US2429648A - Methods of producing alloys low in carbon - Google Patents

Description

Patented Oct. 28, 1947 93 81 tmrnobs QFiBQD N eet eww 1N 'QARBON 1 iver llennerie t and B0 Mika illl'e-K filtgi himsshp et SW den 'Nq flrawing. gllpplicafion Aprils,-1943, SrialNo.

' 482,890. InSweden March.11,i-1 939 t-Cleims- (c1.17528 efiferm-c'chrome e'ailoyss-andhas particular ref: fi

erence :to the treatment .ozfisuch alloys-in which the ferrous :metal content :is relatively .high,;a-s for example alloys-obtainable hy reduction oflovv grade;chrcune ores not adaptedto be con-.

verteditb usable ierro-chriome of-low.carhon:con.-. o

tent Joy prior -refiningitreahhents.

, Heretofore'chmmaex es having achrome content 'of the order Jof at least 60% to 70% have been considered theionly desirable gradesusable for theupreductiongefiferroechromemfiow:carbon 15 fled e th r wither ..p e -rto1the contentand theproduetidn at this material from are ofseventhisliquality:involves expensive refining: processes'rwhichisinceithey are well sknowniin the art needinot :he deseribed here in. =detail.

Qne :of the required steps,- in such refinement is 29 decarbonization of thiealley to reduce the carbon content to an acceptabte valuegparticula'rly when the;alipy istosbeiusedvinlthe'.prodnetionrof stainlesssteel.

.A relatively isiuiizle crsheaap Ilecarburizing 216 process .for mrodnei ng .L-low :earbbn non-alloyed ferrcns meta'hforflnelting'stock and thelike has heretofore. been-promisedxfiennertelt 11.. S. .Pate ent lilp. :;2,1!7!0,1 58) amiinfthefiisclosure of that process; it has been-. smtedzthatthe process .may 80 be.employed;v toflecarhmiizarameng. ether 'materi-.

als, :ferro echromesailoys.

We have.,lhowever,cdiscovered. that.ihe..'e1ecar.-.

burization of ferro-chrome alloys of the :kind now ordinarily producedis.::not.,;easib1e..hy :means of 3-5 the process disclosed ;-;:*by.-.Rermer,fe1t, which is based essential-1y uponttreatmentin an oxidizing gas atmospheremtjhighltemneizature of the metal to be :deca-rburizedmiththe.metal in subdivided erzpellet :f r=h1;=in torderztemrevide the ratio f 40 s rfac area ateelume-e -l ee e teenable 1; gas to act efiectively, he reason for this s hat in such m s he 31 chro e. which a ahi h eiygen, s ap d oxidized as wen as the carbon. The-loss of the valuable it.

chrome'al loy is thatfthe o'xidized chrome forms 50 anenyeldpin'g of airtime oxide covering the surface of the particle under. treatment, and the nature loft-his oxide is (such that itforms in effect a :film-l-ike shield which-is impervious to the theusua .ierrwh eme a e issebieeted o the, ahQ e-tre tme tf here imii r ee iq h q- @ueti e en le Id arhe ha ien t. t e. u fac e pthehar el ea iesi yth imu an u erma ion et hreme e. .1? film w iehest i l lfi ntesressi elv-ssiows dew. dziinell eets. to 1 stentiail step th his. arm-M e p film acting to substantially stop the rea .W h ve hesv r di ee ered het the. deter: bur-w ns ee eh eme alley unsuccessfu l saseousexidizihs m thqd if the't e tmentses .ere e e ette ht daremed w them-a.- terial i edtq the t eatment erwith re pec to th s eps eta-the treatm nt, erheth,

We have li l ipdi th the iron c ntent. of a fer wchreme alley" errsuhstantia'lh in reased as c m ar d-withthermaximum content hereto:

fore cons dered practica ly ilsahle, the character of t e ,oxiderfilm iie m d \when the material .is subjected to. an oxidizingeas atmQspherea-p. parently changes .rvery radicall in i nat r Particularly .WithiI'QSlJfiOt to its reaction to; the activitym theoxidizingeas. .Not only is the content of chromium the oxide reduced incase of an.:alloy5high;.in;iron:lcontent but werbelieve that, the nature :of the oxide, which is. a complex ironichromeeoxygen compound, possesses essentially different chemical characteristics from those of a chrome oxide of thekin'd formed when a-lfighichrome terroe'chrome alloy is subjectedto oxidizing treatment.

This .adifferenceiin the characteristics of the oxide. surface filmappears to, afiectthe gaseous equilibrium zthe 'nxidizihg gasesat. the surface so that the-carbon is the element primarily 0X11? diz'ed. rand-while a certain-amount of chrome is oxidized: the wery 1 much ..:gre.ater 1 velocity of diffusion 30f Ethe zlcarihonsirnmfthe:jinterior of the particle. toitheesurfaeei than. that .of the chrome enables .Ldecarhurizatienl torbe effectively. accomplishedxwithout. undue Iloss ofs-chrome.

Even more imfiortant than. this, however, is that when the awn content of thea'lloyis materially increased as. npmapared with that .in the alloys I heretofore: considered practically disable, the nature of the chrome-iron oxide film changes from the iusual?:hamlamdv impervious nature of a chrome oxide toiagieryious hat-uiie which permits the lozidizingigases to lcontinuecto reactiwith the material-.lbeing. treated.

action ofthesoxidining zas. .a. resultrwhen s5 t v,:wi-th a ieh i-ren eontentallrfl decarburization can be carried out with gaseous oxidation to an extent enabling the carbon content of the alloy to be brought down to very low values.

We have found that a ferro-chrome alloy having from 20 to 25% chrome, with the remainder mainly iron and carbon, may readily be decar-- burized in a gaseous oxidizing atmosphere to an" 4 or hydrogen atmosphere. Thus we have found it suitable to decarburize a 20% chrome alloy having a 5 carbon content down to about 0.25% carbon in an oxidizing atmosphere, thereafter treating the material in a hydrogen atmosphere, by which the carbon content may be brought down to 0.05% or less, while at the same time the oxide film is reduced.

As previously noted the ability to successfully decarburize by reaction with a gaseous oxidizing atmosphere is apparently due to the pervious character of the oxide formed at the surface of an alloy high in iron content and to that end be done by reducing a suitable mixture of chrome U and iron ores with carbon in a furnace, which may be electric or a common blast furnace. The iron required to produce the desired, high iron content analysis may also be added in metallic form. Obviously, the extent to which iron will be required to be added will depend upon the quality of the chrome ore employed and in the case of ores of extremely low quality additions of iron (either in the form of ore or as metal) may not be required in order to obtain the high iron content alloy suitable for the decarburizing treatment.

I For the small particle size suitable for the gaseous decarburizing treatment, the alloy may advantageously be liquified and then granulated and cooled in water, or it may also be brought to suitable size which is an average particle size of 1.0 to 1.5 millimeters, by crushing. Preferably, particularly'if crushing is employed, the carbon content ofthe initial alloy is relatively high, that is to say, of the order of 3% to 5%.

The temperature at which decarburization is carried out should be as high as practically possible without risk of sintering and consequently sticking of the particles to each other or the furnace wall. Thus the temperature should not materially exceed 1200 C. and a temperature of the order of 1100" C. is more to be preferred. Decarburization may advantageously be effected by bringing the heated material into contact with a gas mixture containing 00 and CO2 in suitable proportions, preferably, if the atmosphere contains only these gases, in the ratio of approximatelyv 25%-75%. It may be advantageous to oxidize in vacuo or to dilute the C0-- C02 mixture with other gases such as H2 or N2, in order to obtain the most favorable gaseous equilibrium. However, too high a concentration of nitrogen should be avoided since we have found that this gas may become absorbed in the metal. Hydrogen, however, increases the velocity of the reaction, and may withadvantage be supplied to the furnace. In order to supply the quantity of oxygen required to decarburize, without at the same time introducing a supply of nitrogen, it has been found advantageous to introduce steam or water vapor into the reaction zone, the dissociation of which also acting to enrich the H2 content of the atmosphere. This produces a gas containing H2, H2O, CO, and CO2, andthis gas may be burned with air in the cooler zones of the furnace for preheating the charge without risk of absorption of nitrogen into the alloy.

Because of the fact that an oxide film is formed during the reaction, it may in any case be advantageous not to carry the decarburization to the ultimate degree desired in an oxidizing atmosphere, but to finish the process in a reducing the chrome content of the alloy prepared for decarburizing treatment is advantageously not over about 30% and is preferably of the order Alloys of higher chrome content are, however, susceptible to gaseous oxidation treatment for decarburizing purposes if the nature of the oxide formed. from the higher chrome content alloys is taken into account and the treatment modified accordingly.

Generally speaking, it appears that the higher the chrome contentof the .alloy the more the oxide takes'on the hard tenacious and impervious qualities of a true chrome oxide. Consequently, if the alloy is relatively high in chrome. content the reaction, if not entirely stopped by the oxide film, is retarded to an .extent making the process undesirable if not wholly impractical. -In such instances the difficultymay, however, be overcome by introducing. what may be termed a washing step into the process. Assuming a relatively high chrome alloy to be treated, the initial gaseous treatment may be the same as for a low chrome contentmaterial. This. will result in a progressive slowing .down of the reaction due to the formation of relatively less pervious film, the rate at which the reaction is slowed down depending upon the chrome content of the alloy. When this occurs the character of the gaseous atmosphere is altered by the introduction of a film removing or washing agent of suitable nature which may for example be chlorine. Removal of the oxide film by the washing agent permits the decarburizing reaction to be resumed, and, by washing. as required, the desired degree of decarburizationmay be accomplished.

Various forms of furnace may be employed and in many instances it may be found desirable to mechanically agitate the material during the decarburizing treatment, particularly in the case of treatment of high iron content alloys.

What is claimed;

1. The method of producing low carbon ferrochrome from a high content chrome ore which consists in preparing in finely divided state an alloy including iron and chromium, reducing said highcontent chromeore including the step of adding iron in suitable form to bring the chrome content of the prepared material within the range of substantially from 20% to 30% chrome for subsequent decarburization, and decarburizing said alloy at a temperature below the melting point thereof.

2. The method of producing lowcarbon ferrochrome which consists in preparing in finely divided state an alloy including iron and chromium having a carbon content substantially higher than'that desired in the end product, subjecting theresulting material at a temperature below the melting point thereof to decarburization until the decarburization is'inhibited by an oxide film, removing said oxide film by the use of chlorine, and thereafter further decarburizing the allow at below the melting point thereof.

3. The method of producing low carbon ferrochrome which consists in preparing in finely divided state an alloy including iron and chromium having a carbon content substantially higher than that desired in'the end product, subjecting the resulting material at a temperature below the melting point thereof to decarburization until the decarburization is inhibited by an oxide film, removing said oxide film by means of an oxide film removing agent, and thereafter further decarburizing the alloy at below the melting point thereof.

4. The method of producing low carbon ferrochrome from a high content chrome ore, which consists in preparing in finely divided state an alloy including iron and chromium, reducing said high content chrome ore including the step of adding iron in suitable form to the ore to reduce the percentage of the chrome content of the prepared material to the desired low value for subsequent decarburization, and decarburizing said alloy at a temperature below the melting point thereof to reduce its carbon content.

5. The method of producing low carbon ferrochrome which consists in utilizing in finely divided REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,170,158 Rennerfelt Aug. 22, 1939 96,633 Thoma Nov. 9, 1869 314,004 Edwards Mar. 17, 1885 1,086,019 Bucher Feb. 3, 1914 FOREIGN PATENTS Number Country Date 817,471 France May 24, 1937