US2101426A - Method of producing low carbon high chromium cast iron - Google Patents

Description

Patented Dec. 7, 1937 UNITED STATES.

PATENT OFFICE METHOD OF PRODUCING LOW CARBON HIGH CHBIOMIUM CAST IRON No Drawing.

Application March 14, 1935,

Serial No. 11,074 v 6 Claims. (01. 75-427) This invention relates to the manufacture of iron castings, and more particularly to a method of producing, by cupola melting, high chromium cast irons of relatively low carbon content, and

to an addition alloy used in said method.

The advantageous combination of physical and chemical properties of high chromium cast irons has led in recent years to their widespread use and'it has been found that high carbon content 10 has a deleterious effect upon the desired physical and chemical properties of such cast irons. Specifically, high carbon content greatly lowers the resistance to mechanical or thermal shock, decreases machinability, and lowers the physical strength. Because of the Strong afinity of chromium for carbon, such cast irons, if produced by melting in the ordinary foundry cupola with carbonaceous fuel, absorb large amounts of carbon and are for many purposes quite useless. For these reasons, it has been necessary to producehigh chromium cast irons of relatively low carbon content by methods wherein no substantial amount of solid carbon is in contact with the molten metal-such as melting in electric furnaces, oil fired furnaces, or the like.

An object of the present invention is to provide means whereby high chromium cast iron of relatively low carbon content and possessing the desired physical and chemical properties can be produced by melting the raw materials in an ordinary coke fired foundry cupola.

Another object of the invention is to improve the recovery of metallic chromium in the cupola melting process,-which has the further advantage that the melting rate can be maintained or increased, even when melting high chromium alloys, without entailing prohibitively large losses of chromium.

High chromium cast iron containing, for example, about 15% chromium and about 2.5% silicon, produced by ordinary foundry cuploa melting methods, has a carbon content of upwardsof 3.50%, and usually well over 3.75%. The 45 desirable upper limit of carbon content of such an alloy is about 3.00%. Hence, a method of decreasing the carbon content effectively without adversely affecting the physical and chemical,

properties of the product would be of great value.

It is known that the use of a large proportion of steel in the charge and the use of high blast pressure tend to lower the carbon content of cupola cast mm, but these expedients do not suffice to lower the carbon content of chromium u irons to the desired maximum percentage.

Because of its effect in lowering the solubility of carbon in iron alloys, it might be expected that an increase in silicon content of the product would solve the problem; but in: practice the amount of silicon required to inhibit pick-up of carbon, to any useful degree, is so great that the entire character of the cast iron product is altered.

This invention is based on my discovery that if silicon and copper are melted together in close association with chromium, the inhibiting effect on carbon pick-up by the chromium is so pronounced that castings of required chromium, carbon, and silicon contents can be produced, while the physical and chemical properties of the castings are improved through the alloying efiects of the silicon and copper.

The invention comprises adding chromium in the form of a complex alloy or metallic mixture containing in close association the required proportions of chromium, silicon, and a relatively noble graphite-inducing metal such as copper or nickel which inhibits oxidation under the conditions'obtained in the cupola. This addition alloy or mixture is introduced with the metallic charge as it is fed to the cupola, and melts with the charge in the melting zone of the cupola, alloying with the molten iron as it is formed, It is desirable to use a low carbon iron charge and high blast pressure,'but these expedients are not usually essential to the obtaining of a product suiliciently low in carbon if the alloy is added in Per cent Cr Si Remainder substantially iron Composition of castings t Pu'oont Pu'cont Pcosnt Pcoont Cr Cu s1 Mn Physical properties Transverse strength t m aw. Hm NO. n

.in. Strength, Dellocem Dodcolbafer tion, lbagr tion,

sq. 11. inches sq. inches The decided lowering of carbon content, and the great increase in desirable physical properties brought about by the method of the invention, are obvious. If in this instance the efiect upon carbon solubflity of silicon alloyed with cast iron had alone been depended upon to lower the carbon content, it would have been necessary to raise the silicon from about 2.5% to about 6.5% which would have completely altered the physical properties of the castings and made them practically useless. These low carbon castings, actually made in the cupola, have in general the properties and composition, including silicon content, of similar castings made by electric furnace methods, but they differ very greatly from such castings made by usual cupola melting methods.

The following data from tests with a small cupola further illustrate the advantages of the invention:

cated by the above data is an important advantage of the method of the invention. Theloss of chromium in such operations, if the chromium is added as free lump ferrochromium, is ordinarily on the order of thirty percent, and with high blast pressure it may increase to as much as seventy percent, as indicated by the foregoing data. I have found that with the complex alloy of the invention, the presence of copper and silicon together is efiective in decreasing the oxidation loss of both silicon and chromium, to such an extent that for given blast pressure the loss is reduced approximately by half. This oxidation-resistant characteristic of the addition alloy of the invention makes it possible to operate the cupola at high blast pressure without excessive loss of chromium, which is an important factor in obtaining lowest possible total carbon content, as well as in economic operation of the cupola.

That copper exerts estimate and distinct influence upon the inhibition of carbon pick-up was demonstrated by two melts, otherwise similar, in

the nrst of which chromium silicon alloy without copper was used, while in the second melt the preferred proportion of copper was added. The first melt showed 2.20%and the second 1.8% carbon pick-up.

In the development of the method of the invention I have'i'ound that the addition alloy may take the form of a true alloy of the constituent metals, or mechanical mixtures of them or of alloys containing them. In the case of mechanical mixtures the metals may be bonded in any suitable manner or placed in metal containers to assure melting in proximity to one another.

The preferred range of composition of the addition alloy or metallic mixture is within approximate limits:

Cr=40 to 75% Si=5 to 15% Cu=5 to 15% C=less than 2% Fe=25 to 50% (remainder) As previously indicated, a true alloy of the metals is somewhat preferable but by no means essential, a suitable addition comprising a briquetted mixture of ferrochromium, ferrosilicon andcopper, or copper silicon, chromium silicon, or the like. Low carbon content is preferred in all cases.

While the invention has been described in terms of the use of copper as an alloying agent it is to be understood that within the scope of the invention copper and nickel are substantial equivalents.

I claim:

1. As an article of manufacture for adding chromium toa cupola melting charge, a briquetted metallic mixture comprising a binder and metallic ingredients having the approximate composition range: 40% to 75% chromium; 5% to 15% silicon; 5% to 15% copper; carbon in an amount less than 2%; remainder iron.

2. As an addition alloy for adding chromium to a cupola melting charge, a briquetted metallic mixture comprising a binder and a metallic alloy of the approximate composition range: 40% to 75% chromium; 5% to 15% silicon; 5% to 15% copper; carbon in an amount less than 2%; re-

mainder iron.

3. As an article 01 manufacture for adding chromium to a cupola melting charge, a briquetted metallic mixture comprising a binder and metallic ingredients having the approximate composition range: 40% to 75% chromium, 5% to 15% silicon; 5% to 15% nickel; carbon in an amount less than 2%; remainder iron.

4. As an addition alloy for adding chromium to a cupola melting charge, a briquetted metallic mixture comprising a binder and a metallic alloy of the approximate composition range: 40% to 75% chromium; 5% to 15% silicon; 5% to 15% nickel; carbon in an amount less than 2%; remainder iron.

5. Method of producing,'by direct melting in a carbonaceous fuel-fired cupola, high-chromium cast iron having an abnormally low carbon content; I which method comprises adding to the charge to be melted, at least a major fraction 01' the chromium in the form of briquets containing metallic chromium, silicon, and metal selected fromthe group consisting of copper and nickel; the metallic ingredients of said briquets being so proportioned that the percentage of metallic 75 2,101,426 chromium is greater than the sum of the perof closely associated metals, which mixture has the approximate composition: 40% to 75% ohm mium; 5% to 15% silicon; 5% to 15% metal selected from the group consisting of copper and nickel; carbon in an amount less thanv2%; re- 5 mainder iron.

8 O. BURGESS.