US2290552A - Heat treating furnace - Google Patents

Description

July 2l, 1942. J. R. GIER, JR

HEAT TREATING FURNACE Filed Sept. 26, 1939 ATTORNEY July 21, 1942- J. R. GIER, JR 2,290,552

HEAT TREATING FURNACE Filed Sept. 26, 1939 2 Sheets-Sheet 2 INVENTOR L/o/m G/erLz//f BYM/m/ ATTORNEY WITNESSES:

Patented Julyy21, 1942 HEAT TREATING FURNA'CE Joh-1i R.. Gier, Jr., Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania vApplication September 26, 1939, Serial No. 296,587

(Cl. 266-5) v 4 Claims.

My invention relates generally to electric furnaces, and more particularly to furnaces especially adapted for the heat-treatment, at elevated temperatures, of tool steels in the form of tools, dies, and the like, while enveloped by an inert or protective atmosphere for the purpose of uniformly hardening the article heat treated. 1

One of the primary purposes of my invention is to provide a furnace in which tool steel articles including those made of high speed tool steels may be hardened for subsequent use, free from any of the types of damage heretofore usually experienced, the more important of which are oxidation, decarburization, distortion and cracking.

Oxidation and decarburization are caused by chemical reaction of elements in the steel with certain elements in the furnace atmosphere at high temperatures. Oxidation produces an undesirable surface scale and decarburization produces a soft surface layer, the metal thereof being deficient in carbon. Through the use of certain protective reducing gas atmospheres steels are now commonly prevented from scaling but these atmospheres do not ordinarily prevent decarburization.

To prevent both oxidation and changes in surface carbon content on all types ofl tool steel during heat-treatment requires a reducing atmosphere such as a mixture of hydrogen and nitrogen which may contain CH4 up to 2% and not more than traces of CO2 and H2O. Ordinary controlled atmosphere hardening furnaces are incapable of maintaining a protective atmosphere of the necessary purity and exactness of composition.

It is a purpose of this invention to provide a furnace in which protective atmospheres of high' purity and accurately controlled composition may be maintained to prevent both oxidation and decarburization of all tool steels during heattreatment.

In the widely used and more expensive tool steels which must be heated to maximum temperatures in the range of 1700 F. to 2300* F., more or less, for hardening, decarburization can be extremely rapid due more to high temperatures required for carbide solution than to any direct eiect of alloy content. AI-Ieretofore, it has been the practice to allow for surface damage as well as for distortion by leaving extra stock on the tool which had to be removed after hardening, by one or more finishing operations, such as grinding and hand stoning for ne interior contours; all of which add greatly to the cost of the tools and dies.

It is, accordingly, an object of my invention to provide a furnace for the heat-treatment of tool steels, especially at elevated temperatures 1n the range of 1700. F. and higher, from which oxidizing agents can be eiectively kept out of contact lwith the charge, whether such agents come from the air or are occluded gases contained in the charge, or fire brick of the furnace, or come from other sources. l

Damage by distortion includes the irregular or unpredictable changes in dimensions or contour of the tool in excess of the normal and deter-l visions for hardening tool steels which include the step of cooling in air (gas). This restriction, however, is not at all serious or objectionable since it parallels the previous requirement, namely, that the steels considered are those requiring temperatures in the neighborhood of 1700 F. or even higher for the hardening process. Such steels in practically all cases are capable of, and preferably are, hardened while enveloped by a protective gas and can be successfully hardened without damage in block sizes up to 2" thick and even more, and invlengths limited only by the capacity of the furnace.

Accordingly, it is a further object of my invention to provide a furnace for the heat-treatment of toolr steels at temperatures in the neighborhood of 1700 F. and above in which the charge can be completely enveloped by a nonoxidizing, non-decarburizing gas, which furnace .has provisions for permitting the cooling of the charge while enveloped by the same atmosphere, to prevent distortion and cracking. The furnace of my invention has several basic features and innovations which enable me to harden tool steels of the character aforesaid without surface contamination, with dimensional changes confined to those which are predictable, and without decarburization. In other words, I have devised a furnace in which I can harden tool steels with clean and fully hardened surfaces having the wanted dimensions and free from cracks, warpage, or high internal stresses so that' the article can be used immediately after hardening with the minimum of further grinding, or stoning, or other finishing treatment, and in most instances without the -need of any finishing treatment.

A furnace following my invention, adapted to achieve the heat-treatment of tool steels without damage, should have provisions for heating and cooling the charge within a controlled atmosphere, and for moving theA charge through the freely but without distortion from sagging under its own weight; and since the furnace is for use in commercial fields, it must be low in initial cost and long of life, low in upkeep expense and cost of operation, easy to manipulate, and simple to operate. Broadly, therefore, it is among the objects of my invention to devise a simple means in the form of a commercially-acceptable furnace adapted to the heating and cooling of tool steels in a protective gas so that the steels will be hardened without damage.

It is an object of my invention to provide a furnace in which the widely used and more expensive tool steels can be heat-treated whereby the necessity for machining, grinding, or stoning after the heat-treatment is kept to a minimum, and in most cases entirely eliminated.

The type of furnace I prefer for carrying out the heat-treatment of tool steels as aforesaid employs an elongated metallic muiile having a closed end with the other end open but closed by a suitable door means. The munie preferably is inclined with the closed end upward so that when the lighter-than-air protective gas is admitted to the upper end of the muflle it will fill completely the upper portions of the muille and be forced downward toward the door by the pressure of more incoming gas. When the door for the open, lower end of the muille is opened, contaminating air can and will enter the muiile, and combine with the protective atmosphere, which if combustible, will burn therewith to form products of combustion which are confined to the lower end of the muille from whence the products can be purged through a suitable gas-outlet pipe thereat and before they intermingle with the protective atmosphere in the upper part of the muflle. If the protective atmosphere is non-combustible, then the entering air will be purged. The incline of the muille, therefore, encourages fractionating between the protective gases on the one hand, and the undesirable or harmful contaminating gases, on the other hand, and which usually are heavier than the protective gases customarily employed. Because of the incline of the muflle, the undesirable gases will remain at the lower level near the door whereas the protective gas will be entrapped in the closed upper end of the mufile. flning the heating zone to the upper portion of the muille the charge will be enveloped by the practically pure protective atmosphere although contaminating gases might be present within the lower end of the muiile. lin furtherance of these results, the'protective gas-inlet is located at or near the top of the muille and by providing a continuous flow of gas from the gas-inlet at the top of the muilie, downwardly through the muille, and then out through the gas-outlet at the bottom of the muiile, la clean and pure protective atmosphere will always envelope the charge because the downward flow of pure gas opposes the upward transport of contaminating gas by diffu- Consequently, by con` sion, and -because the ow of pure gas flushes out any occluded gas given off by the charge.

Accordingly, it is at once perceived that I deem vit to be a very important feature of my invention to provide in a furnace for heat treating tool steels while enveloped by a protective atmosphere, an inclined heat-treatment chamber closed at its upper end and having its charge insertion and removal openings at its lower end so that the important possible sources of contaminating gases are confined to the lower end of the munie.

This furnace may also be used to advantage in hardening steels that require liquid quenching. Several pieces may be heated together in the protective gas and may then be withdrawn one at a time for quenching without contaminating the atmosphere around the work remaining in the furnace. 'I'his is not possible in conventional types of controlled atmosphere hardening furnaces having but a single door and a horizontal hearth.

The tilted muiille or hearth is an important feature of my invention since this enables me to move work directly into the heating chamber.

There are many other features and detalls to my invention in addition to those more specifically mentioned above which will be apparent from the following description thereof taken in connection with the drawings, in which;

Figure 1 is a vertical, longitudinal view, partly in section and partly in elevation, of the furnace builtin accordance with my invention, and in which the heating means is in the form of nonmetallic resistor rods of the silicon carbide type. This figure further shows a charging tray in its l position in the upper part of the muilie;

Fig. 2 is a cross-sectional view, taken substantially on the line II-II of Fig. 1 of the furnace, with the furnace proper adjusted to horizontal position;

Fig. 3 is a view similar to Fig. 2 but showing in a modication wherein electrical ribbon resistors are employed for heating the furnace; and

Fig. 4 is an elevational view of a tray for supporting a charge inside the muiile of the furnace during the heat-treatment process.

In the preferred embodiment of my invention,

I employ a furnace withits heat treating chamber inclined but to make the furnace adaptable to uses other than the heat-treatment of tool steels, I construct the furnace in the shape of a rectangular prism elongated in a general horizontal direction with provisions for pivoting one end and raising or lowering the other whereby the heating chamber may be inclined as preferred, or horizontal for other uses, although it is obvious that the heat-treatment chamber might be inclined by other expedients. In general, and referring more particularly to Figs. 1 and 2, the furnace! comprises refractory and insulating walls defining a heating chamber. The walls are constructed of re brick and refractory tile, and perhaps other heat-insulating material, in any customary manner, and include a top wall l, a bottom wall 6, side walls 8 and I0 respectively, a closed end wall I2, and, lastly, an end wall I4 provided with an opening through which a muflle, indicated in its entirety by the numeral I6, extends.

The furnace walls constitute the furnace proper and are encased in a metallic shell. I8 which completely encases the furnace proper except for such openings as are necessary for In the particular embodiment shown, the bottom of the metallic shell and"the`upslianding sides are welded into an open-topped box having-..

a flange completely around its surface near the upper edge. The shell further includes a metallic top 22 suitably bolted to the flange 20 through the medium of a sufficient number of bolt and nut devices 24 so that, in conjunction with a gasket 26 interposed between the edge of the box and the top, the shell is made gas-tight. However, the end of the shell contiguous to the end wall I4 is also provided with an opening approximately fitting the inutile I6 so that the muille may be inserted into and withdrawn from the furnace without tearing down the walls.

The 'mufiie itself comprises two contiguous sections in open communication, one of which is within the furnace proper and includes the heating zone for the charge, while the other section extends outwardly from the furnace proper and provides a combined charging and cooling zone.

The inner section 28 of the muilie is preferably elongated and includes an integral portion which extends almost to the end wall I2 of the furnace. The other, lower end of the section 28 terminates in a flange 30 outside of the shell I8, which extends completely around the periphery of this muiile and provides a means by which the muffle is secured to the furnace proper. The second or outer section 32 of the muiile has interior walls 34 aligned with the inner section of the munie, and the upper end of this section 32 is also provided with a liange 36 completely around its outer periphery at one edge thereof.

To secure the complete muille in place the inside of the shell about the furnace opening is provided with a reenforcing frame ring 38 which may be welded or otherwise secured to the shell about the opening through which the muiile passes. 'Ihis frame ring is apertured to receive a plurality of bolts 4l)P extending through corresponding apertures in the shell and flanges illV and 86. A gasket 42 is interposed between the outside of the shell and the flange 30 while a second gasket 44 is placed between the flanges 30 and 36. By suitably tightening the nuts 46 on the bolts 46 the muflie structure is gas-tightly secured to the furnace proper.

In the embodiment shown in Figs. 1 and 2, the bottom wall 6 of the furnace has longitudinally-running spaced piers 50 apertured to receive lower heating rods 52 which are nonmetallic resistors of the silicon carbide type available to the art, it being understood that the .side walls 8 and I0 and the shell I8 have suitable provisionsto receive these rods.

Supported on the spaced piers 50 is a nonmetallic refractory base plate 54 having a high thermal conductivity, and the inner section 28 'of the muilie rests on this base plate. Upper `heating rods 52 may also be provided to aid minal structures 56 which are preferably such as to render the outer shell gas-tight in spite of 'the apertures in the metallic shell. Terminals of this character are well known to the art and any appropriate type may be employed.

In order to provide the necessary tilt or incline to the furnace, one end of the furnace, and

,- preferably that end having the opening through which the muiile extends, is pivotally mounted at its bottom on the stationary shaft 58 secured in a furnace-supporting frame work 60, while the other end of the furnace bottom is provided witlia downward-facing angle 62 into which fits a small roller`64 of a pneumatic or hydraulic jack, schematically indicated at 66, and having its lower end pivotally fixed to the frame work 60.

From the structure thus far described, it can be at once perceived that by suitable manipulation of the jack 66 the furnace proper may be inclined to any desired angle with respect to the horizontal.

For a. munie having an inner section 36 inches long and an outer section of approximately the same length, with a cross-section having a '10 inch width and 5 inch height, I have found that, for the purposes-of my invention, an incline of about 20 with the horizontal has enabled me to heat treat vtool steels without damage. With these dimensions the top of the lower opening'in the muiiie section 32 is on a level which is below the muflie section 28 and below a very substantial part of the mufe section 32.

When the furnace is so inclined, the closed end wall I2 is obviously raised with respect to the other end wall I4, and the same holds true for the adjacent ends of the muilie. The upper end of the inutile section 28 is closed by an end plate 68 which may be suitably gas-tightly welded thereto so that a protective atmosphere admitted to the mufe will tend to rise and occupy the upper portions thereof although it is to be understood that in actual operation suicient gas is admitted to the mufile to permeate its entire interior. Y l

The structure of the inner section 28 of the muilie is completed by a floor plate 10 which is preferably formed of a non-metallic refractory such as alumina with a suitable binder. floor plate 10 is provided with a recess 12 starting at about the mid-point of the section 28 and extending upwardly to the-upper end of the floor plate which terminates a very short distance, perhaps an inch or two from the end plate 68 of the muille. This recess is provided for the reception of a thermal-responsive device by which the temperature within the mule can be measured and the heating of the furnace contrlled by proper appurtenances.

The temperature-responsive means is combined with gas-inlet means for admitting a protective or controlled atmosphere to the inside of the muiiie. To this end, the end plate 68 is provided with a small aperture or opening 13 in line with the recess 12 of the floor plate and a suitable pipe 14 is gas-tightly welded to the muie about this aperture, the pipe extending upwardly and outwardly of the furnace through suitablyformed holes in the end wall I2 and shel1 I8. The protective atmosphere is admitted to this pipe through an inlet 16 which is gas-tightly but removably provided on a portion of the pipe 14 outside of the furnace proper. The protruding part of the pipe 14 has an open threaded end 18 into which is insertable a pipe 80 of a length sufcient to extend substantially to the bottom of the recess 12, with the end of the pipe at this point closed. The pipe contains a thermo-cou- The l ple junction 82 from which wires 84 run for the length of the pipe and beyond to any suitable indicating or controlling apparatus. The pipe 80 is necessarily of an exterior dimension smaller than the interior dimension of the pipe 14 so that the incoming gas from the inlet 16 may ilow through the annular space between the two pipes and into the muille by way of-the aperture 13.

Any suitable gas-tight packing. Jointv 86 can be level with the top of the exposed opening at the end oi' the mume section 32, through which the charge passes. Accordingly the products of combustion and any unburned air do not rise .to any considerable height in the munie but are confined to the lower positions from where they are immediately swept out of the mume through the outlet-pipe |00. Since no contaminating 'sas gets into the heating zone no extensive purging is required in the charging chamber after lthe furnace has been prepared for operation, and an intermediate door between the contiguous heat-treatment zones of the munie is unnecessary, so that the zones are in open communica'- tion.

Assuming a charge to be heat treated has been placed in the section 32 and door l0! closed, the

mullie may be then purged by having the profor these elements for reasons described in my copending application, Serial No. 296,586 filed concurrently herewith, entitled Heat-treating furnace to which reference may be made for further details. This copending application is also assigned to the Westinghouse Electric 8: Manufacturing Company. I also prefer to permeate the inside of the furnace proper by a protective atmosphere to pervent oxidation of the outside walls of the nickel mullle and to this end the upper part of the shell I8 is provided with a gas inlet |00, and the lower part is provided with a gas outlet |02, the -gas outlet having a downwardly-ex'tending .pipe terminating considerably below the bottom of the furnace proper.

- The outer section 32 of the mullle serves a dual purpose in my furnace, as a charging or purging chamber `for a charge `first inserted into the muille, and as a cooling chamber for a charge after it has been heated in the furnace proper. This section 32 has an end opening controlled by a raisable'door |04 having door-operating mechanisms preferably-constructed and arranged as described in my aforementioned application, a1-l though any appropriate door-operating mechanism can be utilized. At a munie inclination of 20 the top of the opening through which a chargeis inserted into or removed from the mufile through the end opening for the section 32, whether the top be dened by the door in fully open or in partially open position, will be considerably below the inner munie-section 28 and below most of the outer muille-section 32,

In operating my furnace the 'inutile is first purged by admitting the protective atmosphere through the gas inlet 16 from where it flows through the muille and is exhausted through a gas outlet pipe |06 at the bottom end of the muflle, althougha certain amount of this gas, which is under a slight pressure, will seep out through any crevices that might exist between the door |04 and its frame. Additionally a small vent may be provided fpr exhausting protective atmosphere. In actual practice the protective atmosphere may be dissociated ammonia or pure hydrogen and since these are combustible, it is preferable to ignite the gas coming out of the vent or through the crevices aforesaid.

When the door |04-is opened to insert or remove a charge any air entering the muille wil! react with combustible protectiveatmosphere in contact therewith and a name may be observed on a substantially horizontal plane substantially tective atmosphere flow through it f or a short time, at, if necessary, an increased rate of flow,

but because no contaminating gases reenter the heating chamber with the insertion of the charge, it can be almost immediately pushed into the heating chamber. Moreover, any occluded gases in the charge are swept out o`f the heating chamber by the normal flow of the atmosphere before the charge has attained a temperature at which it would be damaged by them. The heat ing zone constitutes the major length of the section 20 at the higher end where the heat generated by the rods 52 and 52 is concentrated.

After the proper heat-treatment, the charge is brought back into the section 32 which now acts as a cooling chamber, being provided with a water jacket |00 having av water inlet H0 and outlet H2 to facilitate and control the cooling. After the charge has been subjected to a suitable cooling treatment in the charge-cooling zone of the mume section 32, the door |04 may be again opened and the charge removed. The furnace is then ready for the heat-treatment of another The tilt of the muflie is preferably such that clean and pure atmosphere will always occupy the heating zone within the mullle I6, but it must not be so great that the charge inside the muille will slide downwardly due to its own weight. The oor plate 10 in the muille has a high coefficient of friction so that the angle of tilt might be somewhat greater than that which would be obtained if a metallic iioor plate alone y were provided instead. Additionally, the nonmetallic refractory floor plate 10 prevents the charge from sticking to the iloor of the muille since this furnace necessarily is operated at relatively high temperatures. inside the mullie by any suitable pusher or puller rod projected througha normally closed hole in the door |04.

In order to provide a means upon which the articles to be heat treated may be conveyed from place to place in the muille, I provide a tray which has features cooperating with the mullle for improving its efliciency. The details of this tray are more fully described in my, aforesaid application,. but generally comprise an open framework bottom HO of a length substantially that of the`main heating zone in the muflle, which can be approximately two-thirds of the length The charge is movedy through the wire screen, so that the articles on the tray, which are being heat-treated, are substantially completely enveloped by the gas. To each of the ends of the bottom H6 are secured upright, spaced radiation shields H8 whose contour corresponds to the effective open area in the muiiie proper but being slightly smaller. These shields improve the heat distribution about the article or articles on the tray and aid in confining the heat of the heating zone to the length of the tray. Consequently, the heat in the chargetreating zone between the sets of radiation shields at the ends of the tray, is relatively evenly distributed.

Fig. 3 shows a modified form of my invention', the muflle section 28 being provided with helical resistor ribbons wound around it. In this construction a layer of non-metallic electrically-insulating refractory paste is iirst placed on the outside of the muille section. 'I'he helical resistor ribbon is then wound around the mule and a second layer of paste placed about it, The refractory is then solidified and the helical resistor is, in effect, embedded in the refractory. Suitable terminals are, of course, provided.

I have very successfully hardened tool steels in the furnace described by the use of a suitable protective atmosphere. The controlled r protective atmosphere must be, of course, judiciously chosen, based on the chemical laws of mass action, and there are three basic types of atmospheres which could be adapted to the hardening of tool steels without surface damage. These are (1) mixtures containing 20 to 40% CO which tend to be actively carburizing or decarburizing unless their CO2 content is closely regulated, (2) pure Hz which slowly decarburizes and is actively reducing to all steels, and (3) pure Na which is totally inertso far as oxidation and decarburization are concerned.

In selecting an atmosphere for tool hardening from this group the primary considerations are:

(l) The gas should preferably be capable of protecting all steels against loss or gain in sur-'- face carbon and not require adjustment of composition to suit various steels.

(2) The gas should be strongly reducing if high chromium steels are to be heat-treated without oxidation.

(3) The gas should be simple to prepare and not require elaborate processing.

Active gases containing a. high percentage oi CO are not entirely satisfactory because their carbon pressure and reducing power are both de pendent on the CO-COz ratio and, therefore, cannot be separately controlled. This characteristic is not objectionable in treating plain carbon and low alloy steels fo'r which the reducing power of this type of gas is adequate over the entire working range of carbon pressures. However, for high chromium steels of low carbon content this gas is unsuited for bright hardening as it must be made highly reducing to prevent tarnish and this would require a high CO-COz ratio such that the gas would be actively carburizing. y

Nitrogen lacks the reducing character needed in a protective atmosphere; moreover, it is expensive When purchased in bottles and must be further puried. When it is prepared from combusted gases it is much cheaper, but expensive processing equipment is required. This leaves pure hydrogen as the most favorable type of gas atmosphere. It is, however, not acceptable be- 1.5% carbon steel soaked in it at 1850 F. (.1010" Y C.) for one hour and its composition does not need adjustment to the requirements ofthe steel to be hardened. Its reducing character permits hardening of stainless steel without tarnish.

Although dissociated ammonia does decarburize slowly, any carbon lost from the surface of the ,steel subjected to the treatment is replaced by carbon which is diffused to the surface from the interior of the steel thereby preventing serious impoverishment of the surface layer. This is possible because of the slow rate of carbon transfer from the steel to the gas. When even this small loss is objectionable, as in treating very thin sections, the gas may readily bemade completely non-decarburizing or actually carburizing by the addition of small proportions of hydrocarbon gas. Thus it is possible to regulate the carbon pressure of the gas between wide limits without decreasing its reducing power. This is not lpossible in the type of atmosphere containing high percentages of CO in which the carbon pressure and oxygen pressure are not independent.

Dissociated ammonia must bevery dry for successful use in a protective atmosphere for tool hardening. A moisture content of 0.3% by volume (dew point 30 F. or 35 C.) at the work surface is generally satisfactory, but a drier gas is preferred since oxygen combines with hydrogen at the furnace temperatures to form water vapors which damage the steel. Additionally, the rate of decarburization decreases rapidly with increased moisture content.

It is not sufficient-to put a dry gas into 'a furnace; it must be maintained dry While it is there in contact with the work. This means that oxygen and oxygen yielding compounds from the air, from the furnace brckwork, or from any other source must be excluded, or removed as by ushing so that the work will not be damaged. My 'furnace is exceptionally well-suited for the purpose.

The furnace described herein has been emlployed for the hardening of blanking and forming dies, as well as miscellaneous tools. Practically all the requirements of these dies or tools can be met by selection of the following types of tool steels, all of which harden by gas cooling even in large sections.

Cr Mo V Percent Percent Percent purivwhat longer.

steel containing 12% Cr and .3% carbon and retained a blight slu'iace.` This is an illustration of the exceptional performance of my furnace when using dissociated ammonia for the protective atmosphere.

As an example of the heat-treatment of tool steels I have successfully hardened a die block made from high Cr steel, No. 3 of the above table. The die was machined to finish size with no allowance for clean-up after hardening. It was set on the tray and placed in the entrance chamber which was purged and ushed forabout 15 to .20 minutes. The work was then'moved into the heating chamber where it was heated to between 1300 and 1400, F. for a short period of time, and then was soaked at temperatures vbetween 1832-1868 F. for about 1 hour orsome- Since the danger of decarburization is practically eliminated by the use of the suitable controlled atmosphere, soaking times may be as long as desired without damaging the steel surface. After this soaking, the workwas withdrawn into the charging chamber where it cooled naturallyunder the protection of the gas to a point at which the work could be taken out of the furnace without objectionable tarnish.

The total time in the furnace for a particular job runs from 1 to 4 hours depending on the size of the work. Hardened in this manner, the die had a scale-free surface that was as hard as the underlying metal, and had a uniform hardness of 745-770 D. P. H. (63-64 Rc). I have also hardened No. 2 steel in a total heat treating time in the furnace of about 40 minutes equally divided between soaking and cooling, the soaking being at a temperature of 2200 F. The article so hardened was a lathe tool and came out bright and clean with a uniform hardness of 64 Rc. The protective atmosphere was dry dissociated ammonia with no hydrocarbons added.

In the fully hardened state the steel will have a slightly greater volume than before hardening, the volume decreasing with tempering and approaching the initial volume as a limit. As these volume changes are characteristic and definite in amount for each steel, their elfect on dimensions may be predetermined and compensated for in machining the tool before hardening so that its final dimensions after hardening will be correct and thus the need for finish grinding is eliminated.

By adding one or two percent of natural gas or other hydrocarbon gas to the furnace atmosphere it is made carburizing, although its action will be quite slow. In such an atmosphere high speed steels may be given the equivalent of a carburizing pack-hardening treatment. For example, a specimen of 18-4-1 high speed steel was heated at 1875" F. (1020 C.) for 2 hours in dissociated ammonia containing 1.4% methane and then cooled naturally in the gas. The surface hardness was 880 D. P. H. (67.5 Rc) and the core hardness 645 D. P. H. (58 Rc). This treatment is useful for producing high superficial hardness on high speed steel tools for use where maximum heat resistance is not required.

While I have described my invention in a preferred form it should be understood that its methods of operation, purposes and results are also parallel to those more particularly described and claimed in my aforesaid copending application.

I claim as my invention:

l. A bright-hardening metal-treating furnace of the type described for bright-hardening chromium-containing steels, comprising an inclined tube-like metal muflle having a lower end provided with a lower opening, and an upper end comprising a closing means whereby the upper end of the muille is closed against extraneous gasfiow; movable closure means for said lower opening; means in said furnace for providing a heating zone in the upper part of said muille, whereby the steels may be heated to temperatures for hardening; means for providing a cooling zonein a lower part of said muille, whereby the steels may be cooled after being heated in said heating zone; movable metallic heat-barrier means between said cooling zone and said heating zone, said heat-barrier means permitting a gas-flow between the zones; and gas-passage means for permeating said munie with a generally downwardly flowing lighter-than-air controlled atmosphere; the incline of the muille being such that the top of the opening through which a charge may be inserted into or withdrawn from the muille when the said movable closure means is in opening position, is on a level which will tend to cause contaminating gases resulting from the now of air into the munie to be confined to the lower en d of the muifle.

2. A bright-hardening metal-treating electric furnace of the type described, comprising a tubelike metallic munie, means for heating a portion of said muie to provide a charge-heating zone therein, means for cooling a second portion of said muille to provide a charge-cooling zone therein, means for supporting saidl munie at an incline with the charge-heating zone higher than the charge-cooling zone, said muille having a lower opening through which a charge may be inserted and withdrawn, movable closure means for said lower opening, the incline of the muille being such that the t pofthe opening of said muiile, as exposed when the closure means is moved to opening position, is on a level below the charge-heating zone and a substantial part of the charge-cooling zone, closing means gastightly secured to thefupper end of said munie, gas-inlet means at the upper part of said muflle for supplying a controlled atmosphere to said muffle, and gas-outlet means at the lower part of said mullle, below the top of said lower opening, for venting gases from the munle, whereby the controlled atmosphere in said mume ows generally downwardly.

3. A metal-treating munie-furnace of the type described adapted for substantially damage-free hardening of chromium-containing steels, comprising a tube-like metallic muiile, means for supporting said munie at an incline, said muilie having closing means at its upper end whereby the upper end of the muiile is closed against extraneous gas flow, the lower end of said muille being provided with an opening through which a charge may be inserted into and withdrawn from the munie, movable closure means for said opening, means for heating one portion of said muille, means for cooling a second portion of said muille, said one portion and said second portion of said munie being in open communication, gas-passage means, comprising a gas-inlet at the upper part of said muflle, whereby the inside of said mume may be permeated with a generally downwardly flowing lighter-than-air controlled atmosphere.

the incline of said munie being such that vthev top of the opening through which a charge mayI be inserted or withdrawn from the munie when the said movable closure means is in opening position, is on 'a level which will tend to cause contaminating gases resulting from the flow of air into the muilie when said movable closure means is in opening position, to be confined to the lower end of said muiiie, said gas passage means including a gas-vent at said lower end of said muiile.

4. A metal-treating furnace of the type described adapted for substantially damage-free hardening of chromium-containing steels, comprising non-metallic insulating and refractory walls dening a heating space, an outer metal casing for said walls, an inclined metal mufe portion and said lower portion of said muiile being in open communication, closing me: ns gas-tightly secured to the upper end of said munie, said walls having an end-wall adjacent to the closed upper having a lower open end through which a charge end of the muflie, movable closure means for said lower open end of said muflie, gas-inlet means at the upper end of said muffle, gas-venting means at said lower end of said mufe whereby the mule may be gas-permeated with a generally downwardly flowing lighter-than-,air controlled atmosphere, means for admitting a controlled atmosphere between said casing and said upper portion of said mule, and means for venting the controlled atmosphere therefrom, the incline of the muiTle being such that the top of the opening at said lower end of the muiile when said movable closure means is in opening position, is below said upper portion of said muliie and below a substantial part, of said lower portion of said munie.

I JOHN R. GIER, JR.

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