US2625473A - Lithium modified magnesium treatment of cast iron - Google Patents

Description

Patented Jan. 13, 1953 LKTHIUM MODIFIED MAGNESIUM TREATMENT OF CAST IRON Oliver Smalley, Larchmont, N. 51., assignor to Meehanite Metal Corporation, a corporation of Tennessee No Drawing. Application September 8, 1950, Serial No. 183,915

4 Claims. 1

This invention relates to cast iron practice in general, and relates specifically to the improved method of incorporating magnesium into molten iron.

This invention is a oontinuation-in-part of my application Serial No. 71,387 now Patent No. 2,527,037.

Much work has been don to improve the physi cal structure of cast iron by the addition of magnesium. This work was begun many years ago by Meehan, and has been the center of intensive interest by nietallurgists in recent years.

The effect of magnesium alone upon the physical structure of cast iron is now fairly well known and understood. This invention is based upon th discovery that improved results, with much less difficulty and danger, are possible by modifying the action and effect of magnesium with lithium.

An object of this invention is to provide a molten iron for casting, which molten iron has an improved fluidity and fluid life.

Another object of this invention is to provide a gray cast iron in which at least a substantial part of the graphite exists in nodular form in the original casting without further treatment.

Still another object of this invention is to provide a gray cast iron having improved founding properties together with improved physical and mechanical properties even in the presence of relatively low or relatively high carbon content, and without regard to removal of sulphur.

Yet another object of this invention is toreduce, or substantially eliminate, the pyrotechnic dangers of adding magnesium to molten iron.

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, being 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.

The eiiect of magnesium upon the physical and mechanical properties of a base cast iron is rather well known. A casting treated with magnesium will have a more compact form of graph ite, some even in nodular form. The exact efiect andresulting form of the carbon l'gmmagnesium treatment alone is of only passing interest insofar as this invention is concerned.

The elTect of lithium upon the physical and mechanical properties of a base cast iron has been fully explained in Smalley application Serial No. 181,129.

This invention provides a method of improved treatment of base cast iron to create a superior casting through simple treatment of the base cast iron with magnesium modified with lithium. The magnesium and lithium are alloyed together and may be added to the molten iron either in the spout or in the ladle. The iron is materially afiected in a beneficial manner to improve both the castability and physical properties of the castings made. The beneficial properties of each additive material are retained and enhanced, whereas the inherent disadvantages of the use of magnesium are suppressed.

The present invention involves in its simplest form only the addition of an appropriate amount of lithium and magnesium to a molten cast iron of appropriate composition, and solution of the lithium and magnesium in the iron, each employing the other as a carrier metal.

It has been found that both lithium and magnesium independently have a whitening effect on gray cast iron. Thus, a gray cast iron melt containing lithium and magnesium will usually produce castings having a cementite network structure unless graphitization is employed following the lithium-magnesium addition. The graphitization step may be omitted if an improved white iron is desired, This invention is applicabl to substantially any cast iron base material. Those skilled in the science of metallurgy will apply any number of additional treatments to raise or lower the content of sulphur, silicon, carbon; and other normal constituents of gray cast iron to achieve properties for specific purposes. They will also understand the normal procedure for obtaining special'rcsults by the addition of alloying agents such as nickel, chromium, molybdenum, etc. Each one, or any combination of special treatments, or alloyadditions, can be carried on in addition to the process of this invention for improving the castability and physical properties by effecting the carbide structure. The effects of the alloy additions are actually improved by the process of this invention.

The lithium-magnesium containing improved gray cast iron of this invention will generally contain over 2% and less than 4.5% of carbon. Silicon content of preferred gray oast irons in accordance with the invention, will be at least 1% and usually fall within the range of 1.5% to 3.5%, but generally 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 .1 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 phosphideeutectic 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, chrome, and manganese, etc. The nickel may be present in amounts up to 40 molybdenum in amounts up to 2%, copper in amounts up to 5%, manganese in amounts up to 2 and chrome in amounts up to 2%.

Lithium metal has a melting point of 367 F. and a boiling point of 2437 F. It may be introduced into molten cast iron without danger of explosions and pyrotechnic display. On the other hand magnesium has a melting point of 1204 1 but a boiling point of about 2050' F. and therefore will burn violently when an attempt is made to add it tomolten iron.

Lithium has a remarkable qui'eting effect upon magnesium even when incorporated in percentages as low as 3% of a. magnesium-lithium alloy. The alloy is useful with the opposite extreme of 98% lithium and 3% magnesium. Some quieting efiect was to be expected, but the remarkable effect of the lithium upon the magnesium sets this particular alloy apart. Furtherfore, the effect of the lithium-magnesium alloy upon a cast iron melt was surprising. This alloy can be added in small amount and have more effect than a much larger addition would be expected to have.

A further advantage which was discovered is that there is no extreme short critical time limit after adding this new alloy in which. the casting must be poured. Reports have been made that after inoculating with magnesium, the casting must be poured within three minutes- Not so with cast iron treated with an alloy of lithiummagnesium. A normal pouring period may be used.

Further, because of the quieting elfect of the lithium upon the magnesium, and the mutual effect of the two metals, the desired results upon the physical properties is obtained with a considerably smaller addition; of total alloy metals.

Retained magnesium is said to be an important factor in the control of physical properties of cast iron. But in practice prior to this invention there Was no practical suggestion of how to add a calculated percentage of magnesium and make it stay in the cast iron. It has been found that all of the magnesium in the alloy of this invention is retained by the melt, and therefor the percentages can be calculated prior to the addition.

The following example is. given;

An alloy of 15% lithium and. 85% magnesiu was used.

The cast iron treated contained:

Si 2.2 Mn .8 S .064 P a .08

Balance substantially iron.

and was melted in a 42" cupola.

Four ladles were taken. To one was added 1 of the alloy; to another A396; to another and to another The alloy was added in lump form 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 Si) were made. The metal stirred until action ceased and test bars and castings were poured.

The action Was different from that of magneslum bearing alloys. It was quiet and suliiciently slow to control physical changes. The a 'iOLlllt 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 N o 1 2 3 1 Amount 01 Alloy Addcd..pcrcent 1% it t .4 Silicon Added .pcrcent 5 .5 5 5 Type of graphite: M icrostruc- Flake Flake ture from 1" sample. V Total Carbon l 3. 6 3. 52 3. 5 3. 5! Silicon 2. 2 2 l9 19 2.1!! Manganesm I 78 (E8 68 61 Sulphur beta .061. 5 .075 .075 Sulphur aiter 3-1 .040 we 2, 303 6 .f 23 24, 000 21, 000 176 176 1 All Nodular. 3 N odular with Trace of Flake.

It has also been found, with this and other tests, that with a base metal sulphur content of .06-.08, at least 78% of this alloy is necessary to promote nodularity. A striking feature of these various tests is that full nodularity has been obtained with under .04'% residual magnesium and under 015% lithium in the presence of a residual sulphur content as high as 04%. It has been discovered that a nodular-spherulitic' structure can be obtained with lithium retained in a range from as low as .002% up to .05% in the finished casting, and the magnesium ranging from less than .04 up to as much as .2

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 processfor the production of malleable cast iron.

To enable a better understanding of the inventionand the mechanism of graphite change a brief discussion on graphite form and its effect on the physical properties of gray cast iron might be helpful.

Mechanism of the formation of graphite in cast iron In high-carbon castiron (hyper eutectic) microscopic graphite specks may exist in the molten iron. When this ispresent, these inclusions of graphite act as nuclei for the growth of 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 pine tree 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:

1. Massive 6. Eutectiform Lamellar 7. Interdendritic 3. Random 3. Nodular i. Rosette 9. Sphereulitic 5. Intergranular 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 there is 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 or 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 understoodthat has confused the subject so long and made graphite control so diflicult of understanding.

In all metastable systems-in which the ironcarbon system belongs-the process of solidification requires a certain time to start, but if the rate of cooling is speeded up, whether by chemical additions, superheating, or by speeding up solidification, a temperature below the anticipated solidification temperature may be reached before solidification actually begins. This phenomenon is called surfusion or undercooling, etc. The amount of undercooling or supercooling varies for different metals and alloys. In the case of alloys of the eutectic compositions, undercooling may shift the eutectic composition in either direction 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 continued abstraction of heat and lowering temperature of the molten mass results in a continued decrease in the extent and rapidity of vibration of the moleculesof the liquid iron, until at the solidification temperature the molecules fall into relatively fixed positions with respect to each other. These positions might perhaps better be called centers of oscillation, for although the random to-and-fro motion of molecules characterizing the vapor and liquid states has disappeared, still all motion of the molecules has by no means ceased and continues all through the freezing range of 2210 F. to 2060 F. and even after solidification. It is merely reduced to oscillation about a fixed point.

It is not difficult to understand, therefore, that if the liquid iron drops below its true freezing point before crystals become visible and then graphitization is induced during this process of undercooling, an entirely new and difierent 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.

Efiect 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 manganese, etc., contaminate the molten metal, it is not possible to effect controlled undercooling. There is, for example, definite evidence that sulphur contamination is a frequent cause of coarse flake graphite. Actually, we do not know sufiicient about the effect of foreign nuclei on the formation and structure of the graphite, and for that matter on the eifect of dissolved gases and chemical reactions taking place in the molten cast iron during the process of solidification.

As all solution and deposition of crystals require time, the rate of fall of temperature will obviously affect the completeness of carbon deposition and the degree of undercooling or supersaturation obtained. In general, with rapid cooling we promote a condition of supersaturation, which may eventually be offset by the deposition of carbon in the form of iron carbide.

Where there is a superficiency of nuclei present, the tendency will be to throw graphite out or solution early in the solidifying cycle, thus reducing undercooling.

However, whether cast iron be of the hypo or hyper eutectic variety, whether it is pure or impure, whether it is subject to a nucleating treatment (graphitization) whether it is cooled slowly or quickly, and whetheixit is or is not subject to a nodularizing treatment, it must solidify through a wide range "of temperature; hence 7 the mechanism of the formation of graphite is always complex; andrarelyisit possible to produce' an iron" casting: wherein the graphite assum'es only one of the types enumerated above.

The'extent to'wliich the other forms of graphite'appear is often aggravatedby the design of the casting itself, particularly in complex designs that create" hot spot conditions and in designs of marked sectional variation.

In this invention, therefore, no claim is made to produce onlyqone particular type of graphite, nor is it claimed that" nodules of graphite are wholly spherulitic;.consisin'ng of an aggregation of'graphitecrystallites'radiating from a common center'ornucleus. In-fact, many so-called spherulit'esiare'truly nodules, that is, they are aggregates of graphite assuming" more or less spheroidal or ovalzform. Nor is this too important, forit has been discoveredthat so far as the ultimate physical properties and service behavior oflthecasting'are concerned, they exert much the same effect.

In conclusion, it may" be said that, as cast, the casting'will'have greatly-improved physical characteristics with a' combined lithium and magnesium content wherein the relative quantity of litlii'um" to magnesium is substantially in a ratio of 1 part lithium" to 30 parts magnesium, to 1 part lithium to 3 parts magnesium, but the retained lithium shall not exceed .05% and the retained magnesium shall be under 2%.

Although the invention has been described in its'preferred form with a certain degree of particularity, it is understood that the present disclosure ofthepreferred form has been made only byway of example. and that numerous changes in the details ofconstruction 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, a casting which if untreated with magnesium and lithium would have resulted in a gray iron casting having a: common flake graphite structure, said casting being characterized by a. substantially nodular graphite microstructure in the presence of at least .0].% retained lithium in conjunction with 134% retained magnesium;

22 An article of manufacture, comprising, a cast iron containing magnesium and lithium which if untreated with said: magnesium and lithium would have resulted in a gray iron castinghaving a flake graphite structure, said castingbeing characterized by asubstantially nodular graphite microstructure in the presence of at least .002% retained lithium: in conjunction with up to 2% retained magnesium.

3. An articleof manufacture comprising, a-cast iron' casting in its condition as originally cast containing magnesium and lithium which if untreated with said magnesium and lithium would have resulted in a gray iron casting having a flake graphite structure, said casting containing 2.0'%-4.5% carbon, 1%-6% silicon, 0.1%-2% manganese, under 1.5% phosphorus, nickel in a maximum amount of 40%, copper in a maximum amount of 5%, molybdenum in a maximum amount of 2%, chromium in a maximum amount of 2%, said gray cast iron casting having a substantial part of the graphite in nodular-spherulitic form in the presence of a sulphur content up to .08%, at least .0021ithium but not more than .05'% lithium, and up-to 2% retained magnesium;

4. .An article of manufacture comprising, a cast iron casting in its condition as originally cast ccntaining'magnesium and lithium which if untreated'with said magnesium and lithium would have resulted in a gray iron casting having a flake graphite structure, said casting containing 2.0%-4.5% carbon, 1 %-6% silicon, 0.1%2% manganese, under 1.5% phosphorus, nickel in a maximum amount of 40%, copper in a maximum amount of 5%, molybdenum in a maximum amount of 2%, chromium-in a maximum amount of 2%, said gray iron casting having a substantial part of the graphite in nodular-spherulitic form in thepresenceof a sulphur content up to .08%, at least .002 lithium but not more than .05% lithium, and up to 2% retained magnesium andv with the relative quantity of lithium to magnesium substantially in a range. having one limit being, av ratio of 1 part lithium to 30 parts magnesium; and the other limit being 1 part lithium to 3 parts magnesium;

OLIVER SMALLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,073,515 Fischer Mar. 9, 1937 2,453,444 Loonam Nov. 9, 1948 2,513,240 Hill June 27, 1950 2516,52 3: Millis etal July 25, 1950 FOREIGN PATENTS Number Country Date 649,47 5 Germany Aug. 25, 1937 553,197 Germany Mar. 24, 1938 OTHER REFERENCES American Foundryman, May 1950, pages 75, 78 and 80.

Claims (1)

1. AS AN ARTICLE OF MANUFACTURE, A CASTING WHICH IF UNTREATED WITH MAGNESIUM AND LITHIUM WOULD HAVE RESULTED IN A GRAY IRON CASTING HAVING A COMMON FLAKE GRAPHITE STRUCTURE, SAID CASTING BEING CHARACTERIZED BY A SUBSTANTIALLY NODULAR GRAPHITE MICROSTRUCTURE IN THE PRESENCE OF AT LEAST .01% RETAINED LITHIUM IN CONJUNCTION WITH .04% RETAINED MAGNESIUM.