US3617259A

US3617259A - Process of making cast iron of improved strength and machining properties

Info

Publication number: US3617259A
Application number: US808301A
Authority: US
Inventors: Heinz-Ulrich Doliwa
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1968-03-20
Filing date: 1969-03-18
Publication date: 1971-11-02
Anticipated expiration: 1988-11-02
Also published as: GB1222798A; FR2004340A1; SU364169A3; NL6903748A; DE1758004B1; FR2004340B1; BE730051A; AT309489B; SE365545B; CH523325A

Abstract

For making cast iron, a nucleating agent is added to the superheated melt consisting of highly dispersed silica or an alumina-silica or titaniumdioxide-silica mixed oxide containing at least 90 percent silica. A cast iron of improved strength and machining properties is obtained.

Description

Inventor Heinz-Ulrich Doliwa Friedberg, Hessin, Germany Appl. No. 808,301 Filed Mar. 18, 1969 Patented Nov. 2, 1971 Assignee Deutsche Gold-Und Silber Scheideanstalt vormals Roessler Frankfurt am Main, Germany Priority Mar. 20, 1968 Germany P 17 58 004.0

PROCESS 01F MAKING CAST IRON OF IMPROVED STRENGTH AND MACHINING PROPERTIES 10 Claims, No Drawings Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Joseph E. Legru Attorney-Michael S. Striker ABSTRACT: For making cast iron, a nucleating agent is added to the superheated melt consisting of highly dispersed silica or an alumina-silica or titaniumdioxide-silica mixed oxide containing at least 90 percent silica. A cast iron of improved strength and machining properties is obtained.

PROCESS OF MAKING CAST IRON OF IMPROVED STRENGTH AND MACHINING PROPERTIES BACKGROUND OF THE INVENTION The invention relates to a process of making cast iron of improved strength and machining properties, and more specifically to the use of specific nucleating agents in such process.

The properties of cast iron are determined, on one hand, by the graphite formation, and, on the other hand, by the structure of the metallic base. Both items depend largely on the composition and on the melting, casting, and cooling conditions. in order to understand the crystallization processes taking place in cast iron, it is important to bear in mind that the modifications of the iron-carbon phase diagram which are caused by the usual contents of cast iron in silicon and phosphorus cannot be disregarded as may be the case with carbon steel. It is, rather, necessary to rely on the corresponding three or more component-phase diagram, in particular the iron-carbon-silicon diagram and the iron-carbon-phosphorus phase diagram. For the crystallization of the graphite from the melt, it is of great importance to determine the subeutectic or supereutectic condition of the cast iron involved. Based on a eutectic melt, the saturation degree S of the alloy can be numerically defined by the abridged formula used generally in industrial practice, which reads as follows:

Si 4.23 3 In this formula, C is the carbon content and Si is the silicon contact of the alloy expressed in percentages, while the number 4.23 indicates the carbon content of the binary graphite eutecticum. With S, l, the cast iron is subeutectic, and with S, l it is supereutectic. In case of graphite formation by way of laminae or flakes, the graphite usually occurs in the form of more or less coarse irregularly curved flakes which are frequently arranged in pockets. With subeutectic alloys it is no longer possible to discern a clear limit between the primarily eliminated 'y-mixed crystals and the eutecticum in the solidified structure. However, the primary precipitate of refined foam graphite, in case of a supereutectic composition, is clearly set off by its particularly coarse structure from the much finer structure of the graphite of the eutecticum. The graphite eutecticum shows the fine distribution of the two components which is characteristic for a eutecticum only if an increased cooling rate and a correspondingly stronger supercooling is effected. The reason. for the tendency to form coarse graphite flakes in the eutecticum is that the graphite as the dominating crystal structure in the eutecticum is strongly affected in its crystallization by nucleating forces.

lt is also well known that metallic additives that are soluble in the melt cause changes in the grain size in the direction of a finer structure or also of a grain enlargement in the solidifying melt. On the other hand, foreign elements or metallic or nonmetallic compositions that are insoluble in the base metal show up in the final product as a separate phase in the form of spheres, crystallites, or thin films which are disposed at the grain boundaries or within the grains of the base metal. The finely dispersed insoluble components in that case often act as stimulating nuclei and they add their action to spontaneous nuclei which may already be present and in general cause a finer grain structure. Foreign nuclei useful for inoculation are both metallic precipitates and nonmetallic occlusions, such as oxides, nitrides, sulfides, silicates, etc.

The composition of the cast iron has to be selected to secure a graphitic solidification under the cooling conditions employed as they result from wall thickness Cut the casting and the type of mold, such as sand mold or ingot mold, cold or preheated mold, etc. This is accomplished particularly by a predetermined selection of the silicon contents which favors the graphitic solidification. An increase of the carbon contents likewise acts in the same direction while, on the other hand, a greater increase of manganese contents has the opposite effeet. Thus, with increasing cooling rate there is obtained an increasingly finer structure of the graphite formation.

The metallurgical pretreatment of the metal prior to casting likewise has a substantial bearing on the graphite formation. The reason is that a superheating causes the crystallization nuclei to be put into solution to a greater extent. This, in turn, causes more undercooling and thus a finer graphite structure.

A fine-grained solidification and fine dispersion of the graphite precipitation in the melt is therefore of greatest importance for high-grade cast iron types such as are required particularly for the thin-walled products which have come more and more into use in recent practice.

Numerous processes which have the purpose to cause the cast iron melts to solidify in a fine-grained structure and to cause a finely dispersed graphite precipitation have been proposed on the basis of the work of E. Piwowarsky and his associates. These proposals are based on the addition of predetermined amounts of suitable nucleating agents after superheating of the melt. This superheating and inoculation leads to high-strength cast iron types with excellent machining properties.

The inoculating agents used are predominantly FeSi, CaSi or other silicon base mixed alloys. These agents can be added in the form of loose granulates or in prepacked form. These agents have a deoxidizing effect, partly also having desulfurizing action and furthermore improve the eutectic structure of the cast iron. The amounts of additive are, in general, for instance 4 kg. of CaSi per ton of iron.

However, the so far proposed additives for an inoculation treatment of the melt no longer meet the stringent requirements of modern casting techniques, since the formation of a desired number of local crystallization centers is limited by the lack of uniformity of the particle size of the additives and by the differential amounts of additives used up for deoxidation and desulfurization. The obtained graphite laminae or flakes thus have different size dimensions.

It is therefore an object of the present invention to provide for a type of nucleating agent in connection with the process described which will result in a particularly high number of local crystallization centers and thus in a finely dispersed graphite. In a broader sense, it is an object of the present invention to provide for an improvement in the process of making cast iron whereby a cast iron of better strength and better machining properties can be obtained when adding predetermined amounts of nucleating agents which are no longer liquid when the melt solidifies.

SUMMARY OF THE INVENTION These objects are accomplished by a process wherein a nucleating agent is added to the superimposed melt comprising a highly dispersed silica or an alumina-silica or titanium dioxide-silica mixed oxide containing at least percent of silica.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The highly dispersed silica employed in this invention may be an amorphous silica obtained in the gas phase by a pyrogenic process or may be a wet precipitated silica. The highly dispersed SiO because of its fine subdivision meets the requirement that the nucleating agent should result in the formation of as many local crystallization centers as possible and should correspond to the lattice structure of the metal crystals. The inoculation with superfine SiO, results in a structure having a finely dispersed graphite and in addition favors the graphite precipitation to an extent that the edge hardness which is caused by the preferential heat discharge at the edges of the casting is completely eliminated. It is this edge hardness which causes subseque t difficulties in machining the metal.

A similar effect is obtainable by inoculation with mixed oxides. preferably containing from 90 to 98 percent SiO and from 2 to 10 percent A1 0 or TiO, The following is offered as an explanation for the effects of the superfine amorphous silica or the mentioned mixed oxides without intention to restrict the inventors to this theory. It appears that the superfinely dispersed SiO, particles exert some kind of a directional force on the melt during cooling and that by wetting of the SiO: particles by the melt and by the presence of free surface forces, an increased activity of the nucleating agent is provoked.

Regarding the making and properties of the pyrogenic process silica see British Pats. No. 1,003,957 and No. 752,654 as well as the copending application Ser. No. 189,236 filed Apr. 17, 1962. See furthermore British Pats. No. 1,031,764 and No. 1,110,331 and copending application Ser. No. 268,302, filed Mar. 27, 1963.

Regarding the precipitated silica reference is made to U. S. Pat. No. 3,235,331.

It has been found that for accomplishing the effects of the invention, an addition of about to 300 g. preferably 100 to 200 g. SiO, per ton of cast iron is adequate. That such small amounts of additives are sufficient is surprising. An explanation may be the fact that the highly dispersed silicic acids used as the nucleating agents are present during the solidification of the eutecticum in solid and not in liquid form. A further reason may be that, contrary to the mechanism in case of the addition of CaSi or FeSi alloys, no uncontrolled formation of deoxidation or desulfurization products takes place.

The silica or mixed oxides may be used in the form of a prepacked composition, for instance it may be prepacked in a tin can. In order to assure a good distribution of the silica throughout the ladle, the package may be weighted down by materials such as FeMn, dry iron filings, etc. The package will thus drop to the ground in the ladle. A propelling agent may also be added, for instance in the form of a nitrogen-generating mixture, in order to cause a whirling effect with consequent thorough mixing and distribution ofthe silica.

The following example further illustrates the process of the invention.

The cast iron used in this example contained 3 percent carbon and 1.6 percent silicon and had a saturation degree (S of 0.89.

EXAMPLE A highly dispersed silica in an amount of about 100 g./t. of melt was added to the melt in the form of a prepackage in a tin can. The tin can in addition included weighting agents and a propelling agent. The weighting agents were dry iron filings, and the propelling agent was a nitrogen-generating salt mixture.

This type of prepackage was thrown into the ladle, where it immediately dropped to the bottom of the mold. The propelling agent had an intense whirling effect and thus provided a uniform distribution of the silica throughout the entire ladle.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a process of making cast iron of improved strength and machining properties wherein a nucleating agent is added to the preheated and superheated melt whereupon the melt solidifies, the use, as the nucleating agent, of a highly dispersed silica or an alumina-silica or titanium dioxde-silica mixed oxide, the mixed oxide containing at least percent silica.

2. The process of claim 1, wherein the silica is a silica obtained in the gas phase by a pyrogenic process.

3. The process of claim 1, wherein the silica is a wet precipitated silica.

4. The process of claim 1, wherein the size of the primary particles of the silica is between 5 and mm.

5. The process of claim 1, wherein the mixed oxide contains about 90 to 18 percent Si0 and 2 to 10 percent Al,0 or "H0,-

6. The process ofclaim 1, wherein the silica or mixed oxide is added to the melt in prepackaged form.

7. The process of claim 6, wherein the prepackage includes a wei hting agent. I

8. The process ofclaim 6, wherein the prepackage includes a propelling agent.

9. The process of claim 1, wherein the amount of silica or mixed oxide added to the melt is from about 10 to 300 g./t. of cast iron.

10. The process of claim 1, wherein the amount of silica or mixed oxide added to the melt is from about 100 to 200 g./t. of cast iron.

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Claims

2. The process of claim 1, wherein the silica is a silica obtained in the gas phase by a pyrogenic process.
3. The process of claim 1, wherein the silica is a wet precipitated silica.
4. The process of claim 1, wherein the size of the primary particles of the silica is between 5 and 100 mm.
5. The process of claim 1, wherein the mixed oxide contains about 90 to 18 percent Si02 and 2 to 10 percent A1203 or Ti02.
6. The process of claim 1, wherein the silica or mixed oxide is added to the melt in prepackaged form.
7. The process of claim 6, wherein the prepackage includes a weighting agent.
8. The process of claim 6, wherein the prepackage includes a propelling agent.
9. The process of claim 1, wherein the amount of silica or mixed oxide added to the melt is from about 10 to 300 g./t. of cast iron.
10. The process of claim 1, wherein the amount of silica or mixed oxide added to the melt is from about 100 to 200 g./t. of cast iron.