US3501291A

US3501291A - Method for introducing lithium into high melting alloys and steels

Info

Publication number: US3501291A
Application number: US675766A
Authority: US
Inventors: Hans Schneider
Original assignee: Sulzer AG
Current assignee: Sulzer AG
Priority date: 1966-10-19
Filing date: 1967-10-17
Publication date: 1970-03-17
Anticipated expiration: 1987-03-17
Also published as: CH485024A; BE705324A

Description

3,501,291 METHOD FOR INTRODUCING LITHIUM INTO HIGH MELTIN G ALLOYS AND STEELS Hans Schneider, Winterthnr, Switzerland, assignor to Snlzer Brothers, Ltd., Winterthur, Switzerland, a corporation of Switzerland No Drawing. Filed Oct. 17, 1967, Ser. No. 675,766 Claims priority, application Switzerland, Oct. 19, 1966, 15,125/ 66 Int. Cl. C21c 7/00; C22b 9/00; C22c 33/00 U.S. Cl. 75-53 23 Claims ABSTRACT OF THE DISCLOSURE Lithium is added to the metal melt after placement of the melt in a treatment vessel lined with a material which is more stable than lithium oxide at the melt temperature in order to scavange out excess oxygen. Also, the melt can first be formed by skull melting techniques before the lithium is added. The lithium can be added to the melt by means of a porous carrier body containing the lithium in distributed form.

This invention relates to a method of introducing lithium into high melting alloys and steel. More particularly, this invention relates to a method for treating metal alloys having oxygen afiinity.

In order to improve mold filling capacity and purity, lithium has been introduced into melts of high melting alloys and steel. The added lithium in such instances acts as a reducing agent for normal slag-forming agents so that very pure metal melts can be produced. Further, at the prevailing temperatures, the lithium reacts to produce a lithium oxide which is the only liquid metal oxide and thus facilitates segregation from the alloy or steel melt.

Additionally, during the teeming of a metal melt into a mold a thin film of metal oxide instantaneously forms on the surface of the melt. This film is usually solid such that flowing of the metal melt into a mold is accompanied by a solid-on-solid sliding rather than liquid-on-solid.- Thus, the coefficient of friction of the flow is substantially higher than it would otherwise be with a liquid-on-solid sliding. However, when added to the melt, the lithium metal present is first oxidized to produce a liquid lithium oxide film which covers the surface of the melt. Thus, when the lithium treated steel or alloy melt is teemed into a mold, a liquid-on-solid sliding is obtained so as to improve the mold filling capacity.

Heretofore, in treating metal melts with lithium the lithium has been introduced or poured into the stream of molten metal when the melt is teemed into a ladle. However, because of the vapor pressure which amounts to several atmospheres in the temperature range of the treated melts, from approximately 1400 C. to 1700 C., the introduction of the lithium into the melts to be treated has been accompanied by great difl'lculties. For example, upon introducing lithium into the stream of molten metal in the above methods, the greater part of the introduced lithium is lost by evaporation. Also, since lithium has a relatively great reactivity, a considerable part of the lithium fraction which has been added into the melt 'by pouring-in has frequently reacted with the lining of the crucible containing the melt. This latter reaction has been undesirable since the crucible lining has usually been rapidly destroyed while the proportion of added lithium available for treatment of the melt has been further reduced. Accordingly, metering of the lithium actually entering into a melt has not been possible owing to the above described side effects. Practical performance of an effective lithium treatment in the heretofore known methods has therefore been difiicult because, by far, the

3,501,291 Patented Mar. 17, 1970 greater proportion of added lithium evaporates or reacts with the crucible lining.

Accordingly, it is an object of the invention to provide a technically feasible lithium treatment of steel and high melting alloys which can be performed in practice.

It is another object of the invention to reduce the possibility of the added lithium attacking the lining of a crucible containing a melt.

It is another object of the invention to meter the amount of lithium added to a melt in a controlled manner.

It is another object of the invention to treat metal alloys having oxygen affinity.

It is another object of the invention to remove as much oxygen, nitrogen and hydrogen as possible from a reactive metal melt prior to teeming.

Briefly, the invention provides a method of lithium treatment of a steel or high melting alloy melt wherein the melt is contained in a vessel lined with a material, such as crystalline lime, which is more stable than lithium oxide at the temperature of the melt.

In one embodiment, the lithium is introduced into the alloy or steel melt as in the alloying of magnesium while the melt is contained in an autoclave within a protective atmosphere of argon or helium, of a mixture of both, or of carbon monoxide maintained at a pressure which exceeds the vapor pressure of the lithium at the temperature of the melt. The lithium is added by the immersion of one or more metallic bodies containing lithium, for example, such commercially available bodies as copper cartridges filled with lithium powder. Alternatively, the lithium powder can be blown into the autoclave by means of a carrier gas, such as, argon, helium or carbon monoxide.

In another embodiment, the lithium treatment is repeated several times so as to improve the purity of the melt by reducing and then re-establishing the gas pressure in the autoclave between individual treaments.

In still another embodiment, vaporized lithium can be blown into the melt. To effect this, lithium or an intermediate lithium compound, for example, lithium silicide, is heated and vaporized under a protective atmosphere by means of an induction coil and in an autoclave vessel. In this instance, the underside of the vessel is provided with nozzle apertures which are initially closed. This vapor pressure which increases with increasing temperature causes lithium vapor to be discharged in powerful jets from the nozzles after opening of the nozzles. The jets then penetrate into the melt disposed below the vessel. Because of the high velocity of the lithium vapor jets, it is sometimes possible to operate without an autoclave but only in a protective atmosphere when adding the lithium. That is, it is not necessary to maintain a pressure over the melt which is higher than the vapor pressure of lithium at the temperature of the melt.

In still another embodiment, the lithium treatment is performed in a melting vessel of a rare gas plasma furnace or of an electron beam furnace with the lithium being added to the melt in a lithium compound, for example, lithium chloride or lithium aluminum silicate, of solid form.

The penetration of the lithium into the melt can be improved still further by rabbling the alloy or steel melt during the lithium treatment by means of a low frequncy multiphase alternating current generated in rab'bling coils.

Where nitrogen is to be removed from the melt in addition to the removal of oxygen by the addition of lithium a metallic calcium and/or barium and/Or strontium is also introduced into the alloy or steel melt. The calcium, barium and strontium is introduced in the same manner as is the lithium and can also take place simultaneously with the introduction of the lithium.

The method of the invention can also be used to treat 3 metal alloys having oxygen afiinity, in particular titanium alloys, which are also referred to as reactive metals. The method effects the removal to a relatively large extent of contaminants such as oxygen, nitrogen and hydrogen in the melts of these reactive metals before teaming into molds.

In this latter usage, an alloy is initially melted by the skull melting tethod under a protective atmosphere. Thereafter, the melt is subjected to a lithium and/or calcium treatment in the skull formed from the solidified alloy during melting in a manner as above with or without the addition of a supplemental amount of lithium and/ or calcium, strontium and/or barium.

Skull melting refers to a melting and teeming operation in which a material is melted in a preferably watercooled metal crucible. The necessary energy for melting the material is generated by means of a total loss elec trode, by means of an electric arc with permanent electrodes, or by means of a plasma arc. Initially, a solid skin of the alloy to be melted, also referred to as a skull, is deposited on the cooled crucible. The liquid melt thus becomes disposed in a crucible lined with the material of the melt. This method, which is particularly employed for winning titanium, usually has the alloy melted under a protective atmosphere under a pressure of 0.5 to 1 atmosphere. The protective atmosphere is usually argon or helium, or a mixture of both and generally necessitates the accommodation of the crucible, the electrodes and the molds for subsequent teeming in an enclosed furnace which is sealed relative to the ambient air. For further details of the skull melting method, reference is made to Vacuum Metallurgy by R. F. Bunshah, Reinhold Publishing Corporation, New York, 1958.

Additionally, the melt can be connected at normal or at elevated pressure with one electrode of a plasma arc and a lithium compound, for example, lithium chloride, or if the introduction of silicon and aluminum is permissible, lithium aluminum silicate, and/or calcium can be added in solid form. Further the alloy to be treated can be represented by total-loss electrodes in an arc furnace while a lithium salt and/or calcium covers the surface of the treated alloy. Still further, the treatment vessel can be a water cooled crucible, for example of copper, which is lined with a thin film of crystalline lime or thorium oxide.

These and other objects and advantages of the invention will become more apparent from the following detailed description.

In order to practice the invention, insofar as a melt is to be subjected only to the addition of lithium to remove impurities, the treatment vessel is first prepared. In this regard the treatment vessel is lined with a crystalline lime. As disclosed from a report by Dynamit Nobel A. G. Feldmuhle, Lulsdorf, Germany, entitled Kristall-Kalk-Kolloquium, dated Dec. 7, 1965, crystalline lime refers to that calcium oxide crystallized from the melt which has an improved hydration stability and may therefore be employed for a crucible lining. Alternately, thorium oxide can also serve as the material for the vessel lining. For small quantities of a melt to be treated, for example, quantities of the order of magnitude required for precision casting, the vessel which is usually constructed of steel is rammed with granular material of crystalline lime. The material is pulverized to granular consistency with grain diameters of up to a few millimeters.

A few percent of aluminum silicon ester can be added as a bonding agent to the ramming compound; however, the granular crystalline lime can also be rammed in the dry form with a graphite body being initially inserted into the hollow space for the melt. By means of a medium frequency induction coil, or by some other means, the lining is then heated in a rare gas atmosphere to a temperature near the melting point of the crystalline lime, that is, to approximately 2500 C. The internal surface of the ramming compound is thus sintered so that a film of fritted calcium oxide is produced on the surface of the treatment vessel. Calcium carbide produced by the interaction with the graphite is mechanically removed after cooling of the treatment vessel. If it is desired to avoid carburization of the melts to be treated at a subsequent stage, the graphite body required for heating the crystalline lime lining may be replaced by a molybdenum or tungsten body.

To obtain fritting at the internal surface of the calcium oxide, a preliminary alloy, for example, of steel is prepared in a steel mold which is inserted into the lined treatment vessel, and then is slowly and carefully melted. As soon as the preliminary alloy is melted, the template mold also melts so that the calcium oxide surface is sintered by the hot melt.

Far larger treatment vessels, the steel vessels are lined with bricks or crystalline lime; calcium aluminum cement being used as bonding agent between the bricks.

The treatment vessel is thus lined to permit the remainder of the treatment to be carried out.

In order to further describe the invention reference will be made to the following examples:

EXAMPLE I A 13% chromium steel is treated with lithium. The treatment serves to improve the purity of the steel by bonding any oxygen in a melt of the steel to the lithium while also serving to improve the mold filling capacity in subsequent teeming into the molds commonly used in precision casting.

Initially, the steel is melted in air in a medium frequency induction furnace having a MgAl spinel lining or in any other known manner and with any other suitable lining. The furnace has a capacity of approximately 30 kg. of steel. Next, the melt of steel is teemed into the prepared treatment vessel. After reheating and rab bling coils are applied, the treatment vessel is inserted into an autoclave. The autoclave is then evacuated and subsequently loaded with a protective atmosphere comprising argon, helium, a mixture of argon and helium, or carbon monoxide which has a vapor pressure exceeding that of lithium. For example, since the melt has a temperature of about 1650 C., the autoclave is loaded at a pressure exceeding 6 atmospheres.

Lithium is then added to the melt in the autoclave.

In one instance, pulverized lithium is blown into the melt by means of known blowing apparatus while using a rare gas stream as an entrainment medium. In another instance, commercially obtainable copper cartridges filled with lithium powder are immersed in the melt. This latter method of adding the lithium advantageously requires no separate apparatus as compared to blowing.

Additionally, in order to simultaneously remove nitrogen from the melt, a pulverized lithium/ calcium or lithium/ barium or lithium/strontium mixture or a mixture of all the metals in powder form is blown into the autoclave nstead of only the lithium by the blowing apparatus. In using the second method above, suitable bodies contalnlng calcium and/ or strontium and/ or barium powder are immersed into the melt with the lithium.

The amount of lithium which is introduced into the melt amounts to approximately 0.1 to 0.2% of the weight of steel. For example, in the present instance, the quantity of lithium is approximately 30 to 60 g. Similarly, the quantity of calcium, strontium or barium introduced into the steel melt is approximately the same.

The lithium treatment of the melt can be repeatedly performed in the above manner. In this method, the gas pressure in the autoclave is reduced between individual treatments and built up again before the next batch of additives is introduced. This results in boiling of the melt so that the molecular gases are driven out. Accordingly,

I: U the fiushing-through of the melt is improved to ensure more intensive mixing of the lithium with the melt while providing better expulsion of the gases.

During lithium treatment, the melt is maintained at the required temperature by a medium frequency induction coil while the melt is rabbled by a rotating electric field, produced in known manner by means of a lowfrequency multiphase alternating current of, for example, 20 to 30 c./s.

After treatment, the material can be teemed into molds also within the autoclave by methods commonly employed in precision casting, the protective atmosphere press re being reduced to approximately 1 atm.

The combined lithium and calcium and/or strontium and/or barium treatment described heretofore enables nitrogen, sulphur and oxygen concentrations in the melt to be substantially reduced by the formation of calcium nitride and/ or strontium and/or barium nitride, lithium oxide and, in some circumstances, lithium sulphide, the

nitrogen content of the melt dropping, for example, from 200 to 100 p.p.m. and the oxygen content being reduced from 120 to approximately 60 ppm. In order to supply the energy required for keeping the melt at the required temperature, means other than induction coils can be used, for example, electrodes immersed in the melt or a plasma arc.

EXAMPLE II An alloy 713 C whose basic constituent is nickel and in which titanium and aluminum are persent as precipitation hardness is recovered from Waste and purified.

Waste of this alloy in the form of solid scrap is placed in a prepared treatment vessel with a lining as above of crystalline lime. The vessel in this example simultaneously functions as the melt container of a plasma furnace. A water cooled copper electrode is introduced from be low into the melt chamber of the treatment vessel while a mating electrode of the plasma furnace, for example, of tungsten, is extended from above into the melt chamber. In addition, helium or argon is employed for producing the plasma of the furnace arc. The waste is initially melted to form an impure alloy melt, to which small quantities of solid lithium chloride, or some other llthlum compound, for example, lithium aluminum s1l1cate, 1f the introduction of silicon or aluminum into the melt is permissible, are added once or repeatedly. The melt is thus produced and treated in the same treatment Vessel as above constructed. Alternatively, an electron beam furnace can be used instead of the plasma furnace.

During treatment, the lithium compound melts at t he temperature of the melt and reacts with the impunties in the plasma arc.

Where the treatment is prolonged, adequate quantitles of oxygen and to some extent sulphur are removed from the melt.

As a high lithium vapor pressure prevails at the temperature of the melt, a part of the added lithium thus escapes into the vapor chamber. However, the resultant lithium loss relative to the purification reactlon can be compensated by the repeated addition of small quantities of lithium salt. Further, in some cases, the plasma furnace can be accommodated in an autoclave while the lithium treatment is performed under a high gas pressure as described above in Example I. Also, the melt can be rabbled during treatment by means of a multiphase, low-frequency alternating current.

Where metallic calcium, strontium or barium are additionally added to the electron beam or plasma furnace, the nitrogen content of the melt is substantlally reduced, the procedure being accompanied by the formation of the corresponding nitrides.

The lithium, calcium, and/ or strontium and/ or barium treatment of the aforementioned alloy enables waste thereof to be processed by the method described heretofore.

so that the casting teemed from the melt by conventional methods, correspond in their mechanical properties and f gas content to those produced from freshly melted alloys.

Insofar as lithium is introduced into a metal alloy having oxygen afiinity after skull melting of the alloy, reference is made to the following example:

EXAMPLE III A titanium alloy to which are alloyed 8% aluminum and 1% each of molybdenum and vanadium, is melted by the skull melting method in an argon or helium atmosphere or in an atmosphere comprising a mixture of both gases at a pressure of approximately 0.5 atm. A jacketed copper crucible, cooled with Water, functions as treatment vessel in this case.

In order to obtain better utilization of the treatment vessel capacity, the vessel can be lined with a thin ceramic film in the manner characteristic of the skull melting method. Since a large number of ceramic compounds of lithium are attacked, the lining selected in this case consists of crystalline lime or thorium oxide applied in known manner as above or, for example, by a plasma spray method.

Treatment of the melt with lithium and/ or calcium After the material is melted, the pressure of the protective atmosphere in the furnace is increased to above 6 atm. because the melt has a temperature of approximately 1650 C. The pressure of the protective atmosphere is therefore higher than the vapor pressure of the lithium and of the calcium at this temperature.

Lithium and calcium is then added in either of two different manners. In one case, pulverized or vaporized lithium and/or calcium is blown into the melt by means of blowing apparatus using a stream of rare gas as an entrainment medium. In the other case, the lithium and/ or calcium is immersed in the melt by means of commercially available cartridges filled with lithium and/or calcium powder.

Alternatively, if the melt is to be simultaneously treated with strontium and/ or barium in order to remove nitrogen, a mixture of lithium/calcium/strontium or a mixture of all four metals is blown into the melt instead of the lithium.

With immersion, suitable bodies containing strontium and/or barium powder can also be simultaneously immersed into the melt.

The quantity of lithium and/or calcium introduced into the melt amounts approximately to 0.1 to 0.2% of the alloy weight. The quantity of strontium or barium thus introduced is approximately the same.

The melt can then be treated so that the gas pressure in the furnace is reduced between individual treatments and re-established before the next batch of material is added. This method of operation results in boiling of the melt by virtue of which the molecular gases are driven out and thus ensures an improved flushing of the melt, so that intensive mixing of the lithium and of the calcium with the melt and better driving out of the gases is obtained.

The advantage of using immersing bodies to introduce the material relative to blowing in of the material is that it requires no separate apparatus.

During treatment, the melt is stirred by a rotating electric field generated in known manner as above by means of a low-frequency, multiphase alternating current of, for example, 20-30 c./s.

A further manner for introducing the lithium into the alloy melt consists in the connection of the treatment vessel and therefore of the melt to the electrode of a plasma arc. Small quantities of solid lithium chloride, or some other lithium compound, or calcium, are then added either once or several times. The lithium ions migrating into the melt react with the impurities thereof, in particular with oxygen. In this way, oxygen and to some extent sulphur and nitrogen are removed to a sufficient extent from the melt during a prolonged treatment. The

high vapor pressure particularly of lithium, which prevails at the temperature of the melt causes part of the introduced lithium and, to a smaller extent, of the calcium, to escape into the vapor chamber the resultant loss of lithium and calcium for the purpose of purification reactions may be compensated by the repeated addition of small quantities of lithium salts and calcium to the melt.

With thismanner of adding the material, the pressure in the furnace need not always be raised to the aforementioned high value; nevertheless, it is also possible to operate at high pressure. Furthermore, as already described heretofore, the melt may be stirred during treatment by means of multiphase, low-frequency alternating current.

Metallic strontium or barium can be additionally supplied to the plasma furnace in order to considerably reduce the nitrogen content of the melt while appropriate nitrides are formed.

Alternatively, an electron beam furnace can be used instead of the plasma furnace.

Furthermore, the contaminated alloy to be treated can also be present in the form of total loss electrodes of an electric arc. The alloy melt will then drip from the electrodes through a film of lithium salt and/or calcium, which covers the surface of the purified alloy in the treatment vessel. On passing through the aforementioned film, the impurities are bonded by the lithium and the calcium.

In the next manner for adding the material, it is possible for the treatment to be performed at the pressure applied for melting the titanium alloy. In this method, in which lithium is added by means of a ceramic or metallic carrier body, a ceramic carrier body is used.

Preparation and charging of the ceramic carrier body The carrier body is prepared from a calcium oxide sponge body in a manner similar to the aluminum oxide body as described in my copending patent application Ser. No. 675,762, filed Oct. 17, 1967 and entitled Method for Alloying Highly Reactive Alloying Constituents.

A porous body of a metal having a high specific gravity, for example, nickel, molybdenum, cobalt or tungsten, can also be used as the carrier body. As already mentioned, a metallic carrier body is selected depending upon whether its parent substance is permissible or even desirable for the melt concerned, in terms of its alloying characteristics. Furthermore, its specific gravity should exceed that of the melt so that the body can descend into the melt without any separate aids.

Where the carrier body is made of tungsten or nickel, such is prepared in a similar manner to that described in the above copending patent application.

Treatment of the melt In order to introduce into the melt the desired quantity of lithium and/or calcium, for example, approximately 0.1 to 0.2% of the alloy weight, one or more of the carrier bodies prepared and charged in the manner heretofore described are introduced by means of the cooled copper pipes or by virtue of their weight into the melt, lithium and calcium detaching from the pores of the carrier body owing to the high temperature and therefore being able to migrate into the alloy melt in the manner as described in the above copending patent application.

Similarly, strontium and/ or barium can be added to the melt by means of carrier bodies to remove nitrogen.

Heretofore, in order to satisfy the purity and quality requirements relative to the aforemetnioned steels and alloys, it was necessary to employ vacuum metallurgy techniques. Since the use of vacuum involves considerable difliculties in the sealing of the vacuum vessel and in the protection of the vacuum plant against the ingress of solid impurities condensed from the vapor phase, operation in vacuum is awkward and time consuming and requires a high technical and economic effort.

The present invention, by means of which effective lithium treatment is made possible, provides a means for avoiding the difficulties due to operation in vacuum without the need for tolerating a loss of quality. The lithium treatment is therefore suitable to replace, at least substantially, vacuum metallurgy techniques in the preparation and processing of high grade steel and alloys.

The invention thus provides a method of effectively and economically utilizing lithium treatment of high melting alloys and steel in practice. Further, with the lining utilized in the method, undesired side reactions of the lining with the lithium can be substantially or completely avoided.

It is noted that the introduction of the lithium into the alloy or steel melt contained in the treatmen vessel can be performed in other ways than those set forth above. Likewise, other known methods can be partially used for the addition of the other materials.

As already briefly mentioned, the purity of the alloy or steel melt is further improved by the lithium treatment. Moreover, this treatment also improves the mold filling capacity in teeming, particularly in castings where the spout usually used in teeming is endangered. These additional effects are due to the fact that lithium produces Very pure metal melts. Furthermore, as lithium oxide is the only metal oxide which is liquid at the prevailing temperatures, segregation from the alloy or steel melt is facilitated. Finally, on teeming the treated alloy or steel melt of the invention into a mold, any lithium metal still present will first be oxidized so that a liquid lithium oxide film coats the surface. Owing to this circumstance, teeming of lithium-treated steel into a mold is accompanied by liquid-on-solid sliding, thus improving the mold filling capacity.

\Vhat is claimed is:

1. A method of treating high melting alloys and steel comprising the steps of forming a melt of the alloy or steel in a vessel having a lining of a material more chemically stable than lithium oxide at the temperature of said melt and selected from the group consisting of crystalline lime and thorium oxide and subsequently introducing lithium into said melt.

2. A method as set forth in claim 1 wherein said step of introducing lithium is performed while said melt is maintained in a protective atmosphere having a predetermined pressure exceeding the vapor pressure of the introduced lithium at the temperature of said melt.

3. A method as set forth in claim 2 wherein the lithium is immersed into said melt within porous carrier bodies containing metallic lithium.

4. A method as set forth in claim 2 wherein an inert entrainment gas stream is blown into said melt to carry the lithium in the form of a powder into said melt.

5. A method as set forth in claim 1 wherein said vessel forms a melting vessel of a rare gas plasma furnace and wherein the lithium is added in a solid form.

6. A method as set forth in claim 1 wherein said vessel forms a melting vessel of an electron beam furnace and wherein the lithium is added in a solid salt form.

7. A method as set forth in claim 2 which further includes the step of alternately reducing and re-establishing said predetermined pressure of said protective atmosphere while subsequently adding lithium after re-establishment of said predetermined pressure to increase the purity of said melt.

8. A method as set forth in claim 1 wherein the lithium is blown into said melt in a vaporized form.

9. A method as set forth in claim 1 which further comprises the step of rabbling said melt under a low frequency multiphase alternating current subsequent to said step of introducing the lithium.

10. A method as set forth in claim 1 which further comprises the step of introducing a metal from the group consisting of calcium, barium and strontium into said melt.

11. A method as set forth in claim 10 wherein said metal is introduced simultaneously with the lithium.

12. A method as set forth in claim 1 wherein the lithium is introduced into said melt when said melt is at a temperature of at least approximately 1400 C.

13. A method as set forth in claim wherein the lithium is added in a salt form.

14. A method of treating metal alloys having affinity to oxygen comprising the steps of skull melting the metal alloy under a protective atmosphere in a crucible lined with a thin film of material selected from the group consisting of crystalline lime and thorium oxide to form a melt of the metal alloy within a skull of the metal alloy, and subsequently introducing a metal from the group consisting of lithium and calcium into said melt.

15. A method as set forth in claim 14 which further comprises the step of introducing a metal from the group consisting of strontium and barium into said melt to remove nitrogen from the metal alloy in said melt.

16. A method as set forth in claim 14 wherein said step of introducing metal into said melt is performed simultaneously with the step of placing said melt under a pressure in excess of the vapor pressure of the introduced metal at the temperature of said melt.

17. A method as set forth in claim 16 wherein the metal is introduced in a number of successive steps and said pressure on said melt is alternately reduced between successive steps.

18. A method as set forth in claim 14 wherein the metal is introduced by at least one immersion into said melt.

19. A method as set forth in claim 14 wherein the metal is introduced by blowing of a powdered form of the metal into said melt within an inert entrainment gas stream.

20. A method as set forth in claim 14 wherein the metal is introduced by blowing of a vaporized form of the metal into said melt.

21. A method as set forth in claim 14 wherein the metal is introduced in a solid form and wherein said melt is connected to an electrode of a plasma are.

22. A method as set forth in claim 14 wherein the metal is introduced from a total loss electrode in an arc furnace and wherein a salt of said metal is formed on the surface of said melt.

23. A method as set forth in claim 14 wherein said step of skull melting is performed in a water cooled copper crucible.

References Cited UNITED STATES PATENTS 2,181,092 11/1939 Ness 95 2,181,096 11/1939 Ness 7545 2,181,097 11/1939 Ness 7595 3,116,998 1/1964 Pagonis 7545 3,321,304 5/1967 Snow 75--53 3,342,250 9/1967 Treppschuh et al. 7549 3,367,646 2/1968 Robertson et al. 75129 3,393,996 7/1968 Robertson et al. 7553 L. DEWAYNE RUTLEDGE, Primary Examiner T. R. FRYE, Assistant Examiner US. Cl. X.R. 7558, 95, 129