US1842272A

US1842272A - Process and apparatus for treating steel

Info

Publication number: US1842272A
Application number: US305667A
Authority: US
Inventors: Horace C Knerr
Original assignee: Ajax Electrothermic Corp
Current assignee: Ajax Electrothermic Corp
Priority date: 1928-09-13
Filing date: 1928-09-13
Publication date: 1932-01-19
Anticipated expiration: 1949-01-19

Description

Jan. 19, 1932. H, Q KNERR 1,842,272

PROCESS AND APPARATUS FOR TREATING STEEL 14/ LJ@ fnvejzo" Jfaface lzefz;

Jan. 19, 1932. H, Q KNERR 1,842,272

PROCESS AND APPARATUS FOR TREATING STEEL Filed Sept. l5, 1928 3 Sheets-Sheet 2 i .57 u @@QM JMW.

Jan. 19, 1932. H C, KNERR 1,842,272'

PROCESS AND APPARATUS FOR TREA'IING STEEL Filed Sept. 13, 1928 3 Sheets-Sheet 3 jfaface' zerf;

Marne/Ly Patented 19, 1932 UNITED STATES PATENT oFFl'ncE HORACE e. ENEER, or GEmuNrowN, PHILADELPHIA, PENNsYLvANA, nsan'nir'oiz. v ro aux ELEcTEo'rnEnnrc conroEA'rIoN, 0E TEENroN, NEW JERSEY, .a couronn- TION F NEW JERSEY PROCESS .AND APPARATUS FOB TREATINGSTEEL Application led. September 13, 1928. Serial No. 305,'6-67.

My invention relates to methods and apparatus for heating steels which require that they bev raised to specially high temperatures, so far best exem lified in the treatment of high speed steel t at it is hereinafter used as typical.

A purpose is to use a pot of carbonaceous material to react lupon the bath to remove oxygen from the bath.

A further purpose is to protect a furnace pot of material which is attacked by oxygen atliigh temperature from the atmosphere by a shield reaching down within the content of the pot and to cool the pot artiiicially .at

the upper end to further protect 'the pot from taking up oxygen.

A further purpose is to protect the exterior of a heat-treating pot formed of material which takes up oxygen at high temperature '20 from the atmosphere, by a surrounding powdered refractory and to alter the character of the refractor at the upper end to give better heat con uctivit there than below, with or without artilical y cooling the refractory so as to reduce chemical activity between the bath and the parts which it affects.

A further purpose is to protect the exterior of a potv holding a bath formed of material which takes up oxygen at high temperature v30 from the atmosphere, by a surrounding.' owdered refractory, to protect the interior a ove the bath by a shield and to artiicially cool the exterior of` the upper end of the pot through the refractory.

A further purpose is to use a graphite furnace pot to hold a heat-treating salt bath and to protect the graphite by an interior seal extending into the salt and by artitically reducing the temperature of the upper part of the graphite.V Y

A further purpose is to protect a graphite pot holding a salt bath from oxidation at the upper range of the pot, above the bath by artificial cooling.

A further purpose is to artificially cool the upper portion of a crucible holding a salt bath, diiiicult of use at high temperatures, in order to permit the use to advantage of protective means.

59 A further purpose is to protect a graphite crucible heating a salt bath by a metallic shield reaching below the salt level and to protect the shield by reducing the temperature at and above the salt bath level to prevent attack by carbon from the crucible.

A further purpose is to reduce lthe chemical activity of the upper end of the pot and its contents by reducing its temperature.

A further purpose is to induce an electric current in the lower part of a graphite cruci-l ble holding a salt bath. and to conduct the heat away from the upper part of the crucible so aseto permit the use of a protective shield dippinginto the salt bath.

In the use of a graphite crucible to contain a salt bath, a further purpose is to protect the crucible by a shield having heat-resisting properties lying back out of line with the walls of the crucible and to reduce the temperature of the crucible and, through it the temperature of the upper part of the bath and the shield to a temperature below that at which the shield and crucible will be seriously attacked.

A further purpose is to protect a graphite crucible heating a salt hath by a metallic shield located out of line of the circulation ofthe salt bath up along the walls of the crucible.

A further purposeis to enlarge the upper end of a salt bath-containing crucible, whether heated by an inductor-or otherwise, to provide an annular pocket -and to shield the crucible -from the atmosphere by a sleeve fitting within the pocket and hence out of the path of circulation of the salt.

A further purpose is to reduce the attack of a salt bath upon a crucible protecting shield by increasing the heat gradient through the shield thus reducing the tem erature of the shield on the side where the ath would attack it.

A further purpose is to use the lower part of a graphite heat-treating pot holding the charge as a deoxidizing agent to absorb oxygen from the bath and to cool'the pot above the charge to prevent deoxidizing action of the pot above this range.

A further purpose is to provide an ini exterior heat and electrical insulation and to make the crucible and its insulation bodil rerlnovable from an inductor coil within which it `es.

A further purpose is to protect the surface of a bath by a llame using a neutral llame for protection alone and an oxidizing or reducing iame for the accomplishment of special purposes.

A further purpose is to protect the interior of a pot holding a heat-treatingbath by a liner introduced within the surface of the bath and to protect the liner from action of the bath at the water-line by the use of a llame.

A further purpose is to determine the temperature of the bath beneath its surface by the use of a tube having the lower end closed and introduced into the bath, and to use an optical pyrometer to indicate the temperature of the bottom of the tube.

A further purpose is to provide for rapid dumping of a pot carrying a bath.

A further purpose is to use a low carbon steel pot for heat treatment within a graphite ot servinvI as a seconda for a ool-surrounding coil carrying alternating current and to protect the inner partof the graphite pot with a non-carbonaceous refractory to prevent carburization of the pot.

Further purposes will appear in the speciication and in the claims.

My invention relates both to the methods and to apparatus by which the methods may be carried out.

I have preferred to illustrate my invention by two forms only, varying slightly, selecting forms which are practical, elfeetive, convenient and thoroughly reliable but which have been selected because they at the same time particularly well illustrate the principles of the invention.

Figures 1 and 2 are vertical longitudinal sections showing the two forms selected.

Figure 3 is a side elevation showinlg a tilting mechanism for the pot shown in igures 1 and 2.

In the drawings similar numerals indicate like parts.

My methods and apparatus are intended for heat treatment of metals and as such are applicable to a number of metals and to a number of purposes in the heat treatment.

Of these the finest example of which I know-because it has the highest utility at the present time-occurs in the art of heat treatment of high speed steels preliminary to quenching, which, in turn, is preliminary to tempering.

Taking up the illustration in the high speed steel art In the tool art, the term high speed steel refers to that group of alloy steels which have the property of retaining their cutting eiiiciency at a dull red heat, permitting the machines in which the actual cutter is placed to be run at speeds and at feeds at which carbon steels and other alloy steels quickly overheat and burn or soften, losing their cutting ability. The capacity to retain its hardness and strength at elevated temperatures has given high speed steel diverseother uses than for cutting purposes, also for cuts where other steels fail at ordinary speeds.

Usually high speed steel contains tungsten, chromium and vanadium. One formula in large use includes tungsten 18%, chromium 4%, vanadium 1% and carbon 0.65%.

High speed steel is used not only for the rougher cutting operations, but for a variety of finished tools such as drills, milling cutters, thread cutters, files, hack saws, dies and the like, where little or no final grindin may be done after heat treatment and in wich it is therefore necessary to avoid decarburization or damage to the inished surfaces or fino cutting edges of the tools during the heat treatment. For this reason a very -high degree of protection to the surfaces becomes necessary if the tools are to give fine performance and have long life.

Approved ractice, as I have found it, for hardening ofp high speed steel may be summarized as follows The tools are preheated slowly and uniformly to about 1500 F. to avoid the sudden shock which would occur if the tools were heated at once rapidly to the quenchin temperature. They are then transferre to a furnace havin a temperature of 2250 to 2400" F. in w 'ch they are heated rapidly and held at maximum temperature but a short time, after which they are quenched in oil or in an air blast, or are cooled to about 1100 F. in a bath of molten salt or molten lead and afterward quenched in oil or air. They are iinally tempered by reheating and soaking for an hour or so at a temperature of 1050 to 1150o F. and are allowed to cool in air.

The very high temperature, in the neighborhood of 24:00o F. is necessary toinsure the maximum solid solution of the. hardening constituents (namely carbidesetc.) in the steel. Failure to reach sufficiently high temperature means failure to secure maximum hardness and maximum cutting eiiiciency.

The tendency for grain growth at the high temperature required, with consequent embrittlement, limits the time at which the steel nia be kept at the high temperature.

For tlie purpose of avoiding damage to finished tool edges and surfaces at the high temperature, the quality of tools havin line cutting edges is often sacrificed by not eating them to suiliciently high temperatures, stopping at 2300o F. or 2250 F., sacrificing service to surface.

Molten salts have been used for a heating medium but have not proved wholly satis-- factory at the very high temperatures, with the result that lower temperatures have been employed, usually if not uniformly, when salts have been used.

I have invented or discovered a salt bath which is fully effective and which permits free vuse -of the higher temperatures disclosed in my ap lication for tempering bath, Serial No. 382,0 3, filed July 29, 1929. The present invention is directed to a. method of protection and a furnace capable of operating satisfactorily with my new bath and also much better than previously, though perhaps not up to Afull hardening temperature, with various other-baths which have been proposed and which have given dliculty in the past by fuming, attack on the container or attack on the steel being treated in the bath. My invention can be used for heating other baths not salts.

In other words my furnace is designed to protect from objectionable atmospheric attacks upon a carbonaceous furnace pot, protecting onthe outside throughout the entire length, and above the bath upon the inside of the pot; also to avoid oxidation of the inside of a .protective shield, and carburization of the outside of the protective shield. All of the sources of injury become more highly injurious with increased temperature causing the pot and shield in prior practice to deteriorate rapidly and necessitating frequent and expensive replacement. My various means of protection make it possible for my furnace to handle not onl my new bath but with prolonged life o pot and shield, to handle even existing salts at temperatures high enough to have given serious trouble in the past.'

One feature of my invention might therefore be described as a furnace for a salt bath capable of opera-tion with minimum injury at temperatures higher thanhave been attainable with safety to the apparatus and (or) tools with existing salt baths and furnaces.

My new molten bath, when uncontaminated, is neutral to steel and has a strong af- -finity for oxygen and oxides, as have many other salts.

Whatever the bath, I find that it is very desirable to protect the surface from oxygen and at the same time to apply strong convection circulation within the body of the molten salt to stir it well and thus to secure temperature uniformity within the bath.

For preheating the tools it is necessary to haveA a. bath which is liquid below the preheating temperature (1500o which is sufticiently fluid at this temperature to insure temperature uniformity by convection and which does not harmfully `contaminate the high temperature bath when carried over into it as film adhering to preheated work.

' I have developed a bath disclosed in said application which consists of a modification of the high temperature bath, which melts at approximately 1450o F. and in which when cold work is inserted, a At-hick layer of the salts immediately freezes around the tool. This layer melts off slowly so that sudden temperature shock is prevented. This preheating bath may be contained in an ordinary pressed steel pot, or preferably in a pot of heat-resisting alloy, and heated by electricity, or by gas, oil orfuel.

I'n heat-treatment by a salt bath, particularly where it is desirable to attainA uite high temperatures, of hi h speed steel or example, in preparation Ior quenching, it is quite desirable to use a graphite potyto hold the fused salt, within which the 'actual heating of the tools is to be effected. -However, this has proved so difficult because of the'highly c-xidizable character of pure carbon that inventors have, been seeking` ways of avoiding use of graphite rather than ways of using it.

One of the difficulties which has been met lies in the action of the fused salt in attacking the interior wall of the pot at and above the surface of the bath. At high temperatures graphite has a -very high affinity for oxygen, and as a result of this aflinity carbon from the interior wall of the graphite pot disused with a cylindrical shield to cut off the atmosphere from the interior above the bath with but little exposure of the cuter surface of the shield to attack by carbon from the pot.

A sleeve or cylinder has been used to protect the upper interior wall of the pot from the atmosphere at and'above the surface of the bath, the space between the cylinder and the pot being closed and the cylinder dipping below the surface of the bath to avoid passage of air from the otherwise open end of the cylinder. However, this was not in connection with a graphite pot and it was assumed that such a pot was impracticable.

Under normal circumstances the use of a steel shield of the character indicated, placed within a graphite pot, ,would result in very rapid deterioration of the shield by reason of the attack upon it of carbon iioating through to it upon the bath from the opposite inner face of the graphite Crucible. Part of the purpose of my invention is to suit these two desirable features to operation together so that I may obtain the benefits of the graphite pot or Crucible without excessive destruction of the shield by carbon attack from thc pot.

With acrucible or pot made of carbonaceous material such as graphite, when used in connection with a shield of heat-resisting material which is capable of attack by carbon, such as an alloy of nickel and chromium, there is serious attack upon either the pot or the shield at the higher temperatures. At the higher temperatures also oxygen attacks the inside of the shield and the pot where it is not protected by the salt or by refractory. The inner pot wall above the bath is attacked by the oxygen leaving a fine carbonaceous deposit which is carried across to the shield by the bath and, at the high temperature, attacks and ultimately destroys the shield.

The passage of particles of carbon my be explained by the fiow of the more fluid part of the bath up along the walls of the crucible`\- which, by the ebulition of the bath above the water line, meistens the interior of the crucible above the water line with a consequent washing down of particles off the graphite.

The use of electricity as a source of heat, whether it be inductively or conductively delivered to the carbonaceous resistor, makes it possible for me to avoid those objections in previous operation which have been based upon loss of efficiency due to a high sensible heat in escaping products of combustion when 4burning gas c-r oil, and deterioration and unpleasant working conditions due to the fumes, gases and heat from these products of combustion. A

The graphite of the pot is a poor conductor of electricity at low temperatures but a much better'ccnductor of electricity at higher temperatures, so that it can be heated by alternating current induced in it by an inductor primary.

As my invention is best describedin conjunction with an illustration I will proceed to describe the construction and operation 0l' the form of the invention seen in Figure l.

In this figure I have shown the graphite Crucible 5 located within an electric, preferably but not necessarily high frequency inductor coil 6 supplied with current from a generator 7 through any suitable leads 8 and 9 and permissibly shunted by condenser l0 for purposes of correcting-the power factor o l' the supply.

The coilhas been shown as made up of hollow conductors of flattened cross-section at 11 for the purpose of accommodating water cooling, but, as with the question of frequency, the detail of the coil need not be emphasized as it is not vital provided the coil suitably induces electric current in the Crucible. I find a pot-surrounding inductor coil advantageous as compared with other 00 heating means, both in the uniformity of heating effected by it and in the freedom thus secured from objectionable products of combustion.

The coil is lined by any retaining insulating material 12, such as mica. which will stand the temperature and which will hold in the preferably powdered refractory 13 by which the pot is spaced and heat-insulated from the coil. This lining and all that lies within it can be withdrawn bodily from the coil, which is tapered for this purpose.

The refractory rests upon a heat-insulat-v at 21 to receive a shield 22 which reaches down into the salt bath and protects the upper inner surface of the pot. At the top the shield is centered and spaced from the pot by a ring 23 which may be supported upon the crucible if desired and which is laterally supported by packing 24 within insulation 12. The'shield is sealed at the bottom by the salt and at the top is sealed against air admission by the

contacts

25, 25 and 252 between it and ring 23, and between the packing and the rings 23 and 18 respectively. A cover 26 protects the upper surface mechanically and can be used to assist also in protection from air inlet between the shield and the pot.

The shield is preferably made of a metal which is highly heat-resistant such as an alloy steel, and is apertured at 27 to provide communication with the space 28 for a further purpose of protecting) against flow of oxygen to this space as will subsequently.

In order to give as high a heat gradient as possible through the shield I illustrate it as being made up of laminations, i. e., of two

shells

29 and 30 so that the joint between the two shells will reduce the rate of heat transfer between. When laminated, whatever the number of thicknesses, the layer or layers of smaller diameter may be made of highly heatresisting alloys and the layer of larger diameter of a material resistant at high temperatures to carburization.

Above the coil 11 I use a filling between the shell 12 and the upper part of the carbonaceous (here shown as graphite) pot which filling is heat-conductive, such as carborundum grits, shown at 31 and cool this material, and thus cool the upper part of the pot, artifically. This artificial cooling is shown in the form of a hollow spiral 32 through which cool water is circulated. I may mount this spiral upon the shell 12 and take it as well as the ring 18 and cover 26 out wit-h the shell and shell contents. Whether this be separately supported and allowed to remain or taken out with the shell is not material to the main parts of my invention.

I show one pyrometer among many by which the temperature of the bath may be e more fully explained A determined, in the tube 33' extending down into the bath where the bottom 34 may be observed through any suitable instrument 35, comprising on optical pyrometer.

In order to protect the heat-resisting interior above the salt bath level from attack by oxygen which is destructive at the high temperatures to which the shield will be raised I rovide a protective `gas content through a gunsen. burner 36 an nozzle37 supplied with gas through pipe 38.

The gas iame may be made merely protective by the use of a neutral flame or may be made oxidizing or reducing as preferred. This same gas content can be used also to fill the space between the top of the shield and the top of the pot to prevent access of air to the space 28 and through the apertures in the shield vma, moreover be used to fill the space between t e shield and ot, more fully protecting against the ob]ectionable oxygencarrying content of this space than can be done by attempted closure alone.

The apertures 27 may be placed at any desired height in the shield and will permit flow of gas into the space 28 to exclude the air without making a vacuum, and to provide a neutral atmosphere when a neutral atmosphere is provided within the shield.

The use of insulation at 39 and 40 between the shield and the ring 23 makes the character of this ring less important than would otherwise be the case.

In Figure 3 the furnace is shown as mounted bodily upon any suitable support 41, providing pivot points on opposite sides at 42 about which the furnace may be tilted and by which the return position of the furnace may be controlled.

The water supply is secured by hose connections 43 and electrical connection is provided through tvvo furnace terminals 44 which normally lie in spaced mercury pools 45 connected to conductors 46 and 47. When the furnace is tilted the electrical circuit is disconnected.

The bath may be emptied into any suitable container 46 and a new'bath substituted.

The forms shown in Figures 1 and 2 differ considerably but have in common the inductor, the graphite inductor secondary 5 (which, however, is an inductor only and not a pot in Figure 2), the shield used for protection of the upper part of the pot so that when the shield is attacked it can be replaced without requiring a new pot, the flame protection treatment within and about the shield, the heat insulation of the inductor, removable with the electrical insulation as a unit from the inductor, the general character of ,the frame and the mounting to provide the electrical connection automatically and disconnection after the circuit has been opened elsewhere, to facilitate dumping.

The graphite inductor secondary is protected by a layer 48 .of material which does not i have a carburizing eect upon the steel pot 49 within it. It need not be fastened to either of thefacin walls and n eed not be cohesive as even san would fill the requirements.

The shield 22 is here arranged to rest upon the upper end of the pot, is apertured as in the case of the shield 22 but is shown as comprising a single layer only.

'Ihe cover 26 is slightly diiferent and is slightly differently connected as compared with the cover 26.

The space 50 between the non-carbonaceous coating of the secondary and the pot is made small in order to secure maximum heat transfer between the secondary and the ot.

The pot is flanged at 51 and this ange rests between insulation 52. and 53 supported upon an annular plate 54 directly beneath an insulating annulus 55.

In operation in the form seen in Figure 1, alternating current applied to the coil induces a corresponding but reverse alternating current in the graphite wall of the crucible, heating the graphite wall and thus in turn heating the salt within the pot to any predetermined temperature which can be retained readily because it is easy to control the quantity of current iovving through the coil.

As the bath is heated from the pot wall, that portion of the bath is most highly heated and is most iluid which lies closest to the wall of the pot. Bubbles may be formed within this outer and more fluid part of the bath and these bubbles along with the convection flow of the molten salt cause a relatively rapid flow of molten salt closely along and guided by the inner wall of the pot. In a normal pot this would cause a splashing or spraying of the molten bath against the walls of the pot.

To bring the salt up to a temperature of, say, 2400 F. requires that the graphite be raised to a temperature even higher than this. At this high temperature if a graphite crucible be used without my invention particles of graphite fall from the inner surface of the pot at and above the Water line, both because of oxidiation of the graphite through the air and because of spraying of the interior graphite wall with molten salt. This results in distributing the graphite particles over the surface of the molten bath where the engage and combine with steel to be eattreated, fusing the steel and' destroying its finish or edge where this takes place.

The protective shield in itself is old but it has hitherto been deemed impracticable to make use of such a shield with a graphite ot for the reasons that though it reasona ly shuts ofl` the atmosphere from the space bex tion of graphite particles over the surface of the molten bath within this space causes excessive carburization of the metal of the shield.

It will be noted that a shield placed within a pot of the usual type lies inside of the line of the pot as extended, so that the circulation of the bath causes its content to bubble up into the space between the pot and shield, having much the same effect upon the interior of the pot as if the shield were not there except that the supply of -trapped air between the pot and the shield is not unlimited and the ultimate damage is correspondingly reduced.

It will thus be seen that the difficulties in the use of the graphite pot and metal shield within it arise primarily from the intensitication of chemical reaction at the high temperature at which the salt bath is desirably maintained, and at that temperature are due to a variety of causes that might be summarized as resulting from the oxidizing atmosphere inside of the shield and between the shield and pot above the bath, and the turbulence of the bath, particularly of that portion of it lying between the shield and pot. Where the pot is not covered by an atmosphere excluding refractory, there should be added to this list the effect of the oxygen in the atmosphere upon the exterior surface of the pot, but this would be much less objectionable since the carbon freed by it would not react objectionably within the pot.

It will be evident that the chemical activity of the salt bath may not only be reduced by reducing its temperature while fused but that if the reduction of temperatures proceeds far enough-wholly Within the control of the designer-the temperature may be reduced to a point where the portion of the bath close to the edge of the pot freezes wholly, cutting olf circulation and thus increasing the viscosity, stopping physical activity and destructive action. l

My invention overcomes all of these diculties. The inductor coil does not induce current in the pot throughout the whole length, so that the upper part of the pot is heated largely by conduction and is heated less than would otherwise be the case because it is thinner than is the lower part of the pot.

The whole exterior surface of the pot is protected against the atmosphere by the surrounding refractory filling, whether heat-insulating or heat conducting. The upper part of the pot is cooled through the heatconducting" filling material and the watercooling about this upperportion. .This cools that portion of the salt bath between the shield and the pot and cools the outer part of the lower end of the shield.

No electric current need be induced in the shield for the three reasons, that the coil lies A chiefly or entirely below the shield, .that the depth of penetration: of the current induced in the pot is shallow enough 'so that there is not need of inducing current in an content of the pot, and because the shiel even if magnetic at lower temperatures has lost its magnetism at the high temperature of use.

Though the inner part of the shield is heated at its lower end by the bath, conduction through it is reduced by the lamination of the shield; and if this were not so the inside of the lower end of the shield would be cooled by reason of the artificial cooling of the outside of the lower end of the shield.

The neutral atmosphere which I provide within the shield and within the space between the shield and the pot cuts off excess of air and displaces any air which there might otherwise be within and about the shield.

The placing of the shield radially out beyond the line of upward circulation of the molten salt passes this circulation into the interior of the shield and ensures a quiescent bath within the space between the shield and the pot.

When I speak of cooling the upper part of the pot wall I do not mean that it is really made cool but merely that it is kept from becoming so highly heated as to oxidize readily even if there were an unrestricted supply of oxygen present. This keeps low the amount of carbon released from the interior wall of the upper part of the pot about the bath and substantially eliminates carburization of the shield. It further greatly reduces oxidation of the shield. As the amount of cooling is within the control of the operator, the temperature can be kept within any limits desired. A reduction of 200 or 300.0 makes a tremendous difference in the chemical ainity of the pot for oxygen and of the shield for oxygen and for carbon.

The removability of the shield or of the pot with its content and surrounding refractory greatly facilitates repairs and replacements, and the tilting of the pot and its content makes is possible to dump a salt bath quickly and replace it where otherwise the emptying would be much slower and more difficult.

The Bunsen burner air supply is normally adjusted to avoid excess air and to maintain a iiame at the top of the pot Where the excess gas appears. This effectually cuts oli oxygen inlet through the llame.

In the form of my invention shown in Figure 2 I have avoided all of the diiiiculties previously met with and have avoided some of them in a different way from their avoidance in the form shown in Figure 1. The use of the metal pot avoidsthe attack of carbonaceous material upon the shield and makes it unnecessary to use artificial cooling since the oxidation of the pot and of the shield, both inside and outside the shield, can be taken care of readily even at the high temperature desired byl controlof the atmosphere engaging them. I show the same control of the atmosphere in this form as in the other.

While I have not used gra hite for the pot, I have taken advantage o its value as a secondary for the primary inductor heating coil b placing the pot within a graphite secon ary. The pot is protected by the noncarbonaceous materiali in between the ot and the graphite, whic oes not inter ere seriously with lproper heat transfer from the secondaryto t e pot.

It is not my intention to restrict my invention to graphite ots as parts of the invention are applica le to constructions in which no graphite is used, either as the pot itself or as a secondary to heat the pot; nor is it my intention to restrict the invention where graphite pots are used toI raphite pots having a thicker bottom wall t an side walls. Th1s comparative thickness of wall is relatively immaterial except with certain kinds of graphite. I iind that the Wall has a greater tendency to porosity axially of the pot than transversely thereof. The particles of carbon coming up along the wall of the pot will float upon the center of the bath and will be burned in the flame, which can be an oxidizing iiame, to the extent desired at the center and yet be reducing or neutral above the outer part of the bath. This will be evident since the cone of the iiame is itself oxidizing and all that is required is that the cone of the ame shall be brought suiiiciently in contact with the bath at about the center, as shown.

Though' I prefer to produce the oxygenfree atmosphere above the bath by the gas iame, it will be evident that this can be se cured by the use of nitrogen or other gas free from oxygen. This oxygen-free atmosphere is highly desirable whether the shield be used or not, as without the shield protection of the interior of the pot it is even more desirable in both forms of Figure 1 and Figure 2.

Though there is not the same reason for enlarging the upper diameter of the pot in Figure 2, as in Figure 1, since there is no carburization to be guarded against and upward circulation of bath between the shield and the pot is less injurious in the form of Figure 2 than in the Figure 1 form, this upper part of the pot can be enlarged in the form shown in Figure 2 also, if desired, so that the space between the shield and pot will lie out of the path of bath circulation, giving an interior pot contour generally like that in Figure 1. Y

Though there is not the same reason for cooling of the upper part of the pot in the form shown in Figure 2 as in the form shown in Figure 1, since the carburization protected against in the Figure 1 form is much more injurious than theoxidation vto be protected against in the Fi ure 2 form, it may nevertheless be desirab e to water-cool the upper part of the pot in the form shown in Figure 2 and this can .be done by the same character of construction as is shown for this purpose in Figure 1.

Some arts of my invention may be practiced (w ere it is not desired to attain the high temperatures which I aim to attain) by inducin current directly in the metal ot as a secon ary. This is not nearly as deslrable as using the graphite secondary for the inductor and does not lend itself easily to the attaining of high temperatures. Nevertheless, where this is done and the construction otherwise comes within my claims I desire, of course, to include it within such of my claims as do not include the use of a graphite or other carbonaceous pot.

The metal pot has advantage over the graphite pot in permitting use of salts which react unfavorably with graphite, whereas the graphite pot has advantages in electrical and thermal efliciency and in the deoxidation of certain types of salt bath.

Wide differences of frequency can be used with my gradient secondary whether it be the pot or a secondary merely heating an interior pot. However, the frequency is not Wholly immaterial as both high frequency and low frequency haveadvantages for different purposes. With high frequencies the extent of penetration of the current within the pot is reduced, making it possible to use a pot having thin Walls, and a very high rate of heat transfer is secured from a low current; While with low frequencies there is less difliculty in obtaining current of the desired frequency, and for purposes of long pot life the thicker pot then desirable also has advantages.

Though the carburization of which I have treated has been due to washing down of carbon from the interior of the pot wall above the bath, I would call attention to the fact that some carburization also may take place through the Washing of carbon upwardly from the interior wall of the pot below the bath level. With the placing of my shield out of line with the circulation this affects only the radially interior surface of the shield.

In View of my invention and disclosure variations and modiiications to meet individual whim or particular need will doubtless bccome evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the structure shown, and I, therefore, claim all such in so far as they fall within the reasonable spirit 'and scope of my invention.

Having thus described my invention what I claim as new and desire to secure by Let-I tents Patent is 1. T he method of protecting a carbonaceous pot holding a high temperature molten salt bath and having a metal shield extending from above into the bath to cut off air from the upper interior part of the pot, from oxidation of the upper interior of the pot and from carburization of the outer part of the shield above the bath, which consists in surrounding the upper end of the shield with a neutral liame to prevent leakage of air into the space between pool and shell.

2. The method of protecting from oxidation the upper interior of a carbonaceous pot holding a high temperature molten .salt b ath and preventing carburization ofametalshield extending from above into the bath to cut off air from the space between the pot and shield, which consists in conducting the heat outwardly from the upper part of the pot as distinguished from the lower part of the pot and dissipating the heat to reduce the temperature of the pot above the bath.

3. The method of protecting the walls of a space above the bath between a carbonaceous furnace pot holding a molten salt at high temperature and a protective shield inside the pot, at a point above the bath which consists in locating these portions of the pot and shield outside the line of circulation of the salt to render the salt within it quiescent.

4. The method of protecting a carbonaceous pot holding a high temperature molten salt bath and having a metal shield extending from above into the bath from oxidation of the upper interior of the pot and from carburizing of the outer part of the shield above the bath, which consists in lowering the temperature of the upper part of the pot above the bath and rendering the bath between the shield and pot quiescent by withdrawing this portion from the line of circulation of salt up along the pot walls.

5. The method of protecting a carbonaceous pot holding a high temperature molten saltA bath and having a metal shield extending from above into the bath from oxidation of the upper interior of the pot and from carburizing of the outer part of the shield above the bath, which consists in locating the shield out of the line of flow of molten salt along the walls of the pot to render the space between the upper part of the pot and shield above the bath quiescent, in lowering the temperature of the outside of the shield by laminating the shield and by artificially cooling the outside of the. pot above the bath.

6. The method of reducing deterioration of a carbonaceous pot in which a molten salt bath is held at high temperatures, which consists in concentrating induction of electric current in the lower part of the pot below the bath level to heat the pot and thus heat the bath and artificially cooling the pot above the bath to reduce it to a lower temperature,

without reducing the temperature of the lower part.

7. The method of reducing chemical activity in the space between the upper part of a pot holding a salt bath and a shield entering the bath which consists in locatin the space out of the line of circulation o the bath and thus rendering quiescent the portion of the salt bath which enters the space.

8. The method of protecting a metal shield extending into a hot salt bath from chemical attack upon the side opposite that upon which it is chiefly heated which consists in using different layers for the shield to increase the heat gradient.

9. The method of protecting a carbonaceous pot holding a molten salt bath and having a metal shield extending from above into the bath from oxidation of the upper interior of the pot and from carburizing of the outer part of the shield above the bath, which consists in establishing a high heat gradient from the inside to the outside of the shield by using different metals for inner and outer layers of the shield.

10. In a heat-treating furnace, a graphite pot adapted to hold a salt content which is to be heated to high temperatures, means for heating the graphite pot below the salt level and artificial cooling means for cooling the pot above the salt level, comprising a cooling coil and a heat-conductive material between the pot and coil.

l1. In a heat-treating furnace, an inductor coil, a carbonaceous pot within the coil of enlarged interior diameter in the upper part of the pot, as compared with the diameter below, adapted to hold a salt bath to be heated to high temperatures and filling the pot above the lower portion of the part of larger diameter and a shield of diameter larger than the smaller part of the pot entering the bath within the part of larger diameter and spaced from the walls of the pot, whereby the space betwen the shield and the pot is outside the line of circulation of the salt solution up along the walls of the pot.

l2. In a heat-treating furnace, an inductor coil, a carbonaceous pot within the coil counterbored in the upper part of the pot adapted to hold a salt bath to be heated to high temperatures, said salt filling the pot above the bottom of the counterbore, a shield protected at the top between it and the pot against air inlet, entering the salt solution and of diameter such that the space between it and the counterbored part of the pot is out of line of the circulation of molten salt comin up along the walls of the bath and artificlal cooling provision for the counterbored part of the pot.

13. In a. heat-treating furnace, an inductor coil, a carbonaceous pot within the coil counterbored in the upper part of the pot adapted to` hold a salt bath to be heated to high temperatures, said salt filling the pot above the bottom of the counterbore, a shield entering the salt within the counterbored portion of the pot and protected against air leakage at the top between it and the pot, heat insulation between the pot and coil below the counterbore, heat conductive material surrounding the pot about the counterbore and water-cooled coils surrounding the heat conductive material to dissipate the heat from the upper part of the pot.

14. A heat-treating furnace comprising an inductor coil, a graphite pot within the coil adapted to hold a salt bath, a shield sealed with respect to the pot to prevent air passage at the top of 'the shield and entering the salt bath, and artificial cooling means for the part of the pot about the shield.

15. A heat-treating furnace comprising a graphite pot adapted to hold a salt bath, means for heating the lower part of the pot and artificial cooling means for the upper part of the pot.

16. A heat-treating furnace comprising an inductor coil, a pot Within the coil adapted to hold a salt bath, a shield sealed with respect to the pot to prevent air passage at the top of the shield and entering the salt bath, and artificial cooling means for the part of the pot about the shield.

17. A heat-treating furnace comprising a pot adapted to hold a salt bath, means for heating the lower part of the pot, a shield sealed with respect to the pot to prevent air passage at the top of the shield and entering the salt bath, and artificial cooling means for the part of the pot about the shield.

18. A heat-treating furnace comprising an inductor coil, a graphite pot within the coil adapted to hold a salt bath, a shield sealed with respect to the pot to prevent air passage at the top of the shield and entering the salt bath, heat insulation for the lower part of the pot between it and the inductor coil, heat conductive means in contact with the portion of the pot about the shield andan artificially cooled coil about the heat conductive material.

19. A heat-treating furnace comprising an inductor` coil, a graphite pot of larger internal diameter at the top than at the bottom and having its lower portion within the coil adapted to hold a salt bath extending into the part of larger diameter and a shield sealed with respect to the pot to prevent air passage at the top of the shield, entering the salt bath and having a larger diameter than that of the bottom of the pot.

20. A heat-treating furnace comprising an inductor coil. a graphite pot within the coil adapted to hold a salt bath and a shield sealed with respect to they pot to prevent air passage at the top of the shield and entering the salt bath, the `shield heilig of multiple layers to provide heat gradient between the interior of the shield and the outside of the shield.

21. A heat-treating furnace comprising an inductor coil, a graphite pot within the coil adapted to hold a salt bath, a shield sealed with respect to the pot to prevent air passage at the top of the shield and entering the salt bath and a gas flame within the shield protecting the shield from air inlet.

22. A heat-treating furnace comprising an inductor coil, a graphite pot within the coil adapted to hold a salt bath, an apertured shield with respect to the pot to prevent air passage at the top of the shield and entering the salt bath and a gas fiame within the shield filling the space within the shield and the space about the shield above the bath to pretect against oxidation of theshield and interior of the pot.

23. In a heat-treating furnace, a pot, means for heating the pot adapted to hold a salt bath therein, a shield sealed against air entry at the to between it and the pot and entering the Ibath and a neutral gas fiame within the shield protecting against air inlet.

24. A heat-treating furnace comprising a` graphite pot, means for heating the pot adapted to hold a salt bath, a shield sealed with respect to the pot to prevent air passage at the top of the shield and entering the salt bath and a chemically treating ame within the shield.

25. A heat-treating furnace comprising an inductor coil, a graphite pot within the coil adapted to hold a salt bath, a flame applied to the top of the bath, a shield sealed with respect to the pot to prevent air passage at the top and entering the salt bath to protect the shield from other chemical effect than that of the fiame and cooling means for the top of the pot about the shield.

26. An inductor heat-treating furnace comrising an inductor coil, heat and electrical insulation therein, a graphite pot within the insulation adapted to hold a salt bath, and a shield within the pot sealing with the salt at its bottom and against air inlet to the space between the shield and pot at the top and providing a space between the shield and pot out of line with the path of the salt bath fiowing upwardly from the pot.

27. In a heat-treating furnace, an inductorcoil, a graphit-e secondary in the coil, insulation between the coil and secondary, a metal pot within the graphite secondary adapted to hold a salt bath and a refractory between the graphite secondary and the pot.'

28. In a heat-treating furnace, a metal pot. a heat-treating bath within the pot, a shield for the upper part of the pot entering thel bath and sealing at the top against air inlet to the space between the shield and pot and a lgas flame protection for the interior of the shield fillingl the upper part of the shield.

HORACE C. KNERR.