US2513713A - Method of carburizing low carbon strip steel - Google Patents

Description

July 4, 195% F. T. COPE METHOD OF CARBURIZING LOW CARBON STRIP STEEL Filed Nov. 24, 1947 3 Sheets-Sheet 1 ZSnnentor Fran/2 T. @bpe (Itfornegs Filed Nov. 24, 1947 F. T. COPE 25%,,733

METHOD OF CARBURIZING LDW CARBON STRIP STEEL 3 Sheets-Sheet 2 Z1 Z3 5 Z4 Z s i 29 5 j l l i [6 5 I 5 75 p f f4 I 1 2 1/ 33 39 3nventor attornegs e ufiy 4, 1950 F. T. COPE METHOD OF CARBURIZING LOW CARBON STRIP STEEL 3' Sheets-Sheet 3 Filed Nov. 24, 1947 Patented July 4, 1950 UNE'HED sra'rss PATENT orrics METHOD OF CARBURIZING LOW CARBON STRIP STEEL Frank T. Cope, Salem, Ohio, assignor to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Application November 24, 1947, Serial No. 787,798

2 Claims. (01. 148-165) The invention relates to carburizing steel and more particularly to methods of continuously carburizing steel in sheet or strip form during continuous endwise movement of such steel.

There is a demand for light gauge cold rolled high carbon strip steel for use in the manufacture of razor blades, steel measuring tapes, springs, and other similar articles. Heretofore such cold rolled high carbon strip steel has been manufactured from hot rolled high carbon strip steel by cold rolling, pickling, annealing and normalizing operations until the desired finished gauge and characteristics are obtained.

Thus when a carbon concentration of say 1.10% to 1.25% carbon is desired or required in the finished product (say 0.007 x 1%" or 0.007 x 9" cold rolled strip), the carbon concentration in the hot rolled strip and at each stage of reduction must not be below 1.10% to 1.25% carbon, and the processing of high carbon steel is involved at every step, The cold rolling of high carbon steel is initself a more diificult operation than the cold rolling of low carbon steel. A greater number of annealing operations are required in the reduction of high carbon steel from say 0.80 gauge hot rolled strip to 0.00'7or 0.008 gauge finished cold rolled strip than in the similar reduction'of low carbon steel.

Furthermore, the annealing or normalizing or heat treatment of high carbon steel is a more diflicult operation than the annealing or normalizing or heat treatment of low carbon steel. More critical controls are required because of the ever present tendency-to-decarburize high carbon steel during any processing involving heating. For these reasons it is usually always necessary, in annealing, normalizing or heating high carbon steel to use a controlled furnace atmosphere with high carbon pressures in the atmosphere so as to insure against decarburization of the steel, the occurrence of which would prevent obtaining the desired high carbon content in the finished material. On the other hand, in processing low carbon steel, carbon loss is unimportant and no special precautions need be taken to avoid carbon loss.

These factors have added substantially to the cost of making light gauge cold rolled high carbon strip steel. Light gauge cold rolled high carbon strip steel might be produced at substantially lower cost by thelow cost rolling of low carbon strip steel in the usual manner to approiiimately the desired finished gauge, if it is: then possible to carburize'the cold rolled low carbon steel to 2 obtain the desired high carbon concentration in the strip.

Many years ago in comparing the propertiesof gases, to be used as protective atmospheres in the heat treatment of metal, by testing gases by the methods set forth in my Patent No. 2,049,947, I noticed a phenomenon occurring under certain circumstances. At certain temperatures iron carbide was produced on the steel test strip when testing natural gas. In other words, a reaction occurred between the natural as atmosphere and the steel test strip resulting in a carbon pick up by or ready carburization of the strip. Under these circumstances the conditions for carburizing theoretically are ideal. The space around the strip is relatively cold so that cracking of the atmosphere for carburization takes place only on the hot metal surface, the metal being internally resistance heated electrically in the gas testing operation. Nascent carbon cracked from the atmosphere is absorbed by the steel as fast as it is deposited. Theoretically there is no soot deposition on the metal surface which remains bright during the gas testing operation. Under such conditions, a maximum rate of carburization should occur.

These principles so discovered in testing natural gas, applied to the continuous carburizing of light gauge cold rolled low carbon strip steel seem to be the answer to the problem of producing light gauge cold rolled high carbon strip steel. However, repeated attempts to apply these principles in carburizing strip steel led to the discovery that the phenomenon observed in testing the properties of natural gas with avery narrow test strip could not be used to carburize a wider strip, say 1%" wide, and provide a uniform carbon content across the width of the strip.

When it is attempted to increase the strip width from a very narrow strip to say, a 0.008 x 1% strip, and to carburize the same in the manner described, the induced heat in the strip, in addition to heating the strip, is dissipated by radiation which does not occur uniformly in a strip of such width. Therefore, the strip at the time of the carburizing reaction is not uniformly heated and is not uniformly carburized. High carbon concentration occurs in the center of the strip and low carbon concentration occurs at the strip edges.

Attempts made by varying the strip speed (time) and power input (temperature) to obtain the desired amount of carbon, uniformity of carburization throughout the strip width, and

Better diffusion of the carbon can be obtained by keeping the power input (stri temperature) on the low side and by decreasing the speed (increasing carburizing time). Likewise a low power input prevents burning or blistering of the strip.

It was discovered further that ammonia added to the carburizing atmosphere affected'the carbon pressure but it did not correct the unequal carbon distribution across the strip width and it increased the nitrogen content of the steel. Also, the carbon pressure of an endothermic gas is too low, and an endothermic gas must be enriched by hydrocarbon additions. In addition it was discovered that a heated quenching bath preferably lead is necessary to enable the carburized steel to be treated continuously without excessive hardening such as to result in breaking and at the same time to be provided with the desired high carbon content, the quenching temperature being controlled to avoid the excessive hardening of the steel;

Many changes were madein the construction of the equipment in attempts to eliminate or correct the low carbon edge and high carbon center difliculty without results.

Finally I discovered that heating of the carburizing chamber external to the strip is necessaryin conjunction with-internal resistance heating of the strip to obtain a uniform high-carbon content across the width of the strip; that there must bean interrelated'and controlled balance-.be-

tween the external heating temperature and the power input to the strip'to obtain the'uniform carbon content across the width of the strip; that if too high a furnace temperature is used, beading or fusing of the stripedges may result; that'natural gas or natural gas diluted with an endothermic or other gas can be used to produce "a satisfactory atmosphere for continuously -carbu-' riz'in'g light gauge low carbon strip steel to a desired "high carbon value; and that under these conditions low carbon strip steel inwidths rang: ing up 'to 9" or more and gauges rangihg from 0.003" to 0.016" can be carburized'successfully to a desired high carbon value having a uniform carbon content across the width of the stripexcept for a possible higher carboncontent inthe zone comprising about the first to of an'inch at 'each edge of the strip. This narrow high carbon zone at each edge ofthe strip can be slit or shearedthere'fro'm.

It was further discovered that when a continuously moving strip is heated both'by internal resistance heating andfurnace heating external .to the strip, the coordination and accurate interrelated control of the balance between internal and external heating necessary for pr'oducinga uniformly carburized strip also can be used selectively to produce a strip having high carbon edges and a low carbon center or lowcarbon edges and a high carbon center. Thus, the temperature 'of three zones of the strip-one edge, thecenter, and the other edgemust be'c'onsidered and'recognized in connection with the control of heating 4 during carburizing in order to maintain the desired balance of heating.

It is therefore a primary object of the present invention to substantially improve the existing methods of making light gauge cold rolled high carbon strip steel.

Also it is an object of the present invention to provide a way of making high carbon strip steel by first cold rolling low carbon strip steel to substantially the desired finished gauge and by then uniformly carburi'zing the strip.

Also it is an object of the present invention to control therelation between internal and external heatappliedto-the strip so as to obtain uniformcarburization of the strip throughout, in the continuous carburization of low carbon strip steel.

Finally, it is an object of the present invention to provide new methods and steps for the manufacture of light gauge cold rolled high carbon strip steel which are simple to carry out anduse, and are precise, effective and accurate in operation, which reduce the cost of making such material, which overcome the foregoing prior art difiiculties, which solve long standing problems in the art of manufacturing high carbon strip steel, and which obtain many new results and advantages herein set forth.

These and othe'r'ob'jects and advantages apparent to those skilled in'the art from the following description and claims may be obtained, the stated results achieved, and the described difficulties overcome, by the processes, methods, steps, procedures, operations, discoveries, and principles, which comprise the present'invention, the nature of which is set forth in the following general statement, a preferred embodiment of which illustrative of the best mode in which the applicant has contemplated applying the principlesis set forth in the following description, and which are particularly and distinctly pointed out and set forth in the appended claims forming part hereof.

The nature of the improvements in methods of continuously carburizing light gauge low carbon strip steel of the present invention may be stated in general terms as preferably including the steps of passin a strip continuously through a sealed heating chamber in the presence of a carbon containing or carburiz ing gas, electrical resistance heating the strip internally to a strip temperature prefer-ably of about 1800" F. and -,not above approximately 2000 F., heating the chamberv through which the strip passes externally of the-strip to control or avoid radiation heat loss from the strip and obtain :a substantially uniform strip temperature across the width thereof particularly at the zone of maiiimum temperature as the strip continuously passes through the chamber, whereby the carburiz'ing atmosphere in the chamber reacts with the strip to uniformly carburize the continuously moving strip; then passing the strip through a current conducting molten bath, such as molten lead bath, maintainedat-a temperatur in excess of Figure lis a diagrammatic side elevation'of a preferred arrangement of improved 'continuous strip carburizing apparatus; r

Fig. 2 is an enlarged side View with parts in section of the vertical carburizing furnace illustrated in Fig. 1; p

Fig. 3 is similar view of the normalizing furnace illustrated in Fig. 1; q

Figs. 4 and 5 are diagrammatic views illustrating certain heating principles-involved in carrying out the methods of the'present invention; and

Fig. 6 is a diagrammatic sectional "view illustrating control means for the carburizing furnace. Similar numerals refer to similar parts throughout the various figures of the drawings.

The improved carburizing apparatus-is diagrammatically shown in Fig. 1 and includes a vertical, preferably gas fired, mulile type, carburizing furnace generally indicated at l, a'preferably gas fired, radiant tube heated, roller hearth normalizing furnace generally indicated at 2, and cooling tunnel generally indicated Coils 4 of light gauge low carbon cold rolled strip steel may be unreeled at 5, and fed, as indicated generally at- 6, through the carburizing furnace l, the normalizing furnace 2 and cooling tunnel 3. The strip is then rereeledas coils l on reels generally indicated at '8 having suitable drive equipment for pulling the strip B through the carburizing and normalizing operations'as indicated. If the coils 4 have not been cleaned previously or otherwise prepared for the carburizing operation, the strip 6 may be passed through a vapor degreaser generally indicated at 9 to remove roll oil from the strip prior to its' entry into the carburizing furnace I.

Referring more particularly to Fig. 2, the furnace l preferably comprises a vertical circular' shell Ill suitably reinforced and supported by structural steel members II' and I2 and supported thereby on the floorjl3. Thefurnace is preferably lined with suitable insulating refractory material l4 to provide a circular heating chamber l5 within which is located a vertically extending carburizing furnace muffle l5, preferably formed by sections of centrifugally cast heat resisting alloy metal welded together to form -a continuous tubular mufiie, the interior ofv which constitutes the carburizing chamber ll. v

The furnace I may be provided with a structural steel platform I8 around the top of the muffle l6. A furnace muffle entrance seal and housing member I9 is mounted in a suitable manner on the top or entrance end of the tubular mufile l6 and the sam may include a trough portion 20 containing mercury indicated by the liquid level 2 I.

An idling roller 22 is mounted in the trough portion 20 and a mercury wiper 23 is mounted within the housing l9. Another idler roll 24 is mounted on the outside of housing 19 and a third idler roll 25 is mounted within housing I9. The strip 6 in passing into the carburizing muffle it passes over roll 24, through mercury seal 2!,

under idler 22, past wiper 23, over idler roll 25,

and thence vertically downwardly within carburizmg chamber I! in mufiie l6.

The mercury sealing bath 2i also serves as an electric terminal in the resistance circuit later to be described, current being supplied to the mercury by conductor 26. An outlet pipe 21 preferably communicates with entrance housing l9 through which spent carburizing gases :may be exhausted from the carburizing chamber ll.

Suitable inspection and access openings, not shown, may be provided for the entrance housing l9; and the'housing I9 is insulated electrically from the muflle tube H6 at the connection 28 therebetween.

The heating chamber I5 surrounding the muffle It is heated by a number of fuel burnerslfi which preferably are gas burners although any suitable type of fuel may be used. The gas or other fuel, and air for combustion, may be supplied by .any usual suitable equipment, not shown. Control valves associated with the burners and drive mechanisms therefor, operatedby usual automatic temperature control mechanisms are provided so that the temperatures in various zones and throughout the heating chamber 15 may be automatically controlled at any desired degree. Also suitable recording and indicating pyrometers and automatic safety equipment for shutting off the fuel in the event of failure of air and fuel supply may be provided in the usual manner.

Although the carburizing furnace I has been described as being preferably a gas fired muffle type furnace, the furnace carburizing chamber ll thereof and the carburizing gas therein may be heated by radiant tubes in a manner similar to the radiant tube heating of the normalizing furnace 2; or the chamber ll of furnace I may be heated by electric resistance heating elements. The important feature in accordance with the present invention is that the atmosphere or carburizing gas in the carburizing chamber l! of theflcarburizing furnace I must be heated by means external of the strip 6 being carburized therein.

An exit housing 30 is mounted at the discharge of bottom end of the muflie tube It, suitably insulated there-from at 3i preferabl incorporating bafile plates and a receptacle containing lead indicated by the liquid level 32 and an idlerroll 33 immersed in the lead bath. The exit housing 30 also is provided with a carburizing atmosphere gas inlet pipe 34. The lead bath also serves as an electric terminal in the strip resistance heat ing circuit and current may be supplied thereto through electrical conductor 35.

A tubular extension 36 of the exit housing 30 communicates with the entrance tube 3! of the normalizing furnace 2, an idler roll 38 being provided in the entrance tube 31, as indicated. The exit housing 3%! may be provided with tubular heating members 39 fired .b burners Ml to heat the lead 32 therein to the desired temperature;

or the burners 46 may be turned off and the lead may be cooled by blowing air through the tubes 39 so as to accurately control the temperatur of the molten lead sealing bath 32.

The lead bath is automatically heated or cooled to maintain a constant selected temperature in excess of about 750 F. through suitable control apparatus including pyrometers, control valves,

thermocouples and air and gas valves and valve operating mechanism of a usual type, not shown.

Suitable peep holes such as ll and Ma are provided in the exit housing 30 between the exit end of the mufile l6 and the lead sealing bath 32 and in the lower end of the muffle it to permit observation of the strip temperatures with optical or radiation pyrometers; and suitable clean out and inspection openings, not shown, may be provided in the exit housing 30.

The strip in passing through :furna'ce mufile [6 is mternall-y heated byielectricaltres'istance by current supplied to'the strip through-conductors 26 :and :35 leading, respectively, to :the -.mercury and :lea'd baths 2i :and 32 which serve asterminals for the electric resistance circuit in: addition to their function as gas .seals forithe mufflebha her [1. The supply of power for :therresistance heating of .the strip in themufile :may be controlledin the. mannerflater describedsso *that the temperature .of the strip, due to internal resistance .heatlng,.:may :be accurately controlled.

Wiper -means 4'2 ;-may..b.e provided in the:tubular extension36 for removing leadlfrom the surfaces of the'carburizedistrip 'before it passes into the normalizingtfurnacel.

. Referring particularly to Fig. the normalizing furnace preferably :includes :a I shell 4.3 suitably reinforced with structural steel members 44 andsupported onfioor zl3. The shell is lined with a suitable refractorymaterialfi to form a furnace chamber 46 heated by means f radiant tube-heating elements 46a. The normalizing furnace 2 is preferably provided with a hearth consisting of tubular heat resisting alloy rollers diagrammatically indicated at 41 extending crosswise of the chamber 46 and supported on bearings outside of the furnace shell 43. The hearth rollers 41 may be driven by suitable drive mechanism coordinated and synchronized with the drive for the rereeling equipment 8.

The entrance tube -31 communicates with the entrance opening 48 of the normalizing furnace and'the cooling-tunnel 3 extends fromtheexit opening 4-9 of the normalizing furnace. 'Ihezcooling hood or tunnel 3 is preferably of a water jacketed type equipped with suitable pipinglfor the circulation of water between the inner and outer jackets to provide for cooling the strip before-its'discharge to room temperature afternormalizing. The co'oling'tunnel 3 also maybe supplied with driven rollers diagrammatically indicated at 50 for conveying the strip -.through the cooling tunnel.

The usual pyrometers, control valve, valve drive mechanisms, thermocouples,combustion air blowers,vgas lines and safety equipment therefor are provided for controlling'th'e heatingof the strip in the-normalizing ifurnace -2 in the usual manner and the normalizingfurnace chamber 46 may .be supplied with a blend of endothermic-gas and natural gas or any other suitable gas to prevent decarburization 'of thestrip during the.

normalizing operation. Such gas also .fills the cooling chamber or tunnel 3 to preventzinfiltra- .tion of. air.

A gasgenerator, not shown, may .be provided 8 .propersynchronismwith the speed of the .strip through thesequipmentp r The carburizing chamber [1 in -mufiie i6 is supplied through pipe 34 withsuitable quantities Ora-carbon containing .gas' for carburizing the strip as its passes through the carburizing chamber ll, and the spent .atmospherefis exhausted through pipe 21. The-gasmay be raw natural gas; or .a blend of raw natural gas and endothermic .gas; or a blend of .raw natural gas, endothermic-gas and ammoniagas; or a blend of raw gas and ammonia gas; or any other suitable gaseous atmosphere having a .sufficient-carburizingepotential .or carbon pressure therein for properlycarburizing the strip steel.

.The strip steel .6 as .it continuously passes through .the .carburiz'ing chamber I1 is electric resistanceheated preferably to a temperature of about 1800 F. by power supplied by conductors 26 .and 35-. temperatureis believed to be themost favorable .car-burizing temperature 'for most accurate control of the continuous carburizing operation 'but lit may be varied somewhat as later described.

The .muflle [6 .of the carburizing furnace also is maintained at a temperature substantially equal to that of thestrip by means of the gas burners 29 and the automatic temperature control equipment therefor. This muflie heating control equipment is manual1y,.mechanically or electrically coordinated with and interrelated to the temperature controlby controlof the power input provided in the electric resistance heating circuit. The control equipment in the resistance heating circuit comprises usual transformers, .voltage regulators and other electrical apparatus" whereby maximum strip temperature incident to resistance 'heatingmay be accurately controlled.

.The ultimate concentration of carbonin the strip'as aresult of the carburizing'operation is a function of time, temperature, and carburizing medium used. An increase 'in the time of carburization (a decrease in the strip speed) Without change inthe .strip'temperature or the carburizing 'medium, will 'increase'the amount of for generating an endothermic gas to .be used as a component of the atmosphere in the :carburizing and normalizing furnace chambers. Such endothermic gas will be later describedzand when used constitutes a protective atmosphere of low carbon dioxide content.

The strip .6, upon being unreeled at 5 ;-passes over idler 24, through mercur .seal M, .under idler 22, over idler 25, through muflle 16 wherein it is carburized, throughlead.sealI:32 ,-.under. idler roll 33, through lead wiper 42, over idler '3 8,'into and through normalizing Ifurnace'chamber 48, through cooling tunnel .3,:-and torereeling: equipment 8. As indicatedthe same-drivaequipment that drives the rereeling.deviceefiextends ito'zdrive the hearth rollers 41 in thenormalizingfurnace and the rollers 50 .inthe 'ccolingltunnel; with all surface speeds of the rollers-maintained in (7.6

strip carburization. An increase in'the strip temperature, without'changing thetime (strip speed) or the carburizing medium will increase the carboncon'centration in the resulting product. If "the strip speed and temperature remain the same, andtheca'rbon pressure of the carburizing gas is increased, the concentration of carbon in the resulting product is increase'd.

The following is an example -of conditions under which a strip maybe carburized in the apparatus. Cold rolled low carbon strip steel i0'.008 x-i9" may :be runithrough the-equipment at-aspeed-of 1-5 fe'et per minute-andiheated to a :stripitemperatureof1806" Fqto provide a uniform carbon concentration of about 1.25% in the 'finished-ima'terial. -By varying the time/temperature and carburizing medium, different gauges of strip, suchwas strip of'an-y width vary- .ing:in gauges say ranging .from 0.003" to 0.016 .may 'be -carburized tovhave carbon concentra- "tions' ranging from 0.60% to 1.3%.

"Variousgases or-mixtures of gases have been indicated for supplying the carburizing atmosphere in the muiiiechamber l1. Preferably the gas may be raw naturalq-gasand the methane (CH4) .in the gas is .theprincipal source of carbon taken up by thesteel. Atypical raw natural gas .maycontain-:85% CH4,:;13% CzHs and 2% nitrogen. -If cit .iS .found underyioperating j conditions that the raw gas has an excessive carburizing potential, which sometimes may result in a deposit of excessive amounts of free carbon in the mufile, the raw gas may be diluted with endothermic gas. A typical endothermic gas made by reacting raw natural gas with only a sufiicient amount of air to burn the carbon in the hydrocarbons to CO is as follows: C02, .2 to .5%; CO, 18.8%; H2, 38.5%; CH4, 1.2% and N2, balance.

However, instead of using raw natural gas or a blend of the same, other hydrocarbons such as propane, or an artificial gas such as carburated water gas may be used. In any case, the carbon in the hydrocarbon constituents of the gas is the principal source of carbon taken up by the steel.

It has been indicated that in heating a relatively wide strip in a carburizing atmosphere by internal heating alone, such as by electric resistance heating, the center zone of the strip is carburized to a substantially greater degree than the edge portions of the strip. Higher center zone strip carburization occurs because of a higher temperature in the center zone than in the edge zones of the strip.

When a similar strip is heated in a carburizing atmosphere solely by heating means external to the strip, the edge zones are carburized to a greater degree than the center zone because the edge zones of the strip are heated to a higher temperature than the center zone.

Figs. 4 and illustrate diagrammatically factors which are believed to explain the differentials in strip temperature between the center and edge zones of the strip when internally-or externally heated. I

Referring to Fig. 4, a tube is indicated at 5| externally insulated at 52. The tube 5| forms a chamber filled with a carburizing gas through which a strip S is being passed and the strip S is heated solely by electric resistance heating, there being no other source of heat. Even though the tube 5| is insulated, there is always some heat which escapes through any known type of insulation, as indicated by arrows 53.

Consider points P1, P2 and P3 which are representative and lie on the inner surface of the tube 5!, point P1 being in a plane normal to the strip surface, point P2 being in a plane at an angle to the strip surface, and point P3 being opposite the strip edge in a plane passing transversely of the strip and parallel to the strip surfaces.

Point P1 sees the strip through angle A1 which is relatively large. The corresponding angle A2 for point P2 is smaller and angle A3 for point P3 is substantially zero, being subtended only by the thickness of the strip.

The strip S in being internally heated only loses heat from its surfaces. Point P1 receives a great deal more heat through angle A1 than is received by point P3 through angle A3. Point P1 thus reaches equilibrium at a higher temperature than points P2 and P3. Even though points P2 and P3 receive additional heat by reradiation, they nevertheless will have a relatively lower temperature than point P1. In consequence, the edges of strip S likewise have a lower temperature than the center zone of the strip. Under such conditions of operation, low carbon edges necessarily are produced.

Now referring to Fig. 5, an externally heated mufile is indicated at 54 and the strip being carburized therein is indicated at S. lhe entire heat demand of the strip is transmitted through the muffle as indicated by arrows 55.

Considering the same points P1, P2 and P3 as considered in Fig. 4, point P1 sees the strip through amaximum angle B1, point P2 through a smaller angle B2, and point P3 through an angle Ba substantially zero. The heating duty for transmitting heat from the rnufiie to the strip is relatively highat point P1 due to large angle B1 and the heating duty at point P3 is substantially zero. Point P1 therefore will be cooled to a relatively great extent and point P3 scarcely any. Some heat will reach points P1 and P2 by reradiation fromother points but this is insuihcient to correct the condition fully.

Thus, under the factors of external heating shownin Fig. 5, the edges of the strip are heated to a higher temperature, or more quickly, or both, than the center zone of the strip; and high carbon edges result. Recognizing these factors and the further fact thta in order to obtain uniform carburization, the strip must be heated to a uniform temperature throughout itswidth and particularly at the zone of maximum temperature in the muffle, it is clear that these factors and conditions must be carefully controlledand coordinated.

In accordance with the present invention the internal resistance heating is considered as the primarysource of heat, because theoretically the conditions forstrip carburization are ideal when Therefore, the

the strip is heated internally. power input to the strip is regulated 'with respect to the strip speed and the carbon pressure of the carburizing atmosphere so that a preferred predetermined maximum strip temperature is maintained, preferably about 1800 F. In accordance with the present invention the heating of the carticularly at the zone of maximum strip tem- 1 perature.

Thus a balance between internal and external heating is established resulting inuniform strip temperature throughout its width. In order to accomplish this coordination and control of heating, the heating temperature external to the strip within. the mufile is maintained very close to the desired -1800 F. strip temperature.

For some-undetermined reason, uniform temperature, uniform carburizing and the best results have been obtained when the furnace temperature is maintained at a uniform temperature slightl below the strip temperature, say, about 1765 F. It is possible that this situation reflects the existence "of a mufile temperature, in the upper portion or heating-up zone of the muflie chamber, higher than the strip temperature, for which reason a slightly reduced mufiie temperature in the maximum temperature zone tends to compensate for the initial lack of equality.

Because of the carburizing reactions constantly taking place, variations in gauge, and temperature changes due to the progress of carburization, the strip temperature due to resistance heating may vary somewhat from the preferred strip temperature'oi 1800" F. However, the resistance heating is controlled by a constant and uniform power input to the strip to maintain the desired strip temperature as nearly as possible. This internal resistance heating temperature control is manually or automatically coordinated with the automatic temperature responsive temperature controls for the muille so that when the-strip '11 temperature changes, or variations occur in the strip temperature across its width, the temperature due to internal heating is changed, or the muffle temperature is changed, or both the internal and external heating temperatures are changed, either manually or automatically, until the desired temperature equilibrium has been reestablished uniformly throughout thestrip width.

Referring to Fig.6, means is shown therein for automatically controlling the-balance between'internal and external heating to maintain the desired temperature-equilibrium The carburizing furnace l and muffle I 6 thereof may be provided with two peep hole passages 56 and -1, the passage 56 being directed toward the center of the strip 6 and the passage 51' being directed toward edge of the strip 6. "The peep hole passages 56 and 51 are preferabl tapered, as shown, to provide a maximum rangeofvisibility for the eyes of radiation heads"58.and 59 of optical or radiation pyrometers. Thus the pyrometer 58 sees the center zone of the strip 6 as indicated by the dot dash lines 60 and the pyrometer head 59 sees the edge portion of the strip as indicated by the dot dash lines Bl.

The radiation heads .58 and '59 are each connected through suitable wiring 62 and 63,.with differential pyrometer instrument 64 such that the millivoltages produced by .each ofthe two radiation heads 58 and 59 are. balanced against each'other so that the pointer 65.01 theinstrumentv reads zero when both heads are exposed to identical temperatures, regardless of'what these temperatures are. Thus instrument 64 reads the center temperature relative to the edge temperature of the strip 6, the indication being plus if thecenter is higher and minus if the center is lower.

Under normal circumstances the edge temperature closely approximates the muffle temperature. If the pointer 65 of instrument 64 reads plus, as indicated in Fig. 6, the center temperature of the strip is higher than the edge temperatureand a decrease in the internal heating of the strip is indicated to ."re-establish uniform strip temperature throughout the width of' the strip.

For this purpose, three impulse lines 65 may lead from instrument 6! to agcontrol-actuating device for the voltage regulator of internal resistance heating circuit whereby the -voltage is adjusted to automatically make such correction as is necessary to re-establish the desired temperature equilibrium condition with uniform strip temperature throughout the width of the strip.

As indicated below, the automatic control may act upon the external heating instead of the internal strip resistance heating, in which event, an increase in the'center temperature of the strip relative to the edge would call for a higher rate of external heating.

In maintaining the temperature equilibrium, the internal heating can be'maintained fixed and the external heat varied to establish and maintain equilibrium, or the external heating: can be maintained at a fixed temperature, and the internal heating varied. With either of these modes of control, a further control. by change of strip speed can be exercised to maintain thezone of maximum temperature at the desired degree.

Therefore, in accordance. with the :presentinvention the relation ofrinternal heating :to external heating is controlled'to-establishathermal equilibrium resulting in uniform :strip temperature throughout the width of the strip to obtain uniform carburization. Thus, the temperature of carburizing-the temperature maintained at equilibrium and uniform and which will give a uniform carbon content across the Width of the stripe-is dependent upon the amount of internal heating by electric resistance.

.A preferred strip temperature for oarburization of about 1800" F. has been indicated and an example has been given of a strip speed for a strip of certain gauge to be carburized at such temperature. However, under certain circumstances it may be desired to increase the speed of carburization which may be accomplished by increasing the controlled carburizing temperature "to I850 F. to 2000 F. but not above approximately 2000 F. accompanied by an increase in the speed of the strip so that the relative balance of speed and temperature conditions are maintained.

Regardless of the controls exercised and the width of the strip, it has been discovered that when uniform carburization is obtained substantially throughout the strip width, sometimes there is an extremely narrow zone of about to A; of an inch at each edge of the strip that is car- 'buri'zed to a higher degree than the remainder of; the strip. However, this is not a serious difliculty since it is usually always desirable or necessary for other purposes to slit or shear the edges of a finished strip. Such a slitting or shearing operation can thus be used to remove the narrow band of high carbon edges. In a 9" strip, for instance, only about 3% of the width of the strip need be removed to obtain a finished strip having uniform carburization throughout.

The lead quenching bath at the exit end of the carburizing muflle is also an important aspect of the present invention. By utilizing such a bath andmaintaining the same in excess of 750 F. the car'burized materialis quenched to such temperature, thereby providing material soft enough to be'h-andled without difficulty. The lead wiper indicated at 4'2 for removing lead from the surfaces of the strip may be any suitable type of wiper construction such as a charcoal Wiper.

The particular lead quenching, sealing and electrical contact bath 32 shown in the drawings is relatively small but it is understood that this bath may be increased in size or the amount of strip travel within the bath may be substantially increased, if desired, in order to control the time that the strip is held in the bath at quenching temperature.

In accordance with the present invention, light gauge low carbon strip steel is reduced in the usual manner by cold rolling and annealing operations from low carbon hot rolled strip to provide substantially the desired finished gauge therein. The strip is then passed through the carburizing furnace l and normalizing furnace 2 to obtain'the desired high carbon content therein and to form the desired heat treated structure. Thereafter, the normalized strip may be annealed if desired and finally subjected to a skin pass to provide, flatness and temper. The edges then may be slit to form the finished light gauge high carbon strip steel.

Accordingly, the present invention provides a radically new and different procedure for the manufacture of light gauge cold rolled high carbon strip steel; substantially reduces the cost of making such material; provides for continuously uniformly carburizinglow carbon strip steel which has been cold rolled to substantially the desiredfinished light gauge; provides a method and means for uniformly heating continuously moving strip steel in a carbon containing atmosphere whereby the steel may be carburized substantially uniformly throughout its width; provides for utilizing substantially theoretically ideal carburizing conditions in continuously carburizing strip steel; provides for establishing and maintaining an equilibrium or balance between internal and external heat supplied to the strip to obtain uniform carburization of the strip throughout in the continuous carburization of low carbon strip steel; eliminates the necessity of taking special precautions to avoid carbon loss in the manufacture of light gauge cold rolled high carbon strip steel; simplifies the character and reduces the number of operations carried out in the manufacture of light gauge cold rolled high carbon strip steel; and overcomes prior art difiiculties and solves long standing problems in the art.

In the foregoing description certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art because such terms are utilized for descriptive purposes herein and not for the purpose of limitation and are intended to be broadly construed.

Moreover, the description of the improvements is by way of example, and the scope of the present invention is not limited to the exact details, percentages, analyses, strip Widths and thicknesses, and carbon concentrations set forth.

In the foregoing description, a carburizing operation is described and is accomplished through the use of a carburizing gas having the desired carburizing potential. It has been indicated that ammonia gas may be blended with raw natural gas to form the carburizing atmosphere. By increasing the relative amount of ammonia gas in the carburizing gas, some nitriding of the strip may occur. Thus the strip may be subjected to a combined carburizing and nitriding operation.

When the terms externally heating and separately heating are used herein and in the appended claims with relation to the heating of the carburizing chamber, these terms are used in a relative sense with respect to the strips or to the internal heating of the strip; because the heating of the mufile or of the carburizing gas in the muflie is performed separately of the heat developed in the strip by internal heating, and

14 by heat applied to the muffle or to the atmosphere within the muffle externally of the strip.

Having now described the features, discoveries and principles of the invention, the operation and procedures of preferred method steps thereof, the arrangements of new apparatus, and the advantageous, new and useful results obtained thereby; the new and useful processes, methods, steps, procedures, operations, discoveries, and principles, and mechanical equivalents obvious to those skilled in the art, are set forth in the appended claims.

I claim:

1. The method of continuously uniformly carburizing strip steel including the steps of passing light gauge cold rolled low carbon strip steel continuously through a sealed chamber containing carburizing gas, electric resistance heating the strip internally to carburizing temperature as it passes through said chamber, applying heat to the strip edges from a source external of the strip in a greater degree than to the center zone thereof to balance the higher radiation loss of internal heat from the strip at the strip edges than from the center zone thereof thereby preventing differentials in the radiation heat loss from the strip across the width thereof and establishing a thermal equilibrium between the internal and external heating, maintaining the thermal equilibrium thus established to provide substantially uniform strip temperature and carburization across the width of the strip, and continuing the internal and external heating for such time as to obtain diffusion of the carbon throughout the strip thickness.

2. The method set forth in claim 1 including the further step of quenching the strip from carburizing temperature to a temperature in excess of about 750 F. to provide a high carbon steel structure free from brittleness.

FRANK T. COPE.

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

UNITED STATES PATENTS Number Name Date 1,552,041 Crapo Sept. 1, 1925 1,704,036 Cope Mar. 5, 1929 1,932,032 Cowan Oct. 24, 1933 1,942,025 Frost Jan. 2{, 1934 2,279,684 Johnson Apr. 14, 1942

Claims (1)

1. THE METHOD OF CONTINUOUSLY UNIFORMLY CARBURIZING STRIP STEEL INCLUDING THE STEPS OF PASSING LIGHT GUAGE COLD ROLLED LOW CARBON STRIP STEEL CONTINUOUSLY THROUGH A SEALED CHAMBER CONTAINING CARBURIZING GAS, ELECTRIC RESISTANCE HEATING THE STRIP INTERNALLY TO CARBURIZING TEMPERATURE AS IT PASSES THROUGH SAID CHAMBER, APPLYING HEAT TO THE STRIP EDGES FROM A SOURCE EXTERNAL OF THE STRIP IN A GREATER DEGREE THAN TO THE CENTER ZONE THEREOF TO BALANCE THE HIGHER RADIATION LOSS OF INTERNAL HEAT FROM THE STRIP AT THE STRIP EDGES THAN FROM THE CENTER ZONE THEREOF THEREBY PRE-

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