Recent European Developments in Consumable Electrode Melting

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Recent European Developments in Consumable Electrode Melting WHEN visiting Europe, it is quite apparent that there is a growing interest in and use of consumable electrode melting. This is evidenced by the increase in the number of companies involved, both the number and size of furnaces being used, and by the number of alloys being melted. As the furnaces become larger, there is a trend on the part of the users toward buying furnaces from equipment manufacturers rather than building them as was done in the earlier development stages. These installations are generally fairly complex, with automatic controls, and they are often placed in massive vaults for protection of personnel against any possible explosion. The last general review of European consumable electrode melting was made by Gruber in 1957.' More recent papers dealing with developments in England are by Child and Harris2 and Cook and S~ainson.~ In Germany, there was at first some lag in industrial use of consumable electrode melting, undoubtedly because most largescale use of this form of melting was for the production of titanium. Since Germany has had no post-war aircraft industry, there was much less support for the development of a large titanium industry. It would also appear that much of the effort of vacuum technology in the Germany steel industry was concentrated on treatments for vacuum degassing of large quantities of steel subsequent to air melting. The resulting improvement in the steel was primarily due to hydrogen removal. However, there is now a definite increase in consumable electrode melting in Ger- 1 References are on page r 6. 7 many, both for steel and reactive metals. In addition, German vacuum equipment manufacturers are now building large furnaces, both for domestic and foreign use. In England, only one major steel company has its own consumable electrode melting plant. This was originally developed for titanium melting and is still usctl more for titanium than for steel. However, a number of other steel companies are having development work done for them, and this will, without doubt, eventually lead to the building of more plants. In France and Italy, there are rather recently built consumable electrode melting plants. In France, the prime emphasis is on titanium production, although the governmental atomic energy groups are doing some consumable electrode melting. Fig I shows a furnace installed at Deutsche Edclstahlmerke at Krefeld, Germany. It was built by Degussa-Wolfgang and is one of several similar furnaces. It is capable of melting up to 500 kg of steel with ingots up to 10-in, diam. Ball-bearing, ferritic, and austcnitic stainless and tool steels and hardenahle alloy steels requiring high impact properties arc included in alloys being melted. The largest single co~lsumable electrode furnace in Europe was installed this fall at the A. B. Bofors Co, at Bofors, Sweden. It is capable of melting up to 6 tons of steel. While primarily built for steel melting, there is also interest in using it for the melting of titanium and zirconium. The largest and most complete consumabie electrode plant in Western Europe is

8 PROCEEDINGS OF ELECTRIC FURNACE CONFERENCE, 1959 that of the Imperial Chemical Industries at Birmingham, England. The furnaces have been replaced several times, the present units having been built by the W. C. der vacuum of either, or both, the furnace and the crucible when changing crucibles. A general view of the furnace area is shown in Fig 2. For safety, the three fur- FIG I-DEGUSSA FURNACE INSTALLED AT DEUTSCHE EUELSTAHLWERKE FOR MELTING STEEL. Heraeus Co. and put into operation early in 1958. This installation consists of three furnaces, each designed to melt ingots up to 24 in. in diameter, giving a maximum ingot weight of 2 tons of titanium. These furnaces have several unique features, the one principal aim being to increase the overall productivity of the units. A most significant addition is the incorporation of allock valve between the furnace chamber and the crucible. This permits independent isolation and maintenance un- naces are installed in concrete cubicles with steel doors. Fig 2 shows the entrances into these cubicles and the control room can be seen above the doors. Electrodes for titanium alloys are prepared by welding together previously pressed compacts 11 in. by 13 in., using titanium plates for joining as shown in Fig 3. These electrodes are transported to the furnace in a crucible truck (Fig 2), centered in the furnace by holding jigs, and welded to a titanium stub mounted on the end of

CONSUMADLE 1q:T.ECTRODE MEL L [NC 9 the electrode holder. One furnace is shown in Fig 4 just as an electrode is being raisrd into the furnace before welding. Welding is done in vacuum by striking front of the operator. He is thus able to observe the arc at all times and make any adjustments that may be needed. Thc rlectrade feed is controlled automat- FIG ~-F~RN.\cE INSTALLATION.\T IJIPERIAL C~~rnc.1~ IKDUSTIUES. an arc between the stub and the top of the electrode, using the normal furnace power supply. When a pool of molten metal of sufficient size is formed, the electrode stub is driven down into the molten pool and allowed to freeze. In similar fashion, cast or forged steel electrodes can be furnacewelded to the electrode stub. This arc for welding is stabilized by a magnetic field produced by a direct current in a coil surrounding the electrode at the point where welding is done. The operators' control room is shown in Fig 5 and a close-up of a control desk in Fig 6. At the desk, the operator has all principal controls together with indicating meters. Recording meters are near (Fig 5). A dual optical system projects two overlapping images of the two sides of the arc on ground-glass screens mounted directly in ically by means of a rather complex electronic control, which operates two d.~. motors connected to a differential drive. This control is designed to give high speed of response and short arc lengths. It also compensates for changes in voltage drop along the electrode during melting. A schematic drawing of one of the furnaces is given in Fig 7. The initial stage of pumping is done by a large Roots pump in parallel with a vapor booster pump. This is backed by two smaller Roots pumps and a water-ring pump, a11 in series. This parallel operation of the Iarge pumps gives the advantages of both types of pumps. The Roots pump gives a rapid pumpdown and, because of its fast pumping rate at high pressures, it is said to minimize the danger of explosion by preventing the build-up of steam pressure if a crucible is punctured.

I0 PROCEEDINGS OF ELECTRIC FURNACE CONFERENCE, 1959 The vapor pump is particularly good for handling large volumes of hydrogen at low pressures. The lock valve is shown in this drawing into the furnace chamber. The furnace is then capped so that it can be maintained under vacuum. Air can then be immediately introduced into the lock valve and the first- FIG 3-TITANIUM ELECTRODES PREPARED BY WELDING PRESSED COMPACTS. (Fig 7), between the furnace chamber and the crucible. Gruber4 has shown a more detailed schematic diagram of this valve, which is reproduced as Fig 8. The lock valve is employed in the follo\ving way. At the end of a first melt of titanium, the stub of the electrode is plunged into the molten pool. In a few minutes the pool is completely solidified and the electrode is withdrawn, lifting the ingot with it melt crucible removed, without waiting for the ingot to cool further. A crucible of larger diameter for the second melt is then placed in position, the lock valve and crucible are evacuated, and the furnace chamber is uncapped. The electrode is lowered and secondary melting is started, the ingot from the primary melt being the electrode for the secondary melt. The use of the lock valve in this manner greatly decreases the

CONSUMABLE ELECTRODE MELTING I I time lost between primary and secondary melts, with a consequent increase in productivity for the furnace. Since the primary ingot retains much of time for cooling the ingot is saved. The crucible truck shown in Fig 2 has a built-in circulating pump and water tank, so that some flow of cooling water can be main- E'IC 4-EJ.ECTRODE BElSC. PLACED Ih' FTJRh'ACE. its heat in this change-over, it can be remelted 20 pct faster than if it were cold at the start of the second melt, which is an unexpected benefit of this method of handling..4fter the second melt has becn complcted, the crucible is capped with a lid, thus maintaining it under vacuum so that the whole crucible assembly can be removed for cooling and immediately freeing the furnace for the next melt. This greatly increases the output of the furnace, since the tained while the crucible is being transferred to the cooling station. When melting high-strength titanium alloys or hardenable steels, which are susceptible to cracking on cooling, the ingots are removed from the crucible when they are still fairly hot and placed in an insulated muffle for slow cooling. Fig g shows a completed 2-ton titanium ingot 24 in. in diameter. The primary melt would have been made in a tapered crucible 10 in. to 20 in. in diameter. For a second

12 PROCEEDINGS OF ELECTRIC FURNACE CONFERENCE, 1959

CONSUMABLE ELI.:CTRODE MELTING I3 melt ingot 20 in, in diameter, the crucible used for the primary melt would be tapered from 16 in. to 17 in. in diameter. ing. This segregation is much more gf a problem with high-carbon steels than with low-carbon steels. Since this segregation in- M! BACKING PUMP UNIT I I \\\ 1! 1 L---J "-\ 1 I \I\\ ---_I-- RESEARCH AND DEVELOPMENT Segreg*ion' and pooz Depth Problems arising from segregation during solidification in arc melting have been of considerable concern, particularly in steel melt- creases as the diameter of the ingot is increased, this problem becomes greater as larger - and larger ingots - are being melted. Segregation generally increases with the depth of the molten pool. Since high melting rates, which are desirable from a pro-

14 PROCEEDINGS OF ELECTRIC FURNACE CONFERENCE, 1959 furnace capping Lid crucible capping Lid FIG 8-LOCK VALVE ARRANGEMENT. (Gruber.4) FIG 9-TWO-TON TITANIUM INGOT, 24 INCHES IN DIAMETER.

CONSUMABLE ELECTRODE MELTING Is duction standpoint, increase the pool depth and therefore the degree of segregation, there is a very definite problem to be dealt with in melting certain steels in large quantities. For these reasons, there has been an interest in research on methods of studying the effects of melting variables and crucible geometry on the depth and shape of the molten pool and the rate and manner of freezing. Some work has been done in the past on the use of radioactive tracers to determine pool depth. 11 simpler method employed by several German research groups is to introduce sulphur into the molten metal. A sulphur print taken of the sectioned ingot clearly reveals the outline of the molten pool at the moment this sulphur was introduced. A slight rise in pressure when the sulphur is introduced gives an indication on the pressure chart of the time when the sulphur was added. Additional sulphur charges can be added at different times to show the change in the pool contour during the melt. A variant on this technique is to employ a high-sulphur, free-machining steel. The resulting macrostructure, as seen from a sulphur print of the ingot, reveals the general shape of the pool during solidification. Another method being employed to outline pool depths near the end of this melt is to use a high-carbon steel on the end of the consumable electrode. When the ingot is etched, the high-carbon area outlines the pool shape at the moment meiting of the high-carbon steel began. Arc Control Equipment Much improvement has been made in developing automatic controls for maintaining proper arc length and stable melting conditions. These generally give very satisfactory operating performance. However, there is occasionally still some problem with unstable arcs and "glow discharge," and there is a lack of full understanding of this phenomenon. This is particularly true dur- ing hot-topping operations at the end of the melt when the current is reduced. Further study of control systems and effects of operating variables appears very desirable. Much larger furnaces for melting steel are in the design stage with furnaces melting ingots up to 40 in. in diameter. Still larger ones are being planned. Atomic Energy Applications In the United Kingdom, since most uranium for fuel element fabrication has been cast, there has been little interest in arcmelting uranium. However, considerable amounts of zirconium for reactor use have been arc-melted in ingots up to IS in. in diameter and weighing up to 3 tons. To prevent contamination, this zirconium is melted in a separate plant from titanium. Its uses are primarily planned for watermoderated and water-cooled reactors. Kiobium and vanadium are being arcmelted with potential interest for liquidmetal-cooled reactors. Beryllium, while fabricated by powder metallurgy techniques and by vacuum induction melting, is also being arc-melted experimentally with promising results. Hafnium, used as control rods because of its high neutron absorption, is also melted by consumable electrode melting. There is a definite interest in the consurnable electrode melting of steel for atomic energy purposes. This interest centers primarily in producing steels with low nonmetallic inclusion content and with superior mechanical properties. CONCLUSION While they are somewhat limited in size and number, western Europe has some of the finest consumable electrode melting plants. Current developments indicate an increasing interest in the use of the consumable electrode process for melting a wide variety of steeis, titanium alloys, and metals of direct interest for nuclear power application.

I 6 PROCEEDINGS OF ELECTRIC FURNACE CONFERENCE, 1959 REFERENCES I. Gruber, H.: Survey of European Arc Melting. In Vacuum Metallurgy, 138; edited by R. F. Bunshah. New York, 1958. Reinhold Pub. Corp. z. Child. H. C.. and G. T. Harris: Vacuum Melting of Steels. J. Iron and Steel Inst., (1959) 1909 414. 3. Cook, Maurice, and E. Smainson: Arc Melting of Reactive and Refractory Metals. J. Inst. Melals (1959) 87 (6), 161. 4. Gruber. H.: Consumable Electrode Vacuum Arc Melting. Elec. Fur. Steel Proc., AIME DISCUSSION E. R. SAUNDERS, CHAIRMAN-We have a few moments available for discussion from the floor. I should like to take this opportunity to express my appreciation to the gentlemen from Case Institute of Technology who are operating the microphones and the slide projector. The session is now open to discussion of Dr. Childs' presentation. W. W. SCHEEL-YOU touched briefly on the hot-topping techniques. Could you expand on that and tell us what they are doing? Do they use a nonconsumable electrode for hot topping at any point, or what specific thing do they use? W. J. CHILDS-Perhaps I did not clarify that as much as I should have. Obviously, in order to get a sound ingot, it is necessary to program the melting current so that toward the end of the melt one puts less heat into the crucible. This is done still using the consumable electrode. It is merely a decrease in the power level and the melting rate, so that the shrinkage is minimized at the end. Difficulty experienced by some of these operators appears to arise from the fact that a decrease in current does not necessarily result in the same decrease in heat to the pool itself. This is a function primarily of lack of stability and lack of a linear relationship between melting current and heat input to the pool itself. I do not think they have solved the problem. They feel it is hard to get consistent results and program it exactly, so they will always get ideal results. E. S. SMITH-1 wonder if you care to comment on vacuum range and what they are using to measure the vacuum in the system? W. J. CHILDS-I do not think there is anything unusual. Gauges are used. The pressures that are being obtained depend on what is melted, titanium being on the order of 10 to 30 microns pressure. Some of the larger steel companies are able to operate somewhat less than a micron, between 10-4 to 1oP3 mm. E. R. SAUNDERS, CHAIRMAN-I am sure that Dr. Childs will be happy to meet with any one of you later on in the day if you have some additional points to cover with him. Having had a general review of this method of melting, we are now in a position to become more specific in the subject matter. One of the advantages of dealing with a subject that is relatively new is that, right at the outset, we can apply our knowledge of the theory of metals in order to better understand mechanisms, and this means that we are in a position to develop a technology based on fundamentals rather than on empirical relationships. If n7e are dealing with fundamentals, obviously we ought to pay attention to the arc itself in the consumable electrode process. Our next speaker is a man who has spent the last three or four years in research work and is going to discuss some of his findings. You may suspect from his accent that he was educated in the United Kingdom. He received his doctorate from the University of Wales in 1953. At the present time he is a member of the Metals Research Laboratory, Union Carbide Metals Co., Niagara Falls, New York. It is with a great deal of pleasure I present our second speaker, Dr. R. P. Morgan. His co-author, Mr. T. E. Butler, is also with the Metals Research Laboratory.