US1748750A - Process of making low-carbon ferro-alloys - Google Patents

Description

Patented Feb. 25, 1930 UNITED STATES PATENT orl-lcs FREDERICK I. BECKET, OF NEW YORK, N. Y., ASSIGNOR TO ELEC'IRO METALLURGICAL COMPANY, A CORPORATION OF WEST VIRGINIA.

PROCESS OF MAKING LOW-CARBON FERRO-ALLOYS 1T0 Drawing.

This invention relates to the art of making low-carbon ferro-alloys typified by rustless iron, low-carbon term-manganese, etc. The object of the invention is to provide a 5 novel and economical process for preparing such alloys.

Rustless iron for the purposes of this invention" may be defined as an alloy consisting essentially of iron and chromium, with upward of about 9 percent of chromium and less than 0.2 percent of carbon. Nickel may also be, present in varying proportions replacing the-iron. A preferred composition contains about 12-14 percent of chromium 15 with 0.12 percent or less of carbon. Another excellent composition contains around 16-18 percent of chromium, with 7 -8 percent nickel and carbon around 0.12 percent or less.

Such alloys have heretofore been made by 20 various processes, such as the introduction of low-carbon ferrochrome or chromium metal into a bath of low-carbon iron; or by subjecting an iron-chromium alloy containing more than'the desired proportion of carbon to an oxidizing treatment to eliminate a portion of the carbon, and thereafter deoxidizing the resulting low-carbon product, as disclosed in U. S. Patent 1,365,091 to Clement.

It has likewise been proposed to reduce the carbon content of ferrochromium by blowing the molten alloy with air or other oxidizing gas by a procedure somewhat analogous to the Bessemerization of steel, as described for instance in United States Patent 1,063,280

to Morehead and United States, Patent 1,063,341 to Edmands: but this procedure, if

continued until the carbon content is reduced to the very low proportions herein contemplated, involves heavy losses of chromium, which oxidizes at increasing rate as the carbon content of: the molten bath is diminished.

It has likewise been proposed to subject molten ferrochrome, at a temperature .of

about 15001600 C. to the action of a mixture of steam and hydrogen, to which mixture other gases such as carbon dioxid, carbon 1 nonoxid or air may be added, it being considered that the hydrogen or other re- Application filed February 25, 1927. Serial No. 171,078.

ducing gas would exert a restraining action on the oxidation of the metals of the molten alloy. However both hydrogen and steam exert a strong cooling effect upon molten ferrochrome, so that suitable operating conditions are not readily maintained. 1

Hydrogen is known to be effective to some degree to remove carbon from metals in the sol1d state under certain conditions, but attempts to reduce the carbon content of molten ferrochrome from sa 2.0 to less than 0.2 percent by means of ydrogen, without application of external heat, were not successful: when the hydrogen was blown beneath the surface of the melt the latter quickly solidified, the reaction heat being insuificient to maintain fluidity; and introduced above the surface it proved quite ineifective to remove the carbon. On the other hand carbon can be quickly removed by oxygen, intromaintained or even increased by the strong- 1y exothermic reaction: and thereafter or 1n some cases simultaneously the highly heated metal is blown with hydrogen to assist, in reducing its carbon content. By proceeding in this way the heat developed during the oxygen blow is availed of to keep the metal molten during the hydrogen blow: and by properly controlling the respective blows a product of almost any desired carbon content down to a few hundredths of one percent can be prepared. The use of hydrogen to efiect the final decarburization to the desired limit presents the great industrial advantage, as compared with the use of oxygen for the same purpose, that the elimination of carbon is accomplished with relatively small losses of chromium.

Following is an illustrative example of a procedure in accordance with my invention,

it bein clearly understood that the invention is not limited to the treatment of term alloys likewise oxidized during this blow, and the highly exothermic reactions (oxidation of carbon, chromium, iron, and silicon 1f present) result in a very highly heated and fimd melt. For convenience I will refer to this highly heated melt as superheated. The oxygen is then cut off and hydrogen immediately introduced beneath the surface of the superheated metal, and this is blown, (under falling temperature conditions) until the carbon is further reduced to the desired point, for example to 0.10 or 0.12% or lower.

Certain other typical and specific instances to which however the invention is by no means limited, are as follows:

A heat containing 19.36% Cr and 1.78% C was blown with pure oxygen, and yielded an alloy containing 16.03% Cr and .08% C, the chromium recovery bein 73.5%. A second heat containing 16.97% r and 1.15% C was similarly blown with oxygen, yielding an alloy containing 12.93% Cr and 0.06% C, with a chromium recovery of 70.8%. These are typical of the results obtainable with pure ox n.

This average of two runs employing oxygen and hydrogen successively was as follows:

The alloy treated contained 17.90% Cr with 2.04% C. At the conclusion of the oxygen blow its composition was 15.75% Cr and 0.17% C. This was then blown with hydrogen and yielded an alloy containing 15.75% Cr and .05% C. The total chromium recovery was 84%.

In another instance the compositions of the original charge, and of the alloy after the oxygen and hydrogen blows respectively were as follows:

Alter After orlgmal oxygen hydrogen Per cm! Per cent Per cent Chromium 21. 47 8. 53 18. 84 Ni 7. 64 a 24 8. 27 2. 01 0. 16 0. 10 0. 12

The total recovery of chromium was in this case approximately 80% and of nickel approximately 98%.

Since the losses of chromium occur mainly during the oxygen blow it is of course advantageous, from the point of view of chromium recovery, to arrest the oxygen blow at a somewhat earlier point than was here the case, say when the carbon content is about 0.2 to 0.3% ,having due regard to the necessity above explained of a sufiicient superheat under the particular operating conditlons to permit the subsequent hydrogen blow to be continued until the desired final carbon specification is attained.

External heat might of course be applied during the hydrogen blow to assist in maintaining the necessary temperature, and under these conditions it would not be essential to superheat the metal b an oxygen blow in order to provide suitable conditions for the hydrogenblow. I believe myself the first to accomplish the substantial decarburization of a molten ferro alloy by means of hydrogen, and this invention contemplates such decarburization broadly, the term decarburization being herein used to indicate a reduction of the carbon content to a point not substantially in excess of about 0.2 percent, or in other words, to a point within the permissible rustless iron range.

The general principle involved in this invention in its preferred embodiment is the use of an oxygen (pure or diluted) blast to maintain, and in part at least to provide, the very high temperature of the melt which is essential to efficient decarburization by hydrogen, the decarburizing action of oxygen being supplemented by that of hydrogen. Within this principle the procedure may be variously modified. For instance the hydrogen and oxygen may be introduced simultaneously, although preferably not in admixture with each other, the oxygen being preferably supplied above the surface of the inelt and the hydrogen below the surface. The ratio of oxygen and hydrogen may be varied as the blow pro-.

gresses to maintain any desired temperature conditions. One very desirable modlfication is to blow first with oxygen, applied above the surface, and then to continue the treatment with both oxygen and hydrogen, applied respectively above and below the surface of the metal.

The processes as described are applicable with appropriate modifications to the manufacture of low-carbon ferroalloys other than rustless iron; and in particular to the manu facture of low-carbon ferromanganese.

Certain alloys, including iron-chromium alloys, after treatment with hydrogen as described herein exhibit a porosity due to blowholes. Where this effect is so pronounced as to be objectionable it may be overcome by treatment with appropriate reagents, among which I now prefer to use a lead-sodium alloy, alone or in conjunction with the usual finishing reagents such as silicon and manganese; or a solid metal may be obtained by subjecting the melt to a very short or indeed momentary blow with oxygen, following the hydrogen blow, in which case it is desirable to follow this by the well known deoxidizing treatments. Other treatments of like efiect ma of course be used.

claim:

1. The process of decarburizing ferroalloys which comprises blowing the molten alloy with chemically uncombined oxygen to remove carbon and raise the temperature of the alloy; blowing hydrogen, substantially free from admixed oxygen compounds through the molten alloy to remove additional uantities of carbon; and continuing the hy rogen blast after the carbon content is reduced below 0.2%.

2. The process of making rustless iron from an iron-chromium alloy containing excess carbon which comprises blowing the mol- 0 tenalloy with chemically uncombined oxygen to remove carbon and raise the temperature of the alloy; blowing hydrogen, substantiallyfree from admixed oxygen compounds, through the molten alloy to remove additional uantities of carbon; and continuing the hy rogen blast after the carbon content is reduced below 0.2%. v

3. The process of making rustless iron from an iron-chromium alloy containing ex- 80 cess carbon which comprises blowing the molten alloy with a gas containing uncombined oxygen in a higher proportion than is present in air to remove carbon and raise the temerature of the alloy; and then removing a 5 rther quantity of carbon by blowing w1th hydrogen in theabsence of substantial quantities of gaseous oxygen-containing substances, the carbon content during the latter part of the hydrogen blowbeing less than 4. The process of making rustless iron from an iron-chromium alloy containing excess carbon which comprises blowing oxygen onto the surface of the molten alloy to raise its temperature and remove carbon; and then blowing hydrogen substantially free from gaseous oxygen containing substances through the molten alloy to remove more carbon, the carbon content of the alloy being below 0.2% during the latter part of the hydrogen blow.

In testimony whereof, I afiix my signature.

' FREDERICK M. .BECKET.