US2894834A - Method for producing ferrous metal castings - Google Patents

Description

United 2,894,834 7 rdyLE'IHQD FoRrRoDUCmGFE-RROUSM TAL CASTINGS Stanislaw T. 'Jazwinski, Camp Hill, -Pa., assignor -t Nova-Cast Incorporated, a corporation of Ohio No Drawing. Application August 13,4952 Serial No, 304,221

3 Claims. (Cl: 7 5 130) '-=This invention relates to arnethod forcontrollingthe graphite-formation in ferrous metal castings, particularly ""in' oastirQns, and to novel'ferrous-metal castings-produced-by said method.

fFOl' many years metallurgists and foundrymen-have been developing new procedures -and tech niques aimed at improving specific characteristics of ferrou's metal castings or making such castings suitable for particular uses.

recent development having a special significance in "the-field of cast irons provides a method 'for making castings in which at least part of the-free-graphite present appears inthe form of spheroidsor nodules? dispersed throughout the matrix, rather than in the familiar flake form. The physical properties of cast irons having free carbon in such-form may approach those of-mildsteels, particularly from the standpoint of tensile strength, ductility,- hardness and yield point. Becauseoftheshape of the carbon particles, cast iron having "free carbon -in the form of nodules or spheroidshas been given-the;name

cast iron of magnesium, preferably in theform ofrna gnesiurn-contain ing alloys, within critical narrow limitsto .provide a residual magnesium content within narrow limits would produce.v nodular iron. Although the use of magnesium in the form of magnesium-containing alloys .toproduce nodular ironvofier's certain advantages over the use of cerium-for this purpose, a substantial amount of magnesiurn is required to, produce nodular iron, with theresult that such production is relatively expensive, and involves the formation of substantial amounts of -drossfa term used herein and in the art to designate included magnesium oxide. nodular iron castings worthless because of its detrimental efiect upon physical properties. Furthermore, it is reportedthatjrnany foundries have difficulties in repeating Dross sometimes renders results. Apparently an addition of a magnesium -conraining, alloy ina proportion proper to producenodular iron Jina given melt, may be either ;too small ortoo greatto produce nodular iron in another melt, even one 'produced under conditionsthought tobe identical. The

L.a t er it was discovered-that the addition-tormolten Patent being used,for example, magnesium. Thus, insome instances, generally crescent-shaped carbon particles have preciably more than theamount of an agent required to form car-bo n nodules isadded to a cast iron melt, casting'sirom that-melt will have essentially the charhon-nodulesoriginally formed. These three stages of carbon form in iron-castings have 'beenobservedby ;;rnic1oscopicexamination,-aswell as various combina- 7 2,894,834 Patented, July, 14,. 19.5.9

Ice

-mechanism by which cerium and magnesium control; the carbon formation -ina melt to produce nodular iron'does not seem'to be understood bytheart, so that no clear theoretical basis is available for predicting, what amount of magnesium will be required with any given cast iron melt. :Consistencyof results and duplication fthe'reof are not possible.

'I' have found "that -varyingefiects in the 'form :of the :free graphite-in iron castings can be achieved by incorporation in the melt of varying amounts of the-agent been observed to result from the addition of a relatively small amount of aningredient that will produce nodular carbon when used in larger amounts. Further, if apacteristics oiordinary gray cast iron,- which phenomenon is attributed to the disintegration or explosion" of car- -tionsthereof. The term"-compacted graphite isused herein, and in the appendedclaims, in itsusual sense, namely, to indicate free carbon in a formmore compacted than the familiar flake graphite, for example, generally crescent-shaped carbon particles and carbon nodules or spheroids, as discussedabove. Compacted graphite formations are, therefore distinguished on the one' 'hanld from ordinary 'fiake;graphite,; and, on the other/from over treated'castings 'infwhioh-nodules have disintegrated appreciably. In general -any iron casting containing graphite in '-a compacted form'has'been found to have'physical properties superior-to those of a casting identical except'that the graphite is in flake form. Similarly, the physical properties of acasting containing compacted graphite are superior to those of a casting identical exccpbthat appreciablenoduledisintegration has occurred. Accordirigiy, the presence of compacted graphite is an advantageous structureforiron castings, and advantageous oven-either ordinary gfiake graphite or disintegrated nodules; theoptimurn physical properties of a casting V containing compacted {graphite being present when' the particular'form of compacting, is nodular, other factors being equal.

'l-lowever, castings containing enerally crescent shaped carbon particles canusually be produced withless care and, frequently, with less expense than can castings with fully nodular carbon structures so that their superior physical properties relativeto ordi- -nary gray cast irons are highlysignificant, because such castings are desirable f 'r many applications where "the extremely highphysical properties of nodular iron are not required and where -the expense of nodular iron; is

not warranted.

The present invention is basedupon the discovery'=of a -novel method for -produ cing ferrous metal castings "characterized by compacted graphite structures, and' of tnovel iferrous metal castings produce'd by such method.

Iron castings ;-containing compactediree graphite' formations -are produced according L to -the invention. -A n 0.1 percent, and the titanium content is not greater than 0.25 percent is first effected under such conditions that contact of air with the melt is minimized. If a pearlitic matrix is desired and the manganese content is from about 0.5 to about 0.7 percent a final silicon content not greater than about 2.8 percent is necessary. With a manganese content higher than about 0.7 percent a pearlitic matrix is achieved with proportionately more silicon. Similarly, with a manganese content lower than about 0.5 percent, the maximum silicon content for a pearlitic matrix is proportionately decreased. If a ferritic matrix is desired the final silicon content must be above that which produces a pearlitic matrix and can be as high as 3.3 percent if a ductile iron is desired, or even higher, up to 7 or even 8 percent if a matrix of silicon ferrite is desired. Similarly, strong carbide stabilizers such as chromium, if present, should be limited to traces. A zirconium-containing ingredient which will decompose or readily dissolve in molten iron, with or Without a graphitizing agent, is then introduced into the melt. For best results, when the zirconium-containing ingredient decomposes in molten iron, such decomposition should occur at a temperature from 1500 F. to 2200 F. The resulting composition is, next agitated to effect thorough mixing between the molten iron and the additions, and cast.

It has been found to be important in producing ferrous' metal castings containing free graphite in compacted form according to the invention that contact between the molten iron and air be minimized during the melting operation. For example, when melting is carried out in a cupola, as is preferred for economic reasons in producing ferrous metal castings according to the invention, the air flow through the tuyeres to the cupola is minimized throughout the melting operation to as low a rate as is commensurate with maintaining an adequate temperature in the iron melt and the required melting rate. It will be apparent that similar precautions can be taken even more readily and more effectively when melting is carried out in an electric furnace, or the like.

To produce ferrous metal castings containing free graphite in compacted form it is also important that the sulfur content and the titanium content be within the limis hereinbefore stated. Zirconium is a powerful desulfurizing agent, and will react with any sulfur present in the melt, thereby reducing the amount of zirconium available as a compacting or nodularizing agent. Titanium above the stated amount is disadvantageous as it seems actually to interfere with compacting of the graphite formations.

In general, any zirconium-containing ingredient suitable for introducing zirconium into an iron melt, that is, one which decomposes in molten iron or readily dissolves therein can be used as a compacting agent, with or Without a graphitizing agent. It is essential that the zircomum-containing ingredient decompose or dissolve, as stated, in order to introduce zirconium into the melt. Silicon-manganese-zirconium, a material containing 60 to 65 percent of silicon, 5 to 7 percent of manganese, and

5 to 7 percent of zirconium, balance iron, is a preferred ingredient which is commercially available in convenient form, and is readily dissolved in an iron melt. The terms percent and parts are used herein and in the appended claims to mean percent and parts by weight, unless otherwise indicated. Other zirconium-containing ingredients can be used in place of silicon-manganesezirconium, for example, zirconium halides such as zirconium fluoride, zirconium chloride, zirconium bromide and zirconium iodide, mixed or double halides of zirconium and other metals, preferably alkali or alkaline earth 'metals, such as mixtures or double salts of zirconium halides and alkali metal halides or alkaline earth halides. It has been found that the amount of acompacting ingredient required to effect a certain extent of compaction of carbon in arcast iron is a function of the condition of the molten iron prior to treatment. Thus, for example, zirconium added to a melt acts as a deoxidizer and as a desulfurizer. The deoxidizing action and the desulfurizing action precede the nodularizing action, so that sufiicient zirconium for substantial reaction with any sulfur in the melt, and with any dissolved oxygen, in addition to any zirconium lost must be added, together with sufficient to effect compacting of graphite structures. It has been found that from 0.01 percent to 0.15 percent of added zirconium, in addition to the amount required for deoxidization and for desulfurization, is sufficient to effect compacting of graphite structures. Ordinarily, not more than 0.15 percent of zirconium need be added to a cast iron melt prepared for the production of castings to have a compacted graphite structure in order to accomplish deoxidation and desulfurization and, in some cases, virtually none is required.

It is usually preferred that a graphitizing agent be added to a melt to produce a compacted graphite structure according to the invention. Silicon is an advantageous graphitizing agent, so that silicon-manganese-zirconium provides both the compacting agent and the graphitizing agent. When a compacting agent other than siliconmanganese-zirconium is employed according to the invention, the graphitizing agent can'be added to the melt either together with the compacting agent, or subsequently. The amount of graphitizing agent added is usually not critical, but, in the case of ferrosilicon, based on silicon content, should be not more than about 0.6 percent, but should be such that the final silicon content of a casting produced from the melt is within the previously discussed ranges. Other graphitizing agents than silicon, either as a zirconium alley or in other form, which can be used include nickel and graphite in pure form.

When the graphitizing agent is ferrosilicon it is preferred that the silicon content thereof be from 75 to percent. Silicon carbide is an advantageous graphitizing agent, and may, in some cases, be a compacting agent.

According to a preferred embodiment of the invention the cast iron melt is pretreated to accomplish at least partial deoxidation or desulfurization, or both, prior to the introduction thereinto of the zirconium-bearing ingredient and the graphitizing agent, if used. Such pretreatment can be accomplished by melting in a unit operated under reducing conditions, such as for instance a closed top cupola, or by adding an alloy or other material to the molten iron, or by introducing a neutral gas or a gas that forms oxides at the temperature of the molten iron. Such pretreatment can be effected by introduction into the melt of an alkali metal protected against oxidation prior to addition to the melt, a rare earth metal, or an alkali metal compound stable at room temperature, but which decomposes in the molten iron to release the metal. Alkali metal oxides and carbonates, where the oxygen or CO group acts as a carrier, are examples of such compounds, sodium oxide or carbonate being preferred for economic reasons. Under certain conditions and certain states of oxidization and sulfurization of a melt the pretreatment described above may yield compacted graphite. It has been demonstrated experimentally that, from a single ladle of molten cast iron, greater compacting is effected with a given addition of silicon-manganese-zirconium when treatment with soda ash precedes the compacting addition. Similar results can be shown with other alkali metal oxides. Similarly, it has been shown experimentally that bubbling through a melt an inert gas, or a gas that will combine with oxygen at the temperature of molten iron, for example, argon, helium, neon, krypton and nitrogen prior to the addition of the zirconium-containing ingredient has a like effect. The same effect'can be achieved by the use of nitrides. Alkali metal nitrides decompose to the metals and nitrogen when added to an iron melt, and thus provide alkali metals for pretreatment or compacting, and, in addition, nitrogen for pretreatment. The combination gscassi of both pretreatments is especially advantageous as complementary results are achieved. It *will'be apparent that pretreatment as described in this paragraph will be equall-y advantageous when compacting agents other {than .zirconium-containing ingredients are used.

The following examples are intended further to illustrate and disclose the invention, but are not to be construedas limitations thereon:

EXAMPLE 1 v .A cupola was loaded inth'e usual manner, and charged to obtain 'base metal of the following chemical composition: C, 4.07%;S, 0.044%; P, 0.022%;Si, 035%; and Mn, 0.16%. Throughout the melting operation the air flow through the tuyeres to the cupola was maintained at as low a rate as was commensurate with an adequate melting rate and maintaining an adequate temperature of the iron melt in order to minimize oxidation thereof.

Approximately a 20 pound portion of molten iron produced as described in the preceding paragraph was collected in a hand ladle to which had been charged approximately one pound of silicon-mangense-zirconium. The iron melt was then stirred, and a test casting was poured into a sand mold. This casting showed a nodular structure.

EXAMPLE 2 A series of experiments was carried out using a cast iron melt substantially identical with that described in the first paragraph of Example 1 in order to demonstrate the eifect of an alkali metal oxide pretreatment prior to an addition of a zirconium-containing ingredient.

The procedure followed was generally the same in each instance, and involved adding an amount of soda ash to a ladle, drawing about twenty pounds of the molten iron into the ladle, adding an amount of silicon-manganesezirconium to the ladle, stirring, and pouring a test casting in a baked sand mold. The structure was determined by microscopic examination of etched and polished specimens cut from the casting. The results of this series of tests is reported in Table I, below.

Table I Treatment Sample No. Mterostructure Soda Ash,

SMZ, Percent Percent Curly. Curly and nodular (5-10 percent). (Jurlfi and nodular (7-12 percent).

Curly and nodular (10-15 percent).

surly and nodular (10-20 percent).

ur y.

the advantage thereof.

EXAMPLE 3 In order to demonstrate the beneficial effect of calcium used with a zirconium-containing ingredient to produce a compacted graphite formation in a cast iron 4.5 pounds ofcalcium silicide and 4.5 pounds of silicon-manganesezirconium were added to a heated bull or receiving ladle; approximately 300 pounds of iron melted by a procedure generally identical with that described in the first paragraph of Example 1 was drawn into the ladle; the melt was stirred; and a test casting was poured into a sand mold. Microscopic inspection of a polished and etched sample cut from the resulting casting showed exploded nodules, indicating an excess of treating agents over the amount required to produce fully nodular iron. It is clear fromthis experimental test that compacted graphite, including nodular, can be produced by small additions of calcium and a zirconium-containing ingredient. For purposes of comparison, but not in accordance with the invention, about 20 pounds of molten iron prepared by substantially the same procedure was drawn into a ladle to which had been charged 0.8 pound of calcium silicide; the melt was then stirred and inoculated with 0.05 pound of 75 percent ferrosilicon, and a test casting poured into a sand mold. Only .curly graphite was observed upon microscopic examination of a polished and etched sample.

EXAMPLE 4 In order to show some of the latitude possible in practicing the method of the invention numerous procedures were carried out, some of which are outlined in the following paragraphs, and all of which produced curly graphitic structures, and, in some instances, a few well defined nodules or spheroids (in every instance the cast iron melt was produced by a procedure substantially identical with that described in the first paragraph of Example 1) Approximately a 300 pound sample of cast iron was treated with N for about 45 seconds in a receiving ladle. After this treatment about a nine pound charge of siliconmanganese-zirconium was added to the ladle, the metal was stirred, and castings were poured.

Approximately a 300 pound sample of cast iron was treated with N for about 1 minute and 45 seconds in a receiving ladle. After this treatment about a nine pound charge of silicon-manganese-zirconium was added to the ladle, the metal was stirred, and castings were poured.

Approximately a 20 pound sample of cast iron was drawn into a ladle into which had been charged about 0.6 pound of silicon-manganese-zirconium. The metal was stirred, and about 0.08 pound of 75 percent ferrosilicon was added to the melt. After further stirring a test piece was cast.

About a 20 pound sample of cast iron was drawn into a ladle into which had been charged about 0.2 pound of silicon-manganese-zirconium, approximately 0.2 pound of calcium silicide, and approximately 0.4 pound of calcium carbide. The molten melt in the ladle was then stirred, and about 0.05 pound of 75 percent ferrosilicon was added. After further stirring a test piece was cast.

It will be apparent that various changes and modifications can be made from the specific details described without departing from the spirit of the invention.

Having described the invention, I claim:

1. A method for producing iron castings wherein compacting of free graphite formations is effected that comprises introducing nitrogen into a cast iron melt, prior to introducing a zirconium-bearing ingredient suitable for introducing zirconium into an iron melt into the pretreated melt in proportions such that the percentage of zirconium introduced into said melt is within the range from 0.01 percent to 0.3 percent, mechanically mixing the cast iron melt, and pouring the resulting molten composition into a mold.

2. A method for producing iron castings wherein compacting of free graphite formations is effected that comprises introducing an inert gas into a cast iron melt, prior to introducing a graphitizing agent and a zirconiumbearing ingredient suitable for introducing zirconium into an iron melt into the pretreated melt in proportions such that the percentage of zirconium introduced into said melt is within the range from 0.01 percent to 0.3 percent, mechanically mixing the cast iron melt, and pouring the resulting molten composition into a mold.

3. A method for producing iron castings wherein cornpacting of free graphite formations is effected that comprises melting an iron charge so proportioned that the sulfur content is not greater than 0.1 percent, and the titanium content is not greater than 0.25 percent, under conditions such that contact between the melt and air is minimized, treating the melt with a pre-treating agent of the group consisting of alkali metals, alkali metal carbonates, alkali metal oxides, alkali metal nitrides, rare earth metals, and rare earth nitrides, introducing a zirconium-containing ingredient suitable for introducing zirconium into an iron melt into the resulting iron melt, agitating the resulting composition to effect thorough mixing between the molten iron and the additions, and casting the resulting molten composition.

References Cited in the file of this patent UNITED STATES PATENTS Muskat May 17, Offenhauer Ian. 16, Smalley n Nov. 13, Wynne Feb. 2 6, Jordan Aug. 5, Crome Dec. 15,

FOREIGN PATENTS Great Britain Aug. 3, Great Britain June 7,

Great Britain Dec. 9,

Claims (1)

1. A METHOD FOR PRODUCING IRON CASTING WHEREIN COMPACTING OF FREE GRAPHITE FORMATIONS IS EFFECTED THAT COMPRISES INTRODUCING NITROGEN INTO A CAST IRON MELT PRIOR TO INTRODUCING A ZIRCONIUM-BEARING INGREDIENT SUITABLE FOR INTRODUCING ZIRCONIUM INTO AN IRON MELT INTO THE PRETREATED MELT IN PROPORTIONS SUCH THAT THE PERCENTAGE OF ZIRCONIUM INTRODUCED INTO SAID MELLT IS WITHIN THE RANGE FROM 0.01 PERCENT TO 0.3 PERCENT, MECHANICALLY MIXING THE CAST IRON MELT, AND POURING THE RESULTING MOLTEN COMPOSITION INTO A MOLD.