US 2271242 A
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Jan. 27, 1942. c, L. ALTEN'BURGER 2,271,242
METHOD OF MAKING NONAGING STEEL Filed May 23, 1940 Sheets-Sheet 1 Jan. 27, 1942- c. L. ALTENBURGER 2,271,242
METHOD OF MAKING NONAGING STEEL Filed May 25, 1940 :s Sheets- Sheet 2 .E- I INVENTOR 6/4/2161. .fllieniur yan ATI'ORNEY Jan. 27, 1942. c. L. ALTENBURGERT 2,271,242
METHOD MAKING NONAGING' STEEL Filed May 25, 1940 3 Sheets Sheet 5 INVENTOR JreycelJVliaziwan BY 7LMMqfluz ATTORNEYS.
' Patented Jan. 27,1942 I METHOD MAKING NONAGING STEEL Clarence L. Altenbur'ger, Dearborn, Mich., as-
slgnor to Great Lakes Steel Corporation,
- Ecorse, Mich, a corporation of Delaware Application May 23, 1940, Serial No. 336,705 7 (Cl. 148-16) v 3 Claims.
This invention relates to iron and steel and to a process of annealing steel and iron in atmospheres of controlled analysis whereby such steels and irons as are previously susceptible to aging after strain may be rendered partially or completely free of the aforesaid susceptibility, de-
-a process of annealing iron or steel by the practice of which the nitrogen content of the steel will be reduced sufficiently to render the iron or steel substantially non-aging; the provision of a process of annealing iron or steel by means of which irons or steels, normally having a nitrogen content sufliciently high torender them susceptible to aging will have their nitrogen content so temperature or other temperatures selected for omervation, after having been subjected to cold work. In the specific case of sheet or strip as used for deep drawing such articles as automobile fenders, quarter panels, hood tops, etc., such sheet or strip is given a pass in a temper or skin mill after annealing. This is done primarily to produce flatness and to remove the elongation occurring at the yield point in annealed material which would result. in unsightly stretcher strains or Liider lines in portions of the finished stamping which have been the more lightly stretched by the drawing operation. After such skin or temper rolling, the hardness, tensile strength and yield point increase upon standing. The yield point elongation returns and the sheet in an extraordinarily short time becomes unsuitable or unfit for the fabrication of deeply drawn articles. In steels used for carrying stress such as structural members in either static or moving structures, the ability of the steel to resist stress concentrations at notches or other change in section is greatly reduced after such aging procreduced as to render them non-aging; and the provision of a process for annealing or otherwise heat treating irons and steels including certain steps of operation by means of which the nitrogen content of the steel or iron may be reduced and the amount thereof allowed to remain in the steel may be accurately controlled.
The process of the present invention results in producing a material having imparted to it certain desirable characteristics for commercial purposes herein described. The essential idea thereof being primarily concerned with a practical method of renderinga steel non-aging by removal of the-deleterious nitrogen content as herein described without the steps or necessity of adding alloying or other materials to accomplish generally similar results and attain certain characteristics of steels. v V
In the accompanying drawings: Figs. 1 and 2 are charts which illustrate the effects of hydrogen atmospheres of predeter-- mined characteristics-upon the nitrogen content I of steels as they occur in an annealing oven in accordance with the present invention;
Fig. 3 is a more or less diagrammatic view of an annealing furnace, such as may be employed in connection with the practice of-the present invention together with associated apparatus arranged to control the atmospherev within the 'anncal ng furnace.
As. is well known, certain steels and irons undergo gradual changes in electrical, magnetic and-mechanical properties uponstanding at room ess. Under certain conditions of corrosive environments, cold worked areas in a steel susceptible to aging are subject to failure under load because of the inter-crystalline corrosion which takes place. Such failure may occur in the body of the steel and is by no means limited to the surface defects.
Prior to this invention, agents such as aluminum, titanium, zirconium, etc., have been added to the molten steel'which have produced killed steels stable against strain aging in a greater or lesser degree. I g
I have found that by annealing steels in hydrogen atmospheres containing definite minimum nitrogen contents which depend upon the active nitrogen content of the steel being annealed, and also upon the partial pressure of hydrogen in such atmosphere, steels completely stable against strain aging "can be produced. 'If the annealing time be shortened or the temperature of the material being annealed is lowered, steels or any degree of stability can be produced. I have also found that by annealing steels'normally susceptible to strain aging in atmospheres containing more than the aforesaid minimum of nitrogen as defined by the nitrogen content of the steel and. 5c
pheres, the strain aging susceptibility of a steel, such as basic open hearth steels, not already sat the partial pressure of hydrogen in said atmosurated by active nitrogen, can be increased.
As is well known, the susceptibility of steel to strain aging can be measured by determining the tensile strength of such steel at both room temperature and at a temperature in the neighborhoodof 400-450" F. Steels stable against strain aging give tensile strengths at such elevated tem- I perature. Samples of basic open hearth rimming steel sheet give about 11,000 pounds per square inch higher than at room temperature. Samples of sheet steel killed with aluminum or zirconium give various values as found in my investigations, varying from 2,000 pounds per square inch more tensile strength at about 400 F. than at room temperature, to values as'much as 10,000 pounds per square inch less at 400' F. approximately, than at room temperature.
Examples of annealing in atmospheres containing nitrogen below the aforesaid minima at about 0.5% nitrogen by volume and a partial pressure of hydrogen at about 1.0 atmosphere are as follows on 20 gauge rimming sheet steel, received in the fully cold reduced condition.
Time to gg ii Tensjillle Differ- Time of cool to streng in ence in Annegihgg anneal, room g lbs. per sq. lbs. per me hrs. temp., fg f inch at 400- square hrs. temp. 450 F. inch As box annealed commercially at:
1280 F 16 I 48 47, 000 58, 000 +11, 000 1280 F 6 45, 464 51, 620 +6, 155 1330 F 6 5 42, 918 38, 860 4,058 1360 F 6 5 41, 445 33, 965 7, 480 1380 F e 6 5 13, 575 35, 268 8, 307
73zi lnnealing atmosphere Hz 12%, C0: 4%, C0 0341.07,, N2
The influence of time of annealing when the volume of hydrogen flowing over the steel is in creased proportionately is exemplified by the following values obtained by annealing at 1380 F. of gauge fully cold reduced material.
Tensile Tensile strength in strength in gf gg Time at. 1380 F. lbs. per sq. lbs. persq. 8 g};
inch at inch at 400- room temp. 450 F.
1.5 hrs 44, 850 51, 700 +0, 850 3.0 hrs... 41, 723 35, 855 --5. 868 6.0 hrs 43, 575 35, 268 8, 307
Somewhat more time at the annealing temperature and/or'higher temperatures are advantageous for thicker sheets. The following results were obtained on sheet 0.051 inch thick.
The effect of time at a given temperature on this thickness of sheet was found to be as follows for 1380 F. annealing temperatures.
Tensile Tensile strength in strnegth in Difference Time in hours at 1,380" F. lbs. per sq. lbs. per sq. in lbs. per
inch at inch at 400- square inch room temp. 450 F This process does not confine itself to the temperature ranges and times set forth above. In commercial practice, certain temperature ranges and annealing times will be found suitable for a given product for reasons of economy and to produce characteristics in the steel other than aging stability only. As an example, there is cited below results obtained by annealing the steels of the above examples at 1675 F. These follow for an annealing time of about four hours in H2 atmospheres containing about 0.05% NZ by volume.
These show a considerable improvement over the annealing by present commercial methods which gave differences in the tensile strength at the two temperatures of -+ll,000 and +11,l50, respectively.
The results cited in the foregoing may readily be understood by a discussion of the physicochemical reactions between the hydrogen and nitrogen of the atmosphere on the one hand, and with the hydrogen of the atmosphere and the nitrogen of the steel on the other. i
When iron or steel containing nitrogen in solid solution is exposed at a given temperature to a gaseous environment of pure hydrogen, such hydrogen will react with the nitrogen of the steel or iron to produce ammonia gas, thus removing the nitrogen from the steel. During such reaction the ammonia content of the atmosphere increases at the expense of the nitrogen contained in the steel, which decreases. This reaction continues until definite quantities of ammonia are contained in the gaseous environment and then ceases. At this point there will be a fixed relation between the nitrogen remaining in the steel, .the partial pressure of hydrogen in the atmosphere and the partial pressure of ammonia in the atmosphere. At other temperatures, difi'erent fixed relations exist between the same quantities. If into such system'which is at equilibrium, ammonia gas is injected in an amount however small, increasing the ammonia content of said atmosphere, the reaction will reverse its direction and nitrogen will enter the steel until the fixed relation between the nitrogen content of the steel and the partial pressures of hydrogen and ammonia in the atmosphere is again attained. Such reaction takes place in accordance with the reversible chemical equation:
2Fe4N (dissolved in ferrite) +3H2 (gas)? 8-Fe (solid) 2NH3 (gas) (1) The fixed relation, constant at a given temperature, is:
(P N173)? 2 a constant in the atmosphere and (P11 is the partial pressure of hydrogen in the atmosphere and A in is the activity of the nitrogen, expressed as iron nitride in the steel.
If, on the other hand, hydrogen be injected into such atmosphere which is at equilibrium with the nitrogen of the steel, nitrogenv will be removed from the steel and the amount of ammonia in the atmosphere will increase until said fixed relation is again established.
'If such atmosphere originally contained nitrogen, the hydrogen will combine directly with such nitrogen to produce ammonia and will continue to produce ammonia-at the expense of the hydrogen and nitrogen of the atmosphere until definitely fixed relations at-a given temperature exist between the partial pressures of hydrogen,
nitrogen and ammonia in accordance with the following reversible chemical equation:
' 3Hz+Nz2= 2NH3 The fixed relation being given'byz (P m -constant where (Pr-m is the partial pressure of ammonia,
hand, either hydrogen or nitrogen be extracted from said atmosphere, or their partial pressures be lowered in any way, the ammonia in the gas will break into nitrogen and hydrogen until equilibrium is again established.
If now a nitrogen bearing steel be contained in an atmosphere containing nitrogen, hydrogen and ammonia, both fixed relations for Reactions 1 and 2 will be attained at equilibrium for the system. It is evident that if the nitrogen in the atmosphere be increased, more ammonia will be produced according to Reaction 2. This increase in ammonia content will result in an increase in nitrogen in the steel according to Reaction 1 since both fixed relations must be satisfied for equilibrium. This expressed in yet another way means that if a steel at 1050 Kelvin is at equilibrium with a gas containing a partial pressure of hydrogen of one atmosphere and a partial pressure of l.3 10 atmosphere of ammonia and, hence,-0.005% nitrogen in the steel, be removed from such atmosphere and into another at 1050 Kelvin containing hydrogen at a partial pressure of one atmosphere and nitrogen at partial pressure of 0.010 atmosphere in which the partial pressure of ammonia, hence, is 4.3X10- atmosphere, such steel will gain in nitrogen content since it has been introduced into an environment containing more ammonia, other things being equal, than it can be at equilibrium with, hence, its' nitrogen content is increased at the expense of the ammonia or the atmosphere until both fixed relations are again satisfied.
Also, if a nitrogen bearing steel be at equinitrogen and ammonia at a given pressure and temperature, a reduction of total pressure, or
dilution with an inert, unreactive gas, will cause decreases in the partial pressures of hydrogen. nitrogen and ammonia. In both fixed relations the partialpressure of hydrogen to the third I power and the partial pressure of ammonia to the second power are efiective and, hence ammonia will break up into hydrogen and nitrogen,
and nitrogen will be removed from the steel until both fixed relations are again satisfied. Hence.
the advantage of operation under low partial pressures of hydrogen.
These phenomena may be further clarified by reference to the chart in Fig, 1. The values of the chart in Fig. 1 are for a, temperature of 1050 Kelvin. The upper portion is for an atmosphere in which the partial pressure of hydrogen is kept constant at one atmosphere while the lower portion of the chart is for atmospheres in which the partial pressure of hydrogen is held at 0.10 atmosphere. I Referring tothe upper portion of the chart in Fig. l and to the curve labeled:
Kelvin when the partial pressure of hydrogen is 1.0 atmosphere. Similarly, at 0.003% nitrogen in the steel, there-will be at equilibrium at 1050 Kelvin and one atmosphere partial pressure of hydrogen 8.7)(10- atmospheres partial pressure of ammonia.
If the partial pressure of the hydrogen be reduced by any method, including dilution with the inactive rare gases such as helium,'neon, argon, etc.', or other gases or combinations not affecting the stabilizing reactions, the curve in the bottom portion of the chart in Fig. 1 labeled with the reaction in. question describes the relations then existing. Thus, at 10x10- nitrogen in the steel there will be 8.4 10-" atmospheres partial pressure of ammonia at equilibrium with 0.10 atmosphere partial pressure of hydrogen librium with a gaseous environment of hydrogen,
at 1050 Kelvin, while at 3 l0 nitrogen in the steel the partial pressure of ammonia will be 2.5 10*" atmospheres. As is evident, these two curves represent the equilibrium conditions of the reactionunder discussion. In the chart. shown in Fig. 2 this curve is drawn for a temperature 01 1173 Kelvin and 'one atmosphere partial pressure 'of hydrogen. Thus, it can be seen that both temperatures and the partial pressure of hydrogen affect the equilibrium concentrations of ammonia in the atmosphere at a given nitrogen content of the steel since the partial pressure of ammonia in equilibrium with the steel at a given nitrogen content is slightly decreased. m, i i
When a mixture of hydrogen and nitrogen gases-are maintained at a given temperature they react to form ammonia until a definite amount of ammonia is produced after which there is no further reaction. This amount of ammonia isit's equilibrium'concentration and .will vary with temperature, the partial pressure of hydrogen, and the partial pressure of nitrogen in the atmosphere in accordance with the following chemical equation: N2+3H2=2NH3.
Referring to the chart shown in Fig. 1, upper section, and the horizontal line marked Pn =1.O, Pn =0.01 we find that when the partial pressure of hydrogen, P112115 1.0 atmosphere and the partial pressure of nitrogen is 0.01 atmosphere, the partial pressure of ammonia in equilibrium with this atmosphereat 1050 Kelvin is found to be about 4.3x 10 atmospheres by reading the ordinate to the left. This amount of ammonia is greater than that amount which can be in equilibrium with iron or steel containing nitrogen as FeiN as can be seen at a glance and from our foregoing discussion of the reaction by which iron absorbs nitrogen, from to 0.012% nitrogen in the steel. Hence, if hydrogen at a partial pressure of one atmosphere containing nitrogen at a partial pressure of 0.01 atmosphere be led over steel, such steel will absorb nitrogen until the percent nitrogen in the steel is somewhat greater than 0.012%. (Extension not plotted.)
If on the other hand, a gas containing hydrogen at 1.0 atmosphere partial pressure and nitrogen at 0.001 atmosphere partial pressure be maintained at 1050 Kelvin at such concentrations, the partial pressure of ammonia in equilibrium with such atmosphere will be about 1.4 10- atmospheres. At nitrogen contents in the steel above about 0.005% this quantity of ammonia is less than that required for equilibrium with iron and, hence, if irons containing more than 0.005%
nitrogen be maintained in the aforesaid atmosphere, such iron will lose nitrogen and become more stable toward strain aging. At nitrogen in the steel below about 0.005% this quantity of ammonia is greater than can be maintained at equilibrium under the pressure and temperatureconditions defined and, consequently, the steel will absorb nitrogen and, hence, become less stable against strain aging. At 0.005% nitrogen in the steel, its ageability will not be affected.
Referring to the lower portion of the chart shown in Fig. 1 wherein the partial pressure of.
hydrogen is maintained at 0.10 atmosphere, the horizontal line marked Pn =0.10, PN2=0.001 is interpreted in the same manner as the horizontal lines of the upper part of the chart shown in Fig. 1. This line calls for approximately 4.3x atmospheres of ammonia for equilibrium as can be seen by reading the ordinate at the left. Al-
though, the ratio of hydrogen to nitrogen in the atmosphere is the same as that in the upper part of the chart in Fig. 1 marked Pn =1;0, PN2=0.01, steels containing more than about 0.0051% nitrogen can be rendered more stable against strain aging, while those containing less than about 0.0051% nitrogen are rendered less stable. This constitutes an advantage in that hydrogens relatively high in nitrogen can be operated under reduced partial pressure either by reducing the total pressure or by dilution with gases inert to the steel or atmospheres as respects nitrogen. If the system be operated with an atmosphere containing a partial pressure of hydrogen, P1: of-
0.10 atmosphere and a partial pressure of nitrogen of 0.0001 atmosphere which can be obtained by dilution at atmospheres wherein the ratio of the partial pressures of hydrogen to nitrogen is 1000 as in the case of the atmosphere of the lower horizontal line in the upper portion of the chart is plotted for a temperature of 1173 Kelvin and a partial pressure of hydrogen which in all cases is 1.0 atmosphere, it can be seen by comparing the upper p'ortion of Fig. 1 with the plot in Fig. 2 that the various horizontal lines across the iron reaction curves at lower nitrogen contents of the steel. Thus, at a temperature of 1173 Kelvin, one atmosphere containing a partial pressure of hydrogen of 1.0 atmosphere and a partial pressure of nitrogen of 0.001 atmosphere will render steels of more than 0.0035 nitrogen more stable whereas at 1050 Kelvin, -the same partial pressures would render steels of more than 0.005% nitrogen more stable against strain aging. Thus, higher temperatures increase the ability of a given atmosphere to remove nitrogen from a steel when such atmosphere contains less than that amount of ammonia which can be at equilibrium with a. solid solution of iron nitride in iron.
In order to derive the benefits of the invention in the finished article it is, of course, necessary to cool the article in the controlled atmosphere to a temperature below that at which the article will absorb nitrogen when again subjected to air, before removing the article from the furnace. Ordinarily when the article has been cooled .to approximately 300 F. it will be safe enough to remove it from the furnace and the gases therein without material damage of absorption of additional amounts of nitrogen in the absence of further heating.
From the above it will be understood that by varying the relative proportion of nitrogen and hydrogen, the partial pressures of the nitrogen and hydrogen, and/or the temperatures of the iron or steel during treatment thereby, the amount of nitrogen in the finished iron or steel may be relatively closely controlled. Experience indicates that in the majority of cases if it is desired to insure that the iron or steel being treated is positively free of strain aging, the nitrogen content of the steel should not exceed 0.001% al-' though the percentage of nitrogen in the iron or steel may be as high as 0.0015% without exhibiting any marked tendency towards strain aging and in some instances may be asgreat as 0.002% without showing the effects of strain aging to any material extent. It also appears that the presence of nitrogen in rimmed steels should be at a lower percentage than in killed steels although it will be understood that even in killed steels and particularly those killed by the addition of silicon the percentage of included nitrogen is an important factor from the standpoint of strain aging.
In any event, if the amount of nitrogen absorbed by the hydrogen surrounding the steel in the furnace is not allowed to materially,exceed that amount thereof as calculated by the following formula, a product satisfactorily low in nitrogen content will result.
where Pu is the partial pressure of nitrogen in the annealing atmosphere in atmospheres and T is temperature in degrees Kelvin of the'annealing atmosphere.
Although the method of this invention can be applied to either rimmed or killed steels, it will be especially useful in the arts in the stabilizing of rimmed steel sheets. Although stee.s stable against strain aging have been produced for many years they have always been made by the addition of relatively large quantities of elements strain aging. Such killed steels are more expener is thereby pushed out of the furnace. Re-.- ferring to Fig. 3, with valves A and D open and valves C and B closed. and with the blower quiet, CO: is allowed to flow through the pipe line 42. past the valve A, through the connection 38, blower 36 and pipe line 34 into the annealing furnace 20, q the air being displaced therefrom through the pipeline 22, connection 24, pipe line 23 and valve D. When the air is displaced, valve D is closed and valve C is opened to permit entry of hydrogen from the line 23. The light hydrosive to make and the rejections of finished prodnot such as sheet or strip, for poor surface conditions is very much higher than for rimming steels. Low carbon steels killed with aluminum are considerably more difficult-to roll in the modem high speed continuous wide strip mill, both roll breakage and spindle breakage increasing, resulting in more frequent shut downfor repairs and, consequently, uneconomical operation. The
difliculty of producing deep drawing steel to meet the stringent requirements of modern manufacturing has grown to the point where rimmed steel, deep drawing sheet has become inadequate because of its rapid increase in hardness and loss of deep drawing properties upon standing a few method of this invention will relieve these conditions.
In the practical application of this process, it
will, in general, be found advisable to re-circulate' the hydrogen through finely divided zirconium, titanium or iron at a lower temperature than the annealing temperature, or through some other satisfactory denitrifying agent. Inthis way, it
.will not be necessary to continually supply. fresh, low nitrogen hydrogen. Also, the partial pressure of hydrogen can be maintained at low values, enhancing its ability to remove nitrogen from the steel and to increase the safety of operation.
The above described process may be carried out by any suitable apparatus. The. apparatus illustrated more or less diagrammatically in Fig. 3
is illustrative of one form which such apparatus Referring to Fig. 3, indicates an maytake. annealing furnace of any suitable or conventional type heated in any suitable or conventional manner and capable of being substantially hermetically sealed during operation. A line of pipe 22 leads out of the top of the furnace 20 and extends to a four-way connection 24 to which is connected an upwardly directed line of piping 26 in which a shut-off valve D is provided, a hori'- zontally extending line of piping 28 in which a shut-off valve C is provided, and a downwardly extending line of piping 30in which a valve B is provided. The line oif piping 30 beyond the valve 3 from the connection 24 extends into a denitrifying tower 32. A line of piping 34 extends downwardly from the bottom of the furnace 20. The pipe line 34 connects with the discharge side of a blower 36 the inlet side ofwhich is connected to a connection .38 from which one gen then flows through the connection 24 and line 22 to the furnace 20 and pushes the heavy CO2 out of the furnace through valve A. When the CO1 content of said furnace has been lowered to satisfactory values, valve A is closed, valve B is opened and hydrogen, diluted or undiluted is led through thefurnace by means of' the blower which operates in a direction such that the hydrogen is'drawn through the denitrifying tower and thence to the annealing furnace. The tower days after being processed in the steel mill. The J line 40 extends to the bottom end of the denitrifying tower 32 and from the other of which extends a line of piping 42 provided'with a valve A therein. At the beginning of the annealing cycle the air volume of the covered annealing box must be removed, sincemere dilution with hydrogen cannot reduce the nitrogen contents to low enough values withineconomical volumes of hydrogen. Such air may be conveniently displaced by C0: introduced slowly into the annealingbox, avoiding turbulence at the point where the CO2 enters said box- The air within the cov- 32 will contain more satisfactory denitrifying agent such as one of those previously described. Whenthe volume of hydrogen drawn into the system is sufficient, valve C is closed, the blower serving to re-circulate the annealing atmosphere through the furnace until annealing is .complete.
The heating of the steel in the furnace ordinarily will begin when the CO: has been lowered to a convenient value Prior to the process of this invention, hydrogen was used in gaseous atmospheres for bright annealing, that is, for the prevention of iron oxide scale from being produced on the surface of the material being annealed. To my knowledge no of the steel to strainage, and commercial formsof hydrogen available for such purpose have had too large a percentage of nitrogen to be effective for the purposes of the present invention. It is essential for the successful working of the process of this invention to closely control nitrogen concentrations in the annealing atmosphere. The control of such nitrogen in the annealing atmos-' phere refers not only to elemental nitrogen, but all compounds of nitrogen, such as for instance, ammonia or nitric oxide,.capabe of affecting the nitrogen content of'the steel, and hence. its sus-. ceptibility to strain age.
It will be understood that broadly. I do not wish to limit myself to the precise values of nitrogen content set forth in the charts and discussion, but to that nitrogen content which under any condition of temperature and pressure is able to affect the strain aging susceptibility of' m e cial irons and steels. The charts and discussion regarding samev are based on thermodynamic calculations which suffer in precision from lack of knowledge regarding the deviation of thermodynamic activity from the mol fraction of iron nitride contained in the steel. Also, the nature -of chemical compound of iron nitride is not yet settled. The charts, therefore, are qualitative only and are useful as a guide.
'- broad invention, thescope of which is commensurate with the appended claims.
What 'I'claim is:
1. In the manufacture'of iron, steel or like ferrous material, a method of rendering said material free from strain aging comprising the steps of placing said material in a furnace, displacing the gas in said furnace with a gas having as the active constituent thereof substantially only hydrogen, causing said material in said furnace to be heated to a temperature sufiicient to effect the usual desired annealing thereof, and removing from said hydrogen any amounts of nitrogen absorbed thereby from said material in excess of those amounts present when the nitrogen content of the material is in excess of (2.002%.
2. In the manufacture of iron, steel or like ferrous material, a method. of rendering said material free from strain aging comprising the steps of placing said material in a furnace, displacing the gas in said furnace with a gas having as the active constituent thereof substantially only hydrogen, causing said material in said furnace to be heated to a temperature sufficient to effect annealing thereof, and circulating the gases within said furnace over a nitrogen absorbing material whereby to abstract that portion of such ni-' trogen as may be absorbed by said hydrogen from said material.
3. In the manufacture of iron, steel or other ferrous material, rendering such material free' from strain agingby introducing said material into a closed furnace, displacing air within said furnace by the introduction of gas inert in its action upon said material, displacing said inert gas by hydrogen, heating said material in said furnace, and maintaining the amount of nitrogen absorbed by said hydrogen from the material at a value not greater than that calculated by the formula Log PN2= +0.3013
where PN. is partial pressure of nitrogen in the annealing atmosphere and T is temperature in degrees Kelvin.
CLARENCE L. AL'I'ENBURGER.