US3360364A - Process for producing nodular graphite in a metal - Google Patents

Description

United States Patent Oflice 3,360,364 Patented Dec. 26, 1967 3,360,364 PROCESS FOR PRODUCING NODULAR GRAPHITE IN A METAL Kenneth H. Ivey and Haskiel R. Shell, Norris, Tenn., as-

signors to the United States of America as represented by the Secretary of the Interior No Drawing. Filed May 25, 1965, Ser. No. 458,793

9 Claims. (Cl. 75-130) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to production of nodular graphite metals, particularly iron.

Cast iron is a ferrous alloy containing carbon and usually smaller percentages of silicon. It contains an excess of carbon over that which can remain in the continuous ferrous phase (unless very rapidly chilled); the excess must be and is precipitated as some form of carbon.

If the excess carbon of the cast iron is precipitated as flake graphite during cooling, the resulting ferrous prodnot is known as gray cast iron. Gray cast iron is machinable and relatively soft but has many inferior properties: lack of toughness, insuflicient ductility and strength, liability to failure under stress, and unsuitably for use under low temperature conditions.

White cast iron is harder than the ordinary gray variety, but it is also quite brittle and may be made somewhat ductile only by long annealing time. White cast iron result-s when most of the carbon remains in the combined state. This material has only limited uses.

Recent developments in ferrous metallurgy have yielded a third variety of cast iron known as nodular or ductile or spheroidal graphite iron. They have the same basic compositions as common gray irons but diifer from the latter in the shape of the graphite particles. In gray iron, the graphite occurs as flakes while in this third variety the graphite is nodular or spheroidal. This change has a great effect on the mechanical properties, and the nodular iron is much more ductile than gray iron. In general, nodular iron is cast at the same temperatures and with the same procedures used for ordinary gray iron.

Nodularizing agents such as magnesium, cerium, or their. alloys with other metals are added to induce the graphite to separate in nodular form rather than as flakes. Nodular iron has high fluidity, improved ductility, im-' proved strength at high or low temperatures and has found wide acceptance in the industry. In many respects it resembles steel (for example, heat treatment can yield quite high strengths). On the other hand, nodular iron can be cast at the same low temperatures and with the same procedures used for ordinary gray iron. Nodular iron, then, has the desirable properties of toughness,

ductility and strength, especially impact strength.

Recent research has shown that for the graphite to crystallize as spherulites the iron must be considerably supercooled. Supercooling can take place only in the absence of nuclei, and ithas been further demonstrated that the impurities in cast iron are the source of such nuclei. Sulfurous inclusions such as (Fe, Mn)S are one such impurity. These impurities prevent the iron from being supercooled since they produce the nuclei for flake graphite crystallization. Of especial significance is the fact that in pure iron melts containing pure carbon and silicon the graphite separates in nodular or spherulitic form. The modifiers act by purifying the melt from dissolved sulfur and oxygen and destroying (Fe, Mn)S inclusions. Thus, by removal of potential nuclei the supercooling required for spheroidal graphite formation is produced. In addition to purification of the melt, the modifiers may facilitate supercooling because the iron adsorbed on the graphite impedes carbon atom access.

A Wide variety of nodularizing agents have been employed in prior art processes, e.g., magnesium or cerium (US. Patent 2,841,488), magnesium alloy and calcium carbide (US. Patent 2,867,555), mica (US. 'Patents 2,92,.5 64 and 2,932,567), magnesium-silicon alloy and tellurium (U,S, Patent 2,933,385), sodium halides (US. Patent 2,948,605) and lanthanum (U.S. Patent 2,970,902). Though generally effective, these prior art agents are subject to a variety of deficiencies such as high volatility, high cost, low fluidity (at elevated temperature), etc.

It has now been found that these disadvantages are overcome by the use of an amphibole, a humite or a miX- ture of the two, with or without an inorganic fluoride, as nodularizing agent. In addition, these nodularizing agents of the present invention have an advantage over certain prior art agents, such as mica, in that they may be used directly in an arc-furnace or c-upola. Thus, when the melt is tapped from the furnace further treatment is unnecessary. Amphiboles are a class of silicate minerals containing hydroxyl or fluorine and may be represented by the general formula W X Y (Z O (OH,F) in which the coordination of the cations to anions is W=l2, X: 8, Y=6, and 2:4. The amphibole structure is a double chain or band silicate whose structure will shift or change to accommodate a great variety of cations. The W position may be vacant or occupied in Whole or in part by cations'of ionic radius from 0.6 to 1.33 Angstroms; the X position has cations of atomic radius from 0.6 to 1.1; the Y position has cations from 0.5 to 0.8; and the Z position has cations from 0.2 to 0.6 Angstroms. In the natural minerals the hydroxyl ion is largely present in the amphiboles and humites, but in the synthetics this position may be entirely composed of fluoride. Typical synthetic fluoramphiboles are: fluortremolite, which has the formula amples of the many thousands of combinations possible by changing the occupants of the W, X, Y, and Z cationic .positions. Non-exclusive examples of cations that may be Norbergite", MgF -Mg SiO Chondrodite MgF -2Mg SiO Humite MgF -3Mg SiO Clinohumite MgF -4Mg SiO These four minerals, natural or synthetic, are known collectively as the humite group and have melting temperatures in the range of 1400 C.1600 C.

Any of the inorganic fluorides of the prior art may be used in combination with the amphibole or humite or with amphibole and humite. Alkali or alkaline earth fluorides such as those of magnesium, calcium, strontium, sodium and aluminum have been found very effective. Non-limiting examples of inorganic fluorides are MgF CaF SrF NaF, AlF

The fluoramphiboles and humite minerals, or combinations thereof, with or without inorganic fluorides, may be easily synthesized by electric furnace melting methods described in the prior art. A common method is melting in an internal resistance electric furnace. This type of melting utilizes very simple equipment, and is amenable to small or large scale production (from a few pounds to hundreds of tons). The product so obtained is massive and coherent. Low loss is, therefore, obtained in cast iron furnace or ladle additions. The product may be easily crushed to any desired size.

Without departing from the basic fluoramphibolehumite-inorganic fluoride (F-H-I) combinations, additions of elements, oxides, silicates, or other inorganic compounds, may be made to the batch to yield any desired alloying or reacting constituent in the treating material. For example, zirconium ion does not enter into the fluoramphibole-humite structure to any appreciable extent, but it may be included in the batch yielding its own oxide or silicate or complex fluoride compound. A wide range of elements may be similarly obtained in the batch without destroying the Wetting and scavenging effect of the fluoramphibole-humite-inorganic fluoride base product. Among these are the oxide, silicate, or other compound of Ti, rare earths, Mo, W, Nb, Ta, Hf, or even the noble metals.

It has also been found that the rapidity of formation of F-M-I is such that for some applications the powdered raw materials may be intimately mixed, cold-pressed, and added directly to the metal to be treated.

In practice, the fluoride-silicate treatment compounds, being composed as described plus desired added compounds, are added to the cupola, arc furnace, or ladle containing the molten iron. Alternately, the treatment material may be added to the ladle prior to the metal. The amount of treatment material may vary from about 0.1 percent to percent or more depending on the amount of scavenging or nodularization to be accomplished, the amount of surface to be wetted, the amount of blowing or mixing to be done and amount and type of metal to be treated. Temperature of treatment will vary from about 1000 C. to 1500 0, depending again on the variables listed above.

Before pouring, an inoculant or inoculating agent may be added. These are known to the prior art, and may be such materials as calcium silicide, ferrosilicon, manganese silicon, zirconium silicon. These are known to have a nodularizing effect on the carbon, but it is in combination with the fluoramphibole-humite-inorganic fluoride compositions that the most startling and pronounced effect is obtained. It is believed that the combination has a synergistic effect, that is, the effect is greater than the sum total of each separate addition agent. Possibly, action as a solvent for both inoculant and materials to be scavenged is one of the effects of the fluoramphibole-humite-inorganic fluoride additive. For some iron compositions the inoculator is not needed. Also, the inoculator may in some instances be incorporated into the F-H-I treatment compounds, so that only one addition is required.

At high temperatures the fluorides of the treatment agents described in this invention are excellent hydrogen removal agents. Hence, an additional effect on the cast metal may be one of hydrogen removal. However, the method and product presented in this invention is not to be bound by any theory or theories concerning the action thereof.

The metallurgy pertaining to obtaining the metals as elemental or alloyed substances in molten form is well founded in the prior science and art. The present invention pertains to improving the properties of the metals after they are obtained in the molten state. Among such metals and alloys amenable to treatment are most of those of commerce, including magnesium, aluminum and its alloys, titanium, vanadium, chromium, manganese, iron and alloy steels, including stainless, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, cadmium, tin, the rare earths, hafnium, tantalum, tungsten, lead.

The density of the fluoramphibole-humite inorganic fluoride compositions is usually in the range 2.9 to 3.3 as compared to water=l. The melting point may be varied from about 900 C. to 1400 C. or even higher if desired. Also, by controlled fluoride additions, a given composition may be lowered in melting temperature. Being less dense than the molten iron, the fluoramphibolehumite-inorganic fluoride agent will float on top, but because of its extreme wetting ability it is taken into the iron in amounts demanded by the impurities of the iron, or with alloying constituents such as Ni, Co, etc., to the extent allowed by time, stirring, and other variables which are controlled by the operator.

The fluoramphibole-humite-inorganic fluoride agent, with or without additions of secondary elements, functions as a surface protective agent, as a wetting agent, as a scavenging or purifying agent, as a nodular graphiteforming agent and as an oxygen and hydrogen removing agent. It, and an inoculating agent if used, may be added separately to the cast iron or other metal, or they may be combined to give a maximum of inoculation, wetting, and scavenging in one treatment. In the latter instance, the inoculating agent is admixed with the F-H-I or is added to the furnace during preparation of the F-H-I.

As indicated above, different combinations of fluoramphiboles, humites, and inorganic fluorides may be used to give the desired melting point or treatment composition. For example, fluoramphiboles alone or containing inorganic fluorides would give the lowest melting compositions. The compositions may be altered to give greater wetting ability, more alloying constituents, etc.

Experiment has shown that the more volatile constituents of the fluoramphibole-humite systems are SiF ME, and alkali fluoride, such as NaF. MgF is not present in appreciable amount in the vapor state at the temperatures involved, but magnesium is of course readily available from the liquid phase. The compositions described in this invention, being or lesser density than the iron melt, will become the top layer except for entrapments. It is desirable that the temperature of the systems be high enough to melt the fluoramphibole-humiteeinorganic fluoride compounds, or conversely, that the correct melting composition be selected and used. If the metal temperature is too low for any desirable combination of F-H-I, then powdered F-H-I may be used. It is then taken into the liquid metal by solution. The rate of solution will depend on particle size, with about mesh being particularly desirable.

The following examples will serve to more specifically illustrate the invention.

Example 1 In this example gray cast iron from a commercial casting and containing carbon in the form of graphite flakes with a lesser amount of silicon was used, and the treating compound was a fluoramphibole with a formula Of N3. Ca-2 Mg -AlSi .O F

Grams Untreated case iron slag 1000 Fluoramphibole 100 The fluoramphibole was placed in the bottom of the crucible, and the untreated gray cast iron placed on top of this compound. The crucible was then placed in a furnace, and the temperature brought up to 1350 C., and held for one hour. The material was then removed from the furnace and cast into a fire clay crucible. There was no residue of note on top of the melt. The material cast very well and remained liquid several minutes after casting. After the iron cooled, the casting was hit with a hammer many times and did not break but only dented, thus showing malleability and ductility. The product was then welded both with are and with gas welding techniques; it welded with a strong bond. There were no pits or blow holes in the material, so the casting was good.

Microscopic analysis showed there were no graphite flakesthe graphite had taken a nodular form.

Example 2 Cast iron containing graphite flakes with lesser amounts of silicon was treated with 8.0 percent of a humite compound, norbergite. The process was the same as in Example 1 except the temperature was increased to 1450 C. The results were much the same except not all of the graphite flakes were nodular. It was felt too little of this particular treating compound was used. The product was not brittle; it was malleable and could be welded with ease, but was not quite as ductile as Example 1.

Example 3 A cast iron slag containing graphite flakes with silicon, and the raw materials to form a fluoramphibole (NaF, CaO, MgO, A1 and SiO were melted, the fluoramphibole raw materials were blended together, then cold pressed into pellets. The pellets in turn were placed in the crucible along with the iron slag. Twelve percent of the treating compounds was used. The materials were melted at about 1250" C. and soaked for one hour, then removed from the furnace. On top of the melt a very small amount of residue was found. The melt was then processed as in Example 1; the product was nodular iron and was found to have properties equal to or better than that of Example 1.

Example 4 In this example a commercial gray cast iron, 5 percent fluoramphibole, 5 percent humite, and 5 percent of potassium fluoride were used. The melting temperature was 1400 0., otherwise the process was the same as in Example 1. A large percentage of scum or residue on top of the melt was formed and was removed. The product looked like steel ,and was cast with ease. It welded with a strong bond and was malleable. There was no trace of graphite flakes.

Not being bound by any theory as to its effect, the fact is that cast iron treated with the fluoramphibole-humitefluoride compositions has been profoundly and favorably affected. The molten metal has lowered viscosity; hence it pours well. The grain structure of the product is more continuous, resulting in low porosity, ductile castings. Resultant effects of the continuous structure containing spherulitic graphite is that the cast iron is highly machinable, may be welded and in fact may be used as a product equal or superior to many steels. The process, as adapted to a given composition of cast iron and foundry conditions, is reproducible.

What is claimed is:

1. A method for preparing a metal containing graphite in nodular form comprising treating the graphite-containing molten metal with a nodularizing agent comprising a silicate from the group consisting of an amphibole and ahurnite.

2. Method of claim 1 in which the proportion of nodularizing agent is from about 1 to about 15 percent by weight of the metal.

3. Method of claim 1 in which the metal is cast iron.

4. Method of claim 1 in which the nodularizing agent comprises a fluoramphibole.

5. Method of claim 4 in which the nodularizing agent comprises a fluoramphibole and a humite.

6. Method of claim 4 in which the nodularizing agent comprises a fluoramphibole and an inorganic fluoride.

7. Method of claim 4 in which the nodularizing agent comprises a fiuoramphibole, a humite and an inorganic fluoride.

8. Method of claim 4 in which the fluoramphibole is prepared in situ in the molten metal by reaction of batch materials capable of forming the fluoramphibole.

9. Method of claim 7 in which the nodularizing agent also comprises an inoculant from the group consisting of calcium silicide, ferrosilicon, manganese silicon and zirconium silicon.

References Cited UNITED STATES PATENTS 2,932,564 4/1960 Evans -53 2,932,567 4/1960 Evans 75-l30 X 3,197,306 7/1965 Osborn et a1. 75l30 DAVID L. RECK, Primary Examiner. H. M. TARRING, Assistant Examiner.

Claims (1)

1. A METHOD FOR PREPARING A METAL CONTAINING GRAPHITE IN NODULAR FORM COMPRISING TREATING THE GRAPHITE-CONTAINING MOLTEN METALWITH A NODULARIZING AGENT COMPRISING A SILICATE FROM THE GROUP CONSISTING OF AN AMPHIBOLE AND A HUMITE.