by Charles W. Finkl some form of positive stirring is required to bring undegassed metal to the surface and vacuum environment.

Transcription

1 Vacuum Arc Degassing by Charles W. Finkl New Developments 109 ring. They work quite well, have costs. and are now fairlv well sorted temperature. TIME AND TEMPERATURE Time of degassing, and any hope of an end-point, is limited by the permissible temperature drop between superheat in the furnace and ficilt furnace maintenance levels and overheating may increase gas absorption. Time of degassing has been stretched slightly by heat saving devices such as radiation shields, preheated insulated ladles, and continuously heated separate vacuum vessels or chambers. But, still the process must be stopped when the melt or bath reaches the required teeming temperature. Practically all of us have cast dirty ingots at one time or another by going slightly beyond the safe teeming temperature trying to extend the degassing cycle. Now time and temperature are controllable through the combination of existing proven processes. We call it Vacuum Arc Degassing; a combination of: a) Argon Degassing; b) Ladle Degassing; c) Purge/ Stirring; and d) Three Phase AC Arc Heating. The general arrangement of a vacuum arc degasser is shown in Fig. 1. The electrical control panel is shown in Fig. 2. Explanation of the Four Basic Processes a) Argon Degassing as now practiced requires cu ft of argon per ton of steel degassed. It works better in a sealed chamber where the atomized metal above the bath doesn't get a chance to mix with oxygen. Flake-free hydrogen is generally not obtainable by this process alone. Temperature drop and cost is about the same as with Ladle Vacuum Degassing using Purge/Stirring. b) Lad,le Degassing is simply placing a ladle of molten metal in a vacuum to remove the deleterious gasses. When the ferrostatic head exceeds the effective vacuum to degas the entire depth of the melt, CHARLES W. FlNKL is president of A. Finkl and Sons Co., Chicago, Ill. I Fig. 1-General some form of positive stirring is required to bring undegassed metal to the surface and vacuum environment. c) Purge/Stirring is the addition of small quantities of a gaseous stirring agent near the bottom of the ladle to flush undegassed metal to the surface while taking advantage of diffusion and dilution of the Partial Pressures as in (a) above. Gas auantities are rtreatlv reduced as compared to (a), however, using only 1-2 cu ft of gas per ton. Purge/Stirring is also effective for efficiently mixing alloy additions, breaking the slag cover for vacuum degassing, and flushing the slag away from the stopper rod to reduce slag attack on the rod. Purge/Stirring with vacuum is now practiced by close to 40 companies throughout the world. d) Three-Phase Arc Heating is identical to the practice used in air-arc furnaces today. Proper kilowatt input is extremely important to match ladle degassing temperature losses and alloy addition requirements. All processes used are now existing, proved techniques. Only their combination is new. All processes can occur simultaneously in the ladle thereby providing greater utilization of the VAD (Vacuum Arc Degamer). The equipment can be designed to operate either in or out of the glow range. arrangement of a vacuum arc degasser. well as a combination of shielding and cooling. Thanks to the additional degassing effect of Purge/Stirring, operation outside of the glow range is comparable to operation within the glow range, but at a lower electrode cost. Glow Range operation is illustrated in Fig. 3. Staying outside of the shaded area provides Glow-free o~eration and the use of standard GLOW AND NON-GLOW OPERATION When operating out of the glow range, normal arc furnace screwed connector electrodes are used. When operating in the glow range, shielded electrodes must be used. Water cooled electrodes have also Fig. 2-Electrical control panel for a vacuum been used in the glow range as arc degasser.

2 110 Proceedings of Electric Furnace Conference, 1968 M.M. Hg. Absolute Fig. 3-Glow graphite electrodes. The curve can be altered by different gases, slags and amount of boil. HISTORY OF EQUIPMENT AND ITS DEVELOPMENT We are now on our fourth stage of development. The first attempt was a single phase arc running off of a bank of welding machines with the bath grounded through the stopper rod and ladle wall. It proved the point that an arc could exist in vacuum and maintain relatively constant current characteristics off a rolling boil. We showed movies of this a few years ago at the Electric Furnace Conference. Our second attempt was a threephase arc with power lines running several hundred feet from our existing furnace transformer to the vacuum tank. It proved several thousand kilovolt-amperes could be put into the bath under vacuum, but we were limited to short cycles due to inductive heating of the long low-voltage supply lines. Our next installation was a standard three-phase 7500 kva arc furnace transformer with water cooled copper designed for 20,000 amps. Circulation of the bath is by Purge/ Stirring providing complete uniformity of the ton heat size. Some 114 heats have been degassed under our third stage of development with the parameters shown in Table I. Operating procedure is completely variable as to time, temperature, and the sequence of vacuum levels depending on the operators desired results. Arc regulation is manual -it can be automatic. We chose to Max. Heat Size range operation. Table I. bring the bath to the electrodes for load balance rather than regulate each electrode individually. All three electrodes are moved together to vary the kilowatt load; and the bath is simply raised or lowered under the individual electrodes through purging variations to balance the electrical load. TWO DIFFERENT TECHNIQUES We have developed two types of processes as with Ladle Degassing. During the 1965 Electric Furnace Conference, we reported two distinct Ladle Degassing techniquesone technique being the most economical form of Ladle Degassing using standard ladle bloating brick; the other technique giving lower final oxygen values using carbon deoxidation and insulated highalumina refractories. A similar situation holds true for Vacuum Arc Degassing with two different techniques available. The most simple technique or procedure (Type A) provides the lowest cost and shortest cycle time possible for degassing without temperature loss. The more sophisticated and likewise more costly procedure (Type B) provides the lowest possible final oxygen values and longer vacuum periods but requires better ladle refractories, special stopper-rods or no stopper at all, and additional vacuum tank cooling at critical connections and seals. A typical Type "A" heat would be as follows: 1. Ladle reaches VAD tank at about the desired teeming temperature. 2. Close tank, start last stage of steam ejector and slowly pull down 70 tons to 100 mm. Turn on purging and arcs as soon as cover is closed. Operate at 4 to 5 mw. 3. If only deoxidation is required process is complete after Yz hr through the combination of argon degassing, Vacuum, purge/stirr7ng and Arc Heating. With a 60-ton heat size you could expect an 0.02% carbon reduction per minute. This would evolve 55 lb of CO per min, or 3300 lb of CO per hr. It would also be equal to about the same pounds per hour of equivalent air at 100 mm and is not an unreasonable load on ordinary ejectors using air or steam or even a Water ring pump. Keep in mind, of course, that at 100 mm the ejectors operate non-condensing so you don't have cooling water problems or waste water problems. Millscale could be used to supply the oxygen necessary for decarbonization but would require about 100 F for 10 min of operation to remove 0.20% carbon. This 100 F of loss can be made up through the use of Vacuum Arc Degassing. Gaseous oxygen can also be used for decarbonization. T. Perry of Republic Steel Corp., and I tried this about 10 years ago, and after a few minutes of oxygen injection into the ladle we melted the porous purging brick. However, magnasite purging bricks are now available for oxygen use, making this an alternative method of carbon reduction. Perhaps the most economical combination for making ELC grades would be to blow down the carbon in the arc furnace to % carbon, being able to hold your oxidizable elements such as chromium during this period. Then transfer to the Vacuum Arc Degasser to remove the remaining points of carbon. This could be done again operating at mm with vacuum arc heating to get the high temperature necessary to hold your chrome and utilize the preferential carbon oxygen reaction. 4. If dehydrogenation is desired turn on the balance of ejectors for a few minutes to reduce the vacuum to under 2 mm Hg Absolute. A typical Type "B" heat would duplicate Type "A" above except the ladle would be lined with higher alumina refractories and possibly a lower heat input would be used. The heat would be held under vacuum longer, further reducing the oxygen content. The longer vacuum time increases the heat recovery time permitting greater alloy additions and/or suif6 reductions. FOURTH STAGE OF DEVELOPMENT Max. Power Input 8to7mw We are now about to try our Max. Heat Time with Stopper-rod 40 min under Vacuum repeated fourth stage of development. We Max. Heat Time without Stopper-rod 1 % hr-reladled are adding additional conductors so Purging Gases Used He, CO CO2, Air, N2. Air Lowest recorded gases on 0.20 carbon steels 0.95 ppm H-10 ppm 0-8 ppm N:, we can reach the rated capacity of Max. Alloys Added Cr and Mn Greatest Temperature Rise 80 ' the transformer. This will make it 7. possible to shorten the cycle time

3 New Developments 11 1 for the basic practice (Type A) of teeming at tapping temperature; or at least match normal non-degassed tapping temperature practice. The latter should take about one-half hour including dehydrogenation for a 70-ton heat, and permits the use of a normal Ladle-Degassing type brick and stopper-rod construction. We are adding a sliding-stopper for the more sophisticated Type "B" treatments and diversified cycles requiring extended vacuum time. The longest vacuum times obtainable with 8-in. to 10-in. stopper rod sleeves is about 40 min of vacuum. For longer cycles the sliding-stopper appears to be the answer. The sliding valve gear and hydraulics should be in operation in February We have also incorporated a Vacuum Immersion Thermocouple permitting any desired after vacuum or teeming temperatures. The sliding stopper was witnessed in operation in Germany and looks great for this application. Alternate stopper rod possibilities are as follows: 1. Larger diameter sleeves: a) requires greater holding devices due to increased floatation; b) requires increased free-height for expansion; and c) spring load stopper-rod mechanism with a tension free steelrod may solve the expansion problem. 2. Water or air cooled rods are possible and are now used on continuous casters. Our air cooled rod didn't last as long as a carefully assembled and well dried 10-in. sleeve. 3. Composite construction using a normal 6-in. clay sleeve covered from the slag-line up with a 10-in. OD x 6%-in. ID high-alumina tube cemented only at the bottom end. This tube should be of the pressure pouring variety. 4. Set a preheated, pre-aligned rod after degassing. 5. For the very long cycles use no "rod" at all and lip-pour into another ladle which could be again Vacuum Treated, and temperature adjustment made. 6. Duplicate 5 above except use a 6-in. nozzle opened with a "Pricker" and reladle in an inert atmosphere. 7. Use a sliding-stopper or bottom pouring device. Call-it-what-youwill, we described such a device and most of the prior art, also in TYPE "A" CYCLE For Normal Tonnage Use Possible cycles using the Vacuum Arc Degasser with a normal stopper rod are: a.) any type of ladle degassing done today but without temperature drop. This includes carbon deoxidation; b.) alloy additions up to several percent; c.) about five points of sulfur reduction; and d.) about five points of carbon reduction using millscale, 0.2% reduction using oxygen. TYPE "B" Possible cycles using the Vacuum Arc Degasser with the sliding stopper or alternate stopper construction are: a) lowest possible gas values; b) sulfur reduction greater than Type A; c) carbon reduction for ELC; d) holding furnace-in inert. atmosphere; e) batching and/ or split heats; and f) unlimited alloy additions. CONCLUSIONS 2. 0, reduction is not seriously affected by variation in vacuum levels between 100 mm and 1 mm Hg Absolute. 3. For O1 reduction alone small steam or air-operated ejectors or water-ring pumps are useable at the lower vacuum levels, and of course, experience less brick wear. 4. Purge/Stirring is a must! Fast efficient circulation and controlled boil is mandatory to protect ladle walls and tank roof from overheating. We all know the dangers of arcing off a glassy dead surface. 5. Lower free board is permissible due to the slower constant rate of deoxidation as compared to the wild 50 mm boil with rapid pump-down. 6. Sulfur reductions are now possible with an arc heated slag added after deoxidation. 7. The VAD can be used as a finishing unit increasing tons per hour of the arc furnace. 8. It can be used as a holding unit-with an inert atmosphere. 9. It approaches arc furnace efficiency of 65 to 70%. 10. Vacuum Arc Degassing provides concurrent heating and dedegassing resulting in: a) shorter cycles; b) less refractory wear; C) the arc blocks the heat from escaping during the boil eliminating pre or post heating; and d) operation is almost Isothermal, reducing damaging temperature peaks. We have degassed our 114th heat. We have added close bo a ton of alloys. We have degassed and raised the bath temperature 80 F. We have had problems and will probably have more as we extend time and temperature. But surprisingly enough, the balancing and main- 1. Extended vacuum periods can taining of the arc is the least of produce even lower Os values. our worries. by W. Wilson The 114 VAD (Vacuum-Arc-Degassed) heats have been of the following types: No. ot Heats Type 5 Carbon 93 Cr-Ni-Mo 6 Cr-Mo 3 5% Cr 7 Others RESIDUAL GASES Upon the hypothesis that VAD allows additional tank time without furnace superheat, the 21 VAD heats of Supplement Table I were selected to analyze the effect of time. Supplement Figs. 1, 2, and 3 show, respectively, the effects of total degassing W. WILSON is director of research, A. Finkl 8 Sons Co., Chicago, Ill. Vacuum Time - Minutes Supplement Fig. 1-Effects of total degassing time on residual hydrogen.

4 112 Proceedings of Electric Furnace Conference, 1968 Vacuum Time -Minutes Vacuum Time - Minutes Supplement Fig. 2-Effects of total degpssing time on residual Supplement Fig. 3-Effects of total degassing time on residual nitrooxyden. gen. Supplement Table I. ~upplement Table II. Rating of Ingot Samples. Tank Ha NP OP Vac-Arc Tlme, (pln), (bomb), (bomb), No. mln. Ppm PPm PPm A.- B C D Slag Practlce Thln Thlck Thln Thlck Thln Thlck Thln Thlck Double Single 2.87' ' S = 0.011% ** S = 0.026% No pins available. Several heats were omitted in this series from Vac-Arc No. 92 to No. 114, those being No. 93 for which no bombs were taken, and No. 96 on which various purge gasses were used. In instances where the tank was opened before the completion of the cycle, the values for the gas samples taken at that time were used. Supplement Table Ill. Ratings According to AMS 2301 D and AMS AM8 2301D No. of Indl- Slag Practice catlons Frequency Severliy Maxima (6 in. x 6 in.) Double Single AMS 2300 Frequency Severlty Maxima Single Slag Hydrogen shows no significant trend beyond 7% min and this might be expected because the equilibrium hydrogen pressure for 1.4 ppm of hydrogen is about 2 Torr. The first 7% min show a decline of hydrogen. Since these short time heats were degassed without arc heating, much of these 7% min represented pumpdown. INCLUSION COUNTS Two 63-ton heats of VAD 8620 were recently compared using J-K counts. One heat was made by a double slag process and the other by a single slag using 30 and 35- min cycles. Rolled samples representing top, middlle, and bottom locations of the first, middle and last ingots (1, 4, and 7) were rated with the average results shown in Supplement Table 11. The single slag heat was comparable to or better than the double slag heat in every instance except for sulfides which are explicable from the sulfur contents. AIRCRAFT QUALITY The same 8620 heats were rated' according to AMS 2301D and AMS 2300 for vacuum arc remelted steels as shown in Supplement Table 11. Clearly the heats passed AMS 2301D with a slight superiority for txe single slag heat. CONCLUSION time on the residual hydrogen, oxy- Oxygen and nitrogen both show The present assessment of VAD gen, and nitrogen. Hydrogen tests a statistically significant decrease is very rudimentary and incomplete. were all from pin samples; oxygen with increasing tank time. These re- Much remains to be done to assess and nitrogen were either from pin sults must -necessarily tail-off at the benefits of VAD and to opor bomb tests. some later time. timize the process. DISCUSSION G. W. WORTH (Republic Steel degassing has great significance for vacuum degassing cycle can now Corp. Research Center, Cleveland, the steel industry. Vacuum arc de- be determined on the basis of prod- Ohio): We have found Mr. Finkl's gassing apparently can alleviate the uct and not temperature loss which paper very interesting and feel his serious problem of temperature con- should result in product quality imdevelopment work on vacuum arc trol during vacuum degassing. The provements and extension of vac-

5 uum degassing to other products and processes. I would like the author to answer the following questions: The data given by the author shows only that extension of degassing time contributes to lower oxygen content. Does the arc contribute to lower oxygen contents as well? C. W. FINKL: The reported reduction in oxygen content due to the extension of degassing time is primarily due to time alone. However, the arc does have an effect on the surface of the bath, in that the arc creates quite a depression in the boil, and flushes the slag away from the area immediately under the arc, thereby providing more bare metal exposed to the vacuum environment. G. W. WORTH: What are the corresponding carbon levels for the oxygen contents that were shown? C. W. FINKL: We have not correlated the carbon levels versus the oxygen contents. However, 90% of the heats shown in Supplement Table I are mostly 55 carbon steels, the lowest carbon heat is 0.17, and the low carbon are somewhat uni- formly dispersed through Table I. We do not feel that the curve is biased by carbon content. Just to pick an example, heat 106 is an 8620, having 11 parts per million of 0 1 after 28 min of vacuum time. G. W. WORTH: What arc gaps are normally used and was any carbon pickup noticed from the arc? What was the effect of pressure on arc stability? C. W. FINKL: The -arc' gap is difficult to determine, because the force of the arc creates an impression on the froth of the boil. Arc length is in effect controlled by amperage, and as mentioned in the context of the paper, the amperage is balanced by raising the froth of the boil to the electrode by increasing the purging rate. We have not experienced any carbon pickup from the electrodes. The effect of pressure on arc stability is clearly shown in Fig. 3, at 1 to 2 mm we can run at a voltage potential of 100. At 100 mm this can be increased to 175 volts, and at 150 mm we can operate at 200 volts. No research has been done in the less than 100 micron range, but we would expect to be able to avoid the glow area New Developments 113 at high voltages if we had equipment that could run less than 100 microns. G. W. WORTH: Did the arc itself contribute to the decarburization reaction? Was the arc operating at time? C. W. FINKL: Decarburization is in theory and we have no experience to report at this time. G. W. WORTH: What was the slag composition and amount used for desulfurization? C. W. FINKL: The desulfurization slag composition would be four parts of lime to one part of bauxite.,g. W. WORTH: Would the author please elaborate on the stopper rod assembly and the means of maintaining electrical contact with the bath during single arc operation. C. W. F~KL: Grounding was done through the stopper rod for a single phase operation, using the graphitic stopper rod end as a conductor to the bath. However, this was found to be inadequate, and a steel plate was inserted in the ladle wall between the refractory, making contact with both the bath and the ladle wall for an effective ground.