US2594876A - Apparatus for carburizing steel - Google Patents

Description

April 29, 1952 F. T. COPE 2,594,876

APPARATUS FOR CARBURIZING STEEL Original Filed Nov. 24. 1947 5 Sheets-Sheet l Snvcntcr April 29, 1952 F. T. COPE APPARATUS FOR CARBURIZING STEEL Original Filed Nov. 24, 1947 5 Sheets-Sheet 2 Frank TCope gm ww (Ittomeg 5 April 1952 F. T. COPE APPARATUS FOR CARBURIZING STEEL 3 Sheets-Sheet 5 Original Filed Nov. 24, 1947 3|meutor Patented Apr. 29, 1952 APPARATUS FOR CARBURIZING STEEL Frank T. Cope, Salem, Ohio, assignor to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Original application November 24, 1947, Serial No. 787,798. Divided and this application Au gust 6, 1948, Serial No. 42,759

2 Claims.

The invention relates to carburizing steel and more particularly to apparatus for continuously carburizing steel in sheet or strip form during continuous endwise movement of such steel; and this application is a division of my copending application Serial No. 787,798, filed November 24, 1947, Patent No. 2,513,713, dated July 4, 1950.

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 steelhas 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 in itself a more difficult 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 0.007 or 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 difiicult 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 toavoidcarbon 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 the low cost rolling of low carbon strip steel in the usual manner to approximately the desired finished gauge, if it is then possible to carburize the cold rolled low carbon steel to obtain the desired high carbon concentration in the strip.

Many years ago in comparing the properties of 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 gas 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 rateof carburiza- -tion should occur.' r r These principles so discovered in testing naturalgas, applied to the continuous carburizing of lightgauge 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 a very 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 stripand 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 proper difiusion of the carbon throughout the strip thickness were not successful in producing uniform carburization throughout the strip width.

Nevertheless, a high carbon content can be obtained in the strip and the carbon gradient from the surface of the strip to the center of the strip may be held to a minimum by a proper balancing between the strip speed and applied voltage.

Better diffusion of the carbon can be obtained by keeping the power input (strip 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 attempts were made to eliminate or correct the low carbon edge and high carbon center difficulty without results. Linings in the mufile in which the strip was heated b electrical resistance were provided, changed, and discarded in an attempt to control heat loss; the side of the muflle chamber was changed; and the supports within the muffle on which the strip rested in passing horizontally therethrough were changed. It was impossible, however, to hold the carbon in the strip at a uniform amount and the carbon content would vary between wide limits.

Other difficulties included the deposition of soot in the muffle chamber causing short circuiting at times, particularly when intermediate supports were used. The elimination of intermediate supports by locating supports only at each end of the muflle prevented cracking of the atmosphere and the deposition of soot at these regions because of low temperatures. circuiting dangers but necessitated a higher strip tension to keep the sagging portion of the horizontally moving strip from touching the bottom of the muffle between end supports. This increased the danger of pulling the extremely light gauge strip apart by the increased pulling tension.

In order to overcome some of these difficulties, an externally heated muffle chamber was resorted to and the theoretical amount of gas required for obtaining the desired carbon concentration in the treated strip was carefully controlled.

These changes led to the discovery that a greater flow of natural gas was required than the theoretical calculated flow, to carburize the strip to the desired carbon value. However, it seemed impossible to determine the conditions that would give a uniform carbon content across the width of the strip being treated.

Then I discovered, by temporarily eliminating internal resistance heating of the strip and depending entirely upon external mufiie heating, that the strip, either when stationary in the muffle or when drawn through the same continuously, had exactly the reverse undesired car- This reduced short 4 burized characteristics, namely, the edges thereof had a higher carbon content than the center.

These discoveries led to the final discovery that heating of the carburizing chamber external to the strip is necessary in conjunction with internal resistance heating of the strip to obtain a imiform high carbon content across the width of the strip; that there must be an interrelated and controlled balance between 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 strip edges 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 carburizing light gauge low carbon strip steel to a desired high carbon value; and that under these conditions low carbon strip steel in Widths ranging up to 9" or more and gauges ranging 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 strip except for a possible higher carbon content in the zone comprising about the first -2; to A; of an inch at each edge of the strip. This narrow high carbon zone at each edge of the strip can be slit or sheared therefrom.

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

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

Furthermore, it is an object of the present invention to substantially reduce the cost of making light gauge cold rolled high carbon strip steel.

Also it is an object of the present invention to provide new equipment for carburizing strip .steel when making high carbon strip steel by first cold rolling low carbon strip steel to substantially the desired finished gauge and by then uniformly carburizing the strip.

Likewise it is an object of the present invention to provide a means of uniformly heating continuously moving strip in a carbon containing atniosphere whereby the strip may be carburized substantially uniformly throughout its width.

Furthermore, it is an object of the present invention to provide apparatus which enables the maintenance of substantially ideal carburizing conditions in carburizing low carbon strip steel continuously.

Also it is an object of the present invention to provide means for controlling the relation between internal and external heat applied to the strip so as to obtain uniform carburization of the strip throughout, in the continuous carburization of low carbon strip steel.

Moreover, it is an object of the present invention to provide a new apparatus for continuously carburizing low carbon strip steel.

Furthermore, it is an object of the present invention to eliminate the necessity of taking special precautions to avoid carbon loss in the manufacture of light gauge cold rolled high carbon strip steel.

Also, it is an object of the present invention to simplify the character and reduce the number of operations carried out in the manufacture of light gauge cold rolled high carbon strip steel. Finally, it is an object of the present invention to provide new apparatus for the manufacture of light gauge cold rolled high carbon strip steel which is simple to use, and is precise, effective and accurate in operation, which reduces the cost or" making such material, which overcomes the foregoing prior art difficulties, which solves long-standing problems in the-art of manufacturing high carbon strip steel, and which obtains many new results and advantages herein set forth.

These and other objects 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 discoveries, principles, apparatus, combinations, parts, sub-combinations and elements which comprise the present invention, the nature of which are set forth in the following general statements, a preferred embodiment of whichillustrative 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 apparatus for continuously carburizing strip steel of the present invention may be stated in general terms as preferably including a vertical preferably externally heated muffle type furnace, a preferably mercury sealing arrangement at the upper or entrance end of the furnace, a molten preferably lead sealing and quenching bath at the exit or lower end of the furnace, means for supplying carburizing gas to the interior of the furnace muffle, means for heating the furnace muffle externally of the strip, a continuous normalizing furnace preferably connected with the lead bath sealing means, means for passing a steel strip continuously in order through said mercury bath seal, said carburizing muille, said lead bath seal, and said carburizing furnace; means for passing electriccurrent through the strip while passing through said carburizing furnace muflle to internally heat the strip, and means for coordinating the balance of heating of the strip internally by electrical resistance and externally by furnace chamber heating to provide a maximum strip temperature uniform throughout the strip width whereby uniform carburization is obtained.

By way of example, a preferred embodiment of improved carburizing apparatus is illustrated in the accompanying drawings forming part hereof wherein:

Figure 1 is a diagrammatic side elevation of a preferred arrangement of improved continuous strip carburizing apparatus;

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

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

. Figs. 4 and 5 are diagrammatic views illus- 6 trating certain heating principles involved in using apparatus 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, muille type, carburizing furnace generally indicated at I, a preferably gas fired, radiant tube heated, roller hearth normalizing furnace generally indicated at 2, and a cooling tunnel generally indicated at 3.

Coils l of light gauge low carbon cold rolled strip steel may be unreeled at 5, and fed, as indicated generally at 5, through the carburizing furnace I, the normalizing furnace 2 and cooling tunnel 3. The strip is then rereeled as coils I on reels generally indicated at 8 having suitable drive equipment for pulling the strip 6 through the car-burizing 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 5 preferably comprises a vertical circular shell as suitably reinforced and supported by structural steel members ii and i2 and supported thereby on the floor I3. The furnace is preferably lined with suitable insulating refractory material hi to provide a circular heating chamber within which is located a vertically extending carburizing furnace mufile I5, preferably formed by sections of centrifugally cast heat resisting alley metal welded together to form a continuous tubular mufile, the interior of which constitutes the carburizing chamber H.

The furnace I may be provided with a structural steel platform 18 around the top of the muffle IS.

A furnace muille entrance seal and housing member 59 is mounted in a suitable manher on the top or entrance end of the tubular muffle i6 and the same may include a trough portion 26 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 99. Another idler roll 24 is mounted on the outside of housing 5 9 and a third idler roll 25 is mounted within housing Id. The strip 6 in passing into the carburizing muffle I6 passes over roll 2 3, through mercury seal 2 I, under idler 22, past wiper 23, over idler roll 25, and thence vertically downwardly within carburizing chamber IT in mufiie I6.

The mercury sealing bath 2! 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 27 preferably communicates with entrance housing IE3 through which spent carburizing gases may be exhausted from the carburizin-g chamber ll.

Suitable inspection and access openings, not shown, may be provided for the entrance housing I9; and the housing I!) is insulated electrically from the muffle tube It at the connection 28 therebetween.

The heating chamber I5 surrounding the muffie I6 is heated by a number of fuel burners 252 swears 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, operated by usual automatic temperature control mechanisms are provided so that the temperatures in various zones and througl'iout the heating chamber l 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 carburizin 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 ii 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 carburizing chamber ll of the carburising furnace I must be heated by means external of the strip 5 being carburized therein.

An exit housing is mounted at the discharge of bottom end of the muffle tube It, suitably insulated therefrom at 3| preferably incorporating baflie plates and a receptacle containing lead indicated by the liquid level 32 and an idler roll 33 immersed in the lead bath. The exit housing 36 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 38 communicates with the entrance tube 3'! of the normalizing furnace 2, an idler roll 38 being proided in the entrance tube 31, as indicated. The exit housing 30 may be provided with tubular heating members 39 fired by burners 40 to heat the lead 32 therein to the desired temperature; or the burners 40 may be turned off and the lead may be cooled by blowing air through the tubes 39 so as to accurately control the temperature 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 mechanisms of a usual type, not shown.

Suitable peep holes such as 4! and 41a are provided in the exit housing 39 between the exit end of the muffle l6 and the lead sealing bath 32 and in the lower end of the mulilc if; to permit observation of the strip temperatures with optical or radiation pyrometers; and suitable cleanout and inspection openings, not shown, may be provided in the exit housing 30.

The strip 6 in passing through furnace muifie i6 is internally heated by electrical resistance by current supplied to the strip through conductors 26 and 35 leading, respectively, to the mercury and lead baths 2! and 32 which serve as terminals for the electric resistance circuit in addition to their function as gas seals for the muffle chamber 7. The supply of power for the resistance heating of the strip in the mufiie may be controlled in the manner later described so that the temperature of the strip, due to internal resistance heating, may be accurately controlled.

Wiper means 42 may be provided in the tubular extension 36 for removin lead from the surfaces of the carburized strip before it passes into the normalizing furnace 2.

Referring particularly to Fig. 3, the normalizing furnace preferably includes a shell 43 suitably reinforced with structural steel members 44 and supported on floor I3. The shell is lined with a suitable refractory material 45 to form a furnace chamber 46 heated by means of radiant tube heating elements 45a. 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 from the exit opening 49 of the normalizing furnace. The cooling hood or tunnel 3 is preferably of a water jacketed type equipped with suitable piping for the circulation of water between the inner and outer jackets to provide for cooling the strip before its discharge to room temperature after normalizing. The cooling tunnel 3 also may be supplied with driven rollers diagrammatically indicated at 50 for conveying the strip through the cooling tunnel.

The usual pyrometers, control valves, valve drive mechanisms, thermocouples, combustion air blowers, gas lines and safety equipment therefor are provided for controlling the heating of the strip in the normalizing furnace 2 in the usual manner and the normalizing furnace chamber 46 may be supplied with a blend of endothermic gas and natural gas or any other suitable gas to prevent decarburization of the strip during the normalizing operation. Such gas also fills the cooling chamber or tunnel 3 to prevent infiltration of air.

A gas generator, not shown, may be provided 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 described and when used constitutes a protective atmosphere of low carbon dioxide content.

The strip 6, upon being unreeled at 5 passes over idler 24, through mercury seal 2|, under idler 22, over idler 25, through muffle [6 wherein it is carburized, through lead seal 32, under idler roll 33, through lead wiper 42, over idler 38, into and through normalizing furnace chamber 45, through cooling tunnel 3, and to rereeling equipment 8. As indicated, the same drive equipment that drives the rereel'ing device 8 extends to drive the hearth rollers 41 in the normalizing furnace and the rollers 50 in the cooling tunnel, with all surface speeds of the rollers maintained in proper synchronism with the speed of the strip through the equipment.

The carburizing chamber ll in muffle I6 is supplied through pipe 34 with suitable quantities of a carbon containing gas for carburizing the strip as it passes through the carburizing chamber I1, and the spent atmosphere is exhausted through pipe 21. The gas may be raw natural gas; or a blend of raw natural gas and endothermic gas; or a blend of raw natural gas, endothermic gas and ammonia gas; or a blend of raw gas and ammonia gas; or any other suitable gaseous atmos- 9 phere having a sufficient carburizing potential or carbon pressure therein for properly carburizing the strip steel.

The strip steel 6 as it continuously passes through the carburizing chamber I1 is electric resistance heated preferably to a temperature of about 1800 F. by power supplied by conductors 2B and 35. This temperature is believed to be the most favorable carburizing temperature for most accurate control of the continuous carburizing operation but it may be varied somewhat as later described.

The mufile 16 of the carburizing furnace also is maintained at a temperature substantially equal to that of the strip by means of the gas burners 29 and the automatic temperature control equipment therefor. This mufile heating control equipment is manually, mechanically or electrically coordinated with and interrelated to the temperature control by control of 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 heating may be accurately controlled.

The ultimate concentration of carbon in the strip as a result 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 in the strip temperature or the carburizing medium, will increase the amount of strip carburization. An increase in the strip temperature, without changing the time (strip speed) or the carburizing medium will increase the carbon concentration in the resulting product. If the strip speed and temperature remain the same, and the carbon pressure of the carburizing gas is increased, the concentration of carbon in the resulting product is increased.

The following is an example of conditions under which a strip may be carburized in the apparatus. Cold rolled low carbon strip steel 0.008 X 9" may be run through the equipment at a speed of 15 feet per minute and heated to a strip temperature of 1800* F. to provide a uniform carbon concentration of about 1.25% in the finished material. By varying the time, temperature and carburizing medium, different gauges of strip, such as strip of any width varying in gauges say ranging from 0.003" to 0.016" may be carburized to have carbon concentrations ranging from 0.60% to 1.3%.

Various gases or mixtures of gases have been indicated for supplying the carburizing atmosphere in the mufiie chamber ll. Preferably the gas may be raw natural gas and the methane QCI-Ir) in the gas is the principal source of carbon taken up by the steel, A typical raw natural gas may contain 85% CH4, 13% CZHG and 2% nitrogen. If it is found under operating 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 Inuille, the raw gas may be diluted with endothermic gas. A typical endothermic gas made by reacting raw natural gas with only a sufficient amount of air to burn the carbon in the hydrocarbons to CO is as follows: C02, .2 to .5%; CO, 18.8%; Hz, 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 suchas carburatecl 10 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 5 illustrate diagrammatically factors which are believed to explain the difierentials in strip temperature between the center and edge zones of the strip when internally or externally heated.

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 muffle is indicated at 54 and the strip being carburized therein is indicated at S. The entire heat demand of the strip is transmitted through the mufiie as indicated by arrows 55.

Considering the same points P1, P2 and P3 as considered in Fig. 4, point P1 sees the strip through a maximum angle B1, point P2 through correct the condition fully.

Thus, under the factors of external heating shown in 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 that in order to obtain uniform carburization, the strip must be heated to a uniform temperature throughout its width andparticularly at the zone ofmaximum strip temperature in the muffle, it is clear that these factors and conditions must be carefully controlled and coordinated.

In accordance with the present invention the internal resistance heating is considered as the primary source of heat, because theoretically the conditions for strip carburization are ideal when the strip is heated internally. Therefore, the 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, prefer-ably about 1800 F. In accordance with the present invention the heating of the carburizing chamber external to the strip is controlled and coordinated with the internal strip heating so that the amount of net heat loss from the strip (considering the strip as being heated internally by resistance heating) is substantially zero throughout the width of the strip and particularly at the zone of maximum strip temperature.

Thus a balance between internal and external heating is established resulting in uniform strip temperature throughout its width. In order to accomplish this coordination and control of heating, the heating temperature external to the strip within the mufiie 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 temper-ature is maintained at a uniform temperature slightly 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 muffle chamber, higher than the strip temperature, for which reason a slightly reduced muffle 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 very somewhat from the preferred strip temperature of 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 mulfle so that when the strip temperature changes, or variations occur in the strip temperature across its width, the temperature due to internal heating is changed, or the mufile temperature is changed, or both the internal and external heating temperatures are changed, either manually or automatically, until the desired temperature equilibrium has been re-established uniformly throughout the strip 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 mufile 56 thereof may be provided with two peep hole passages 56 and 51, 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 preferably tapered, as shown, to provide a maximum range of visibility 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 (H.

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 of the two radiation heads 58 and 59 are balanced against each other so that the pointer 65 of the instrument reads zero when both. heads are exposed to identical temperatures, regardless of what these temperatures are. Thus instrument 65 reads the center temperature relative to the edge temperature of the strip 6, the indication being plus if the center is higher and minus if the center is lower.

Under normal circumstances the edge temperature closely approximates the mulile temperature. If the pointer 65 of instrument 54 reads plus, as indicated in Fig. 6, the center temperature of the strip is higher than the edge temperature and 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 66 may lead from instrument B4 to a control-actuating device for the voltage regulator of internal resistance heating circuit whereby the voltage is adjusted to automaticall 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 the zone of maximum temperature at the desired degree.

Therefore, in accordance with the present invention the relation of internal heating to external heating is controlled to establish a thermal equilibrium resulting in uniform strip temperature throughout the width of the strip to obtain uniform carburization. Thus, the temperature of carburizingthe temperature maintained at equilibrium and uniform and which will give a uniform carbon content across the width of the stripis dependent upon the amount of internal heating by electric resistance.

A preferred strip temperature for carburization 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 1850 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 V of an inch at each edge of the strip that is carburized to a higher degree than the remainder of the strip. However, this is not a serious difficulty 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.

Another important factor in the present invention is the vertical arrangement of the carburizing furnace and mufile. When a strip is heated by internal resistance in a horizontal furnace in a carburizing atmosphere, soot stringers may form and if they contact the strip and muffle wall, short circuiting may occur which cannot be controlled. Furthermore, deposits of carbon or soot may form in the furnace, usually near the place of entry of the carburizing gas and these deposits, if they contact the strip, likewise may cause short circuiting.

Furthermore, in a horizontal furnace, in order to maintain the amount of tension required to pull the strip therethrough at a minimum, it is necessary to support the strip at intervals so that it moves centrally through the furnace. Support means for such purposes cause difficulties.

- The use of a vertical furnace arrangement overcomes all these difliculties as it avoids the problem of supporting the strip and reduces to a minimum short circuiting incident to soot formation or carbon deposit. Furthermore, the introduction of the carburizing gas at the bottom of a vertical furnace may reduce the amount of soot formation and carbon deposit because the strip and furnace atmosphere are both considerably hotter in this region than in other zones of the furnace available for the entry of carburizing gas. a v

The lead quenching bath at the exit end of the carburizing muiiie is also an important aspect'of the present invention. By utilizing such a bath and maintaining the same in excess of 750 F. the carburized material is quenched to such temperature, thereby providing material soft enough to be handled without difficulty. The lead wiper indicated at 42 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 temperature.

Although a lead sealing, quenching and contact bath has been indicated as being preferred, it is to be undersood that other desirable current conducting molten baths, such as molten salt baths, may be used for sealing the carburizing chamber, quenching the carburized strip, and conducting current to the strip.

.Another desirable aspect of the present invention is the location of the normalizing furnace 2 immediately adjacent and communication by sealed passage 36 with the lead seal at the bottom of the carburizing furnace. Normalizing is desirable if not necessary, particularly if any nitrides are formed in the steel as a result of blending ammonia gas with the carburizing gas. By performing the normalizing operation, which is preferably carried out at a temperature of about 1700 F. in chamber 46, immediately after the strip emerges from the quenching and lead sealing bath, the amount of heating for normalizing is reduced by the amount of retained heat in the strip following the carburizing and quenching operation, the strip being at the quenching bath temperature as it emerges from the lead bath.

A further advantage in the particular location of the normalizing furnace is that the same or some of the same gases may be used for the carburizing atmosphere in carburizing chamber I1 and as a protective atmosphere in normalizing chamber 46. Thus, any gas leakage from one furnacechamber to another will not contaminate the atmosphere in either chamber and special seals other than the gas-tight communicating passage between the furnaces need not be provided.

The desirability of performing the normalizing operation immediately after quenching and before the strip has cooled below the quenching temperature, by connecting the normalizing furnace directly to the lead quenching and sealing bath at the exit end of the carburizing furnace, have been described. However, the carburizing operation may be carried out without immediate normalizing by passing a strip through a carburizing furnace such as illustrated in Fig. 1 and coiling the strip after some further cooling after it leaves the lead quenching bath. Thereafter the normalizing operation may be carried out as a separate operation in a separate normalizing furnace. I

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 i 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 thenmay be slit to form the finished light gauge high carbon strip steel.

Accordingly, the present invention provides a radicallynew 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 carburizing low carbon strip steel which has been cold rolled to substantially the desired-finished 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 difficulties and solves long-standing problems in the art.

In the foregoing description certain terms have 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 strip or to the internal heating of the strip; because the heating of the mufile or of the carb-urizing gas in the muffle is performed separately of the heat developed in the strip by internal heating, and 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 discoveries, principles, apparatus, combinations, parts, sub-combinations and elements, and mechanical equivalents obvious to those skilled in the art, are set forth in the appended claims.

I claim:

1. In apparatus for continuously uniformly carburizing strip steel, a furnace having a vertical tubular muffle wall forming a carburizing chamher; an entrance housing mounted at the top of said muffle, a trough associated with said housing and forming therewith an entrance opening to said housing and muffle, a current conductive bath in said trough sealing said entrance opening, a roll in said trough bath, a roll in said housing above the upper end of said muffle; an exit housing mounted at the bottom of said mufile provided with an outlet, a current conductive bath in said exit housing sealing'said outlet, a roll in said exit housing bath below the lower end of said muffle, heating means for said exit housing maintaining said bath therein at a predetermined temperature; means for supplying carburizing gas to the interior of said mufile; means for passing steel strip to be carburized continuously through said trough and beneath said trough roll, then through said entrance opening and over said entrance housing roll, then vertically downward through said muffle, then through said exit housing bath and beneath said exit housing roll, and then through said outlet; means for heating the furnace chamber externally of the strip; means for passing electric current to and from the strip through said current conductive baths and through the strip while it passes through said carburizing chamber to internally heat the strip; electrical insulation means preventing current flow between said current conductive baths other than through said strip; control means electrically connected with the supply of current passed to and from the strip maintaining a constant uniform power input to the strip thereby maintaining a constant strip carburizing temperature due to internal heating; temperature control means connected with the chamber heating means; pyrometer means mounted in the furnace wall including a plurality of heads one directed toward a longitudinally extending center zone of the moving strip and a second directed toward a longitudinally extending edge portion of the moving strip, a differential temperature device connected with said heads indicating strip temperature differentials across its width between the moving strip center zone and edge portion, and means electrically connected with said device and the power input control means actuated upon indication of a temperature differential to operate the power input control means to change the rate of power input 2. In apparatus for continuously uniformly carburizing strip steel, a furnace having a vertical tubular muffle wall forming a carburizing chamber; an entrance housing mounted at the top of said muffle, a trough associated with said housing and forming therewith an entrance opening to said housing and mulile, a current conductive bath in said trough sealing said entrance opening, a roll in said trough bath, a roll in said housing above the upper end of said muffle; an exit housing mounted at the bottom of said muffle provided with an outlet, a current conductive molten lead bath in said exit housing sealing said outlet, a roll in said exit housing bath below the lower end of said muffle, means selectively heating and cooling the molten lead bath in said exit housing, means controlling said heating and cooling means to maintain said molten lead bath at a predetermined temperature in excess of 750 F.; means for supplying carburizing gas to the interior of said muffle; means for passing strip steel to be carburized continuously through said trough and beneath said trough roll, then through said entrance opening and over said entrance hOllSillg roll, then vertically downward through said muffle, then through said exit housing bath and beneath said exit housing roll, and then through said outlet; means for heating the furnace chamber externally of the strip; means for passing electric current to and from the strip through said current conductive baths and through the strip while it passes through said carburizing chamber to internally heat the strip:

electrical insulation means preventing current flow between said current conductive baths other than through said strip; control means electrically connected with the supply of current passed to and from the strip maintaining a constant uniform power input to thestrip thereby maintaining a constant strip carburizing temperature due to internal heating; temperature control means connected with the chamber heating means; pyrometer means mounted in the furnace wall directed toward at least two longitudinally extending zones of the strip moving in the muffle actuated by variations in strip temperature across its width to indicate any such temperature variations as the strip passes through said mufile; and the control of the molten lead bath temperature at a predetermined temperature in excess of 750 F. permitting the carburized strip quenched in said bath to pass around said exit housing roll and out of the inlet.

FRANK T. COPE.

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

UNITED STATES PATENTS

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