US3257197A - Method for adding nitrogen to molten metals - Google Patents

Description

J1me 1966 F. s. DEATH ETAL METHOD FOR ADDING NITROGEN TO MOLTEN METALS 7 Sheets-Sheet 1 Filed April 17, 1963 THE ABSORPTION AND DESORPTION OF NITROGEN FROM INGOT IRON(.O04%C,.07%O) AND O.4|%C STEEL AT ABOUT IGOO'C mi N m 4 mg m m W R col B mm w w m w w M w M 7 ||O M w 7 cm D\ E s flan N N w 47 u- P O 0 N T 00 T H o N 0. AEN B E U 0 A m M Y BM CTN M SD dm 0 r m- ME mm A PM 0- .l A UTF T w o /A By 3 A I" A o P A O l w. m m m m. M. m. D A x H; zo:. m. .ZwuZou zuoom. z

INVENTORS FRANK S. DEATH DAVID A. HAID ATTORNEY J1me 1966 F. s. DEATH ETAL METHOD FOR ADDING NITROGEN TO MOLTEN METALS 5 Filed April 17, 1965 '7 Sheets-Sheet 2 THE INFLUENCE OF A SLAG COVER ON THE ADSORPTION AND DESORPTION OF NITROGEN IN SILICON-IRONS CONTAINING ABOUT l% SILICON LON fislucoN SLAG COVERED 0.92% SILICON NO SLAG ,EUILIBRIUM loflslu IIO w 3 0 m o TIME-MINUTES INVENTORS FRANK S. DEATH DAVID A. HAID BY 9 9 M ATTORNEY June 21, 1966 F. s. DEATH ETAL 3,257,197

METHOD FOR ADDING NITROGEN TO MOLTEN METALS Filed April 17, 1963 '7 Sheets-Sheet 5 THE EFFECT OF SULFUR ADDITIONS ON THE DISSOLVED NITROGEN CONTENT OF I% Si Fe WITH I6.6% NITROGEN IN THE TORCH GAS.

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METHOD FOR ADDING NITROGEN TO MOLTEIN METALS Filed April 17, 1965 7 SheetS -Sheet 4 THE EFFECT OF DISSOLVED OXYGEN ON THE MAXIMUM NITROGEN CONCENTRATION. 40 LB CHARGES,

l6.6% N2 IN TORCH GAS, APPROX. I600'C .IOO

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% O IN BATH INVENTORS FRANK S. DEATH DAVID A. HAID mjfww A T TORNEY June 21, 1966 F. s. DEATH ETAL 3,257,197

METHOD FOR ADDING NITROGEN TO MOLTEN METALS Filed April 17, 1963 '7 Sheets-Sheet 5 THE EFFECT OF CHRDMIUM ON THE MAXIMUM NITROGEN CONCENTRATION. 40 LB. CHARGE,|6.6% N IN TORCH GAS ONE ATMOSPHERE SOLUBILITY- .200

/. EQUlLlBRIUM SOLUBILITY I65 N MAX.% N lN BATH o 4 8 l2 I6 20 WT. CHROMIUM INVENTORS FRANK S. DEATH DAVID A. HAID A T TORNE Y June 21, 1966 F. s. DEATH ETAL 3,257,197

METHOD FOR ADDING NITROGEN TO MOLTEN METALS Filed April 17, 1963 7 SheetS- -Sheet 6 I w .200 /EQUILIBRIUM E O N Z 0 x .100 3 VOLUME N IN TORCH GAS THE VARIATION IN THE MAXIMUM NITROGEN CONTENT WITH CHANGES IN TORCH GAS COMPOSITION FOR A TUNGSTEN-CHROMIUM-MANGANESE IRON BASE ALLOY.

INVENTORS FRANK S. EATH DAVID A. HAID A TTORIIIEY June 21, 1966 F. s. DEATH ETAL METHOD FOR ADDING NITROGEN TO MOLTEN METALS 7 Sheets-Sheet '7 Filed April 17, 1963 FRANK S. DEATH BY DAVID A. RAID ATTORNEY United States Patent 3,257,197 METHOD FOR ADDING NITROGEN T0 MOLTEN METALS Frank 5. Death, Tonawanda, and David A. Haid, Kenmore, N.Y., assignors to Union Carbide Corporation, a corporation of New York Filed Apr. 17, 1963, Ser. No. 273,676 Claims. (Cl. 75-12) This application is a continuation-in-part of our copending application Serial No. 86,425, filed February 1, 1961, now abandoned, -for Method for Adding Nitrogen to Molten Metals.

This invention relates to the use of nitrogen in metal alloy production and more particularly to a method for adding preselected amounts of nitrogen to molten metallic baths wherein the solubility of nitrogen in such metallic bath is unexpectedly increased over that predicted from gas-metal equilibrium laws such as Sieverts Law.

At present, nitrogen has two main applications in steel production. In one such application nitrogen is used to produce a rimming characteristic in fully-killed steels. In the second application nitrogen is used as an alloying element.

In steel making when molten steel cools to the temperature range in which it begins to solidify, the solubility of the gases dissolved in the metal usually decreases, and the proportions of gases that can no longer be held in solution are expelled from the metal. The chemical equilibrium between carbon and oxygen in the steel is changed as the temperature .falls and these two elements react to form carbon monoxide which is evolved from the metal during cooling as the system attempts to establish a new equilibrium. In fully-killed steel, that is steel to which deoxidizers have been added to remove the oxygen, no gas is evolved from the molten metal on solidification since there is essentially no oxygen to react with the carbon. When such fully-killed steels are poured, shrinkage occurs on solidification causing a slightly concave top and also causing a cavity below the top called the pipe. In order to prevent the formation of pipe a certain amount of rimming action that is, entrapment of gas bubbles within the steel during solidification, is'desirable, nitrogen is sometimes used to promote this rimming action. Rimmed steel, when properly made, has a minimum of pipe and a good surface. When the metal in the ingot mold begins to solidify, there is a brisk evolution of gas resulting in an outer ingot skin of relatively pure metal. For many applications, particularly when the surface of the product is important, this steel is used to a considerable extent. When nitrogen is used to produce the rimming characteristics careful control must be exercised so as to avoid excessive rimming 'which would cause undesirable blow holes.

As far as nitrogen as an alloying element in steel is concerned, it is generally considered that nitrogen contributes to deleterious properties, such as embrittlement, in the final product. However, small, controlled amounts of nitrogen are desirable in some grades of steel, such as the American Iron and Steel Institute 200 Series Steel, to enhance particular properties. This series is characterized as consisting of low nickel-high manganese stainless steels. For example, Type 201 specification is as follows: 0.15% max. C, 0.25 max. N, 5.5%-7.5% Mn, 1.0% max. Si, 16%l8% Cr, 3.5%5.5% Ni, 0.30% S, and 0.60% P. Another typical specification of steels in this series, the Type 202 is 0.15% max. C, 0.25% max. N, 7.5%10.0% Mn, 1.0% max. Si, 17%-19% Cr, 4%6 Ni, 0.30% S, and 0.60% P. In stainless steels of this type the nitrogen present in the steel promotes austenite stability and serves as a partial substitute for nickel as long as there is some manganese present.

Finer grain size is also promoted. There is a particularly useful end effect of nitrogen in the case of high chromium (about 15% min.)-low carbon (about 1.2% max.) stainless steels that are completely ferritic. These alloys are single phase except for carbides, hence no grain refinement by phase transformation is possible. A large grain size, once formed by improper heating procedure, will be retained on cooling to room temperature. Only be cold work and recrystallization can the grain size be reduced. In addition, the fact that the high chromium steels are inherently notch-sensitive further aggravates the effects of grain coarsening. :Nitrogen additions have been employed to obtain a finer grain size in these steels.

However, in all of the above instances the danger of blow-hole formation upon solidification persists if excessive amounts of nitrogen are employed. Moreover, since the solubility of nitrogen in the molten steel bath is usually higher than in the solidified metal, molten bath saturation with nitrogen cannot be used to achieve the desired beneficial results of nitrogen addition. Clearly, there exists a need for a nitrogen addition method whereby effective control over the amount of nitrogen added to the molten stainless steel bath is achieved.

Accordingly, it is an object of this invention to provide a process for adding nitrogen to a molten metallic bath in excess of that heretofore predicted by equilibrium laws.

A still further object is to provide an electric arc furnace process wherein hydrogen gas is used to control the adsorption of nitrogen in the molten bath metal being treated.

Another object is to provide a process for producing a metal alloy having a preselected amount of nitrogen as an alloying element.

A further object is to provide an electric arc process 'for introducing nitrogen into a molten metal bath.

Yet another object is to provide a method for introducing nitrogen to a molten metallic bath wherein a directionally stabilized collimated electric arc is employed.

Another object is to provide a process for controlled introduction of nitrogen in a metal bath for the promotion of rimming or as an alloying element.

Still another object is to provide an electric arc furnace process for controlling the amount of nitrogen in the arc plasma.

Other objects will be apparent from the remaining disclosure and drawings in which:

FIGURES 1-6 are graphic illustrations of the effect of various parameters on nitrogen concentration; and

FIGURE 7 is a cross-sectional view of a typical furnace in which the process of the invention may be carried out.

In its broadest aspects, the invention provides a method for adding preselected amounts of nitrogen to molten metallic bath which comprises providing a charge of metal to a suitable vessel, establishing an electric arc in said vessel, heating such charge until the molten metallic bath is formed, introducing a mixture of an inert gas and a preselected amount of a nitrogen bearing material at least partially into said arc, impinging the nitrogen containing are plasma onto a localized zone of a molten metallic bath, this zone acting as an absorbing zone while the remainder of the metallic bath surface acts as a desorption zone, adjusting the ratio of nitrogen bearing material to inert gas to control the absorption-desorption kinetics so as to provide a dynamic equilibrium condition, the maximum nitrogen content at such equilibrium condition being above that predicted by Sieverts Law which is wherein C is the nitrogen concentration in weight percent; P is the partial pressure of nitrogen above the bath in atmospheres; and K is the equilibrium solubility at the bath temperature in weight percent under 1 atmosphere of nitrogen for the particular alloy being treated.

This constant is either tabulated for various particular melt compositions or can be calculated by anyone skilled in the art to a satisfactory degree of accuracy by summation of the effects of the individual alloying elements in a melt on the activity coefiicient of dissolved nitrogen in relation to the presence of various elements. This constant is treated in an article entitled Predicting the Solubility of Nitrogen in Molten Steels by F. C. Langenberg, trans. AIME, vol. 206, p. 1099 (1956).

More specifically the invention provides a process for adding preselected amounts of nitrogen to a molten metallic bath which comprises providing a charge of metal to a suitable vessel, directing a high pressure are to the charge of metal, flowing a mixture of an inert gas and a preselected amount of a nitrogen bearing gas into the high pressure arc, collimating and stabilizing such arc and gas stream by surrounding such stream with a cold surface, impinging the nitrogen containing arc plasma onto a localized zone of a molten metallic bath, the zone acting as an absorbing zone while the remainder of the metallic bath surface acts as a desorption zone, controlling the absorption-desorption kinetics so as to provide a dynamic equilibrium condition, the maximum nitrogen content at such equilibrium condition being above that predicted by Sieverts Law which is wherein C is the nitrogen concentration in weight percent; P is the partial pressure of nitrogen above the bath in atmospheres; and K is the equilibrium solubility at the bath temperature in weight percent under 1 atmosphere of nitrogen for the particular alloy being treated.

The invention is predicated on the discovery that when a nitrogen containing material is added to an arc plasma comprising an inert arc gas and the resulting column is impinged on a molten metal bath the extremely reactive nitrogen is absorbed very rapidly in the area of impingement. Because of this very strong absorption of nitrogen at. the point of plasma impingement the bath becomes supersaturated with respect to bulk metal temperature and nitrogen partial pressure. At this condition, the bath surface not in contact with the arc plasma acts solely as a desorption surface. Thus, the molten bath surface is divided into two zones, the absorption zone in the area of arc plasma impingement and the desorption zone in the area away from plasma impingement.

The invention differs from the prior art in that a dynamic equilibrium supersaturation of molten metal with nitrogen is achieved, and an effect on the kinetics of the dynamic equilibrium is possible by adding preselected amounts of nitrogen bearing material, preferably a nitrogen gas, to an inert gas and using such mixture as an arc gas which forms the arc plasma.

The dynamic equilibrium nitrogen content of the bath is defined as that concentration at which the absorption rate equals the desorption rate. It has been found in all cases studied that this maximum nitrogen concentration, observed at the dynamic equilibrium condition is supersaturated with respect to the nitrogen partial pressure because of the very high rate of absorption at the plasma impingement region.

Discovery of this phenomena presents for the first time an effective method of nitrogen compositional control. This can be accomplished simply by adjusting the arc gas composition so that according to a previously calibrated furnace charge, the desired level of nitrogen concentration will be reached. By use of nitrogen as a component in the torch gas during melt-down, it has been observed that the desired nitrogen concentration is achieved by the time all the charge is melted and brought up to pouring temperature.

In this disclosure, the term inert gas means any gas such as helium, neon, argon, krypton, xenon, and CO. The inert gas acts to adjust the partial pressure of nitrogen by diluting such nitrogen and controls the nitrogen content achieved.

It is felt that this method of nitrogen introduction as an easily controllable alloy element will be useful in all arc furnaces. However, it is also felt that the actual nitrogen solution obtained will change not only with alloy composition but with the actual arc furnace geometry. Changes in bath size, arc length, arc-anode size, will change the absorption and desorption regions. Because the maximum nitrogen obtained is a balance of the absorption-desorption reactions, changes in geometry which have affected the surfaces at which these reactions occur will probably change the actual nitrogen solution for a given nitrogen torch-gas composition. As such, it will be necessary to precalibrate each furnace geometry with a given alloy composition.

Thus, a furnace similar to the one shown in FIGURE 7 was calibrated for a tungsten-chromium-manganese iron base alloy by varying the volume percent of nitrogen in the torch gas and noting the weight percent of nitrogen in the melt. The weight percent of N in the melt was then plotted against the volume percent of nitrogen in the torch gas and the curve of FIGURE 6 obtained. For the particular furnace and composition in question, it is obvious, from FIGURE 6, the weight percent of nitrogen in the melt can be varied and controlled by the ratio of N to argon in the torch gas. For example, referring to FIGURE 6, if it is desired to introduce 0.2 weight percent N into a tungsten-chromium-manganese iron base alloy, the torch gas ratio should be about 20 volume percent nitrogen, the balance argon.

In practicing the invention, an arc torch such as that described in more detail in US. application Serial No. 50,194, filed August 17, 1960, now Patent No. 3,147,329, is employed. Briefly, such torches are characterized by a non-consumable nozzle wall which laterally restricts the arc gases and directs a hot gas effiuent. In such torches a main electrode usually made of tungsten is gas shielded by a gas inert to such electrode and another gas, to be treated by the arc, is injected into the arc plasma. Both transferred and non-transferred are torches may be employed. However, transferred arc torches are preferred. The are torch functions as an injection lance for the nitrogen and also as the heat source for meltdown. However, in some cases, it may be desirable that other heating means be used for melt-down of the charge. In these cases, the arc torch functions mainly to produce the desirable unexpected nitrogen absorption reaction.

In actual operation after the charge is melted down in an arc furnace, nitrogen gas, in controlled amounts, is introduced into the arc column. The nitrogen-containing arc plasma is then directed to the molten metal surface and the nitrogen is absorbed in the area of arc plasma impingement. The rest of the bath surface acts as a desorption surface. At dynamic equilibrium, the maximum nitrogen concentration with respect to the nitrogen partial pressure will be supersaturated.

While it is preferred that the nitrogen-bearing material be injected into the arc plasma, nitrogen gas can be simply used as the furnace atmosphere. In the latter case, the pumping characteristics of the arc are sufiicient to circulate the nitrogen through the arc plasma. Further, the process of the invention may be practiced with a conventional open arc but in this case the nitrogen bearing material must be passed directly through the arc.

It has been discovered that the process of the invention is most effective when the hydrogen content of the arc plasma and furnace atmosphere is kept low, at least below 10%, preferably below 1%. If hydrogen in amounts equal to or greater than 10% is present in the arc plasma or furnace atmosphere, it has been found that nitrogen solubilities are greatly decreased. In fact with 10% hydrogen. added to the argon-nitrogen plasma the nitrogen solubility is reduced to equilibrium levels, It is believed the presence of suflicient hydrogen causes changes in the physical characteristics of the plasma, mainly the thermal conductivity, so that molecular nitrogen rather than dissociated or ionized nitrogen strikes the metal surface. Absorptions then occur only to equilibrium levels. This phenomenon has also been observed with some heliumni-trogen mixture but the effect of helium is not as great as hydrogen.

The effect of hydrogen on the solubility of nitrogen can be beneficial in instants when nitrogen values are desirably low. Hydrogen can be added to the arc gas to control the amount of nitrogen absorbed by the molten bath. The hydrogen can be removed by a final treatment with argon.

Furthermore, it has been found that the composition of the molten bath can affect markedly the maximum nitrogen concentration. Oxygen and sulfur in steels, although they do not appreciably affect the equilibrium nitrogen solubility, strongly. affect the kinetics of absorption and desorption and therefore the maximum nitrogen concentration in the range between 0 and 0.25% 0 and between 0 and 0.35% S. Higher oxygen and sulfur concentrations within these ranges result in much higher nitrogen supersaturation.

The net absorption and desorption rates, and hence the maximum nitrogen concentration, are effected by the amount of nitrogen in the torch gas or furnace atmosphere. Increased percentages of nitrogen above the bath will normally result in increased amounts dissolved in the bath, the amount dissolved always being above the equilibrium solubility predicted by the nitrogen partial pressure and bulk metal temperature.

An example of the unexpected results obtained with the invention method is illustrated by the following discussion of the drawings.

The nitrogen gas-molten steel interaction was studied in a 40 lb. charge arc-torch furnace. The maximum nitrogen concentration was measured by sampling the bath through a sampling port. FIGURE 1 shows the normal nitrogen saturation at about 1600 C. under 16.6% N remainder argon for iron and for 0.41% carbon steel. A second set of curves illustrates the remarkable increase in saturation when 16.6% nitrogen the remainder argon is used as the arc torch gas at 100 c.f.h. flow through the torch. The equilibrium concentration for 16.6% nitrogen for iron is calculated from Sieverts Law C=K /P to be 0.018% where .166 atmospheres is the partial pressure P The observed maximum concentration is shown to be 086%, Curve A. The calculated equilibrium concentration for 0.41% carbon, 0.01% oxygen steel under 16.6% nitrogen is 013%. The observed maximum is 0.029%, Curve B. These data indicate that supersaturation of nitrogen is occurring and that maximum nitrogen observed is truly a dynamic equilibrium characteristic of the absorption and desorption kinetic process.

It has also been observed that the desorption kinetics can be upset by an impedance such as a slag, giving a different maximum nitrogen concentration for otherwise identical conditions. FIGURE 2'illustrates the increase in maximum nitrogen concentration due to the presence of a slag barrier for about 1.00% silicon iron. Again in this case, appreciable supersaturation of nitrogen over the calculated equilibrium of 016% has occurred. With no slag the maximum is 032%; with a slag barrier the maximum is .055

FIGURE 3 shows the effect of sulfur on the maximum nitrogen observed, it is seen that increasing sulfur to 0.28% drastically raises the nitrogen concentration even though little equilibrium solubility change .001%) has occurred. The similar effect of oxygen content on the maximum nitrogen is shown in FIGURE 4.

FIGURE 5 shows the effect of changing the nitrogen solubility by alloying, in this case chromium in iron on the maximum nitrogen concentration. It is obvious that changing solubility, changes the maximum nitrogen observed. It is felt that this is accomplished mainly by changing the desorption kinetics. Again, it is seen that the maximum nitrogen content obtained is above that in equilibrium with the nitrogen partial pressure (16.6% N

FIGURE 6 shows the effect of changing the nitrogen partial pressure in the torch gas on the amount of nitrogen dissolved in the molten metal. The continuity of the curve indicates that a high degree of control over the amount of nitrogen dissolved in the metal is possible even though a high degree of supersaturation of nitrogen relative to the actual partial pressure exists. For example, if an alloy composition containing .200 weight percent N is desired, 20 vol. percent N the remainder argon is provided to the torch. The resulting melt will have the desired composition.

All the data included herein was obtained with the furnace illustrated in FIGURE 7. Such furnace comprises a hearth 1 having a depth E of 3 inches and a width D of 9 inches. Hearth 1 is positioned in an outer casing 3 which has a width F of 16 inches. The furnace is provided with a cover 5 which has a height B of 2 /2 inches making the overall furnace height A 13 /2 inches. A torch 7 is mounted in the cover 5 such that the torch nozzle 9 is about 3" to 4" from the metal bath surface. A bottom electrode 10 is positioned centrally in the bottom of the casing 3 and has a diameter of inch. The furnace is provided with water cooling coils 11.

What is claimed is:

1. A method for adding preselected amounts of nitrogen to a molten metallic bath which comprises establishing an electric arc in a vessel containing said molten metallic bath; introducing a mixture of an inert gas and a preselected amount of a nitrogen bearing material at least partially into said arc, impinging the nitrogen containing arc plasma onto a localized zone of the molten metallic bath, the zone acting as an absorbing zone while the remainder of the metallic bath surface acts as a desorption zone, adjusting the ratio of nitrogen bearing material to inert gas to control the absorption-desorption kinetics so as to provide a dynamic equilibrium condition, the' maximum nitrogen content at such equilibrium condition being above that predicated by Sieverts Law.

2. Method according to claim 1 wherein the nitrogen bearing material is nitrogen gas.

3. Method according to claim 1 wherein the inert gas is argon, xenon, neon, krypton, helium and CO.

4. Method according to claim 1 wherein the nitrogen bearing material is nitrogen gas and the inert gas is argon.

5. A method for adding preselected amounts of nitrogen to molten metallic bath which comprises providing a charge of metal to a suitable vessel, establishing an electric arc in said vessel, heating such charge until the molten metallic bath is formed, introducing a mixture of an inert gas and a preselected amount of a nitrogen bearing material at least partially into said arc, impinging the nitrogen containing arc plasma onto a localized zone of a molten metallic bath, the zone acting as an absorbing zone while the remainder of the metallic bath surface acts as a desorption zone adjusting the ratio of nitrogen bearing material to inert gas to control the absorption-desorption kinetics so as to provide a dynamic equilibrium condition, the maximum nitrogen content at such equilibrium condition being above that predicated by Sieverts Law.

6. A method for adding preselected amount of nitrogen to a molten metallic bath which comprises providing a charge of metal to a suitable vessel, energizing a high pressure are above said charge, heating such metal charge until a molten metallic bath is formed, introducing a mixture of an inert gas and a preselected amount of nitrogen bearing material at least partially into said arc, collimating and stabilizing such are by surrounding such are with a cold surface, impinging such collimated and stabilized nitrogen containing arc plasma onto a localized zone of said molten metallic bath, said zone acting as an absorption zone while the remainder of said surface of said metallic bath acts as a desorption surface, adjusting the ratio of said nitrogen bearing material to said inert gas to control the absorption-desorption kinetics so as to provide a dynamic equilibrium condition, the maximum nitrogen concentration at such equilibrium condition being above that normally predicted by Sieverts Law.

7. A method for adding preselected amounts of nitrogen to a molten metallic bath which comprises establishing an electric arc in a vessel containing said molten metallic bath, introducing a mixture of an inert gas and a preselected amount of a nitrogen bearing material at least partially into said arc, impinging the nitrogen containing arc plasma onto a localized zone of the molten metallic bath, the zone acting as an absorbing zone while the remainder of the-metallic bath surface acts as a desorption zone, adjusting the ratio of nitrogen bearing material to inert gas to control the absorption-desorption kinetics so as to provide a dynamic equilibrium condition,

the maximum nitrogen content at such equilibrium condition being above that predicated by Sieverts Law, and then introducing a preselected amount of hydrogen gas but not more than 10% into said furnace atmosphere to further control N absorbed in the bath.

8. Method according to claim 6 wherein the nitrogen bearing material is nitrogen gas.

9. Method according to claim 6 wherein the inert gas is argon, Xenon, neon, kryton, helium and CO.

10. Method according to claim 6 wherein the nitrogen bearing material is nitrogen gas and the inert gas is argon.

References Cited by the Examiner UNITED STATES PATENTS 2,069,205 2/1937 Arness 7559 2,909,422 10/1959 Schwabe 75-l0 FOREIGN PATENTS 797,339 7/ 1958 Great Britain. 823,428 11/ 1959 Great Britain.

DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

Claims (1)

1. A METHOD FOR ADDING PRESELECTED AMOUNTS OF NITROGEN TO A MOLTEN METALLIC BATH WHICH COMPRISES ESTABLISHING AN ELECTRIC ARC IN A VESSEL CONTAINING SAID MOLTEN METALLIC BATH; INTRODUCING A MXTURE OF AN INERT GAS AND A PRESELECTED AMOUNT OF A NITROGEN BEARING MATERIAL AT LEAST PARTIALLY INTO SAID ARC, IMPINIGING THE NITROGEN CONTAINING ARC PLASMA ONTO A LOCALIZED ZONE OF THE MOLTEN METALLIC BATH, THE ZONE ACTING AS AN ABSORBING ZONE WHILE THE REMAINDER OF THE METALLIC BATH SURFACE ACTS AS A DESORPTION ZONE, ADJUSTING THE RATIO OF NITROGEN BEARING MATERIAL TO INERT GAS TO CONTROL THE ABSORBTION-DESORPTION KINETICS SO AS TO PROVIDE A DYNAMIC EQUILIBRIUM CONDITION, THE MAXIMUM NITROGEN CONTENT AT SUCH EQUILIBRIUM CONDITION BEING ABOVE THAT PREDICATED BY SIEVERT''S LAW.

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