US3138493A - Method of heat treating beryllium copper alloys - Google Patents

Description

E. M. SMITH `lune 23, 1964 METHOD OF HEAT TREATING BERYLLIUM COPPER ALLOYS Filed March 19, 1962 INVENTOR. 2mn/@ew M 9M/7H ATTRNEK mm w Q wN WN NN United States Patent O 3,138,493 METHOD F HEAT TREATING BERYLLUM CPPER ALLOYS Ellsworth M. Smith, Reading, Pa., assiguor to The Brush Beryllium Company, Cleveland, Ohio, a corporation of Ohio Filed Mar. 19, 1962, Ser. No. 180,652 11 Claims. (Cl. 14S-11.5)

This invention relates generally to a method of heat treating beryllium copper alloy sheet material and particularly to a method o f heat treating beryllium copper alloy sheet material in a continuous operation under controlled time and temperature conditions.

The invention is herein described and illustrated in connection with the treatment of beryllium copper alloy sheet. The term beryllium copper alloy is used herein to denote alloy consisting essentially, by weight, of from 1.4% to 2.1% beryllium, 0.17% to 0.42% of metal selected from the group consisting of cobalt, nickel, and mixtures thereof, and the balance copper.

The term sheet is not used herein in a specic sense as defined in technical handbooks and standard specifications, but in a more general sense to include not only metal stock classied as sheet, but also that classified as thin plate, strip, and foil.

It also applies whether or not the stock is in the form of a band, such as is commonly wound into coils as it leaves the rolling mill, or in the form of cut-off lengths of such a band, and whether or not it is in condition for fabrication, or already fabricated into articles.

One of the earliest prior methods of producing such sheets is to form the beryllium copper alloy into sheets, cold work the sheets, and then subject them to a hardening operation by passing them through a mule furnace wherein they are heated to a temperature of about 600 to 650 F. and maintained at that temperature for a relatively long period of about two to three hours. This method is used on the sheets whether they are in flat form as received from the mill or are in the form of articles fabricated therefrom. Due to the great length of the heating period required in this prior process, high internal stresses are developed in the sheets and limit their formability. Additionally, the desired physical and mechanical properties cannot be obtained consistently. The resultant product tends toward brittleness. However, when the material is treated in this manner in sheet form, the sheets must be retained in fixtures during treatment to prevent distortion.

A second method is one wherein the beryllium copper alloy sheets are cold worked in the form of strip to a maximum reduction of 37% after which the strip is heated in coiled condition at a temperature range of about 600 to 850 F. for a relatively long period of from one hour to several hours, to provide the desired tensile strength. The strip resulting from this method evidences an advanced state of gamma growth, particularly at the grain boundaries, and this causes variations in hardness throughout the coil or strip, reduces the endurance life of the strip stock and resultant sheets, and decreases their formability. In addition, the still lengthy heating cycle of the sheets in coil form results in what is commonly referred to as coil set; that is, the sheets tend to retain the curvature which they had While forming a part of the coil. The sheets so curved are detrimental to high speed press performance and result in non-uniformity of the fabricated part.

A third method also is employed. This method comprises cold working of the material in sheet form to between about 37% reduction and 60% reduction to develop a sheet which can be aged at high temperatures in a relatively shorter time than employed in the other two methods. The material is then passed continuously in strip form through a mule furnace, in which a reducing atmosphere is maintained, for a period of about three minutes to fifteen minutes, and at a temperature of from about 725 to 800 F. The method has a number of advantages in that the time required for heat treatment is t greatly reduced and the sheets are free from coil set and have excellent endurance life. However, because of the extensive reduction by cold working, the resultant sheets have high residual internal rolling stresses and unidirectional grain distribution which limit their formability. Likewise, the results produced by the method are not consistent in that the physical properties of the resultant sheets vary frequently and uncontrollably.

Currently, in the commercial preparation of beryllium copper alloy sheets, the beryllium copper alloy materials are cast into ingots. The ingots are first hot rolled and then alternately solution annealed and cold rolled until a desired and predetermined ready to nish thickness is obtained. The strip is finally cold rolled to its nal thickness, and may or may not be annealed thereafter, depending on customer requirements.

Insofar as concerns beryllium copper sheets products commonly sold to the customer for fabrication, no standard of physical properties required has been established by the government or any recognized technical authority other than for beryllium copper alloys containing from 1.80 to 2.05 percent, by weight, of beryllium. This standard established for this alloy is applicable to sheets supplied by the mill in solution annealed and cold rolled tempers, and also to sheets which have been heat treated in either strip or fabricated form at 600 F. for about two to three hours. This method of treating the sheets in the form of fabricated parts is performed by the customer, and while it develops high tensile strength, it also renders the parts brittle. In the event the sheets are given a heat treatment at 600 F. for two to three hours, the

'resulting brittleness would preclude fabrication because the sheet would crack and fracture during forming operations. Thus, the customer is confronted with the problems of heat treating the sheets of fabricated form and of providing and employing various expensive jigging fixtures to minimize distortion during the heat treatment.

It became apparent, therefore, that by improving suiciently the formability of mill heat treated sheets they could be fabricated into parts without fear of developing forming cracks, and without requiring heat treatment by the fabricator before or after fabrication.

A principal object of the present invention is to provide a method of treating beryllium copper alloy sheets of this general character to impart thereto physical properties which sufliciently improve their formability to make possible their fabrication by the fabricator without the necessity of any heat treatment at the fabrication site, either prior or subsequent to the fabrication of the sheets into article form.

Another object is to provide a method of heat treating 3 such sheets so as to improve their elongation, and also, under certain conditions, to improve their tensile strength, or both their tensile strength and their elongation concurrently.

A more specific object is to heat treat such sheets, in either strip or fabricated condition, to effect stress relief and thereby improve the relationship of their elongation to their tensile strength.

Another important object is to improve the physical properties of beryllium copper alloy sheets by heat treatment of the sheets for an extremely short interval, as compared to the relatively long periods heretofore employed, and by quick quenching of the heated sheets.

A further object is to provide such a method which is adapted for both continuous or batch type operations.

Other objects and advantages will become apparent from the following description wherein reference is made to the drawings, in which:

FIG. l is a top plan view of a treating apparatus for practicing the method of the present invention on two strips of metal concurrently, the cover being omitted for clearness in illustration;

FIG. 2 is a side elevation of the apparatus illustrated in FIG. l, parts thereof being shown in section;

FIG. 3 is a fragmentary vertical longitudinal sectional view of the bath container of the apparatus, and is taken on the line 3-3 of FIG. l; and

FIG. 4 is an enlarged vertical cross sectional view taken on the line 4-4 in FIG. 1.

FIG. 5 is a vertical cross sectional view taken on the line 5--5 in FIG. l.

Referring to the drawings, two sheets 1 of beryllium copper alloy, Wrapped into the form of coils 2, are supported on suitable rotatable coaxial pay-off supporting spindles 3 for permitting the sheets to be drawn continuously therefrom through the treating apparatus from right to left, FIG. 1. Each strip 1 is fed from its coil 2 through suitable tension rolls 4, over a suitable guide roll 5, and under a supplemental positioning roll 6, and thence into a bath of molten salt contained in an elongated container 7. The strip is held immersed in the bath by suitable holddown rolls S so as to be maintained at all times below the level of the bath, later to be described.

The strip passes from the left hand holddown roll 8 into a suitable quenching compartment 10 wherein it is cooled by spraying liquid coolant thereon by means of spray nozzles 11. From the cooling compartment 10, the strip passes between pinch rolls 12 over a guide roll 13 and is rewound into a coil 14.

The pinch rolls 12 are driven through a clutch 15 by a sprocket 16, chain 17 and driving sprocket 18. The sprocket 18 is driven by a motor 19 of a recoiler 20. The pinch rolls 12 pull the strip from the coil 2 through the bath, and maintain the strip under tension and in a horizontal plane passing between, and tangent to, the rolls 12, while it is immersed in the bath. Each coil 14 is rewound on its recoiler 20 after passing the pinch rolls.

As mentioned, the bath container 7 is elongated. It is in the form of an open top tank 21 having a removable cover 21a with openings 22 at the ends. Within the tank 21 are two troughs 23 which are laterally spaced from each other, extend lengthwise of the tank 21, and terminate at their ends a short distance from the ends of the tank 21. The troughs 23 have side walls 23a and end walls 23b. Each end wall of each trough has an upper edge 23e and terminates at its upper edge below the upper edges of the side Walls 23a. Each edge 23C provides a weir over which the molten salt bath, being introduced continuously into the troughs, discharges into the end portions of the tank 21.

Beneath the tank 21 is a heating tank 25 wherein are heating coils 26 through which a heating medium from any suitable source is circulated for heating the salt of the bath to the required temperature and liquidity.

Within the tank is a feed pipe 27 which has an inlet end 28 opening into the tank 25 and an outlet end 29 for supplying molten salt to the troughs 23. The molten salt is drawn into the inlet end 28 and forced out of the outlet end 29 by means of a recirculating pump. The pump is in the form of a propeller 30 mounted on a vertical shaft 31 and driven rotatably through the medium of a belt 32 by a motor 33. The pump acts to pump the molten salt of the bath continuously into the troughs 23. A baffle housing 34 is disposed above the outlet end 29 so as to prevent the splashing of the molten salt. The housing has passages 35, as best illustrated in FIG. 4, through which the salt ows from the outlet end 29 into the troughs 23.

The molten salt discharged over the weirs 23e falls into the end portions of the tank 21, which provide catch basins 36, and thence ows from the basin 36 along the bottom of the tank 21, beneath both the troughs 23 and the space therebetween, and discharges from the tank 21 into the heating tank 25 for reheating and recirculation. The bottom wall of the tank 21 slopes downwardly from its ends to the tank 25 to assist in the return flow of the molten salt. The openings 22 are disposed at levels such that the strip 1 can enter the tank 21 through the opening 22 and pass over, and in spaced relation to, the weirs 23C of the troughs 23, below the level of the overflowing bath, and thence through the trough 23, in spaced relation above the weirs 23e at the outlet ends of the troughs into the compartment 10, and therethrough onto the coil 14, preferably being maintained in tensioned condition and horizontal from its point of initial immersion to the pinch rolls 12.

In operation, therefore, the strip is fed from the coil 2 through suitable tension rolls 4 and over a guide roll 5, and under a supplemental roll 6 into the salt bath in the tank 21, wherein it is maintained immersed by virtue of the holddown rolls 8 and the pinch rolls 12. It passes through the bath and into the cooling compartment 10 wherein it is sprayed with coolant. It passes from the compartment 10 through power driven tension rolls 12, over idler guide rolls 13, onto the coil 14 on the recoiler 2t). By the action of the tension rolls 4 and 12, the sheet is held straight, fiat, horizontal, and under tension as it passes through the bath, as hereinbefore described. AS a result, the surface of the coiled sheet is smooth and of straight cross section at all points along its length.

If extremely heavy sheet stock is to be processed, it is not necessary to pass it over the guide rolls 5 and 13. Instead it can pass from the coil 2 through the tension rolls 4 and over the supplemental positioning roll 6, which is lowered for this purpose to guide the strip properly through the inlet opening 22.

The speeds, time and temperatures to be employed will be varied to some extent depending upon the sheet or strip thickness, the particular range of metal ingredients of the alloy being treated, and the physical properties desired in the finished sheet.

In accordance with the present invention, beryllium copper alloy sheets are heated by immersing them in the molten salt bath while the bath is maintained at a temperature ranging from about 525 F. to 825 F. The immersion is for an interval from as little as two seconds up to about six minutes. This can be determined by the speeds of the rolls 12 and by the position of the rolls 8. The closer the spacing of the rolls 3 from each other, the less is the time of exposure of the sheet to the bath.

The heated sheets are quenched in the compartment 10 by sprayed coolant which is maintained at a temperature of about 40 F. to 70 F., this being adequate to cool the sheets almost instantly.

Strip treated in this manner, while thus held under tension by the rolls, is dead Hat; that is, it is substantially planar except insofar as it is subsequently wound into a coil 14, and even after coiling it tends to resume its at condition when uncoiled.

If desired, the sheet stock may be cut into separate, sheets and conveyed through the bath by conventional conveying means, but this is not as satisfactory inasmuch as it must be performed by equipment requiring considerable servicing and further does not permit of tensioning the sheets while they are passing through the bath and which is desirable.

The salt bath employed in the process comprises a mixture of neutral inorganic nitrate and nitrite salts which are employed in molten state and in such state are unreactive with the sheets. Many commercially available neutral salt mixtures used for standard salt baths can be employed, but the nitrate-nitrite mixture is preferred as it will not react with, and has no deleterious effect upon, the beryllium copper alloy sheets. The particular type of such salt chosen is dependent upon the heating range desired.

The quenching bath may comprise a fluid spray, such as air or water. A coolant liquid bath with sheet immersion may be used if desired.

Within the broader ranges of times and temperatures hereinbefore set forth, certain different eifects can be obtained by variations of times and temperatures within narrower ranges, but the most outstanding difference is that resulting from the variations in the time element. For example, the results desired may be stress relief, with an outstanding increase in percent elongation without any diminution or increase in tensile strength. On the other hand, it is quite often desirable to increase greatly the tensile strength and, while this is frequently accompanied by a diminution in elongation, nevertheless a much higher elongation is retained in a higher range of tensile strength than has heretofore been possible. Finally, in some instances, not only can the elongation be increased substantially, but the tensile strength can be greatly increased concurrently therewith.

The various physical properties desired are to some extent controlled by the needs of the customer. However, the important feature is that by the present method the eifects obtained can be preselected within certain narrow limits within the broader limits and consistently. produced within those narrow limits.

The following tables disclose a number of dilerent characteristics of sheets for customer use supplied by the mill in accordance with the present method. The data in all tables below were obtained by testing 0.20 inch thick beryllium copper alloy sheets produced in accordance with the prior method above disclosed, and then heat treated in accordance with the present invention. Similar data and results have been obtained by testing beryllium copper strip ranging from about 0.07 inch to about 0.0005 inch inthiclmess.

Table I Contz'nuous Strand Mill Heat Treating in a Salt Bath Alloy A Alloy B Age Hardening Conditions Be 2.03%, Be 1.61%, Initial Temper Co 0.25%, Co 0.23%,

Balance Copper Balance Copper Temper- Tensile Percent Tensile Percent Time ature, F. Strength, Elong. Strength, Elong. p.s.i. in 2in. p.s.i. in 2in.

73, 000 40 73, 000 35 700 74, 000 42 79, 000 35 700 75, 000 37 84, 000 32 180 sec 700 90,000 24 93, 000 31 30 see 750 73, 000 40 76, 000 36 Annealed sec 750 79, 000 32 86, 000 27 sec 750 110, 000 19 100,000 19 300 sec 750 143, 000 12 136, 000 14 30 see 800 74, 000 39 77, 000 35 60 sec 800 85,000 28 87,000 30 180 sec 800 115,000 12 108, 000 18 ASTM Standard-AT 3 hrs. 4 0 20 85, 000 13 26 90, 000 26 60 sec 700 100,000 21 95,000 22 180 sec 700 157,000 9 112, 000 24 30 sec 750 100, 000 16 93, 000 22 M H10% Reduction in Thickness 60 sec 750 121,000 14 100, 000 18 180 sec 750 169,000 9 120,000 14 8 133, 000 11 14 95, 000 23 14 103, 000 18 7 119, 000 14 ASTM Standard-M HT 3 21 97, 000 21 60 sec 16 104,000 19 180 sec 700 152, 000 9 124,000 16 30 seo 750 108,000 18 103, 000 19 Elf-20% Reduction in Thickness- 60 sec 750 126, 000 14 110,000 15 180 sec 750 169,000 8 132, 000 13 300 sec 800 181,000 6 147,000 10 30 sec 800 121,000 13 105,000 20 60 sec 800 135,000 14 113,000 14 180 sec 800 152,000 10 129, 000 13 ASTM Standard% HT 2 0 3 104, 000 5 9 119, 000 14 6 133, 000 11 4 142, 500 9 6 125, 000 11 Hard-37% Reduction In Thickness., 5 136,000 8 4 157, 000 6 3 163, 000 6 6 131, 000 11 6 151, 000 6 7 154, 000 6 ASTM Standard-HT 1 Table II.--Continuous Strand Stress Relieving zn a Salt Bath-Alloy A Stress Relieving Federal Specifications Conditions QQ-C533 and ASTM Tensile Percent Initial Temper Strength, Elong,

Temperp.s.i. in 2 in. Tensile Percent Time, ature Strength, Elong. scc. I". p.s.i. in 2in.,

nun.

73, 000 40 600 73,000 43 600 73, 000 44 600 73,000 45 600 73,000 45 Annealed 600 73,000 43 60, 000-78, 000 35 5 700 73,000 42 10 700 73, 500 43 15 700 74, 000 44 30 700 74,500 42 45 700 74, 500 39 0 M 85,000 22 5 600 85,500 26 10 600 85, 500 28 15 600 86, 000 30 30 600 87,000 30 A 1Il0% Reduction in Thickness 45 600 88,000 29 75, 000-88,000 10 5 700 86,000 24 10 700 86,500 26 15 700 88, 000 28 30 700 91,000 26 45 700 94, 500 22 0 91, 000 16 5 550 91, 500 18 10 550 92, 000 19 15 550 93, 000 18 30 550 93, 500 20 45 550 95,000 20 s as 92,233 17 93, 21 $4 1I 20% Reduction 1n Thickness. 15 600 93' 000 22 85,000-100, 000 5 30 600 95,000 22 45 600 97, 500 21 5 700 93,000 22 10 700 94,500 23 15 700 94, 500 22 30 700 97,000 21 45 700 101,000 18 0 110,500 3 5 550 111,000 7 10 550 111,000 8 15 550 111, 090 9 30 550 111, 000 9 45 550 112, 500 9 13 se 1121033 1 11 ,5 11 Hard 31% Reduction in Thickness 15 600 111,000 13 100,000 120,000 l Table Ulf-Tensile Strengths Obtaz'nable by Conventional Mill Processing of Alloy A Rolling Reduction, percent Range of Tensile Strength (psi.)

Percent of Elon- Temper Designation gatlon 10D-110, 000 110-120, 000 120-135, 000 135-150, 000 1GO-170, 000

Table IV.-Tensile Strengths Obtanable Only by Salt-Bath Processing of Alloy A Rolling Range of Percent Tempel' Designation Reduction, Tensile 0i Elonpercent Strength gation (ps1.)

XHM "190'Y 10 1GO-170,000 8 XHMS "190 10-20 170190,000 5 Table I, for example, relates generally to sheets in which tensile strength was to be emphasized, either with an improvement, or in some cases a slight diminution, of the present elongation. The results are tabulated for different mill hardened tempers obtained by predetermined reduction by cold rolling.

Table II is directed more to stress relief whereby the elongation is greatly increased while maintaining the tensile strength within the Federal Government and A.S.T.M. specifications for the respective tempers.

Table I covers two alloys, Alloy A containing 2.03% of beryllium and 0.25% of cobalt, balance copper, and the Alloy B containing 1.61% of beryllium and 0.23% of cobalt, balance copper, as noted at the heads of the columns in the tables. Table II relates only to the former one of the alloys of Table I.

It is apparent, therefore, that in accordance with the present method, greatly improved characteristics, particularly in the relation of elongation to tensile strength, can be obtained in heat treating periods considerably shorter than those heretofore employed with like starting sheets. In general, the physical properties described are obtained with sheets in which the reduction generally does not exceed 37%, and usually is considerably below that amount, as compared to prior methods wherein the physical properties are improved by reductions above 37% to as much as 60%.

Table III shows the reductions which are employed in conventional processes and the corresponding tensile strength as to Alloy A only.

Table IV, on the other hand, shows like information, as to Alloy A, resulting from the present process. Therein, tensile strengths in a range of 160,000 to 170,000 pounds per square inch, starting with a sheet having temper of only reduction, are obtained, as compared Ito the extra hard temper of Table III, requiring a 60% reduction in the starting sheet.

By employing 10% to 20% reduction, further increases in the tensile strength into a range of from 170,000 to 190,000 pounds per square inch are obtained by the present method. Thus, by the present method, employing the salt bath, reliance for high tensile strength does not have to be placed on high degrees of reduction in the thickness of the starting sheets, with the corresponding high residual internal rolling stresses resulting therefrom, in the nal sheets. Since residual rolling stresses are maintained at a low level, a corresponding high percentage elongation is obtained after the salt bath heat treating.

Since the process dispenses with the necessity for the heavy reductions and lengthy heat treatments, it makes possible the economical production of materials with much higher elongations at both the high and low tensile strengths than heretofore have been available.

Having thus described my invention, I claim:

1. The method of producing beryllium copper alloy sheets having a predetermined tensile strength and elongation and comprising cold rolling of a sheet consisting, by weight, essentially of from 1.4% to 2.1% beryllium, 0.17% to 0.42% of metal selected from the group consisting essentially of cobalt, nickel, and mixtures thereof, and the balance of copper, to effect a predetermined reduction and thereby to increase the tensile strength, whereby a concurrent reduction in percent elongation occurs, then heating the rolled sheet in a salt bath to a temperature ranging from about 525 F. to 825 F., and maintaining the sheet heated in said temperature range in said bath for a period from about two seconds to about five minutes, then removing the sheet from the bath, and immediately upon removal, quenching the sheet with a liquid bath, while maintaining the sheet under tension continuously throughout the heating and quenching operations.

2. The method according to claim 1 wherein said reduction of said sheet is a maximum of about 37%.

3. The method according to claim 1 wherein said salt bath is non-reactive with the sheets.

4. The method according to claim 3 wherein said salt consists essentially of a mixture of inorganic nitrites and nitrates.

5. The method according to claim l wherein the sheets are conveyed continuously along a predetermined path and, during the continuous travel along said path, are subjected successively to said heating in the salt bath, removal from the bath, and quenching.

6. The method according to claim 1 wherein said period is from two seconds about forty-tive seconds.

7. The method according to claim 1 wherein strength is further increased by maintaining the material in said bath for a period of time greater than that required to produce the maximum increase in percent elongation.

8. The method according to claim 7 wherein the longer time of heating is discontinued while the elongation remains greater than it was at the beginning of the heating operation.

9. The method according to claim 1 wherein the alloy consists, by weight, essentially of beryllium about 1.80% to 2.05%, metal from the group consisting of cobalt, nickel, and mixtures thereof, from 0.20% to about 0.27%, and the balance being copper.

10. The method according to claim 1 wherein the alloy consists, by Weight, essentially of beryllium about 1.6% to 1.8%, metal selected from the group consisting of cobalt, nickel, and mixtures thereof, of from about 0.20% to 0.35%, and the balance being copper.

1'1. The method according to claim l wherein the temperature to which the sheet is heated is of from about 700 F. to 800 F.

References Cited in the lile of this patent UNITED STATES PATENTS 1,321,530 MacDonald Nov. 11, 1919 1,570,815 Wylie Jan. 26, 1926 1,635,793 Koref et al. July 12, 1927 1,712,663 Gero May 14, '1929 1,852,528 Kinney Apr. 5, 1932 2,172,639 Werner Sept. 12, 1939 2,192,495 Werner Mar. 5, 1940 2,279,684 Johnson Apr. 14, 1942 2,717,845 Carter Sept. 13, 1955 2,818,075 Dunlevy et al. Dec. 31, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pabenb NQ., 3,138,493 June 23, 1964 Ellsworth lVL, Smith ib -ie hereby Certified that errer appears in tbe above numbered paben't requiring correction and that the said Letters Patent should reedv as corrected below.

Columns 5 and q in Jbhe table, under the heading "l irl-20% Reduction in Thicknessu and opposite "300 sec" in the first ColumnY the figure in the next Columnv under `ehe heading 'l"em1oerature OFI." for "800" read 750 Signed and sealed this 3rd dey of November 1964:7

(SEAL) Amst:

ERNEST W, SWIDERA EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims (1)

1. THE METHOD OF PRODUCING BERYLLIUM COPPER ALLOY SHEETS HAVING A PREDETERMINED TENSILE STRENGTH AND ELONGATION AND COMPRISING COLD ROLLING OF A SHEET CONSISTING, BY WEIGHT, ESSENTIALLY OF FROM 1.4% TO 2.1% BERYLLIUM, 0.17% TO 0.42% OF METAL SELECTED FROM THE GROUP CONSISTING ESSENTIALLY OF COBALT, NICKEL, AND MIXTURES THEREOF, AND THE BALANCE OF COPPER, TO EFFECT A PREDETERMINED REDUCTION AND THEREBY TO INCREASE THE TENSILE STRENGTH, WHEREBY A CONCURRENT REDUCTION IN PRECENT ELONGATION OCCURS, THEN HEATING THE ROLLED SHEET IN A SALT BATH TO A TEMPERATURE RANGING FROM ABOUT 525*F. TO 825*F., AND MAINTAINING THE SHEET HEATED IN SAID TEMPERATURE RANGE IN SAID BATH FOR A PERIOD FROM ABOUT TWO SECONDS TO ABOUT FIVE MINUTES, THEN REMOVING THE SHEET FROM THE BATH, AND IMMEDIATELY UPON REMOVAL, QUENCHING THE SHEET WITH A LIQUID BATH, WHILE MAINTAINING THE SHEET UNDER TENSION CONTINUOUSLY THROUGHOUT THE HEATING AND QUENCHING OPERATIONS.

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