US3905803A - Process for producing ingots by electric resistance melting particulate metal under slag - Google Patents

Abstract

A process for producing metal alloys in the form of large-size ingots, by melting under electro-conductive slag, wherein the material forming the alloy is partly supplied as solid metal and partly as metal in powder, granules or pellets, for conciseness called ''''particulate metal'''' comprising the sequential operations of: A. INTRODUCING INTO A MOLD, THE BOTTOM OF WHICH IS MADE OF ELECTRO-CONDUCTIVE MATERIAL, A CONSUMABLE-ELECTRODE UNIT, UNTIL CONTACTS THE SLAG; B. PARTIALLY FILLING WITH SAID PARTICULATE METAL A ZONE OR RECESS PROVIDED INSIDE SAID ELECTRODE UNIT AT ITS END CONTACTING THE SLAG, CLOSED AT ITS BOTTOM END BY A BOTTOM WALL, AND AT ITS TOP BY A FEEDING PLATE FOR THE POWDER METAL; C. ESTABLISHING IN SAID ZONE A CONTROLLED ATMOSPHERE BY A GAS CIRCULATION; D. INCREASING THE TEMPERATURE UNTIL OBTAINING THE GRADUAL MELTING OF SAID BOTTOM WALL AND THE GRADUAL SIMULTANEOUS SINTERING OF THE PARTICULATE METAL IN SAID ZONE UNTIL FORMING A SINTERED LAYER HAVING A THICKNESS SUFFICIENT TO SUPPORT THE WEIGHT OF THE SUPERPOSED PARTICULATE METAL; E. MELTING THE SO OBTAINED COMPOSITE ELECTRODE COMPRISING BOTH THE ELECTRODIC UNIT AND THE PARTICULATE METAL CONTINUOUSLY FED TO THE MELTING ZONE.

Description

United States Patent Bianchi et al.

[451 Sept. 16, 1975 PROCESS FOR PRODUCING INGOTS BY ELECTRIC RESISTANCE MELTING PARTICULATE lVlETAL UNDER SLAG [75] Inventors: Paolo Bianchi; Aldo Ramacciotti;

Eugenio Repetto, all of Rome, Italy [73] Assignee: Centro Sperimentale Metallurgico S.p.A., Rome, Italy [22] Filed: Nov. 27, 1973 [21] Appl. No.: 419,380

[30] Foreign Application Priority Data Dec. 6, 1972 Italy 54535/72 [52] US. Cl 75/10 C; 13/9; 75/10 R;

[51] Int. Cl...... C22d 7/00; l-IOSb 7/18; B22d 27/02 [58] Field of Search 75/l012, 84;

[56] References Cited UNITED STATES PATENTS 2,541,764 2/1951 Herres 75/10 R 2,782,245 2/1957 Preston 164/252 2,813,921 11/1957 Vordahl 75/10 R 3,234,608 2/1966 Peras 75/10 R Primary Examiner-Peter D. Rosenberg Attorney, Agent, or FirmY0ung 8L Thompson [57] ABSTRACT A process for producing metal alloys in the form of large-size ingots, by melting under electro-conductive slag, wherein the material forming the alloy is partly supplied as solid metal and partly as metal in powder, granules or pellets, for conciseness called particulate metal comprising the sequential operations of:

a. introducing into a mold, the bottom of which is made of electro-conductive material, a consumable-electrode unit, until contacts the slag;

b. partially filling with said particulate metal a zone or recess provided inside said electrode unit at its end contacting the slag, closed at its bottom end by a bottom wall, and at its top by a feeding plate for the powder metal;

c. establishing in said zone a controlled atmosphere by a gas circulation;

(1. increasing the temperature until obtaining the gradual melting of said bottom wall and the gradual simultaneous sintering of the particulate metal in said zone until forming a sintered layer having a thickness sufficient to support the weight of the superposed particulate metal;

e. melting the so obtained composite electrode comprising both the electrodic unit and the particulate metal continuously fed to the melting zone.

10 Claims, 1 Drawing Figure PATENTEU SEP 1 8l975 15%;. 9 05 O3 t eu arlyt hemetals antlthe. Qthe ve em nts formingthe al oy ein supplied the melt ng zon more or less cont uouslyihthefor t uit blyi mixe 'par iel of alloysno o met ording o, :oneo these pro-1 ces a etal: u ed whi h -i.s 1' conti u y shaped to.ai na hdi meter t be, imme iately ebbVe the ingot ,rnoldwhere the melting-process is being care b t; .thistube b side ser ingv the function o sunplying the metal to the alloy,t;s e r ves,alsevthe function-of le trod and bf e nveybr e ce for the metal P 9 9 UHQdPQQdvlUIQ the. t be a t p, moves fre ly th b t bbut the leng ther fyto op tinuously in the hottest zone of the liquid metallpool, anapresis l unde ithe end e ithe ele rod h main ad an a e .h thebretie lly can; be o a n y th process consist inthat, ,it, is possible to control at each moment the composition of the produced ingot by he i l fih f eQin J te fq pb b h P cl and l slsctretle; alsb; produ P t cul y physically-and ehemieally hem gen eus is Obtai at least theoreticall l he a etual drawbaeks a e due: to t e i of the t paratus for forming the strip into a tube, which does not allow .the simultaneous utilization .ofmore than .two or threeelectrodes, thus prnti,ng,. the suitably rapid manufacture 9f h s insets and. to he ratherfrequent ocean??? QfIQ Qgging of the leet ode: du t y the particles, dueto thesm all diametrical dimensions of the tube and to the slowness of theprocess. These W v c I I ,in faetwifthez'electrode tube isclogged bytthepar cles itis; impossible to continuously adjust t he con positron of the, produced ingot,

and moreover du tothe effect of the sudden change ness of the process ,itis impossible to obviate immediately the formation of clogs, as their effects are not immediately detectable, withthe consequence that the zoneshowing adifferentanalysis from'the optimum will greatly enlarge.

It is to benoted, moreover, that by the process known under the ,ESRnameit has been impossible, hitherto, to obtainingots weighing more than a few tons, if metal powders areuscd, [and weighing some tens of tons if an ingot is used as electrode. However, in certain sectors of the industry, for instance in the manufacture of rotors for big alternators or turbines, particularly felt is the need of having available remarkably heavy ingots, of upitosome' hundreds of tons, with an extremely closely controlled composition, and free from holes,inclusions,segregations, etc. Such a kind of ingot, as regards the characteristics ofphysical and chemical, homogeneity, can be hardly obtained .by the conventional methods of melting in a furnace and teeming into an ingot mold; however, it might be obtained with an ESR process if the technical limitations concerning the weight of the so obtained ingots were not present. t Y

A first attempt to solve this problem and' to meet a very=importantrequirement has been made by proposing a process including sequentially the operative stages both of the conventional method of ingot casting and of the ESR process. This method consists in conventionallycasting an ingot having the required weight, in removing the entire inner part along the longitudinal axis which is that generally higher in physical and chemical defects, and in filling the so produced recess, by a process according to the ESR techniques. This proposal appears not to have been accepted as the ingot obtained according to this teaching has an outer zone which is not free from any of the characteristic de fects of the big castings, namely stratifications of composition and physical defects, an inner zone with optimum chemical and physical characteristics, and an intermediate zone between the other two zones having intermediate physical and chemical characteristics. These differences of composition and of physical characteristics in the radial direction and the presence in the outermost layer, the most stressed, of most of the physical and chemical defects have brought the art to consider such an ingot unsuitable for the manufacture of large-sized pieces (rotors for turbines, etc.).

The purpose of this invention is to eliminate the cited drawbacks, connected with the known ESR processes using metal particles, and allowing one to overcome the problem of the low limits of weight in the produced ingots so as to allow ingots to be manufactured with uniform and controlled composition, free-from the'cited physical drawbacks and having a weight of some hundreds of tons.

According to the present invention a process is provided forthe manufacture of metal alloys in the form of large-size ingots, by melting under electroconductive slag, wherein the material forming the alloy is supplied inpart in the form of solid metal and in part underthe form of particulate metal, i.e., powder or grains or pellets, wherein with the term particulate metal includes one or more metal elements or alloys, suitably mixed, said process being characterized by the combination of the following steps:

a. introducing into a mold having its bottom made of electro-conductive material a consumable electrode unit until it contacts the slag,

b. filling partially with said particulate material a zone located inside said electrode unit at the tip thereof contacting the slag, said zone being delimited at its bottom by a bottom wall closing said end and at its top by a feeding plate for the metal particles;

c. establishing in said zone a controlled atmosphere by gas circulation;

d. increasing the temperature until obtaining the gradual melting of said bottom wall and the gradual simultaneous sintering of the metal particles in said zone until forming a layer having a thickness sufficient to support the weight of the superposed column of the particulate material;

e. melting the composite electrode so obtained, comprising both the electrode unit and the metal particles continuously fed to the melting zone.

Theiequipmentwused: according to this invention: for darryingJout this process-includes "a consumable:elect-rode syste m whichischaracterizedgby one or more cy-. lindricalwmetaltubes, preferablyone to four, for feedingxthe metal, each with a diameterivariable from 20 to 90 percent of the diameter of the ingot to be produced, saidrtubesbeing:extendableby means of welding of furtherlportionstof tube to that, or to those, which is, or are: being meltedsthroughout the duration ofthe meltiQgprOCS.. r t r ilhe present invention \will be now described with particular :reference? to the attached drawing' showing by wayqf non lirnitative example one preferred em bodimentof theequipment according to thisinvention, usinga singleelectrode. I l h Rarticularlypin theattached drawing the FIGURE shows a sectional vertical view, of a scheme of a facility for the manufactureof ingots by the process according tothis'invention, 1

--With'=.reference to the FIGURE, therein are shown a mold 1, of conventional ESR type, inside ofwhich has been shown theliquid metal pool, andthe superposed layer 6of molten slag, into which is immersed the lower tipofa metaltubular'electrode 2 supported and guided by technologically suitable means, of a substantially conventional kind. altis tobe noted that said tubular electrode 2 isprovided in its lower part with a recess orzone 5 downwarddelimited, at the start of the re-melting; by 'abottom wall 3 at least 35mm thick; closing the lower end ofthe electrode 2, and at its top, by a feeding plat efl which will be described in greater detail hereinafter;

readysintered metal particles under the" superposed column ofmetal particles which drops downwards in the recess 5,passingthrough the duct 8 and the hole or holes 11 of the plate 4.

As far as the course of the process according to this invention is concerned, initially the lower -tipofthe electrode 2 submerged into the slag is closedby ametal bottom'wall 3 fixed to the walls of the'electro'de2 in side"of which isintro'duced the metal particlesoy a feeding plate 4. Thispla'te and the cited bottomwallB define a recess 5 which is thus partially filled withthe metal particles. lnside said recess a controlled atmosphere will be established by a gas circulation, for instance using an inert gas such as argon. Then through each electrode an alternating or direct current (20 to 100 'V and 5,000 to60,000 A) is caused to pass through the circuit the electrodes of which arethe metal tube and the mold. This current, due to the Joule effect, develops a large amount of heat within the slag6under the electrode 2, causing thus the gradual melting of the bottom wall 3 and simultaneously the gradual sintering of. an ever increasing layer 7 of metal particles, inside said recess 5. When the bottom wall is entirely melted, the sintered layer7 has still a thickness sufficient to support the superposed particle column 10, which is still lflCOl'lI'BnL Said layer 7 serves now the function of bottom wall and the melting process of the sintered layer and of simultaneous sintering of a layer of metal particles assumes a stable operative condition.

The metal tube 2 serving the function of an electrode will be downwards moved at a rate of speed comprised between 1 and 10 cm per min., and the particulate metal is fed so as to keep almost constant its volume within the recess 5.

' The feedingpIateA eonsis ts of aldisk madeleither of metal or orahyaarh'e'rfsuiraule materiaLhaS/inga diameter a little less than theinternaldiameterof the electr0de2; said plate istransected by one or more-holes l lg-provided with-delivery}valvesg into whichare insertedgthe duct and the feeding: components 8 for the metal "particulesf and by one or' morewholes l2 into which are inserted the duct or ducts 9-for feedin g the ga'sfor establishing the controlled atmosphereambient. Thisplateds supported by" means different from said feeding ducts for the powder andfor the gas; and is located in fixed position with respect to the mouth of the mold 1, or insidethe mold itself, and in] thisfcase it: will be slow ly'upvvards moved in'ord'eflto-rhaintain constant its distance from 'themolten metal pool. The=feeding ducts forthe pa rtieules and-for the gas are independent from the electrode, said ducts 'can be disconnected from the plateand removed from the electrode inlorder to; 'allo'w the coupling of the operating electrode tothe subsequent lengths-oftube. 1 1 t 9 The metal which willbd'sup'plid'insidethe cylindrical-"t'ube 2 and which fillsthe'recess 5can be supplied, as afores'aid,=in the form ofpowde r; ofgranules or of pelletsi"* :1: i i lf'only powders ar'e used, the mostsuitable grain sizes are the finest available onwhe-mar'ketyand the lower limit of sizeis' determined only bythe cost of the 'pow ders th emselvesfif I y Itis possible howeveri 'to'use coarse powders, with aLmaximum grainsize of 4mrn,iand intliiscaseit isnecessary to mix said'coarse powders with finer powders in the percent of at least 20 percent byweight soasto obtain for the mixture an apparent 'bulk density of at least- 2.7"grains/cc. i Q" Ontheotherhand, if granules or pellet'sare tobe used itwill be necessary/to mix therewith at least 20percent b'yfweight'of a mixture of powders and srnall gr'anules, the si'ze of wh ichis between 0:01 and rmm, soastoob tainafinal mixture having an apparent bulk density of at-least 2.6 grams/cc Y in the lightof the practical-and theoretical inrdrma: tionavailable before the experiments which ledto this invention, thesol'uti on' here proposed forthe technical problemof the production orbig 'ingbtsivith the ESR teafii ueabneard to be doubtfully applicableflnfact, aset of considerations as to the sintering rates of the non' compressedmetal particles, as tdtherhelting rate whichrn ustberather high in orderto allowbig ingots to bejmariufactured with industrially acceptable speed, asf't othe" mechanical resistance of thefisirrtered metal prodiiets athighternperatures, and as to the thermal arid el ectrical conductivity of the metal powders in incoherent mass, caused the art to consider the process as above describ'ed to be hardly capable o fbeing practiced.

Thesubsequent tests, on the contrary, demonstrated, that other -theoretically more effective solutions were r M, -\;1M., ,i not capable of be ngpractlced foreither economical or composition by weight as follows:

slag havingthe i CaF 40%; Al O 30%; CaO 24%; MgO 6% C 0.25; Mn 0.50; P 0.01; S 0.009; Si 0.25; Ni 3; Cr 0.30; O 60 ppm the balance consisting of Fe and minor impurities.

The used alternating current at 50 H had 50 V and 20,000 A.

In steady condition of operation, the temperature of the slag was 1650 1,700C; the downwards feed rate of the electrode was about 3.3 cm/min., and the supply rate of the powder was 500 kgs/h.

After 24 hours an ingot has been obtained weighting tons and having the composition as follows:

C 0.27; Mn 0.46; P 0.008; Si 0.13; Ni 30; Cr 0.30; S 0.008; 0 50 ppm the balance consisting of Fe and minor impurities.

This composition appeared to be substantially constant throughout the longitudinal and transverse sections of the ingot and also the mechanical properties appeared to be uniform.

The present invention has been described with particular reference to certain specific embodiments thereof, but it is to be understood that modifications might be entered without thereby departing from the scope of the invention as defined by the appended claims.

Having thus described the present invention, what is claimed is:

1. An ESR process for producing metal alloy ingots by melting under electro-conductive slag, comprising establishing in an electro-conductive mold a quantity of slag, introducing into said mold a hollow consumable metal electrode having a closed bottom immersed in said slag, introducing particulate metal into said hollow electrode, establishing an inert gas atmosphere in said hollow electrode, passing an electric current through said electrode and mold to melt said closed end of the electrode and to sinter said particulate metal adjacent said end of the electrode to form a sintered layer having a thickness sufficient to support the weight of the su perposed particulate metal, and supplying further particulate metal to the interior of said electrode while continuing to pass said electric current thereby progressively to melt said particulate metal and said electrode and progressively to build a said ingot in said mold.

2. A process as claimed in claim 1, and feeding said electrode downwardly into said mold at a rate between 1 to 10 cm/min.

3. A process as claimed in claim 2, and welding further hollow electrodes to said hollow electrode to replace the material of the electrode consumed by melting.

4. A process as claimed in claim 1, in which said particulate material has a grain size up to 4 mm.

5. A process as claimed in claim 1, in which the bulk density of said particulate metal is at least 2.7 g/cc.

6. A process as claimed in claim 1, in which said elec tric current is 20 to 100 volts and 5,000 to 60,000 amperes.

7. A process as claimed in claim 1, and maintaining the end of said electrode a constant distance above the level of the metal bath that forms atop said ingot.

8. A process as claimed in claim 1, in which said electrode has a diameter from 20 percent to percent the diameter of the ingot to be produced.

9. A process as claimed in claim 1, said particulate metal and electrode being steel.

10. A process as claimed in claim 1, said particulate metal and said electrode having the same composition. i

Claims (10)

1. AN ESR PROCESS FOR PRODUCING METAL ALLOY INGOTS BY MELTING UNDER ELECTRO-CONDUCTIVE SLAG, COMPRISING ESTABLISHING IN AN ELECTRO-CONDUCTIVE MOLD A QUANTITY OF SLAG, INTROUDCING INTO SAID MOLD A HOLLOW COMSUMABLE METAL ELECTRODE HAVING A CLOSED BOTTOM IMMERSED IN SAID SLAG, INTRODUCING PARTICULATE METAL INTO SAID HOLLOW ELECTRODE, ESTABLISHAS INERT GAS ATMOSPHERE IN SAID HOLLOW ELECTRODE, PASSING AN ELECTRIC CURRENT THROUGH SAID ELECTRODE AND MOLD TO MELT SAID CLOSED END OF THE ELECTRODE AND TO SINTER SAID PARTICULATE METAL ADJACENT SAID END OF THE ELECTRODE TO FROM A SINTERED LAYER HAVING A THICKNESS SUFFICIENT TO SUPPORT THE WEIGHT OF THE SUPERPOSED PARTICULATE METAL, AND SUPPLYING FURTHER PARTICULATE METAL TO THE INTERIOR OF SAID ELECTODE WHILE CONTINUING TO PASS SAID ELECTRIC CURRENT THEREBY PROGRESSIVELY TO MELT SAID PARTICULATE METAL AND SAID ELECTRODE AND PROGRESSIVELY TO BUILD A SAID INGOT IN SAID MOLD.

2. A process as claimed in claim 1, and feeding said electrode downwardly into said mold at a rate between 1 to 10 cm/min.

3. A process as claimed in claim 2, and welding further hollow electrodes to said hollow electrode to replace the material of the electrode consumed by melting.

4. A process as claimed in claim 1, in which said particulate material has a grain size up to 4 mm.

5. A process as claimed in claim 1, in which the bulk density of said particulate metal is at least 2.7 g/cc.

6. A process as claimed in claim 1, in which said electric current is 20 to 100 volts and 5,000 to 60,000 amperes.

7. A process as claimed in claim 1, and maintaining the end of said electrode a constant distance above the level of the metal bath that forms atop said ingot.

8. A process as claimed in claim 1, in which said electrode has a diameter from 20 percent to 90 percent the diameter of the ingot to be produced.

9. A process as claimed in claim 1, said particulate metal and electrode being steel.

10. A process as claimed in claim 1, said particulate metal and said electrode having the same composition.

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