US3393996A - Treating agent for ferrous metals - Google Patents

Description

3,393,996 TREATING AGENT FOR FERROUS METALS John C. Robertson and James L. Nichols, Midland, Mich, assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Filed Nov. 29, 1965, Ser. No. 510,398 6 Claims. (Cl. 7553) ABSTRACT OF THE DISCLOSURE The invention comprises an agent for use in treating ferrous metals which comprises a porous body having the pores thereof impregnated with magnesium silicide (Mg Si). A method of its use in treating molten cast iron also is disclosed. With the present treating agent and nodularizing method, rapid but non-violent introduction of magnesium into molten cast iron, effective graphite nodularization and unexpectedly high retention of magnesium in the cast iron are realized.

This invention relates to an improved agent for the treatment of molten ferrous based metals and more particularly is concerned with the composition of such agent and its use in the manufacture of ductile, i.e., nodular, grey cast iron.

It is known in the art that magnesium in cast iron, over and above the amount required for desulfurization, will cause uncombined carbon present therein to be present in a compacted, and preferably a spherical, form. Such cast iron is known as nodular iron and possesses properties different from and improved over grey cast iron where the uncombined carbon is present as a normal flake graphite.

The terms nodular iron or nodularization as used hereinafter refer to iron having spheroidal, i.e., compacted, graphite inclusions and to the process by which these are formed. 7

A wide variety of processes and apparatus related to the introduction of magnesium into molten cast iron have been set forth in the art. Many of these known techniques employed complicated equipment or use compositions such that undesirable substances are introduced into the molten ferrous bath along with the magnesium, but violent, uncontrolled, and hazardous reaction generally accompany the addition of magnesium into the iron in a majority of such known processes. Now, unexpectedly, a safe, simple, and easy to handle composition and process have been found for introducing magnesium into a ferrous based melt which additionally provides for an increased magnesium retention by the metal.

It is a principal object of the present invention, therefore, to provide a novel nodularizing agent for introducing magnesium into molten ferrous based melts and for producing nodular iron therewith whereby rapid but nonviolent reaction is achieved. A substantially complete nodularization of the graphite in the iron product is produced along with the establishment of a desirable high residual magnesium content in the treated iron. A further object of the present invention is to provide a nodularizing agent which obviates the need for special plunging equipment to introduce the treating agent under the molten iron surface and requires no complicated mechanical application equipment. A still further object of the present invention is to provide a nodularizing composition and process which is safe to use and operate and which obviates the need for covered ladies and other special pressure equipment as are employed in other conventional nodularizing processes. It is a further object of the present invention to provide an agent and process for nodulariz- States Patent ing iron wherein undesirable alloying elements and impurities are not introduced into the melt as in many conventional magnesium treatments. It is another object of the present invention to provide a nodularizing agent which, when used for nodularizing iron, assures substantially no loss of the ferrous base melt because of spilling or other violent action of the melt during treatment. These and other objects and advantages will become apparent from the detailed description presented hereinafter.

In accordance with the present invention, a treating agent for use with ferrous-based metals is prepared by impregnating a porous body such as coke with magnesium silicide at high temperatures and in the absence of atmospheric oxygen and moisture. A composite treating agent is thus produced which may be introduced into a molten ferrous metal to produce nodularization of the carbon therein. No violent action of the ferrous bath is produced by the introduction of such treating agent and a surprisingly high residual magnesium concentration is retained in the ferrous metal.

The porous bodies useful as carriers for the magnesium silicide are materials having up to 70% of their volume composed of interconnecting voids which retain their mechanical strength above the melting point of cast iron and which contribute no undesirable impurities into the molten ferrous metal. Such materials include porous coke, carbon and graphite, sponge iron, porous ceramics and porous compacts made by compressing materials such as silicon carbide under high pressure or by sintering briquets of cast iron chips.

A porous body, as defined above, is impregnated with magnesium silicide by immersing a heated piece of such porous body into molten magnesium silicide and maintaining it therein until the pores are substantially filled. The porous body impregnated with Mg Si may then be cooled to form a solid, particulate treating agent sufficiently stable for storage and shipment and suitable for the treatment of molten ferrous metals. It is desirable, for best results, to heat the porous body to a temperature above that of the molten Mg Si prior to immersion. During the impregnation step, as outlined above, it is necessary that atmospheric moisture and oxygen be eliminated from above the surface of the molten magnesium silicide bath. This may be accomplished by any suitable means such as purging with a dry inert gas.

The magnesium silicide-containing treating agent thus prepared is used to treat molten iron, cast iron or steel by submerging a desired quantity of such treating agent below the surface of the molten iron or steel to be treated. Since the temperature of the molten ferrous metal is greater than the boiling point of magnesium silicide, the magnesium silicide is driven out of or expelled from the pores and enters the iron in a controlled manner without a violent reaction, the rate of expulsion being initially a function of the surface to volume ratio. Since magnesium silicide is expelled at the surface of the treating agent particle and volatilization progresses toward the center of the particle in direct proportion to the heat transfer from the molten iron to the porous body, the amount of magnesium silicide expelled is a function of the total time of immersion and the temperature of the molten ferrous metal being treated. After some or all of the magnesium silicide has been driven out of the porous body, the latter can be removed from the molten iron and resubmerged beneath the surface of the molten magnesium silicide for reimpregnation. The process of treating the molten iron may then be repeated so as to permit recycling.

In the preferred form of the invention, metallurgical coke or sponge iron are employed as the porous body. Metallurgical coke is inexpensive, contains about 50% voids, exhibits good mechanical properties at high temperatures and dissolves slowly in molten cast iron or steel so that only a small amount of carbon is absorbed in such melt. Likewise, sponge iron contains about 60 to 70% voids, has good mechanical properties and contributes no foreign material to a molten iron bath even if immersed therein for a sufliciently long period to cause melting of the sponge iron.

Introduction of the treating agent into the molten ferrous bath is accomplished by conventional means, e.g., by placing a number of pieces or lumps of magnesiumsilicide-impregnated coke of known Mg Si content in a conventional treating basket and submerging the basket below the surface of a molten iron bath until visual evidence of volatilization has ceased. The basket is removed and remaining coke, if any, floats to the surface where it is easily skimmed 01f.

Cast iron treated in accordance with this invention is converted into ductile cast iron by nodularizing the graphite therein. This is accomplished without violent agitation of the ferrous metal and such treatment produces an unexpectedly high residual magnesium content in the treated cast iron. The same treating agents are useful in treating molten steel to reduce the sulfur content thereof. Such treatment is accomplished in the same manner and likewise produces no violent agitation of the molten steel during treatment.

EXAMPLE 1 In order to demonstrate the effectiveness of magnesium silicide impregnated into a porous material, the following comparative experiments were conducted: (a) a commercially available metallurgical coke having an average porosity of about 45 to 50 percent voids by volume was placed in a crucible under an atmosphere of argon and magnesium silicide having a particle size of about 20 mesh (Standard mesh size) was placed over the coke. The crucible was then heated to between 1150 and 1200 C. for a short period to permit the magnesium silicide to melt and flow into the pores of the coke. The mixture was allowed to cool and a sample of the coke impregnated with magnesium silicide was removed from the solid mass and weighed to determine the weight gain of the coke. (b) In the same manner, samples of the same metallurgical coke were impregnated with magnesium. A temperature of only 740 to 750 C. was required, however, to melt the magnesium so that is would flow into the pores of the coke. The mixture was allowed to cool and a sample of the coke impregnated with magnesium was removed and weighed to determine the weight gain of the coke.

Samples taken from these preparations had the following characteristics:

Components Wt. percent Mg in Coke A molten ferrous alloy was employed which had the following nominal analysis: Carbon 3.2 to 3.5 percent by weight, silicon 1.5 to 1.9 percent by weight, manganese 0.2 to 0.35 percent by weight, phosphorus 0.025 to 0.03 percent by weight, sulfur 0.004 to 0.008 percent by weight with iron being the balance. The temperature of the molten iron at time of treatment was between 2700 and 2750 F. in all cases. The impregnated coke briquets were immersed in molten iron until by visual observation magnesium generation had ceased. This period was about 30 seconds for the magnesium-containing samples and about 45 seconds for the magnesium silicide-containing samples. After the coke briquets were removed from the molten iron, a post inoculant of 0.7 weight percent of 85 percent ferro-silicon was added. The molten iron was tapped into a mold and allowed to cool. The ingots were tested to determine the amount of magnesium residual in the iron and the following results were obtained:

Magnesium Magnesium iron impregnated with about 15 to 25 weight percent Mg Si produces similar results both in lack of violent agitation during treatment of the cast iron and in magnesium residual in the treated iron.

We claim:

1. An agent for use in treating ferrous metals comprising a porous body having up to percent of its volume composed of interconnecting voids and further characterized as retaining its mechanical strength above the melting point of cast iron and contributing no undesirable impurities into molten. ferrous metal, said porous body having the pores thereof impregnated with magnesium silicide corresponding in composition to the formula 2. An agent according to claim 1 wherein the treating agent is coke having a porosity of from about 45 to 50 percent voids by volume, said voids being impregnated with said magnesium silicide.

3. An agent according to claim 1 wherein the treating agent is sponge iron having from about 60 to 70 percent voids by volume, said voids being impregnated with said magnesium silicide.

4. In a process for the nodularization of graphite and production of a high residual magnesium content in cast iron which comprises providing a quantity of molten cast iron, immersing in said cast iron a sufiicient quantity of porous body having the pores thereof impregnated with a magnesium base treating agent and removing any residual porous body from the treated molten cast iron the improvement which comprises providing a porous body having up to 70 percent of its volume composed of interconnecting voids and further characterized as retaining its mechanical strength above the melting point of said cast iron and contributing no undesirable impurities into said molten cast iron, impregnating the pores of said porous body with magnesium silicide corresponding in composition to the formula Mg Si, and immersing the magnesium silicide impregnated porous body into molten cast iron, whereupon said magnesium silicide enters into said molten cast iron and undergoes rapid but nonviolent reaction therewith to produce nodularization of the carbon within said cast iron and the retention of a high residual magnesium concentration in the cast iron metal.

5. The process of claim 4 wherein the porous body is coke having a porosity of from about 45 to 50 percent voids by volume.

6. The process of claim 4 wherein the porous body is sponge iron having from about 60 to 70 percent voids by volume.

References Cited UNITED STATES PATENTS 2,563,859 8/1951 Millis et al -170 2,606,112 8/1952 Jordan 75-130 2,823,989 2/ 1958 Deyrup et al. 75-130 2,881,068 4/1959 Bergh 75-130 2,933,385 4/1960 Moore 75-130 3,321,304 5/1967 Snow 75-130 BENJAMIN HENKIN, Primary Examiner.