3. Improves tensile ductility of rotor REFERENCES forgings.

4. Does not affect the ingot structure or segregation characteristics of large ingots.

5 . Deoxidizes some steels, the amount of oxygen removed being related to the composition of the steel.

The authors gratefully acknowledge the cooperation of the Engineering Depart- ment of the Elliott Company; the Research Personnel in the Division of Explosives Technology, Pittsburgh Station, U. S. Bureau of Mines; the Engineering Depart- ment of American Bridge Division, U. S. Steel Corporation; the Engineering, Oper- ating, and Metallurgical Departments of Duquesr~e Works, U. S. Stcrl Corporation; and the Operating and Metallurgical De- partments of Homestead District Works, U. S. Steel Corporation.

I. Zappfe. C. A., and C. E. Sims: Hydrogen. Flakes and Shatter Cracks-a Correlated Abstract. ~Wrlals and Alloys (1940) 11, 145-IjI, 177-184; I2,44-51, 145-148.

2. Baraduc-Mullcr. L. H.: A paratus for Re- moving Gases from ~ o g e n Steel by the Action of a Vacuum. U. S. Patent 1.077,gz 5. November 4. 1914.

3. Hornat, J. N.. and %I. A. Orehoski: Vacuum Casting of Steel. J. Metals. A I M E (July 1958) 10, 471.

4. Basset, J. G.. J. ITT. Dougherty, and G. R. Fitterer: Sampling of Liquid Steel for Hvdronen. Obcn Hearth Proc.. AIME (1956)-39, 4 '52

5 . Huff. G. F., &.-ii..Bailey, and J. H. Rich- ards: Sampling of Liquid Steel for Dis- solved Oxygen. J. Metals, A I M E (1gj2)

N. H. KEYSER, C H A I R M A N - T ~ ~ ~ ~ YOU

for a very nice talk, Mr. Orehoski. We will defer questions to the end of this group of papers.

The next paper is by Mr. A. L. Lehman, Superintendent, Electric Furnace Melting Department, Bethlehem Plant, Bethlehem Steel Co., BethLehem, Pennsylvania.

Vacuum Stream Degassing

DURING the past year, several excellent papers have been prescntcd covering the numerous advantages of the vacuum de- gassing process and the metallurgical qual- ity gains that have been made possible through its use.I.2 Since their coverage of the theoretical aspects of the process, of the removal of gases and minimization of nonmetallic inclusions by the process dur- ing pouring, and of the metallurgical advantages obtained when evaluation of the final product was very thorough, this paper will stress only the unit assembly and the factors irlvolved in actual oper- ation of vacuum degassing units and in production of vacuum degassed ingots.

1 References are on page 93.

Recognizing the multiplicity of prob- lems involved in actual operation of a vacuum stream degassing unit, both from the standpoint of operation and metal- lurgy, it was decided a t the Bethlehem plant of the Bethlehem Steel Co. to con- struct a small development unit to be located a t the 7-ton electric furnace of the tool steel department. This was placed in operation in August 1956.

l l a n y developmental studies were carried out in this unit, including stream degassing into both molds and ladles, and stream dis- persion studies under various tank pres- sures. This work established practices, refractory requirements, temperature con- trols, and pouring techniques that made

PHYSICAL CHEMISTRY OF STEELMAKING 8 5

possible full-scale production when the and in them have been cast the world's larger units in the electric furnace melting largest vacuum degassed ingots. The department were ready for operation. largest ingot, the 120-in. round corrugated

ingot weighing 500,ooo lb, requires the mE 250-T0N V~cuu36 output from five basic electric-arc furnaces.

UNITS Current orders and projected require- The first large 250-ton unit was placed ments to meet the needs of the heavy

in operation on July 2 , 1957, and was forging industry, coupled with crane

FIG I-THE 250-TON VACUZTY DEGASSING UNIT. The first tank is in the center foreground and the second is shown a t the left during an actual

pour. The 36-in. main Line leading to the jet system can be seen at the rear of the first tank. The control panel is a t the extreme right.

followed by a second unit on January 21,

1958. Since July 2, 1957, and as of Novem- ber 14,1958, two hundred and seventy-five large forging ingots have been cast in the degassing tanks. About 28 vacuum de- gassed ingots per month can now be pro- duced, ranging in size from a 62-in. round corrugated ingot weighing 70,000 lb to a 120-in. round corrugated ingot weighing ~00,000 lb.

These two tanks are the world's largest

capacity limitations and the low space beneath existing crane runways, were the chief factors in determining tank capacity and the type of construction to be used.

Thus, a vacuum degassing tank of 250 tons maximum capacity was decided upon. I t had to be dome-type rather than bell- type construction. I t was necessary to dig a pit 32 f t below pit floor level to accom- modate the tank. This depth was 10 f t below the water table, thus requiring

FURNACE- UDLE

special foundation construction. The avail- able overhead space restricted the height of the pony ladle to 48 in.

The overall picture of the present instal- lations are shown in Fig I. Fig 2 shows a cross-sectional view of the tank with a typical mold and sinkhead assembly. The aluminum rupture cup is just below the nozzle of the pony ladle.

The 12%-ton pony ladle is used to obtain uninterrupted pouring of ingots requiring the capacity of more than one electric furnace. The 36-in. main line is a t the right of Fig 2, leading to the jet sys- tem. Observation ports for visual control of pouring, television, or motion-picture cameras are on the periphery of the top of the tank.

The vacuum degassing tank measures 17 f t in diameter by 29 f t 4% in. high;

i t is made in two sections. The stationary part measures 17 it in diameter by 19 ft 11 in. high. The removable dome measures g f t 5% in. high and has a top diameter of 11 f t 6 in., which tapers to a 17-ft diameter in 6 f t 8 in. of its height, or a t a point 2 f t 3% in. above the seal between the dome and the stationary bottom portion of the tank.

A 6-in. flat plate caps the dome, whereas the tank wall construction utilizes a 135-in. thick plate. A ~?k- in . wedge-shaped neo- prene ring is used to make a vacuum seal between the dome and the tank. Also, another r %-in. wedge-shaped neoprene ring is used to make a vacuum seal between the top of the dome and the pony ladle. The machined flanges a t the bottom of the dome and the top of the tank, as well as those a t the bottom of the pony ladle and the top of the dome, are water cooled to protect the neoprene rings.

PHYSICAL CHEMISTRY OF STEELMAKING

SfAGE NO. 3 B o r n

-STAGE NO. 4 EJMOR

-NO. 2 BAROMIRK CONDENSR NO. 1 BAROMETRIC CONMNSER

-VACUUM TANK NO. 2

. . I_'___----J

FIG 3-STEAM JET EJECTORS FOR 250-TON DEGASSING UNITS.

VACUUX SYSTEM stream from the ejectors to permit checking the shutoff pressure, unloading, and shut-

The four-stage evacuator system used ting down the system. When tank I is used,

to obtain vacuum was designed to meet the main valve of tank z is closed, isolating

the following operating conditions: the latter from the system, and vice versa.

Operating pressure. . . . . . . . . rooo microns The first stage of the steam ejector sys- Air capacity.. . . . . . . . . . . . . . 318 Ib per hour tem is adjacent to the main valve. Then, Steam temperature. . . . . . . . . 4 jo°F max. in succession downstream, are the second Steam pressure. . . . . . . . . . . . . 13 j psi stage unit and a 42-in. barometric con- Steam quantity (total). . . . . . 9900 lb per hour denser, where the is by Condenser water quantity water and discharged to the hot well.

(total). . . . . . . . . . . . . . . . . . goo gprn Condenser water tempera- The noncondensable gases are cooled to a

ture, entering.. go°F max. lower temperature and withdrawn from . . . . . . . . . . Leaving.. . . . . . . . . . . . . . 107°F rnax. the condenser by the third steam ejector.

This in turn discharges into a 28-in. baro- A cross-section elevation of the jet metric intercondenser, where the steam

system rising above the tanks is shown and condensable gases are condensed by in Fig 3. The 36-in. main valves are just water and discharged into the hot well. above the tanks a t pit floor level, one for The noncondensable gases are cooled to a each tank. These valves are placed up- lower temperature and withdrawn from

the intercondenser by the fourth-stage is of special design, equipped with two ejector, which discharges its steam plus the nozzles and stopper rods. The inside dimen- noncondensable gases to the atmosphere. sions of the pony ladle after lining are

Steam-jet ejectors operate on a mass- 5 f t 3 in. in diameter by 3 f t 4 in. high, velocity principle. The propelling steam and it has a capacity of 12% tons.

FIG 4-CONTROL PANEL.

expands through a divergent nozzle, con- vcrting its pressure energy into velocity energy, and the mass of high-velocity steam is discharged from the nozzle in a directed flow through an air chamber and into a convergent-divergent diffuser. As the steam passes through the air chamber, i t comes in contact with and entrains a delinite mass of the vapors to be evacuated. I t imparts to this mass a portion of its own velocity by being decelerated and the total mass a t the resultant velocity enters the diffuser. where i ~ s velocity energy is, in the greater part, converted into pressure, thus permitting the mass to be discharged under prcssure into the next chamber. The en- trained mass is thus compressed from a low absolute to some higher absolute pressure.

The two tanks are tied into one four- stage ejector system for removing gases and vapors from the vessels. The tanks are not used simultaneously. The pony ladle

CONTROL PANEL

Fig 4 shows the control panel adjacent to the pouring platform. The instru- mentation (left to right) includes tem- perature recorders for measuring metal temperatures during pouring; television screens for observing the pouring stream and the sinkhead metal rise; vacuum gauges and recorders (all under the clock); switches for admitting argon and for breaking vacuum; gauges for measuring steam consumption, inlet and outlct water temperatures; and control levers operating the 36-in. main valves.

Vacuum is measured by three gauges. One vacuum gauge utilizes the ionization effect of alpha-particle radiations from a small permanent radium source. The other two vacuum gauges are hot-filament, thermocouple-type gauges. All three gauges operate on the principle that the conduc- tivity of a gas changes according to the

PHYSICAL CHEMISTRY OF STEELMAKING 89

quantity of the gas present. One of the The sinkhead, of a floating type, is set gauges is connected to a recorder, which down into the mold for the proper ingot- plots a graph of the vacuum in millimeters body height. Four heavy anchor bolts of mercury during the entire operation. hold the sinkhead flask firmly to the ingot This serves as the permanent record of mold, to permit moving the completed each pour, showing how the unit pumped assembly into the tank.

TABLE I-Dust and Ladle Analyses PER CENT

Material I C I Mn I Si I Ki I Cr I V ( Mo 1 Cu I Fe

Nickel-Chromium-Vanadium-Molybdenum

Ladle. . . . . . . . . . . . . 0 . 3 3 0 .73 0 . 2 5 2 . 8 6 0 . 9 9 0 . 2 2 0 . 5 3 0 . I7 Dust . . . . . . . . . . . I 1 . 6 6 1 46.3 1 1 . 6 3 I 0.18 I 0 . 3 6 I 0 . 0 1 I 0 . 0 5 1 1 . 6 0 1 17.90

Chromium-Vanadium-Molybdenum

Ladle ............. / 0 . 3 3 1 0 . 8 3 1 0 . 2 6 1 0 . 1 , I 1.01 / 0 . 2 3 I 1.21 I 0 . 1 4 1 Dust.. . . . . . . . . . . . 1 . 6 9 47 .7 1 .40 o . 1 3 0 .38 0 . 0 4 0 . 0 9 1 . 2 0 1 5 . 5 0

down, the pressure under which the ingot body was poured, the sinkhead filling technique, and the shutdown procedure.

I n utilizing this equipment as a pro- duction unit, normal working procedures were altered to insure maximum tank utilization in order to obtain maximum production rates.

The sinkhead is bricked well in advance. High-quality alumina brick and mortar capable of withstanding a minimum tem- perature of 3ooo°F and of resisting the erosion of the molten metal are used. Every sinkhead is individually bricked for each ingot, so that a t least 15 in. excess brick is available. This permits pouring the sinkhead under full vacuum con- ditions. The sinkhead is dried from 12 to 48 hr, dependent upon size, with one half of that drying time held to approximately I ~ O F as determined by buried thermo- couples a t the midwall location.

The ingot moId is cleaned of scale and refractory material and then coated with a graphite mold wash. This wash is impor- tant in maintaining clean, scale-free molds for subsequent pours.

Once the packing and anchoring are complete, the assembly is heated and kept hot until ready for use. The proper blocking is placed in the tank; the stool is positioned; the casting pan is packcd; and the entire mold assembly is lifted up and positioned on the stool in the tank. This is a critical phase. I n general, however, adequate de- gassing is accomplished within the ordinary limitations of pouring heights used in the casting of ingots of the various sizes. The maximum drop oblainable without com- plete sinkhead spraying is the ideal. This would mean that adequate degassing is taking place, since the stream is partially tearing apart, and that a minimum of exogenous materials will be carried into the mold, since erosion of the sinkhead brick is minimized. Thus the best obtain- able ingot surface, ingot cleanliness, and degassed metal are obtained.

I t is then time to clean the stationary part of the tank and the dome of the tank. Tank cleanliness cannot be overempha- sized. During vacuum degassing, a deposit of dust settles on the mold, dome, and tank. This dust analyzes approximately 50 pct Mn. The higher the manganese specification, the greater the amount of

9O PROCEEDINGS OF ELECTRIC FURNACE CONFERENCE, 1958

dust formed. The danger of a dust explo- sion should not be overlooked, and tank cleanliness must therefore be thoroughly stressed.

Comparison of dust analyses and the ladle analyses of two kinds of ingots poured are given in Table I .

The clean dome is now placed on the clean tank; the pouring platform is positioned; the high-temperature refrac- tory funnel is set; the aluminum rupture cup is placed on the thoroughly heated pony ladle; the pony ladle is placed on the dome and the system is ready to pump down.

Steam is turned on the fourth stage, which will pull the tank pressure down to 75,000 to 125,000 microns. Water is then turned on the 28-in. intercondenser and the third stage is started. The tank pres- sure should thcn decrease to between IO,WO and 25,ooo microns. These readings are actually taken in inches of mercury by use of a manometer connected to the 42-in. barometric condenser. The water temperature in the 28-in. intercondenser is then adjusted to a rise of 25 Fahrenheit degrees over the incoming water tem- perature. Water is then turned on the 42-in. condenser and the second stage is started. This will pump the tank down to a n absolute pressure of from 3 m to ;ooo microns. The first stage is then started and the tank will blank off a t a pressure of 250 to 400 microns. The temperature of the water in the 42-in. condenser is adjusted to a rise of 17 deg above the incoming water temperature and we are h a l l y ready to pour. This pumping-down time takes approximately 2; minutes.

the weight of the ingot play a controlling role.

The pour of a 108-in. round corrugated ingot weighing 245,ow Ib, which requires the output of two of the so-ton basic arc electric furnaces plus that of a 28-ton basic arc electric furnace is described in the following paragraphs. The steel is a chromium-vanadium-molybdenum analy- sis used for forgings for the electrical industry.

While all the activity to prepare for a vacuum cast was taking place in the pits, the three electric furnaces melted down, went through a vigorous oxidizing period, slagged off, and had alloy additions made just as for a conventional air-cast product. No changes in melting practice have becn made for a vacuum degassed product. The emphasis has been to produce the highest quality steel with the lowest gaseous con- tent possible, regardless of whether pour- ing is in air or in the vacuum degassing tanks.

Knowing the anticipated pouring rate and the furnace charge, i t is possible to plan the tapping sequence accurately and to keep the pit informed about the progress. This is done through group meetings prior to the pour, so that each man involved underst.ands not only his part in the overall picture but also the part every other man plays in this operation. Much of the success attained must be attributed to the training of all personnel by the practice engineer and the operating supervisors.

The heats were then lined up to tap 6 minutes apart. The bottom heat would tap 2 0 deg hotter than the subsequent heats and its carbon content would be 0.03 pct higher than for the second heat and 0.05 pct higher than for the top heat.

The pit crew was told the order of pour, when to anticipate reaching the seal, and the crane service required for each ladle,

Up to this point, the sequence of work for the pony ladle rigging, and for the is identical regardless of the ingot size to dome rigging. Manpower was assigned to be poured. Now, however, ingot size and take care of each operation.

PHYSICAL CHEMISTR .Y OF STEELMAKING 9 I

FIG 5-TYPICAL PRESSURE CURVE FOR 108- INCH ROUND CORRUGATED VACUUM CAST INGOT (245,400 POUNDS).

With the furnaces ready and all per- sonnel familiar with the pouring pro- cedures, the actual pour was ready to begin. The pony ladle was filled and the nozzle

FIG 6-MOLTEN JLETAL TORN APART ISTO TINY DROPLETS IN VACUUM DEGASSING TANK.

used for pouring the heat was opened. The pressure history of the pour was perma- nently recorded on a pressure gauge as shown in Fig 5.

Chart speed is approximately one block per minute exccpt that the interval from "4th stage on" to "start pour" is con- densed into an equivalent of 8 minutes on this chart. The actual time of this section of the curve was I hr 50 minutes. The remainder of the chart is a t the regular speed of approximately one block per minute.

A pressure surge to approximately 1500 microns occurred a t the instant the nozzle was opened, followed by very rapid recov- ery, to approximately 750 microns pouring range. When the molten metal left the nozzle and entered the vacuum degassing tank it was torn apart into millions of tiny droplets, as shown in Fig 6. This shows the stream from the nozzle tearing apart to a diameter of 50 in. in a 24-in. drop inside the vacuum degassing tank. .

Experience a t Bethlehem has indicated that within practical considerations ade- quate and uniform degassing is accom-

plished within pouring ranges as high as 1 2 0 0 microns.

The gases removed from the molten steel during pouring, as taken from the main vacuum line ahead of the steam jets, were analyzed on a mass spectrometer and found to contain approximately 33 pct hydrogen, 33 pct nitrogen, and 33 pct carbon monox- ide (Table 2 ) .

TABLE ;?--Gas Analysis PER CENT

For each heat of steel poured, hydrogen tests were taken from the pony ladle. Also, several immersion thermocouple tempera- tures were taken in the pony ladle metal. When the seal was reached, an expendable buried thermocouple showed the tempera- ture of the metal in the mold and indicated that it was now safe to take the first shutoff. The temperature pattern for this particular 108-in. ingot was a s shown in Table 3.

The sinkhead was poured intermittently.

At each shutoff, the pressure in the tank dropped, since the degassing effect was retarded. The pour was discontinued when the sinkhead was filled to the proper level. I t has proved advantageous to stop pour- ing with several inches of metal still remaining in the pony ladle, to maintain the vacuum.

Then the 36-in, main valve was closed to isolate the tank from the pumping system. Because of vacuum in the tank, degassing continued, which, with some air leakage, built up the pressure to about 10,ooo microns. During this short interval, the jet system was shut down in the reverse order of starting up. Argon gas was injected to dilute the combustible gas mixture prior to breaking vacuum. The vacuum break valve was then opened.

Pit personnel then coordinated their efforts.to remove the pony ladle in time to take hydrogen samples from the molten sinkhead. This completes the series of hydrogen tests, which began in the fur- naces during the oxidizing period for each heat of steel. These hydrogen levels ap- proximate I .Q ppm before slagoff, 3.0 ppm before tap, 3.5 pprn in the pony ladle, and 1.2 pprn from the sinkhead of the vacuum degassed ingot.

After the sinkhead hydrogen tests were taken, the portable platform and dome were quickly removed, to prevent excessive overheating of the upper plate of the dome. Under normal conditions, the heat radi- ated from the top of the sinkhead heats the underside of the dome to approxi- mately goo°F.

Water was then turned on the outside of the tank to remove the heat radiated by the mold, thus preventing tank distortior

The ingot was allowed to solidify in th tank for 50 hr prior to stripping an shipping to the Press Forge Departmen for further processing. With the corn pletion of this operation, the shop rapidly prepares for the pouring of the next vacuum degassed ingot.

PHYSICAL CHEMISTRY OF STEELMAKING 93

At Bethlehem, the transition of this new development to a daily production tool was accomplished rapidly and smoothly. Thus, vacuum degassing has become a n important production unit in the shop.

I. Hornak, J. N.. and M. A. Orehoski: Vacuum Castine of Steel. J. Metals. AIME (Tulv .- - 1958) ;0, 471.

2. Stoll. J. H.: Vacuum Pouring of Ingots for Heavy Forgings. Meeting of Am. Iron and Steel Inst., New York, May 1958.

[The close of Mr. Lehman's presentation was a moving picture showing the operation and the degassing effect of the process.]

N. H. KEYSER, C H A I R ~ N - T ~ ~ ~ ~ YOU,

Mr. Lehman for a very nice paper and a fine picture.

The final prepared paper is by Mr. C. W. Finkl, Vice-President-Engineering, A. Finkl and Sons Co., Chicago, Illinois.

Experiences With Ladle Degassing

BY C. W. FINKL

SOME new terminology used in vacuum WHY LADLE DEGASSING? metallurgy appears in this Paper. When Three reasons influence a company in a ladle degassing is mentioned, i t means that choice of procedures; namely, to produce a ladle is placed in a vacuum tank, the a better product, to reduce overall cost cover is closed, and the tank is subjected of producing that product, or to attain to a vacuum. Degassing is done the better delivery to the customer. Ladle action of the vacuum on the surface of the degassing seemed to offer all three possi- steel in the ladle and diffusion of hydrogen bilities to A. ~ i ~ k l and sons co. to the surface. Sufficient hydrogen should be removed

Ladle stream degassing, often called to reduce the possibility of flaking and ladle to ladle degassing, is a Process in hydrogen embrittlement. These hazards which a ladle is placed in a tank, a transi- are now by proper heat treat- tion instead of a cover is placed on top of ment, but the company felt that if the that tank, and either a pony ladle or the hydrogen could be reduced to a safe level regular tapping ladle is placed on top of an overall better product would be pro- the transition section. The vacuum between duced. ~h~~~ was also the hope that oxygen the top and bottom ladles is sealed and could be removed in sufficient quantities degassing is done by the action of the to reduce the oxide inclusions, thereby vacuum on the stream from the top ladle improving the transverse properties and to the bottom ladle. again obtaining a better product.

The third process considered will be overall cost of production is going that explained in the Papers presented to be difficult to evaluate. The additional earlier;'*2 that is, ingot stream degassing, cost of ladle degassing will have to be where the stream from a Pony ladle going plotted against the savings in heat treat- into a vacuum is degassed as during teem- ment now necessary to permit the hydrogen ing into an ingot mold contained in that to diffuse to a safe level, ~f this heat treat- tank. ing cost can be cut in half, savings can be

applied against the cost of ladle degassing. There is an additional saving, however, in

Orehoski, M. A.. and J. N. Hornak, P. 6 8 , the fact that when the Finkl plant is this volume.

Lehman. A. L.. p. 84, this volume. running a t capacity the limiting production