US2831761A - Method for producing low-carbon ferrochromium - Google Patents

Description

2,831,76l Patented Apr. 22, 1958 METHOD FOR PRODUCING LOW-CARBON FERROCROMIUM Hugh S. Cooper, Cleveland, Ohio,

Walter M. Weil,.Cleveland, Dhio No Drawing. pplication May 10, 1956 Serial N0. 583,894

assignor to 6 Claims.

This invention relates to the production of ferrochromium and particularly to the production of low-carbon ferrochromium by the decarburization of high-carbon ferrochromium. Though the invention is not limited to the production of ferrochromium having any particular ratio ofchromium to iron, industrial uses of ferrochromium for most purposes require a percentage of chromium ofat least 50% and generally at least 65%, a range of 65-75% being most commonly used as a master alloy in the production of stainless steel. Thus, the principal uses forthe inventioninvolve chromium'contents of this order of magnitude.

In the production of many chromium-containing iron alloys, it is highly desirable to employ a ferrochromium.

master alloy having a low carbon content, generally of the order of 0.03% or less in the case of the production of stainless steel, unless the stainless steel also contains columbium or titanium to fix the carbon in the steel in such a manner that higher carbon contents can be tolerated without inter-granular corrosion.

For many years, ferrochromium of low carbon content has been prepared principally by a silicon reduction process in an open-arc electric furnace. This process is expensiveto operate, and many efiorts have been made to find a less expensive process for accomplishing the same end result; r a

More recently, it, has been proposed to produce lowcarbon ferrochromium by decarburizing high-carbon ferrochromium under subatmospheric pressure with asuitable oxidant for the carbon, the'carbon being removed largely as carbon monoxide. Such a process is the subject of U. S. Patents 2,473,019, and 2,473,020 to Erasmus and U. S. Patent 2,473,021 to Spendelow et al. The in, ventions of these patents are presently being exploited commerciallyon a large scale using silica as the oxidant, a pelletized mixture ofhigh-carbon ferrochromium and silica being heated under. a relatively high vacuum and maintained at an elevated temperature sufiicient to effect combination of the carbon with the oxygen of the silica. The silica also assists in removing excess sulfur, though it adds siliconto the alloy in the process. Thus, the resulting product. is a low-carbon ,ferrochromium in pellet form having a relatively-high silicon content. According to the abovementioned patents, the process is preferably conducted ata' temperature above .1150" C. but below the temperature at which the reacting charge fuses. To obtain a practical rate of reaction while avoiding fusion, a temperature above the solidus temperature of. the

high-carbon ferrochromium, but below about 1300 C..

(for 65% "to 75% chromiumcontent), is employed until the carbon. content at the surfaces of the pellets approaches the eutecticcomposition,;whereupon the temperature is reduced to below the eutectic temperature until the carbon concentration throughout the charge has dropped substantially below'the f eutectic percentage. Thereafter, the temperature may .be raised and decarburization continued .at as hignIa. temperature as desired, provided only that the liquidus'temperature of theferrochromium is not reached before the decarburizing reaction is substantially complete.

According to U. S. Patent 2,473,021, the. outer surfaces of the pellets of charge material are decarburiz ed more rapidly than the interiors thereof, so that the outer surfaces approach the eutectic composition before'subst a m tial decarburization of the interiors of the pellets has occurred. Unless the temperature is reduced as the .eutectic'composition is approached in the outer surfaces. of the pellets, these surfaces tend to fuse and form a skin which prevents further effective decarburization of the interiors of the pellets, so that continued verts the interiors of the pellets to a stone-like unreacted mass. To avoid this, it wasrproposed to reduce the temperature to below the eutectic temperature when the surfaces of the pellets approach the eutectic composition and to maintain the lower temperature until substantially the entire mass has passed through the eutectic composition. Thereafter, the temperature may be raised as decarburization proceeds without danger of fusion, provided the liquidus temperature is not too closely approached.

heating con- Decarburization of high-carbon ferrochromium according to this last described process is very slow, at best. According to the three patents referred to above, the process requires heating for upwards of 20 hours to reduce the carbon to the desired low concentration. In actualpractice, when decarburizing many tons of alloy at a time, the heating cycle takes about one week; As noted above, decarburization is performed with the charge material in-the form of compacted pellets, and these pellets retain their form during the process. Thus the final product is in the form of highly porous pellets, which have been found to be objectionable in many cases because of their tendency to float on top of a molten metal bath to which they are added, thus'making it diflicult" to melt the product into the bathbefore it absorbs excessive amounts of nitrogen'from the atmosphere, which gives undesirable properties to the resulting alloy.

The principal object of the present invention is to V ess for decarburizing high-carbon ferrochromiurn by heating with an oxidant without the necessity for compressing and bonding a mixture of the reactants into cemented pellets.

In accordance with the present invention, the rapidity and, therefore, the economy of the decarburizing reaction are greatly enhanced and, if desired, the low-carbon ferrochromium product may readily be produced in the form of'non-porous, massive metal more suitable for many metallurgical usesJ Moreover, in accordance with the present invention, the high sulfur content of the highcarbon starting material may be reduced to a satisfactory value without producing the high silicon content which results from the use of silica as the oxidant.

The present invention is based on the discovery that it is possible to use'iron oxide, chromium oxide, or a mixture thereof as the oxidant, while carrying out the decarburizing reaction at a higher and substantially uniform temperature through the eutectic stage, thus greatly accelerating the reaction to the point where it may be completedin a period of from one to two hours. It has been discovered that, at such higher reaction temperatures, sulfur in the charge material is substantially reduced without the use of silica as an oxidant, and that the decarburizing reaction may be carried out at such higher temperatures without fusion of the reactants and consequent retardation of the reaction, which has heretofore been considered inevitable if the charge passes through the eutectic composition above the eutectic temperature.

' In accordance with the invention, finely comminuted high-carbon ferrochromium is mixed with the metal oxide, also in a finely comminuted form. For convenience in handling, this dry mixture may simply be pressed into lumps 'or cakes of any convenient size without employing any bonding agent.

The mixture of. high-carbon ferrochromium and metal oxide, either in loose comminuted form or lump or brick form, with or without a bonding agent, is first heatedto a temperature in the range of about 800 to 1000 C. in a suitable reaction chamber, while reducing the pressure in the chamber until a high vacuum is obtained, preferably less than 0.1 mm. of mercury absolute pressure. The temperature in this range is maintained long enoughfor the entire charge to have reached such temperature and for air and gas entrained in the reaction mass to be effectively reduced to a negligible quantity. Thereupon the temperature of the mass is preferably raised as quickly as the charge will absorb the heat, to

a point well above the solidus temperature "of the ferrochromium, e. g., about 1325 l425 C., while maintaining the low subatmospheric pressure to the extent that it is practical to do so during the rapid evolution of carbon monoxide resulting from the decarburizing reaction. Under these conditions, the carbon content of the ferrochromium is rapidly reduced to below the eutectic composition with no objectionable tendency of any part of the charge to fuse.

When the carbon content has been reduced to well below the eutectic composition, the temperature may be further raised gradually to about 1500 C. or so until decarburization and evolution of carbon monoxide have ceased, as evidenced by the ability to maintain a substantially stable vacuum in the reaction chamber. Thereupon, if desired, the temperature may be further raised to above the melting temperature to convert the decarburized ferrochromium to a molten condition suitable for pouring from the reaction chamber into molds. Preferably, before raising the temperature above about 1500 C. for this purpose, an inert gas is bled into the reaction chamber so that the gas pressure will inhibit vaporization of chromium duringthe melting operation. ,However, the tendency for such vaporization to occur to any appreciable extent is slight because of the fact that the chromium is present as an alloy with iron and is less rapidly vaporized in this condition.

When the charge material has been preliminarily cornpacted or for-med into cemented pellets, the decarburizing reaction leaves the product in substantially the same pelletized or lump condition, except that it is porous due to the rapid evolution of gas from the mass during the decarburizing reaction. The above melting step converts the product into dense massive metal.

Instead of finally raising the temperature in the reaction chamber above the melting point of the decarburizecl ferrochromium, the temperature may be raised only sufficiently to sinter or partially fuse the material into a coherent mass. Such mass may be broken up and the fragments readily cold pressed into coherent, dense masses of a size and shape suitable for use as an alloying ingredient in the manufacture of steel.

Despite the fact that the charge material, when handled in accordance with the present invention, apparently passes through the eutectic composition at a temperature at which it should theoretically be in a molten condition, it appears that no melting occurs during the process. When the charge has been uniformly heated to well above the solidus temperature before the decarburizing reaction is initiated, it appears that the reaction proceeds at such a rate throughout the mass that the eutectic come position does not exist for a long enough time for any appreciable fusion to occur. Thus, higher reaction temperatures than have heretofore been used may be employed without difiiculty and may be maintained continuously above the eutectic temperature from the time the reaction is initiated until the carbon content has been reduced to well below the eutectic composition. After passing through the eutectic composition, the liquidus curve rises sharply with further reductions in carbon content. Below about 1% carbon, the temperature may be gradually raised to a considerably higher level as the carbon content is further reduced, preferably while avoiding temperatures above the liquidus curve until decarburization is complete.

To illustrate the operation of the invention, three examples are given below of runs made on a high-carbon ferrochromium of the following analysis:

Percent by weight Chromium 68.06 Carb n 4.59 Silicon 1.70 Sulfur 0.055 Iron (-by diiference) 25.595

Example I Iron oxide as an oxidant was mixed with the highcarbon ferrochromium in the ratio of 26.25 parts to parts by weight. The briquetted mixture was placed in the furnace, and the furnace was evacuated to about .002-.003 mm. pressure. The furnace temperature was then raised to about 850 C. and held for about 30 minutes to degas the charge and heat it uniformly'throughout its mass. Because of the release of occluded gases, the pressure in the furnace rose to about .150 mm. during this preheating operation, the rise in pressure being so slight as to indicate that decarburizing had not yet commenced.

The temperature was then quickly raised to about 1375" C., whereupon rapid evolution of carbon monoxide occurred. This further impaired the degree of vacuum that could be maintained in the furnace to such an extent that no vacuum readings could be made on the Me- Leod gauge. After about 2 hours of heating at this temperature, while continuing to pump out the evolved gas, a steady pressure reading of about .003 mm. on the McLeod gauge was again obtained, indicating that decarburization had virtually ceased. The temperature was then quickly raised to 1650 C. to melt the charge, while maintaining the vacuum. Finally, the furnace was allowed to cool, and the charge was removed and analyzed. The analysis of the product was as follows:

Percent by weight Chromium 54.86 Carbon 0.033 Sulfur 0.022

The balance of the product, of course, was made up of the iron and silicon in the original ferrochromium charge and in the iron oxide mixed therewith.

Example 11 Chromium oxide as anoxidant was mixed with the high carbon ferrochromium in the ratio of 20 parts to .100 parts, by weight, and the briquetted mixture Percent by weight Chromium 73.14 Carbon 0.020 Sulfur- Example III 7 A combination of 5 parts of chromium oxide and 7 parts of iron oxide was mixed with 100 partsof the high-carbon ferrochromium, by weight, and the mixture was subjected to the same operations described in Example 1, except that the preliminary heating was conducted by slowly raising the temperature of the entire mass to about 900 C. over ,a period of about /2 hour. 'Ihetemperature was then raised rapidly to about 1300" C., and a decarburizing temperature was maintained between 13l0 and 1375 C. for about 2 hours. The final melting temperature was'about 158S C. The analysis of the product was as follows:

Percent by weight Chromium" 62.52 Carbon 0.022 Sulfuri 0,023-

From the foregoing examples, it will be observed that iron oxide, chromium oxide, and mixtures thereof are substantially equally effective in decarburizing ferrochromium and in reducing the sulfur content. The

choice between the three types of oxidant is governed largely by the desired final chromium-to-iron ratio, which is increased or decreased to the extent that chromium may be present in a greater or lesser ratio in the oxide or mixture of oxides used as the oxidant.

Though, in all three instances, the charge was maintained well above the eutectic temperature of approximately 1275 C. throughout the decarburizing operation,

observation of the charge as the carbon content passed through the eutectic composition showed no visible sign 'of fusion, and the briquettes retained their original form until the final melting step. The great rapidity of the decarburizing operation demonstrated that the formation and escape of carbon monoxide was at least not significantly hindered by fusion of the charge, if any fusion at all occurred.

As noted above, a possible explanation for the rapidity of the decarburizing reaction and for the apparent lack of fusion at temperatures at which. the charge would have been expected to melt, while at or near theeutectic composition, is that the composition of the charge changed so rapidly at the high decarburizing temperatures employed that insulficient time for fusion was available. It is believed that preheating the charge uniformly to a temperature below but close to that at which decarburization commences, for degassing the charge, contributes to the surprising results obtained by making rapid and uniform heating of the entire charge to a temperature above 1300 C. easier to achieve. Whatever the explanation may be, however, the eificiency of the decarburizing operation and the lack of any sign of fusion until the final melting temperature is produced are general, chromium oxide is preferred for this purpose,

as well as being desirable for increasing the chromium vention is not limited to the particular conditions dis closed in the examples and that numerous variations thereof may be employed within the scope of the appended claims.

What is claimed is: i i e 1. The method of decarburizing ferrochromium containing in excess of 3% carbon by weight, comprising mixing said ferrochro'mium and an oxidant, both in comminuted form, the oxidant being selected from the class consisting of iron oxide, chromium oxide, and mixtures thereof and being present at least in the amount required to supply oxygen sufiicient to combine as CO with the carbon to be eliminated from said ferrochromium, heat ing said mixture uniformly and under subatmospheric pressure to a temperature between about 800 and 1000 C. to degas the same, quickly raising the temperature of said mixture to an initial reaction temperature in the range above the eutectic temperature but below the liquidus temperature of said ferrochromium while maintaining said subatmospheric pressure, and maintaining said subatmospheric pressure and a reaction temperature above said eutectic temperature until the carbon remaining in said ferrochromium is below 1% by weight. 2. The method of decarburizing ferrochromium containing from about 65% to'75% chromium and in excess of 3% carbon by weight, comprising mixing said ferrochromium and an oxidant, both in comminuted form, the oxidant being selected from the class consisting of iron oxide, chromium oxide, and mixtures thereof and being present at least in the amount required to supply oxygen sufficient to combine as CO with the carbon to be eliminated from said fe'rrochromium, heating said mixture uniformly and under subatmospheric pressure to a substantiallynon-decarburizing temperature above 800 C. to .degas the same, quickly raising the temperature of said mixture to an initial reaction temperature in the range between 1300" and 1425 C. but below the liquidus temperature of said ferrochromium while maintaining said subatmospheric pressure, maintaining said subatmospheric pressure and a reaction temperature in said range until the carbon remaining in said ferrochromium contrary to what would be expected from the prior art.

In addition, contrary to the teachings of the prior art, it is unnecessary for the oxidant to contain any silica in order to reduce the sulfur content when operating at the higher temperatures employed in accordance with the present invention. Thus, sulfur may be reduced without increasing the silicon content of the ferrochromium. In

is below 1% by weight, and finally raising the temperature of the ferrochromium above its melting point to coalesce the same into a non-porous mass.

3. The method of decarburizing ferrochromium containing in excess of 3% carbon by weight, comprising mixing said ferrochromium and an oxidant, both in comminuted form, the oxidant being selected from the class perature above the eutectic temperature until. the carbon remaining in said ferrochromium is below 1% by weight, surrounding the decarburized ferrochromium with an atmosphere of an inert gas, and finally raising the temperature of the. decarburized .ferrochromium above its melting point to coalesce the same into a non-porous mass. 7

4. The method of decarburizing ferrochromium" containing in excess of 3% carbon by .Weight, comprising mixing said ferrochrornium and an oxidant, both in comminuted form, the oxidant being selected from theclass consisting of iron oxide, chromium oxide, and mixtures thereof and being present at least in the amount required to supply oxygen suflicient to combine as CO with the carbon to be eliminated from said ferrochromium, heating said mixture uniformly and under subatmespheric pressure to a substantially non-decarburizing temperature above 800 C. to degas the same, quickly raising the temperature of said mixture to an initial reaction temperature in a range above the eutectic temperature of said ferrochromium but below 1425 C. while maintaining said subatmospheric pressure, and maintaining said subatmospheric pressure and a reaction temperature in said range until the carbon remaining in said ferrochromium is below 1% by weight.

5. The method of decarburizing ferrochromium containing in excess of 3% carbon by weight, comprising mixing said ferrochromium and an oxidant, both in comminuted form, the oxidant being selected from the class consisting of iron oxide, chromium oxide, and mixtures thereof and being present at least in the amount required to supply oxygen sufiicient to combine as CO with the carbon to be eliminated from said ferrochromium, heating said mixture uniformly and under subatmospheric pressure to a substantially non-decarburizing temperature above 800 C. to degas the same, quickly raising the tem-' perature of said mixture to an initial reaction temperature in a range above the eutectic temperature of said ferrochromium but below 1425 C. while maintaining said subatmospheric pressure, maintaining said subatmospheric pressure and a reaction temperature in said range until the carbon remaining in said ferrochromium is below 1% byweight, surrounding the decarburized ferrochromium with an atmosphere of an inert gas, and'finally raising the temperature of the decarburized ferrochromium above its melting. point to. coalesce the same into a non-porous mass.

6. The method of decarburizing ferrochromium containing from about to chromium and in excess of 3% carbon by weight, comprising mixing said ferrochromium and an oxidant, both in comminuted form, tr e oxidant being selected from the class consisting of iron oxide, chromium oxide, and mixtures thereof and being present at least in the amount required to supply oxygen suificient to combine as CO with the carbon to be eliminated from said ferroc-hromiu-m, heating said mixture uniformly and under subatmospheric pressure to a temperature between about 800 and 1000 C. to degas the same, quickly raising the temperature of said mixture to a reaction temperature in the range between 1300 and 1425 C. below the liquidus temperature of said ferrochromium, maintaining said subatmospheric pressure and a reaction temperature in said range until the carbon remaining in said .ferrochromium is below 1% by weight, surrounding the decarburized ferrochromium with an atmosphere of an inert gas, and finally raising the temperature ,of the decarburized ferrochromiurn above its melting point to coalesce the same into a non-porous mass.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. THE METHOD OF DECARBURIZING FERROCHROMIUM CONTAINING IN EXCESS OF 3% CARBON BY WEIGHT, COMPRISING MIXING SAID FERROCHOMIUM AND AN OXIDANT, BOTH IN COMMINUTED FORM, THE OXIDANT BEING SELECTED FROM THE CLASS CONSISTING OF IRON OXIDE, CHROMIUM OXIDE, AND MIXTURES THEREOF AND BEING PRESENT AT LEAST IN THE AMOUNT REQUIRED TO SUPPLY OXYGEN SUFFICIENT TO COMBINE AS CO WITH THE CARBON TO BE ELIMINATED FROM SAID FERROCHROMIUM, HEATING SAID MIXTURE UNIFORMLY AND UNDER SUBATMOSPHERIC PRESSURE TO A TEMPERATURE BETWEEN ABOUT 800* AND 1000* C. TO DEGAS THE SAME, QUICKLY RAISING THE TEMPERATURE OF SAID MIXTURE TO AN INITIAL REACTION TEMPERATURE IN THE RANGE ABOVE THE EUTECTIC TEMPERATURE BUT BELOW THE LIQUIDUS TEMPERATURE OF SAID FERROCHROMIUM WHILE MAINTAINING SAID SUBATMOSPHERIC PRESSURE, AND MAINTAINING SAID SUBATMOSPHERIC PRESSURE AND A REACTION TEMPERATURE ABOVE SAID EUTECTIC TEMPERATURE UNTIL THE CARBON REMAINING IN SAID FERROCHROMIUM IS BELOW 1% BY WEIGHT.