US2420377A - Method of brightening tinned strip - Google Patents

Description

May 13, 1947. c, JON Es 2,420,377

METHOD OF BRIGHTENING TINNED STRIP Filed Jan. 6, 1945 2 Sheets-Sheet 1 II'HII llI INVENTU/P May 13, 1947. c. G. JONES 2,420,377

METHOD OF BRIGHTENING TINNED STRIP Filed Jan. a, 1945 2 Shets-Sheet 2 C an! 6.1753265,

Patented May 13, 1947 METHOD OF BRIGHTENING TINNED STRIP Carl G. Jones, Youngstown, Ohio Application January 6, 1943, Serial No. 471,407

4 Claims. (01. 204 -3 Because of the existing shortage of tin, the production of hot dipped tin plate for the manufacture of tin cansand other articles has been l i gclyfcurtailed and a distinct impetus has consediieritly been given to the'production of electroplated "tinned strip which requires less tin per unit of area for corresponding corrosion resistaiif'e. However, as'it comes from the plating line, electroplated tinned strip has a grayish mat surface which is not only less attractive than the usual bright surface of 'hotdipped plate but for some; reason isless resistant to fruit and other a c ads sothat electroplated tin plate in its natural state conn'ot be satisfactorily used for the manufacture of cans for thefruit canning industry and the like. 'To render it suitable therefor it is consequently necessary to impart a bright surface finish to it and the present invention is directed tothat end.

A principal object of the invention, therefore, is the'provisi'on ofla novelmethod of brightening tinned strip. Y

Other objects, advantages and novel features comprehended by the invention are hereafter more particularly poi'ntedout or will be readily apparent to those skilled in the art from thefollowing description of the method thereof which is" preferably but not necessarily performed through the medium of the apparatus claimed in my copending application for U. S. Letters Patent, Serial No.' 538,405, filed June 2, 1944, which is a division hereof.

'At'the outset it must be. app t the surface-of electroplated tinned strip is of a very delicate nature, easily marred by contact with other objects and when heated very readily affected by changes of temperature, contact with air, gases and liquids andby other factors, while the strip itself exclusive of the plating usually approximates .010 in thickness and the plating .0QQD36" thus, under. these conditions the total thickness. of the tinnedstrip, which is generally produced in '28," "or greater width, is about 010072 Moreover, these strips are rolled and plated "at, speedsrunning upto 600' perminute and even fhigherso that the problem of handling and treating them tos'ecur'e the requisite bright.- ness and freedom from scratches, oxidized or blue; patchescrystalline patterns, shadows and like. imperfections in their surfaces is a distinctly difficult 'onebefc'ause of the inherent. character of the" material itself and the speed, accuracy and perfection with which the operation mustbe performed to render it. commercially practicable.

Moreover, when tin is heated in intimate con-i tact with iron, as when the tinned strip is heated sufficiently to fuse the tin as an incident of the brightening operation, mutualdiffusion occurs; that is, the tin tends to migrate into the ferrous base and to a lesser extent the iron tends to mi rate into the tin so that a zone of intermingled greater is the amount of the tin which enters this zone or iron-tin boundary as it may be termed, with consequent diminution of the body of tin on the strip surfacewhich, in turn, correspondingly detracts, from the corrosion-resistance of the strip. A small amount of this iron tin boundary is nevertheless desirable in the interests of'coating adherence but the boundary itself is very brittle and therefore if it is too thick incipient; cracks may occur in it when the strip is subse- V quently formed to desired shape; it follows that a satisfactory brighteningprocedure must be of a character such that it will suitably brighten the strip without resulting in the creation of an excessively thick iron-tin boundary which, asstated,

not only decreases the corrosion resistance o'f th'e strip but detracts from its forming qualities.

Commercial tin, such as is used for plating, contrains some impurities which are carried over in the bath to the ferrous strip and are therefore present in the coating. To some extent, depending on their character and amount, these impuri ties afiect its characteristics but under average conditions the plating, as it comes from the plating line, is brittle at all temperatures up to about 437 F.; it thencommences to become plastic and fuses at about 450 F.

I have found, by extended experiment, that as the temperature is raised above 437 F. minute globules begin to form at spaced points in the plating which gradually increase in size and numher as they break through the surface untilthey ultimately flow together at about 450. F. thereby producing a very bright mirror-like surface on which a blue and then a yellowish oxide very shortly forms unless the coating is properly and promptly quenched. Thus by suitably heating and quenching the plated strip, its initial granular mat surface may be transformed to one of uniform brilliance with, additionally, a distinct decrease in porosity thereby adaptingthe finished strip for the manufacture of tin cans intended for canning fruit and thev like as well as for other purposes requiring tin plate having a bright, substantially non-porous finish.

The problem of heating the delicate flexible strip while it is traveling at a high rate of speed so as to bring it to the proper temperature to create the desired brightened surface and then quenching it so as to minimize the formation of the iron-tin boundary without marring it through contact with other objects and without production of discolorations, crystalline patches, shadows, wavy effects and other generally similar surface imperfections therefore involves many factors, to some of the more important of which brief reference has been made.

In accordance with the method of the present invention the strip after plating is caused to follow a predetermined path and progressively heated by electrical inductance or a combination of electrical conductance and inductance in such manner that each point on the strip is at approximately 437 F. on its arrival at a predetermined station on said path. Thereafter the rate of heating is preferably reduced as the strip travels farther along the path until the tin is completely fused and the desired bright surface consequently produced, this condition obtaining when the strip has attained a temperature somewhat in excess of 450 F. which is the approximate melting point of the commercial tin usually employed for plating strip, and I have found that to insure uniform and complete melting it is therefore advisable to carry the temperature of the strip to substantially 485 F. as this allows an ample margin of safety over and above the theoretical melting point. The strip is then initially cooled until the molten tin has hardened or set or, in other words, has attained the lower limit, namely 437 F., of its plastic or transition range between that temperature and 450 F. or thereabouts, and in the interest of prevention of excessive iron-tin boundary this cooling is effected as rapidly as may be without causing surface blemishes, but as the tin during this initial cooling or quenching is in a molten or plastic condition the quenching medium must of necessity be one which will have no inimical effect upon it and therefore in the nature of a gaseous one such as air. However, as soon as the tin has solidified a liquid quenching medium may be utilized to best advantage and the strip is therefore carried into a bath of water or oil, which I have found must be maintained within a definite temperature range as otherwise various surface defects and blemishes appear on the strip surface, and the strip is maintained in this quenching medium until it desirably attains the temperature thereof after which it is carried out of the bath and the liquid adhering to the strip removed preparatory to coiling the strip for transportation and storage. In accordance with my invention these several operations are performed in coordinated sequence in such manner that the finished strip is satisfactorily and uniformly brightened throughout its entire surface and on both sides, and while the said method may be practised by the use of any apparatus suitable for its performance, that now to be described which forms a part of my invention and which is claimed in said divisional application Serial No. 538,405, is well adapted therefor and in consequence I prefer to employ it for the purpose.

To enable those skilled in the art to comprehend the invention I shall first describe the said apparatus, two forms of which are shown in the accompanying drawings, and shall then refer more particularly to its use in the practice of 4 the said method. Referring, therefore, to the drawings,

Fig. 1 is a substantially diagrammatic side view partially in vertical section of apparatus in which the heating of the strip is effected by electrical induction, and

Fig. 1a is a generally corresponding view of apparatus in which the strip heating is effected by a combination of electrical conduction and induction.

Fig. 2 is a fragmentary detail elevation of a portion of the liquid quenching means, and

Fig. 3 is a generally similar detail view of a portion of the means primarily utilized for freeing the brightened strip from the liquid quenching medium.

Throughout the drawing the same characters are employed to designate corresponding parts and it will be understood the devices shown in Figs. 2 and 3 are equally applicable to both forms of apparatus.

That shown in Fig. 1 to which reference will first be made is adapted to be disposed closely adjacent the plating tank from which the strip progressively emerges after receiving on both sides its coating of tin and is designed to define a predetermined path for the strip so that as it moves therealong it is subjected successively to the various operations incident to brightening the coating in accordance with the method until it finally emerges from the apparatus in fully brightened condition and thus ready for any further treatment or operations such as passing it through a branning machine to thoroughly dry it preparatory to coiling. To this end the apparatus includes a series of rolls to guide the strip S (the relative thickness of which is considerably exaggerated for convenience of illustration) as it is received from a pair of bridle rolls I located between the apparatus and the plating tank, said series preferably comprising a roll 2 under which the strip first passes and is thence carried vertically upward and over another roll 3, thence horizontally and down over a third roll 4, under a fourth roll 5 located in a quenching tank 6 hereinafter described, thence between pairs of rolls 1, 8 and 9 in a generally upward direction and finally down and beneath a final roll I!) whence it proceeds in a horizontal direction to the branning machine and coiler. All the rolls are preferably faced with rubber or other suitable material which will not abrade the delicate surfaces of the strip and the rolls 8 or 9 or either of them may be suitably driven so as to pull the strip through the apparatus, or the coiler to which the strip is conducted may be utilized for the purpose as will be readily understood.

Beneath roll 4 which serves to direct the strip vertically downward, is arranged an induction coil C which surrounds the strip with an appropriate clearance at all points and therefore defines a narrow elongated substantially rectangular opening through which the strip can freely pass. Because of the heavy high frequency current employed, this coil is preferably made of a length of copper tubing and a constant circulation of water maintained through it by suitable fiexible connections (not shown) to the free ends of the coil, the current being supplied to and flowing from the coil through leads l2 respectively connected adjacent its ends. Thus when the coil is energized, a current is induced in the strip as it progressively passes therethrough with resulting rapid rise in strip temperature in accordance with the usua1 and well known principles of induction heating. It will be noted, however, that the lower turns in the soil are more widely spaced. than those of its upper-or major portion for a reason hereinafter explained.

The coil is preferably housed in ;a casing I3 arranged in a frame M, the casing at its upper and lower ends being substantially closed save for relatively narrow slotsrespectively designated 15, I6 through which the strip passes, this .arrangement tending to protect the strip from undesirable air currents in the vicinity-of the coil. A pair of small guide rolls I7 is also mounted on'the casing frame above slot l Ewiththeirpass in alignment therewith, these rolls serving :to properly guide the strip :to the coil and minimize any tendency to whip, preventing entrainment of excess air by'the rapidly moving strip as it passes into the coil casing and also decreasing any counter flow of air from the casing, while adjacent the lower end of the latter a pair of bafiles is also attached to thecasing frame for a purpose hereinafter explained. The strip is therefore very accurately guided into the coil which, :in consequence, may be constructed so it will closely surround the-strip with minimum clearance with corresponding increase in heating efliciency,while the air in the coil casing remains substantially quiescent as is essential for most satisfactory results since air disturbance or turbulence at this point leads to lack of uniformity in the brightness of the strip.

As for optimum results it is essential that the time between the point at which the tin attains its maximum fluidity and that at which it is initially subjected to the liquid quench be constant or substantially so, the coil assembly is arranged to permit its vertical adjustment in accordance with the linear speed of the strip, the specific means provided for effectingthisadjustment being a matter of choice. Thus in the drawing frame M is shown as suspended by a cable 2i extending over a sheave 22 above roll 4, thu enabling the assembly to be raised and lowered as desired, the frame, under such conditions, being preferably arranged to travel in stationary guides (not shown) to insureitscorrectalignment with relation tothe strip.

Mention has been made of the quenching tank 6 which is disposed below the lowermost portion of the coil and adapted to contain a relatively considerable body of quenching liquidwhich may be either oil or water, the former being generally preferred. Roll 5 is arranged proximate the bottom of the tank preferably near one endthereof so that as the strip leaves the roll and passes to guide rolls 5 located near the top and toward the other end of the tank, it will-have .a longer run through the liquid than if rolls 1 were more nearly vertically aligned with roll 5,.and .it will be appreciated that if it is desired to increase the length. of this run additional rollsgenerally corresponding to 5 may be arrange'din thetank and the strip carried in a tortuous path thereover and finally to rolls 1 instead of directly thereto as shown. In operation, the level of the liquid L is preferably maintained a little .below rolls 1 substantially as indicated.

A casing 25 the lower end of which is submerged in the liquid extends upwardly from the tank to receive the strip after it emerges from between the baflies Z9 and an air-duct 25 enters the side of this casing at a point-above the tank and communicates with a blower 21 operative toiorce air into the casing in controlled quantities.

ASSO- thepairs of distributing pipes:

l ted wi h th top 0. h c sin h ch i re erably closed-in save for a central slot 25 ifOI-thfi passage of the strip, are pairs of oppositely inwardly directed vertically spaced baliies 23, 2-9, the primary function of these several baffles, which are preferably made adjustable by any suitable means, being to gradually and progressively diminish the velocity of the air expelled upwardly from the slot in the casing and effect an even distribution of it over the strip. When the tin is so highly heated that it is fluid or sub.- stantially so, it is susceptible to disturbance by comparatively light air currents impinging upon it but as it progressively hardens this tendency correspondingly lessens o that the relative velocity between the downwardly moving strip and upwardly moving air may be increased to' accelerate its cooling rate until the tin is hardened sufiiciently to permit a liquid quench to be employed, and the battles are therefore designed to efiect-this result.

However as has been explained, it is advisable to increase the distance between the coil and the quenching tankin correspondence to increases in the linear speed of the strip and, consequently, it is similarly advisable to increase the length of casing 25 so that substantially the same relation between the baiifles and slot 25 and the coil will be preservedat any position of the latter within the limits of its permissible movement as when it is Zin its lowest position. To this end the top of the casing may be made lremovabletopermit the insertion of additional casing sections .125" between it and the nqain easing so that for each increment which the coil assembly is raised, a generally corresponding extension of the casing upwardly maybe efi'ected to maintain substantially-the same relation.betweenthe parts; thus, when the coil-assembly is disposed, for example, in its highest'position, suihcient sections 25 are interposed-to bring the casing top to the position indicated 'in dotted lines.

On opposite sides .of the strip within casing 25 andabove the normal level cf the quenching medium, means a-reprovided for directing streams or :rathersheets of the latter against the strip, said means, which are best illustrated Fig. :2, comprising a pair of pipes-3f) disposed onopposite sides of the strip and eachprov-ided with a longitudinal slotrel :facing generally toward the strip so that :liuid introduced intothe pipes under suitable pressure will be projected through the slots against the proximate faces of the strip. Other=similarpipes Iii-are arranged farther along the .path .of the sheet and beneath the normal. liquid level, thesepipesbeing likewise provided with slctsfS-i. Pipes and 32 on each side of the strip-are respectivelyinterconnected at their ends by risersid from which conduits'sii, Sirespectively extendto a heat exchanger-E in turn interconnected by alargerpipe 3'5 with tank's, apum-p -P. being interposed inlthis pipe and adapted to draw fluid from the tank and force it through theheat exchanger and-conduits Sei, (it to from which it isproiected against the st e valves being arranged adjacent these es for centrolling .the amount of fluid through them. In order-to permitvariatienin theangle at which the fluid strikes thestrip, pipes are arranged sothey are susceptibleof rotative adjustment,.for:eXample and as shown, byproviding them with 3 "fl ges abutting cor-respendin :flanses carried by ris so that removin th .boltsu i ine th flange th m nes 7 can be revolved in either direction in increments determined by the spacing of the bolt holes and then reassembled. However, any other appropriate means for effecting this adjustment may be utilized if preferred.

It will thus be apparent that when pump P is in operation each of pipes 38 is effective to project an unbroken sheet of the quenching medium against the proximate side of the strip at a point above the level of the liquid in the tank and that pipes 32 operate similarly at a point farther along the path of the strip after it has entered the body of the liquid, the primary function of these latter pipes being to break up and force off the sheet any of the quenching medium which tends to adhere thereto and might thus prevent the fresh relatively cool liquid in the tank from directly contacting both sides of the strip, while the primary function of the other pipes 38 is to bring the quenching medium directly against the sheet at the earliest possible moment and, in addition, prevent ignition of the liquid in the tank should the strip accidentally enter at too high a temperature as the curtain of liquid produced by these pipes then prevents the oil in the tank from flashing.

As the strip progressively emerges from the liquid and passes to rolls 1, some of the liquid adheres to it and this it is desirable to remove not only in the interest of conservation but because tinned strip intended for can manufacture must be substantially devoid of all traces of oil, and means are therefore provided for freeing the strip from the major portion of this adherent oil.

In their preferred embodiment, said means comprise one or more pairs of pipes 40, 4! respectively disposed one above the other on opposite sides of the strip above rolls 7 and each provided with a longitudinal slot 42 adjacent the strip adapted to direct air against its proximate face. As best shown in Fig. 3 one end of each pipe is blind while the other is extended to one of a pair of headers 43, 44 respectively connected with the branches of a conduit 45 leading to a blower 46 for supplying air unde suitable pressure. As in the case of fluid pipes 30, 32, pipes 46, 4| are arranged for axial rotative adjustment so as to permit variation of the angle at which the sheets of air which issue from them impinge upon the strip, the effectiveness of the air in removing the liquid film being enhanced if the pipes are inclined oppositely downwardly from their ends toward their centers as shown in Fig. 3 as the air then tends to force the film oppositely outward toward the edges of the strip. Under some conditions it is sufficient to employ but one pipe on each side of the strip though usually two or more pairs are preferred, and after its passage between them the strip is not only substantially free from quenching liquid but is additionally cooled; it then leaves the apparatus through the superj acent rolls with an almost mirror-like finish on its opposite faces.

In the practice of the method with an apparatus of the character just described, the plated sheet is first carried about the various rolls so it will properly follow its predetermined path and the tank filled to the proper level with a suitable quenching liquid such as oil, one having a viscosity between 40 and 125 Saybolt at 100 F. being preferred since oil above 125 Saybolt quenches so slowly the strip has a dull brightness while oil below 40 Saybolt although giving a bright surface tends to form patterns because of the rapidity of the quench. During the brighten- 8 ing operation the oil in tank 6 should be maintained between and F. and a steam heating coil 41 disposed in the bottom of the tank may be availed of to initially bring it within this range and thereafter to maintain it there if necessary.

The strip being thus threaded and the oil properly heated, coil C may be energized, the pumps and blowers put in operation and the strip set in motion so it will progressively pass through the coil, thence through the countercurrent of air from casing 25, thence through the casing and between pipes 30 where it is drenched with the quenching liquid, thence into the body of the latter between pipes 32 which as hitherto explained insure its being contacted with fresh oil and which also serve to keep the oil in the tank in a constant state of agitation, thence about roll 5 to roll I, thence between pipes 40, 4| where adherent oil is removed, and finally under roll ID as it leaves the apparatus.

IAS hitherto explained, the plating on electroplated strip as it comes from the plating tank is relative brittle and hence the strip cannot be bent without likelihood of small cracks being formed in its coating; thus by the tim the strip passes roll 4 it may exhibit these cracks but after the coating is brought to fluid or substantially fluid condition they naturally disappear and have no tendency to re-form so long as the strip is moved in a straight line. Therefore to avoid their reappearance subsequent to the heating step, roll 5 is so positioned that the strip before or just as it reaches the roll has been lowered to a temperature not exceeding 322 as it can then be safely carried about the roll and subsequently manipulated without danger of cracking the plating. To this end and to meet varying conditions of operation, roll 5 is preferably arranged for vertical adjustment relatively to the tank so it can be disposed to accord with the temperature of the strip as it reaches the roll.

As the relatively cold strip reaches guide rolls I! and enters coil C its temperature is rapidly raised by the current induced in it until that point is attained at which the tin begins to soften. The temperature then approximates 437 F. and if the same heating rate with relation to the linear travel of the strip, normally 500 or more feet per minute, is continued, there is nsufiicient time for the tin after it reaches its melting point of 450 F. to form a smooth unbroken film on the ferrous base if it then passes out of the influence of the coil whereas if it remains within its influence until the surface has attained the desired character, the tin becomes too hot and turns permanently bluish, probably from excessive oxidation.

Therefore with a view to'decreasing the rate of heating during the transition or plastic stage of the tin between 437 F. and 450 F. as well as while it is thereafter flowing to form the desired unbroken film, the lower turns of the coil are spaced farther apart than those of its major or upper portion; thus when the spacing of the last two turns, for example, is twice that of the other turns the temperature rise which they impart to the strip will be the same as that imparted by any two of the other turns but will require twice the time for each degree of rise.

For optimum results the maximum temperature of the tin when it leaves the coil should not greatly exceed 485 F., since higher temperatures, as has been noted, render the tin bluish in color and the tin should be cooled as rapidly as possible after this temperature is attained and air or other gaseous medium must be initially employed to reduce its temperature until it is set or hard med, 1. e., has attained a temperature of 437 F. or less, for if the tin is subjected to the action of a liquid quench while molten or plastic its subsequent appearance is utterly ruined. 'On the other hand, air cooling tends to increase the production of oxides on the surface of the tin and of the iron-tin boundary to which reference has been made so that, with a view to employing air cooling for the minimum essential time, the height of the coil assembly is so adjusted with relation to the linear speed of the strip that the distance between the point at which the strip attains its maximum temperature, which is approximately the lower end of the coil, and the point the liquid sprayed from pipes 39 first contacts it, is such that the tinwill have solidified before the latter point is reached; in other words, so that the time required for a point on the strip to pass from the coil to the liquid received from pipe as is just sufficient to reduce the tin temperature from its maximum to 437 F.

Moreover, when the tin is in its condition of maximum fluidity as when it emerges from the coil, it is readily disturbed by air currents and the bafiies 253, 23 and 29 are therefore designed to first protect it from such currents and then progressively increase the relative velocity between the strip and the upwardly moving current of air from slot 25' until it passes into casing 25, the baffles being desirably made adjustable for this purpose so their effect can be coordinated with strip speed and other operating conditions, all with the object of solidifying the tin as rapidly as possible in air to a point at which it can be safely subjected to the liquid quench.

The strip now enters the body of liquid in the tank and, as has been explained, continues without substantial change of direction until its temperature is lowered to 322 or less at which time it can safely be carried about roll so as to change its previous path and carry it to rolls 1, the arrangement being such that it will desirably reach the latter at approximately the temperature of the liquid. If the length of the run between rolls 5 and l is insufficient to obtain this result at higher strip speeds, other rolls (not shown) may be disposed in the tank and the strip carried about them in a tortuous path until it ultimately reaches rolls 1. Just after the strip passes into the body of the liquid, it is subjected to the sheets of liquid projected from pipes 32 which forcibly contact it and break up any film of liquid which may be adhering to it, thus insuring the maximum cooling effect of the quench; these pipes also maintain constant agitation of the body of the liquid which, it has been found, materially enhances the rapidity of the quench over that secured when the liquid is unagitated, and therefore serve a double and important function.

Upon leaving rolls 1 which remove some of the adhering oil, the strip passes between the pairs of pipes M, 4! and the high pressure air they eject against its opposite sides frees it from substantially all the balance and also effects a further reduction in temperature so that the strip ultimately leaves the apparatus at or above the maximum temperature of the liquid quench and is thence passed, preferably at a temperature of around 150 R, either through a branning machine to remove the last traces of the oil and thence to a coiler or else direct to the latter.

By way of example it may be stated that for optimum results a strip .010" thick having a tin coating .000036" thick should be subjected to the air quench for more than .4 second and less than 1 second where the relative maximum velocity between the strip and the air is approximately 900 per minute, that is, a point on the strip should travel from the last turn of the coil to the pipes 36 in from .4 second to 1 second, and that the entire time a point on the strip is subjected to the liquid quench should not exceed 1.5 seconds of which not less than .1 second should be consumed in its traverse from pipes 36 to roll 5. These values therefore dictate to a large extent the dimensions and arrangement of the apparatus.

In the apparatus heretofore described the heating of the strip from room to maximum temperature is effected entirely by induction through the medium of coil C, but in the apparatus shown in Fig. la the strip is preliminarily heated by electrical conduction and under some conditions it may therefore be preferred. This apparatus therefore includes a source of current supply such as a generator G, furnishing direct or alternating current, connected by suitable leads 5,0 with roll 4 and a contact roll 5i located at a suitable point in the vicinity of bridle rolls I so that the current from the generator will enter the strip by this roll and leave it at roll 4 which, of course, is not in this instance rubber faced as it also is employed as acontact roll. Because of the very heavy current required in cpnduction heating, small burned spots tend to appear on the strip unless it is coated with a film of liquid, preferably 011, when initial electrical contact is i made with it; the apparatus therefore includes a relatively small tank 52 containing a roll 53 around which the .strip passes after leaving the bridle rolls and a small pair of squeeze rolls 56 located above the tank in advance of contact roll 5! so that when oil or other liquid is maintained in this tank the strip will pass therethrough before reaching contact roll El, any excess being squeezed on by rolls 5!? and dripping back into the tank. In other respects the apparatus of Fig. la is substantially similar to that of Fig. 1 and requires no further description.

When utilizing the combination of conduction and induction heating to attain the desired maximum temperature, the former is employed to raise the strip from room temperature to not in excess of 322 so that the strip may b carried about roll 3 without the injury which would occur if its direction of travel were materially changed at a higher temperature. The conductively heat ed strip then passes to coil 0 by which it is heated to the final temperature and preparatory to cooling and quenching as hitherto explained. The coil may therefore be designed to raise the temperature of the strip for only some 163 instead of some 415, assuming a starting temperature of and the initial cost of the apparatus as a whole consequently materially reduced; per contra, the combination heating necessarily involves regulation of two separate circuits, higher heat losses and certain operating disadvantages over the all induction heating method.

It will be understood that while I have herein described with considerable particularity one manner of performing the method of my invention by means of certain apparatus I do not thereby desire or intend to confine or restrict myself in any way since modifications and changes may be made in the practice of the method and other apparatus employed if desired, without departing from the spirit and scope of 11 the invention as defined in the appended claims.

Having thus described my invention, I claim and desire to protect by Letters Patent of the United States:

1. The method of brightening tinned strip which comprises progressively moving the strip longitudinally in a predetermined path while progressively heating it until it attains at a point on said path a temperature of approximately 437 F., further heating it to a temperature higher than the melting point of the tin coating but at a slower rate per unit of travel and, on attainment thereof, immediately cooling it in a gaseous medium to approximately the first mentioned temperature, and then quenching it in a liquid quenching medium to a temperature substantially below said first mentioned temperature.

2. The method of brightening tinned strip which comprises progressively moving the strip longitudinally in a predetermined path while progressively heating it until it attains at a point on said path a temperature of approximately 437 F., further heating it to a temperature higher than the melting point of the tin coating but at a slower rate per unit of travel and, on attainment thereof, immediately cooling it in a gaseous medium to approximately the first mentioned temperature, then quenching it in a liquid quenching medium to a temperature substantially below said first mentioned temperature, removing excess of the medium from the strip surface, and then further cooling it in air.

3. The method of brightening tinned strip which comprises the steps of moving the strip along a substantially rectilinear path while progressively heating it until it attains a temperature of approximately 437 F. at a given point on said path, further heating it at a slower rate per unit of travel until at a point farther along said path it attains a temperature between 450 and 485 F., then immediately cooling it in air to a temperature of approximately 437 F., next quenching it in a liquid to a temperature not greater than 322 F., and finally removing it from the liquid and further cooling it in air,

4. The method of brightening tinned strip which comprises the steps of moving the strip along a susbtantially rectilinear path while progressively heating it until it attains a temperature of approximately 437 F. at a given point on said path, further heating it at a slower rate per unit of travel until at a point farther along said path it attains a temperature between 450 and 485 F., then immediately cooling it in air to a temperature of approximately 437 F., next quenching it in a liquid maintained at a temperature between 125 and 175 F. to a temperature not greater than 322 F., then diverting the strip from said path, and finally removing it from the liquid and further cooling it in air.

CARL G. JONES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number

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