US1985553A - Process of making cast iron - Google Patents

Description

Patented Dec. 25, 1934 UNITED STATES PATENT OFFICE 4 Claim.

This invention relates to the production of gray cast iron of high quality. Such irons are characterized by a requisite degree of fluidity in the molten state to insure good pouring qualities, a

5 homogeneous structure with high dispersion of graphite particles, high tensile strength, elongation and shock resistance, relatively low internal stresses and good machining qualities.

The invention has for its object the provision of an improved process for producing such iron that is highly efficient, rapid and, by reason of the close control to which it is susceptible, capable of producing the highest grade, or a predetermined grade, of graphitized cast iron with a high degree of uniformity.

It has long been known that oxygen and silicon play essential parts in the graphitization of iron during solidification thereof. However, the manner in which these elements function has not heretofore been clearly understood. As the result of research in this field I have discovered that oxygen dissolved in the molten metal acts in some way upon the carbon in the metal to create graphite nuclei or centers of crystallization. I have further discovered that silicon alone, in the absence of oxygen, does not cause graphitization and that its only effect, in the absence of oxygen, is to increase the linear rate of crystalline growth of the graphite particles. It oxygen is present in the metal along with the silicon, silica may be formed and cause cementite of the iron to break down with resultant graphitization, while silicon alone dissolved in the iron only increases the rate of graphite particle growth.

tinct functions performed by oxygen and silicon I have based the present improved method of preparing graphitized cast iron in which these functions are performed in such a manner as to permit of a high degree of control of the graphitizing process such as, I believe, has never heretofore been attained.

In carrying out my process I first produce an iron melt characterized by absence of graphite nuclei. I term this state of the iron the zero state because of the absence of nuclei. This zero state" may be defined practically as one in which no graphitization occurs during normal pouring conditions for sand cast specimens up to about two inches diameter. Next I adjust the silicon content of the melt to an amount not less than 0.5%, which I have found to be the minimum amount suitable for the carrying out of the later oxidizing step of the process. The specific amount of silicon employed will depend upon the specific Upon the knowledge of these separate and dis- In Germany September 5, 1932 character of the graphitic structure sought for the cast iron, as will later be more fully explained. During the adjustment of the silicon content of the melt I prefer to maintain a nonoxidizing environment for the metal for reasons 5 which I will later point out. Following the adjustment of the silicon content the melt is subjected to an oxidizing treatment for the purpose of generating graphite nuclei in suitable numbers. This oxidizing treatment can be carried out in 10 various ways. After the oxidizing treatment of the molten metal, the melt is allowed to cool and solidify. During solidification the cementite of the iron is decomposed with resultant graphitization, the character of which is determined by the 15 number and character of the graphite nuclei created by the oxidizing treatment and by the amount of silicon in the melt. If the highest possible quality of the cast iron is to be attained, I subject the solidified casting to a heat treatment of several hours at temperatures preferably between about 675 and 735 C. This latter treatment decomposes the cementite of the perlite of the iron with resultant precipitation of ferrite and graphite, thus increasing both the ferrite content and the graphitization of the iron.

In order that my invention may be more clearly understood, I will now describe the carrying out of the several steps of the process in further de- 30 tail.

The production of the melt without nuclei, giving the zero state referred to, may be carried out in a variety of ways. For example, I may use as a starting material iron already free of graphite nuclei, such as white pig iron. Indeed, where white pig iron is available, I prefer to make use of it in carrying out my process, particularly if it is feasible to carry out the production of castings in connection with a blast furnace producing such white pig iron since the molten metal may be taken direct from the blast furnace with resultant saving of fuel costs.

However, I may, alternatively, start with gray iron instead of white iron. Thus, I may start with a molten bath of normal gray pig iron mixed with scrap in accordance with the common practice in malleable iron foundries. Such a melt may be reduced to the zero state in accordance with my invention in various ways. For example, the carbon and silicon content of the melt may be lowered 0 by burning them out by oxidation of the bath. Again, one may assist toward the end sought by superheating the melt, thus hastening the solution of particles remaining from the solid state, which particles would, if not dissolved, act as a special 5 expedients.

kind of nuclei leading to coarse graphite precipitation. Further, the removal of graphite nuclei may be accomplished by the removal of oxygen from the melt as by heating in a reducing atmosphere, meanwhile tapping off the oxygen-containing'slags that are formed, and superheating of the melt may be of assistance in this connection by facilitating the reduction of oxides in the melt. In removing the graphite nuclei of the gray iron melt I may use any one of the above described expedients, or combinations of two or more of these However, I prefer when possible to avoid the use of superheating because it increases fuel and refractories consumption and heat losses and also tends to cause undesirable dendritic pre cipitation of the solid solution crystals.

The termination of this first step of the process, in which the zero state of the metal is attained,

is readily determined by sprue tests which may be made from time to time and which show the elimination of the graphite nuclei.

After the iron melt is reduced to the zero state i it usually will contain too little silicon. I have found, as previously noted, that a minimum content of 0.5% silicon is essential to the satisfactory carrying out of my graphitization process. As has already been stated, the precise amount of silicon required depends upon the character of the graphitic structure of the metal desired and this, of course, depends upon the use to which the metal is to be put. For example, if castings with thin sections are to be produced, both a large number of nuclei and a high velocity of growth are necessary, because of rapid cooling, to get a section entirely graphitized. But velocity of growth and number of nuclei are also inversely related, because a larger amount of nuclei replaces partly an increased velocity of nuclei growth or an increased velocity of nuclei growth replaces partly a larger number of nuclei. The latter case is especially of importance for thicker casting sections where I prefer a high velocity of growth combined with a low number of nuclei, because a large number of nuclei would change the crystallization temperature from deep undercooling to little undercooling near the melting point of such heavy section iron, which would lead to a coarse graphite structure.

From what has been said, it is clear that the silicon content and the oxidizing treatment of the zero state metal are closely related and more or less interdependent. In carrying out the process a suitable balance between the two is to be attained. I have demonstrated that the presence of silicon alone will not cause graphitizing of cast iron during solidification because it only increases the linear crystallization velocity of the graphite. Graphitization requires also the formation of nuclei or starting centers of crystallization and such nuclei are created by the presence of oxygen. Without oxygen graphitization does not occur even with high percentages of silicon. Thus I have produced in the absence of oxygen white and insuiliciently graphitized mottled iron castings with a silicon content of over 1.7%, which is generally considered about the highest percentage of silicon permissible to secure, with the use of an electric furnace, a white iron casting in malleable cast iron production. Theoretically there seems to be no reason why a silicon content several times as great as this may not be employed in carrying out the present process. As a practical matter, however, I believe that a silicon content in excess of 2 /2 to 3% will rarely be needed and probably lower percentages will ordinarily be satisfactory.

In effecting the silicon adjustment of the melt, I prefer to maintain a deoxidizing environment for the molten bath. If this is not done, creation of graphite nuclei may begin at some indefinite time during the silicon-adjusting step with resultant loss of the precision or accuracy of control which it is possible to attain if the generation of graphite nuclei is reserved to the final oxidizing treatment of the melt. Then, too, if a relatively low silicon content is to be employed it is desirable to avoid oxidation of the silicon as the consequences of such oxidation would be relatively greater because of the small initial amount of silicon.

The final, oxidizing treatment of the melt, prior to solidification, for the purpose of creating a suitable number of graphite nuclei in the metal, can be carried out in a variety of ways. For example, the oxidation treatment can be carried out in the furnace either by maintaining a suitable oxidizing atmosphere therein, or by using an oxidizing slag covering for the melt during the finishing metallurgical treatment before the tapping of the bath, and this can ordinarily be accomplished by addition to the bath of a small amount of oxide ores. Another method of carrying out the oxidizing treatment is by the use of a runner lined with a mixture of clay and oxides, such as iron or manganese oxides,

which give off the requisite amount of oxygen to the molten metal. Again, the oxidizing treatment can be effected by blowing air, steam, or other oxidizing gases, such as carbon dioxide, through the molten metal. This can conveniently be accomplished while the metal is poured from an upper level to a lower one. Obviously the specific oxidizing treatments referred to may be carried out also in a holding or pouring ladle.

When the molten metal, which is given the finishing oxidation treatment, is poured the metal, during solidification, undergoes graphitization with resultant formation of a homogeneous structure with widely dispersed graphite particles, the gra'phitic structure being subject to nice control because of the definite adjustment of the silicon content, the provision of the definite zero state of the molten metal and the definitely controlled generation of graphite nuclei by the oxidizing treatment, and the homogeneous character of the structure with high dispersion of graphite particles being due in part to the fact that crystallization during solidification is effected at a deep under-cooling temperature, which favors the formation of a fine structure. this nice control of the number, distribution and size .of the graphite particles made possible by my process, the quality of the castings produced may be found satisfactory for many purposes without further treatment. However, as has been stated, the attainment of the highest possible quality is to be secured by a suitable heat treatment of the castings.

This treatment, as has been stated, is preferably carried out by heating the castings for several hours at temperatures between about 675 and 735 C. The purpose of this treatment is to destroy the perlite, which is formed after solidification from solid solution crystals, by breaking it down into ferrite and carbon.

Numerous advantages flow from the use of my improved process, as will readily be appreciated by those skilled in such matters. One of the more important advantages of the process is the great possible accuracy of control of the process Because of and the resultant product secured by reducing the graphite nuclei of the iron to a definite zero state, providing a definite silicon content with is made possible by the use of a raw material 'with a low silicon content and the use of reducing atmosphere in the furnace with resultant easier elimination of graphite nuclei, so that it is unnecessary to employ a superheating treatment of the melt, such treatment, as has been pointed out above, being objectionable on account of high fuel and refractories consumption and high heat losses and on account of undesirable dendritic precipitation of solid solution crystals.

I have already briefly mentioned the economic advantage arising from the use of molten white pig iron direct from the blast furnace and this will readily be appreciated without further comment.

The character of the iron castings produced by my process naturally brings the process into comparison with the standard process of producing malleable iron castings. In comparison with the latter process, a distinct advantage attaching to my process consists in the avoidance of the high shrinkage which characterizes the white iron castings which are subjected to the standard malleableizing treatment, such castings having a shrinkage about twice as great as gray iron castings such as are produced in my process. The lower shrinkage which characterizes my process correspondingly diminishes distortion during cooling and internal stresses after cooling which are a chief cause of fracture and resultant losses.

Still another advantage of the present process, in comparison with the standard malleableizing process, is the shortening of the heat treatment of the, casting as carried out in my process. Again, the heat treatment of the casting in my process is carried out at low temperatures, in comparison with the first stage of the standard malleableizing treatment. This lower treating temperature avoids change of the fine structure of the solidification graphite and of course results in lower costs than can be attained where the high temperature treatment is employed.

While, as has been indicated, the use of raw material with a low silicon content may be advantageous in shortening the melting procedure of my process, it is to be observed that it is possible in carrying out my process to increase the silicon content in the cast iron higher than is allowable in malleable iron castings where solidification occurs without graphitization. By using higher silicon content I am able, where desirable,

to reach the highest velocity of nuclei growth with a minimum oxidizing treatment and at the same time attain a maximum decomposition of perlite by the subsequent heat treatment of the castings.

It will be understood that the above described procedures may be modified more or less in carrying out the process and that my invention is not limited to the specific procedures described, ex-

cept as indicated by the appended claims.

What I claim is:

l. The process of making cast iron comprising preparation of a molten iron substantially free from graphite nuclei; adjusting the silicon content of the molten metal to an amount notless than 0.5% proportioned to the desired size of the graphite particles of the solidified metal; and subjecting the melt to a final oxidizing treatment proportioned in amount to the number of graphite particles desired in the solidified metal.

2. The process of making cast iron comprising preparation of a molten iron substantially free from graphite nuclei; adjusting the silicon content of the molten metal to an amount not less than 0.5% proportioned to the desired size of the graphite particles of the solidified metal and meanwhile maintaining a deoxidizing environment for the melt; and subjecting the melt to a final oxidizing treatment proportioned in amount to the number of graphite particles desired in the solidified metal.

3. The process of making cast iron comprising preparation of a melt of white cast iron; adjusting the silicon content of the molten metal to an amount not less than 0.5% proportioned to the desired size of the graphite particles of the solidified metal; and subjecting the melt to a final oxidizing treatment proportioned in amount to the number of graphite particles desired in the solidified metal.

4. The process of making iron castings comprising drawing molten white cast iron from an ore-smelting blast furnace; adjusting the silicon content of the molten metal to an amount not less than 0.5% proportioned to the desired size of the graphite particles of the solidified metal; and subjecting the melt to a final oxidizing treatment proportioned in amount to the number of graphite particles desired in the solidified metal.

WOLFRAM RUFF.