US2870006A - Process for melting metals - Google Patents

Description

- Filed Oct. 20, 1955 J. L. MORNING PROCESS FOR MELTING METALS Jan. 20, 1959 2,870,006

2 Sheets-Sheet 1 ijiq. I

9 959955? 8 GRAPHITE BOWL I A 4 ,E", i ff/Z 3 INVENTOR JOHN L. MORNING ATTORNEY Jan. 20, 1959 J. L. MORNING 2,870,006

PROCESS FOR MELTING METALS Filed Oct. 20, 1955 2 Sheets-Sheet 2 A s c I PmoM ETMon nsmc IMPROVED TEMPERATURE 0F METAL AT SURFACE |5oo-c FURNACE HOLDING TEMP.

POWER -E l l'i2 LM..MEL ME.-

BEACHLINE TEMPERATURE T|ME 0F HEATING-NEW METHOD- 'TIME 0F HEATING-PRIOR METHOD TIME A SUPERHEATING PERIUD (IMPROVED MEN/0D) B POUR PER/00 (NEW MET/1'00) 0 POUR PER/0U (PR/0R MET/1'00) INVENTOR JOHN L. MORNING BY Wgw ATTORNEY PRocEss non MELTING. METALS John L. Morning, Wilmington, DeL, assignor to E. I. dil Pont de Nemours andCompanflwilmington, D'el;, a

corporation of Delaware Application October 2.0, 1955,.Serial No. 541,708

4' Claims. (Cl. 75 -84) This invention relates to. an improved method of melting metals which in themolten state arereactive with refractory materials used in. conventional melting processes.

Many 'of the higher meltingpoint metals. and alloys present special problems with regard to-pouringof cas-t= ingsandingots. The difl'ieulties with. which. this invention is concerned are encountered inthemeltihg process. Titanium and alloys containing a major amount of'ti tanium serve as good examples to illustrate the problems involved and the method of this invention which overcomes them. Molten titanium is reactive with. atmos- ,pheric gases and with. therefractory materials which are used in melting such metals. as iron. For this reason, there. is the problem: of. containing the nolten metal during the melting process. One method. utilizes a titanium metal crucible surroundedby an inert atmosf phere. The. difficultyf presented the. fact thatthe melting point of the. container. ist he same as that of the material being melted is overcome by; externally cooling the titanium cruciblewhileintroducing.radiant heat from a source above the crucibleso that the maximum. heat is absorbed by the surface of the moltemtit'anium con: tained Within the crucible. Theproce'ss is v initiatedfby dropping-particles o f metalinto thecrucible and: allowing them. to become molten. By, proper cooling itis possible to maintain a crucibleof sufiicient. thickness to contain the molten metal within it. The charge. is gradually built up within the. crucible by subsequently dropping particles of titaniuminto thepool of melted metal. This method is isclosed in (re-pending application of. C. H. Winter, In, SerialNo. 387,011, filed October 19, 1953.

With this type of melting process, extreme care must be taken toprevent any part of the. crucible from completely. melting. Furthermore, it-rnightbe expected that the. cruciblethickness would vary during the I melting operation. The. portion ofthe crucible whiehvissub: jected. to the greatest variations. in thickness is at the rim .where the. surface of thepoolcontacts the walls of the crucible. This. rim of contact is known as the beach line. T he temperature of,the-crucible.is greatest at the beach line because the heat is being; radiated from above, thus resulting in maximum temperaturesnear the surface of'the pool. The Winter application realizes this. problem. and special cooling means are maintained about the. crucible at the beach line.

i This invention makesit possible to maintain a sufii cient thickness of the crucible at the beach line, by a process which. comprises applying radiant heat from above. to a pile of metal particles contained with-in a titanium crucible, melting a. portion of said pile .by said radiant heat while introducing additional metal in 2,870,006 Patented Jan. 20, 1959 finally removing the molten metal from the crucible. 7

-When the moltenmmetal which is being treated, hasa tendency to react with atmospheric gases, the above process should be carried out in a nonreacting atmosphere.

The advantages of this invention stem from the fact that the heat is absorbed by the metal particles of the pile rather than by the molten .pool. This pile offers a large surface areaof high emissivity value to the heat source 'as eontra'stedto the prior method of feeding pa'rticle'sof titanium. into a molten pool. This prior method depends upon heat transfer from the liquid metal to the solid feed, and this heat must first be transferred from the radiant heat sourceto the low emissivity molten metal so. that a considerably lower rate of energy interchange is experienced; By using this invention it is possible tome lt titaniumuat temperatures above 1700 C. and: at the same time maintain a temperature of n'otmore than 13009.0 at the beach line area.

Referringn'ow to Figure 1 of the. drawing, thereis shown" a suitable. apparatus for carrying out the process of this invent-ion. 1 hi's apparatus-isa til'table furnace havi'ng a'n outer corrosion+resistant shell 1" of steel, or otherdeSired'metal, internally lined'with a ceramic or other suitable insulating. material 2" within which, as shown, the: furnacing meansprope'r is suitably disposed; This furnaeing means can comprise a bottom bowl secs tion- 3;. of graphite; molybdenum or other refractory.

material an'd anupper roof section 4,. also preferably composedofgraphite; Suitably disposed Within or otherwise. associated: with said roof section is an elec trical-resistor orsimilar. type heatin'gelement 5 having connectionswith a. source of. current (not shown) for h'eatingpan'd maintaining the furnace at any desired terns pe'r'ature above the melting point of the. member alloy 'subj'ected m melting and. purification treatment. A fura na'ce inlet 6 is provided in the upper section i'througli which met'alscan.be' charged to thefurna'ce. A lso. provided in upper" section: iis a decontamination outlet 7 through which vaporized impurities evolved in: the operationcan be readilyrwithdrawn. Usually magnesiuirl andthe magnesiumchloride vapors which are evolved in thelrnelting of titanium are retained in the system to maintain an inert atmosphere. if an excess builds up it is Withdrawn through outlet 7. A crucible 8, consisting of asolid layer, or skull, of titanium metal and function'- ing to retain. the molten metal 9 within the furnace is provided within'and over the interior oib'cwl-S. Dis= posed b'etweenthelowermost part of thereof section l and the upperv limitsof the bowl Sand crucible 81' is a heat dam element it preferably composed of. porous graphitewhich projects inwardly. as shown for a relatively short distance over the beach line 11. to block and thereby minimize? the quantity of" heat reaching the exposed crucible wall above the beach line. Externally disposedaboutthe bowl section 3 at; substantially the beach line area 11 isa suitable cooling element 12, such as a coil or cooling fingers, for circulating therethrough a suitabletcoolin'gfiuid such as water .or other desired coolant.. Thiscoolingelement provides a-higher heat loss. in thisarea 11 so that a solidla'yer of titanium is maintained throughout the melting operation. A pouring particle format a ratesufiicient to maintain a pile within and above the surface of the molten metal and out of contact with the. heat source, said addition of metal being continued at said rate until the complete charge is introduced into the crucible, sustaining the heating until ailthe me'tal'in the crucible is in the molten state and spout or other suitable form of outlet 13 is provided in the bottom section 3'. On tilting 'of the furuaceabout a shaft, not shown in the drawing, molten metal in purified state can be withdrawn to conventional ingotforming or otherdsired solidifying and recovery means.

To form a. titanium metal or alloy cruci'blewithin the above described apparatus, titanium metal "01' alloy sponge such asisrecoveredfrom the reduction oftitanium tetra chloride with magnesium, is 'initially-fed via inlet Ginto; therefractory bowl.3. Radiant heat from heater element 5 is used to melt the metal in an inert atmosphere. The pure molten titanium or alloy which results is then cooled upon the surfaces of bowl 3 by rocking the furnace while simultaneously externally cooling the bowl 3 to a temperature below the melting point of the metal. This procedure is one method of forming the crucible. Another method would be to machine the crucible from an ingot.

In the process of this invention, a further charge of metal is fed into crucible 8 for melting and purification therein at such a rate that pile 14 is kept as large as possible without coming into contact with the roof 4. The metal particles as they approach the melting point sinter and consolidate, thus allowing additional material to be added to the'furnace. Suflicient feed material can 7 be added to the furnace by this method up to the rated capacity of the furnace. Concurrently, the lower portion 'of the shell 1 is water or air cooled externally to withfurnace outlet 7. After the metal is melted, it usually is heated above its melting point. to insure that the metal will remain sufficiently fluid for a short period of time after itleaves the crucible. This additional heating is known as superheating, and it facilitates the handling of the metal in the pouring operation. The purified molten metal is then removed through pouring spout 13.

It may be readily seen that the apparatus could be adapted to 'a continuous process by using spout 13 asan overflow.

The process would comprise applying radiant heat to a continuously fed pile of metal particles extending out of a pool of the molten metal, at a temperature sufficient for pouring, continually melting said pile by said radiant heat while withdrawing the molten metal at a rate which is equivalent to:the rate of the melting of the pile.

Ingot-forming apparatus which may be used to receive the :molten metal is disclosed in U. S. Patent 2,085,450. However, the method of ingot-molding is not considered a part of this invention, and it is apparent that the molten titanium may be handled in accordance with prior art methods after it is removed from the furnace. It must, for obvious reasons, be maintained out of contact with oxygen and nitrogen, whether inside or outside the furnace, and accordingly, it is preferred to maintain an inert atmosphere within the system, argon or helium being suitable for this purpose. A slight pressure of a few millimeters above atmospheric within the furnace is desirable to avoid inleakage of air.

Figures 2, 3 and 4 illustrate graphically the improve ments over the prior art which are obtained by the process of this invention when melting pure titanium.

I Figure 2 illustrates the time-temperature pattern of the upper layer of metal in the furnace. This upper layer is that portion of the metal which is exposed to the radiant heat. It will be seen from the graph that in the prior process where a pool of molten titanium is produced and solid titanium is fed into the pool, the upper surface is superheated over the melting point of titanium metall In the improved process, where there is a pile of metal accepting the heat from the radiant source, the tempera ture is considerably lower, thus showing a more elficient utilization of the heat source. The area on the graph designated as superheat is'that period of the process when the temperature of the metal is raised so that it will not I solidify too quickly on subsequent pouring.

Figure 3 illustrates the amount of power input vs. time of heating, and it shows that under the prior method the power to the resistors of the furnace may be slightly reduced and held at a lower'value after the initial'pool of titanium is obtained. This reduction preventstoo great a superheating, and at the same time it compensates for the poorer heat acceptance which occurs when the pool- Q is formed. The improved method of operation allows a greater input of power to the heated surfaces because of the higher acceptance rate of heat in the mound of solid material. The graph also shows that the total amount of energy required to melt the metal by the new improved process is less than that of the prior process because a shorter time is required as a result of a more eflicient use of the energy supplied.

Figure 4 illustrates how the beach line temperature is decreased by the improved process during most of the heating cycle. By probing the beach line area with a graphite rod at various beach line temperatures and measuring the penetration of the rod it is possible to correlate, for a particular apparatus, beach line temperatures with crucible thickness. Therefore, beach line temperatu're is in effect a'measure of the thickness of the crucible, and it will be seen from Figure 4 that under the method of this invention beach line temperatures are lower for most of the heating cycle, and therefore there V is no danger of completely melting the crucible wall.

The following examples illustrate particular modes of operating the process; these examples are presented in illustration and not in limitation of the invention.

7 Example I p The method of this invention was carried out in askull furnace as shown in Figure l. The furnace had a maximum capacity of about 300 lbs. of molten titanium held in a crucible of solid titanium of about 4 inches in average thickness and about 2" in thickness at the beach line area. 'The maximum molten pool size with a safe crucible thickness was about 16" by 28" with a 4" depth.

The furnace was heated to a temperature of about 2000 C., as determined by an optical pyrometer sighted'on the.

radiant roof, using kilowatts of electrical power. A

residual amount of molten titanium was observed in the.

furnace. Initially 50 lbs. of titanium metal sponge particles were fed at a rate of about 600 lbs. per hour. Feed-. ing was then continued at a rate of 200 lbs. of titanium of 200 lbs. of titanium had been added, and the material 5 was allowed to become molten. Beach line temperatures were carefully watched during a 30 minute holding or superheating period. Optimum beach line temperature to hold a 2" sidewall at the beach line in this experiment, a

was 1250" C. As soon as the predetermined beach line temperature had been reached a depth of pool measurement was made. tion, skull thickness and depth of molten metal allowed a ready estimate of the amount of molten metal in. the 9 furnace. A 187 lb. ingot of metal was poured from this melt. ties for commercial use.

A control run was carried out in the same furnace utilizing the prior art method of forming a molten pool of titanium. and then feeding the solid titanium to this The electrical power input was regulated 1 molten pool. to maintain a beach line temperature of about 1300' C. An initial feed rate of 300 lbs. of titanium per hour was maintained and then a feed rate of about 50 lbs. per hour Y was permitted with a power input of kilowatts. A

200 lb. charge of titanium was added in 190 minutes and the same superheating holding time of 30 minutes was utilized for pouring the ingot. The quality of the ingot was about the same as that obtained by the process described above. A comparison of total melting time and I A knowledge of the furnace configura- The metal ingot has satisfactory physical propern ut ca ene y iz d l m efln adr nta es t the mprove p e 1 Factor Improved Prior Method: Method Titanium Feed Rate, lbs. per hour 200 50 Total Furnacing- Time, minutesincluding heat1ngup.-andpouringm; 270 405 Total .Electrieal Energy in k. w. hours 630 1, 012

Example 11. Acomparison of the improved method and the prior method was carried out in furtheriruns' usingltheprocedures outlined inExample I; In these additional tests the beach line temperatures were depressed by cooling the furnace after pouring an ingot. During the holding period of 30-40 minutes, beach line temperatures of approXimatelyn1000'C. wereutilized. At this beach line temperature a thicker sidewall (up to about 4" in thickness) was obtained, and this resulted in av-small (about 100 lbs.) moltenmetal inventory in the furnace. A comparison of the metal melting tirneand electricalenergy utilized] illustrates the advantages of the improved process:

Factor Improved Prior Method} Method Titanium Feed Rate, lbs. per hour 200 50 Total lhs. Fed; 1G0 100 ot En n ci s me .m ut c ms- 1 1 heating up and pouring 115 160 Total Electrical Energy in k. w. hours 800 400 While Figure 1 illustrates the feeding of titanium and removal of vaporized magnesium chloride through suitable outlets in the furnace dome, feed of solid sponge can also be effected through an opening in the beach line above the melt level but below the heat dam. In this procedure, recourse can be had to a ram or other suitable device for injecting the feed. In a continuous adaptation of this invention, the pile could be maintained against the crucible wall rather than being centrally located as shown in Figure 1. By having the pile in this location it would cover a portion of the crucible wall and thereby protect it against excessive heat.

If desired, vapors of the by-product salt and residual reducing metal which are evolved from the metal being melted, can be removed from the furnace in a stream of inert gas through suitable ports in the Wall of the crucible. These volatile impurities which are evaporated during the melting may be condensed and recovered in the liquid or solid state.

While titanium has been mentioned as a suitable metal for the melting and purification treatment in this process the invention is applicable to the treatment of any metal, and more particularly to high melting point refractory metals such as titanium, zirconium, hafnium, thorium, and alloys containing as a major component at least one of said metals. The term a major component means that at least 50% of the alloy is composed of one or more of the refractory metals. Typical commercial titanium alloys such as 94% Ti, 2% Fe, 2% Cr, 2% Mo; 90.9% Ti, 1.5% Fe, 1.4% Cr, 1.2% Mo, 5% Al; 90% Ti, 6% Al, 4% V; 92% Ti, 8% Mn; 92% Ti, 4% Mn, 4% A1; 92.5% Ti, 5% Al, 2.5% Sn are suitable products for the method of this invention as are experimental alloys such as 64% Ti, 36% Al. Similar alloys of the other mentioned refractory metals are also contemplated. The size of the metal particles which are introduced into the furnace may vary. An average particle size ranging from /2 inch to 100 mesh is suitable and may consist of sponge particles, crystals, pressed pellets, or particles of metal such as turnings, punchings or chips. The crucible used to melt the metal should be substantially non-contaminating to the desired metal product. When pure metal is esired hercruc le should, be m 151 men 3. and of the'purity of the metaljdesiled. Whenalloysfar melted the crucible may be'of the same composition as the alloy being melted or ofja 'composition'which"will not substantially change the eQIhPoSitioii desiredf When diflerent alloycompositions are treated in a series of melt ings inthe same furnace, the use of a" residue of nietal from a previous melting, a slight melting off of metal from the wall of thecrucible, and variationflin the feed compositionare contemplated asmethods" to obtain desiredcompositions of melted products. Minot-changes in the composition of the'metal charge are contemplated using these rnethodsbut by scheduling the series of meltings, crucible compositions may be obtained which mini mize undesirable contamination of the desired product.

Some of the metal charge may be. placed in the furnace during the pre-heatingfperiod whenthe furnace is being brought up to the operating range. However, this pro cedure is 'optionalj and addition of the metal may be delayed until melting temp'eratures arereached; In either case, the charge should'neve'r be'added soffast that the pileof metal builds up and cl ta'c't's" the radiant roof since this mayresult incontaminationof-tliemetal. Induction means'are also suitable for heating the furnace. For ci ample, a graphite" tube may beheatedby induction to 2400 C., and in turn heat would be radiated to a crucible below. Alternating; current of 9600 ,cycles has been successfully used,inthis connection. Of' course, graphite resistors or anhelect'ric arcmay also serve as the heating means." When using a carbon arc, one may enclosethe arcwithin a shielding graphite tube, so that carbon-con tarnination is avoided; The tubewill' act'as a radiatin'g element for heating the charged material. Heating by other means than electricity can be substituted, provided the products of combustion and other contaminating materials do not contact the titanium and provided a surface radiating in excess of about 2000 C. can be obtained.

As previously mentioned, the interior atmosphere of the furnace must be inert; i. e., it must be unreactive with the structural materials within the heated zone as well as with the metal being melted. This can be accomplished by the use of argon, helium, krypton or other inert gases. The vapors of the by-product salt and unreacted reducing metal are also useful in maintaining the non-contaminating atmosphere. These vapors, evolved during the heating operation, can be retained in the system and in many instances there will be a sufficient amount to establish the desired atmosphere. However, if there is a deficiency of these vapors for this purpose, it is convenient to supplement them with one of the above-mentioned inert gases, such as argon. The use of these by-products avoids the necessity of supplying a purchased gas which adds to the cost of the melting operation.

I claim:

1. In a process of melting with radiant heat from a hot surface as the sole heat source, said melting being accomplished by applying said radiant heat to a charge of metal particles situated below said surface in a crucible of metal which is maintained in the solid state by external cooling, an improvement minimizing the melting of the crucible at the beach line area where the crucible contacts the surface of the pool of molten metal formed by melting comprising shading the beach line: area to minimize radiant heat transfer thereto, adding the charge of metal particles at a rate sufiicient to maintain a pile of metal particles large enough to absorb a major portion of the radiant heat being transmitted to both said pile and said pool, thereby preventing appreciable superheat in said pool until the melting step is substantially completed, said addition of metal being continued at said rate until the complete charge is introduced into the crucible, sustaining the application of heat and superheating the molten pool in a period of time which substantially less than the time required for melting, and then removing the molten metal from the-crucible.

2. In. a process of'meltingwith radiant heat from a hot surface as the sole heat source, said melting being accomplished by applying said radiant heat to a charge of'particulate metal selected from the group consisting of titanium, zirconium, hafnium, thorium, and alloys containing as a major component at least one of said metals, situated below said surface in a crucible of the samemetal as that which is to be melted, maintained in the solid state by external cooling, an improvement minimizing the melting of the crucible at the beach line area where the crucible contacts the surface of the pool of molten metal formed by melting comprising shading the beach line area to minimize radiant heat transfer thereto, adding the charge of'metal particles at a rate suflicient to maintain a pile of metal particles large enough to absorb a major portion of the radiant heat being transmitted to 'both said pile and said pool, thereby preventing appreciable superheat in said pool until the melting-step is substantially completed, said addition of metal being continued at said rate until the complete'charge' is introduced into the crucible, sustaining the application of heat and superheating the molten pool in a period of time which is substantially less than the time required for melting, and then removing the molten metal from the particles situated below said surface in a crucible of ti- I tanium which is maintained in the solid state by external cooling, an improvement minimizing the melting of the crucible at the beach line area where'the crucible coni portion of the radiant heat being transmitted to both said pile and said pool, thereby preventing appreciable superheat in said pool until the melting step 'is substantially,completed, said addition of titanium being continued at said rate until the completecharge is introduced into the crucible, sustaining the application of heat and superheating the molten pool in a period of timetwhich is substantially less than the time required for melting,

and then removing the molten titanium from'the crucible.

. References Cited in the file this patent I V I UNITED v STATES PATENTS 2,436,124 Sklenar Feb. 17, 1948 OTHER REFERENCES Metallurgia, June 1949, pp. 69-76.

Transactions of the Electrochemical Society, article by Kroll et al. entitled Melting and Casting Zirconium Metal, September 1949, vol. 96, No. 3, pp. 158169.

The Iron Age, vol. 170, No. 16, Oct. 16, 1952,-pp. 105 113., 7 ,7 f 1 Metal Industry, Feb. 19, 1954, pp. 143-144.

Herres Feb. 14,

Claims (1)

1. IN A PROCESS OF MELTING WITH RADIANT HEAT FROM A HOT SURFACE AS THE SOLE HEAT SOURCE, SAID MELTING BEING ACCOMPLISHED BY APPLYING SAID RADIANT HEAT TO A CHARGE OF METAL PARTICLES SITUATED BELOW SAID SURFACE IN A CRUCIBLE OF METAL WHICH IS MAINTAINED IN THE SOLID STATE BY EXTERNAL COOLING, AN IMPROVEMENT MINIMIZING THE MELTING OF THE CRUCIBLE AT THE BEACH LINE AREA WHERE THE CRUCIBLE CONTACTS THE SURFACE OF THE POOL OF MOLTEN METAL FORMED BY MELTING COMPRISING SHADING THE BEACH LINE AREA IN MINIMIZE RADIANT HEAT TRANSFER THERETO, ADDING THE CHARGE OF METAL PARTICLES AT A RATE SUFFICIENT TO MAINTAIN A PILE OF METAL PARTICLES LARGE ENOUGH TO ABSORB A MAJOR PORTION OF THE RADIANT HEAT-BEING TRANSMITTED TO BOTH SAID PILE AND SAID POOL, THEREBY PREVENTING APPRECIABLE SUPERHEAT IN SAID POOL UNTIL MELTING STEP IS SUBSTANTIALLY

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