium in a range between .005%-.1%, and having a tensile strength of over 50,000 p. s. i.

2. An article of manufacture comprising, a cast iron casting containing lithium which if untreated with said lithium would have resulted in a gray iron casting having a flake graphite structure, said casting being characterized by a substantially nodular-spherulitic form of graphite containing a nodularizing agent consisting of lithium in a range between .005%-.1%, from 2.0%-4.5% total carbon, 1%-6% silicon, .1%-2% manganese, not more than 1% phosphorus, not more than .08% sulphur, nickel in a maximum amount of 40%, copper in a maximum amount of 5%, molybdenum in a maximum amount of 2%, and chromium in a maximum amount of 2%.

OLIVER SMALLEY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,869,497 Osborg Aug. 2, 1932 2,371,564 Smalley et al Mar. 20, 1945 2,485,760 Millis et al. Oct. 25, 1949 2,488,511 Morrogh Nov. 15, 1949 2,527,037 Smalley Oct. 24, 1950 FOREIGN PATENTS Number Country Date 658,197 Germany Mar. 24, 1938 OTHER REFERENCES The American Foundryman, May 1950, pages 75 to 79.

Claims (1)

1. AN ARTICLE OF MANUFACTURE COMPRISING, CAST IRON CASTING CONTAINING LITHIUM AND UP .08% SULPHUR WHICH IF UNTREATED WITH SAID LITHIUM WOULD HAVE RESULTED IN A GRAY CAST IRON HAVING A FLAKE GRAPHITE STRUCTURE, SAID CASTING BEING CHARACTERIZED BY A SUBSTANTIALLY NODULARSPHERULITIC FORM OF GRAPHITE CONTAINING A NODULARIZING AGENT CONSISTING OF LITHIUM IN A RANGE BETWEEN .005%-.1%, AND HAVING A TENSILE STRENGTH OF OVER 50,000 P. S. I.

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