US3078158A - Metallurgical process - Google Patents

Description

Feb. 19, 1963 G. NAESER 3,078,158

METALLURGICAL PROCESS Filed Aug. 7, 1961 CARBON CONTENT v. HEATING TIME HEATING TIME (HRS) 2 a. a d O O o womuamoa Noeavo INVENTOR. Gerhard Naeser BY h/s af/omeys United States Patent 3,078,158 METALLURGHCAL PRUCESS Gerhard Naeser, Dnisburg-Huckingen, Germany, assignor to Mannesmann Aktiengesellschaft, Dusseldorf, Germany, a German company Filed Aug. 7, 1961, Ser. No. 129,707 3 Claims. (Ci. 75--.5)

This invention relates to a process for decarbur-izing ferroalloy powders, especially powdered ferrochromium.

Various elements used in alloying steel, such for example as chromium, manganese, silicon, tungsten, molybdenum and vanadium are normally added to the steel as so-called ferroalloys, i.e. alloys of iron with relatively large proportions of the particular metal to be added. Thus, for example, ferrochromium usually contains 60-75% chromium, the balance being chiefly iron with up to 3% Si.

It is well recognized that these ferroalloys must have as low a carbon content as possible. However, when the alloys are made according to conventional techniques, they have a high carbon content, e.g. up to carbon, and must therefore be decarburized. Several processes have hitherto been proposed for carrying out this decarbunization.

In one prior process, finely ground ferrochromium powder is heated under oxidizing conditions so that most of the carbon is removed as carbon monoxide. However, in this process oxidation of the metal occurs simultaneously with oxidation of the carbon and it is therefore necessary subsequently to subject the partially oxidized alloy to heating in a reducing atmosphere. It is not generally possible to reduce the carbon content of alloys much below 1% by this process.

In another conventional process a finely powdered ferrochromium rich in carbon is partially oxidized to obtain a ratio of oxygen to carbon above 1.3 and is then heated at 1300 C. under reduced pressure for an extended time, i.e. about hours. The reaction between the carbon and oxygen results in an alloy containing only about 0.02% carbon.

It has also been suggested to mix powdered ferrochromium rich in carbon with finely powdered chromium oxide or iron oxide in proportions such that the amount of oxidizing agent is stoichiometrically adjusted to the carbon content. The subsequent extended heating in vacuo at about 1300 C. results in ferrochromium containing about 0.01% carbon.

Finally, it is known to atomize molten ferrochrome by means of compressed air, steam or other oxidizing gases in such a way that the resultant ferrochrome powder is partially oxidized and the degree of oxidation and carbon content of the powder are such that the oxygen content is sufiicient to convert the larger part of the carbon, upon subsequent heating under reduced pressure, to carbon monoxide.

It will be appreciated that while the several processes described above which involve heating under reduced pressure can give quite low carbon contents, the time involved to achieve this is objectionably long.

It has now been found that still lower carbon contents can be obtained with much shorter heating times if to the ferro-alloy powder, before heating, there is added a small amount of an alkali compound.

The invention therefore comprises, in a process for decarburizing powdered ferroalloys having high carbon contents by heating or roasting under sub-atmospheric pressure, the improvement of adding an alkali compound to the powder before heating.

The drawing is a graph showing carbon content as a function of heating time for two alloy powders under- 3,078,158 Patented Feb. 19, 1963 "ice going decarburization. One of the powders has an alkali compound added in accordance with the invention, while the other does not.

Any ferroalloy can be treated according to the inven tion. Preferably, however, the powder is one which has been made by atomizing a melt of the alloy with compressed air, steam or other oxidizing gas and which contains an amount of oxygen equivalent to its carbon content. The invention is especially suitable for decarburizing ferrochromium.

The alkali compound added may be any suitable compound of the alkali metals. Potassium fluoride and potassium carbonate have been found especially useful. The amounts added are very low, normally not more than about 5%. Thus, for example, for potassium fluoride an addition of 0.05 to 5%, preferably 0.1 to 0.3% by weight of the alloy powder is prescribed.

The technique used to add the alkali compound may vary. Thus, for example, it may itself be reduced to a finely divided powder and physically mixed with the alloy powder; or a solution of the alkali compound may be used to impregnate the alloy powder.

The invention will be further described with reference to the following specific example which is given for purposes of illustration only and is not to be taken as in any way limiting the invention beyond the scope of the appended claims.

Example A ferrochrome melt containing, by weight, 70% Cr, 1% Si, and 8.5% C, the balance being iron, was atomized with compressed air to give a powder in which the weight ratio of carbon to oxygen was about 1:13. This powder was then impregnated with a potassium fluoride solution so that the total amount of potassium fluoride deposited on the powder amounted to 0.2% by weight of the powder. The powder was then roasted at 1300 C. at a reduced pressure of between 0.01 and 30 mm. Hg vacuum. The course of the decarbur-ization is shown graphically in the drawing. Referring to curve B, the carbon content after only one hour had already fallen to 0.014%. After three hours it was 0.004% and after 10 hours the carbon content could no longer be determined analytically.

In contrast, curve A, shows the course of decarburization of the same powder without alkali addition at the same temperature and pressure. As will appear, in this operation it was impossible, even after 20 hours of heating, to attain the extremely low carbon content obtained in the prior run.

I claim:

1. A method for the decarburization of ferrochrome powder which has been reduced to powdered form by atomization with air and contains equivalent amounts of carbon and oxygen, which comprises impregnating the powder with potassium fluoride and then roasting the impregnated powder under reduced pressure to achieve substantially complete decarburization.

2. The process claimed in claim 1 wherein the potassium fluoride is used in a proportion between about 0.05% and about 5% by weight of the alloy powder.

3. The process claimed in claim 1 wherein the potassium fluoride is used in a proportion of between about 0.1 and about 0.3% by weight of the alloy powder.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A METHOD FOR THE DECARBURIZATION OF FERROCHROME POWDER WHICH HAS BEEN REDUCED TO POWDERED FORM BY ATOMIZATION WITH AIR AND CONTAINS EQUIVALENT AMOUNTS OF CARBON AND OXYGEN, WHICH COMPRISES IMPREGNATING THE POWDER WITH POTASSIUM FLUORIDE AND THEN ROASTING THE IMPREGNATED POWDER UNDER REDUCED PRESSURE TO ACHIEVE SUBSTANTIALLY COMPLETE DECARBURIZATION.

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