US2778755A - Method for the controlled cooling of steel forgings - Google Patents

Description

Jan. 22, 1957 v. BROWN ETAL 2,778,755

METHOD FOR THE CONTROLLEO COOLING OF STEEL FORGINGS` Filed Jan. 27, 1953 5 Sheets-Sheel'I 2 agg 2% MJ rml/36 'w'ml @figg Jan. 22, 1957 v. BROWN ETAL METHOD FOR THE CONTROLLED COOLING OF STEEL FORGINGS Filed Jan. 27, 1955 3 Sheets-Sheet 3 @vani-:2:75: w'cior .Brown fon HJVeZson.

QQ/W Hij-57E United States Patent@ FOR THE CONTROLLEDV COOLING 'METHOD t 0F STEEL FoRGlNGs p Victor Brown, Elmhurst, and John H. Nelson, Palos Park,

Ill., assignors to Kropp Forge Company, Chicago, lll., acorporation of Illinois f v Application January, 195s, serial No. 333,468

` 1 Claim. (cl. 14s-21.5)

Thisinvention relates to the controlled cooling of y gineering uses, are subjected to some form of heat treattuentv for the purpose olf improving their internal structure and mechanical properties. Although the instant invention is adapted for use in heat treatment (or production of suitably heat treated) non-ferrous metals and alloys, the instant invention is also adapted for such purposes with ferrous metals, particularly steel forging, and the production and'treatment of ferrous metal articles is of particular importance to industry at the present time. Certain .specific details of the instant invention will', therefore, be described particularly in connection withsuch ferrous metal articles, although it will be appreciated by those skilled in the lart that the principles land concepts of the instant invention will be applicable lto the production and heat treatment of nonfferrous metals and alloys.

' ln general, the problems and considerations involved in the heat treatment of ferrous metals revolve around lll the peculiar characteristics of these metals to undergo system is cooled to below about 700 C., the iron changes from the gamma state 'to the 'alpha state, and alpha iron is capable of maintaining only very minute amounts of carbon in solid solution. Accordingly, duringthe transition from the gamma to the alpha state, the carbon is caused to yseparate from the iron, presumably in the form of iron carbide or cementite and the resulting alpha iron system is generically referred to as pearlite. This transition or transformation is generally referred to as the austenite-pearlite transition or transformation. The rate at which this transition takes place has a 4 very important effect upon the internal structure and mechanical properties of the resulting metal article. lf thetransition is effected through extremely slow cooling, the resulting steel article is soft and ductile; Whereas if the cooling is effected very rapidly, as by quenching, the resulting steel article is hard and perhaps brittle because of the high vtemperatures involved. However, the controlled cooling of such metal articles in order to obtain a desired set of properties in the final articles is greatly complicated. Vln addition, collateral problems arev in- Patented Jan. 22,1957

ICC

volved such as the excessive oxidation ott the metal articles during prolonged exposure to air at high temperatures. Particularly in the case of steel production, the problem is also complicated by the ditiiculties in handling and transporting large pieces of hot metal from one location to another in a plant.

lt has been suggested that metal articles may be cooled or heated under controlled conditions by moving the same through an elongated furnace-like structure wherein these are several different temperature stages. For eX ample, the desired cooling stage of a hot metal article might be obtained by cooling such article in a series of furnace zones, each heated to obtain a slightlylower temperature than the previous one. The use of such a substantial amount of furnace space and equipment. is',- however, expensive and the problems of atmosphere control and the like are quite complicated.

On the other hand, if a hot ferrous metal article is quenched in a suitable oil or the like cooling medium, the cooling of the articletakes place so rapidly that it is usually necessary to subject the article to a subsequent heat treatment such as annealing in order to put the article in suitable condition for subsequent metal working or machining operation. The same is often true if the article is cooled merely by exposure to the air. An article such as a hot steel forging, when exposed to thek air, loses heat very rapidly, not only by convection, but also by radiation.

The instant invention provides a unique and extremely simple method and apparatus for effectively controlling the cooling of a hot metal article, so that the desired nal internal structure may be obtained using Vas a starting material a hot-worked or hot-forged article. The instant invention provides a means and method for handling the hot article, immediately after forging or the like operations, and for effectively accomplishing a heat treat-l ment of this article, without the usual step of cooling and then reheating, merely by effectively controlling the cooling of the article.

It is, therefore, an important object of the instant invention to provide an improved method and apparatus for controlled cooling of a hot metal article.

It is a further object of the invention to provide a method of cooling a hot metal article, which comprises positioning the hot article in a metal chamber closely spaced therefrom, closing the chamber to preventconvection cooling from the atmosphere and surrounding the chamber with an insulator to control the rate of heat loss through the chamber walls.

It is another object of my invention to provide an improved method of cooling a hot metalV article, which comprises surrounding the article with a solid radiator'- conductor closely spaced therefrom to seal the article in a non-absorbing non-conducting atmosphere, and embedding the solid radiator-conductor in a particulateretractory insulator to control the rate of heat transfer through the solid.

It is still a further object of the invention to provide an improved method of cooling a hot steel forging in the austenite stage, which comprises sealing the forging in a steel tube closely spaced therefrom to prevent conduction and convection cooling and effectively delay heat loss through radiation from the forging to eifect the austenite-pearlite transition in the forging at 650 C. y

It is still another object of the invention to provide an improved apparatus for controlled cooling of hot metal articles, comprising a bed of insulating material having a plurality of spaced separate'chambers therein, an insulator cover for each said chambers that is removable to selectively open and close the chamber, and each voffsaid chambers being adapted to receive and removably mount one of saidY metal articles in closely spaced relation to the chamberwalls.

It is yet a further object of the invention to provide an improved apparatus for controlled cooling of hot metal articles, comprising an open top concrete pit support beams extendingthereacross in spaced relation to each other, a bed of particulate insulating material filling said pit and the spaces between said beams, a plurality of metal Sleeves spacedly mounted on said beams and susp ended in said insulating bed, mounting means in each said sleeves for removably supporting one of said metal articles in closely spaced relation to the sleeve, and a removable insulating closure for each said sleeve sealingly cooperating with said insulating bed to enclose said article in said sleeve during the cooling thereof.

Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following disclosure of preferred embodiments thereof'in the specification and in the accompanying draw ings.

On the drawings:

Figure l is a table showing diagrammatically the geueral character of the austenite-pearlite transition at dif ferent cooling rates;

Figure 2 is a sectional elevational view of an apparatus embodying the instant invention;

Figure 3 is a fragmentary top plan view of the apparatus shown in Figure 2; and

Figure 4 is a sectional elevational enlarged view of a top closure used in the apparatus of the instant invention.

As shown on the drawings:

In connection with Figure l, it should be pointed out that those skilled in the art generally refer to the hot ferrous metal comprising a soli-d solution of carbon in gamma iron as austenite or austenitic iron and the cold alpha iron, with carbon deposits therein, which is stable at room temperature as pearlite or pearlitic iron. lt so happens that there are several so-called transition constituents called-martensite, troostite and Sorbite, which are presumably formed during the austenite-pearlite transition, and one or more of which may be present in the final stable product, depending upon the speed of cooling through the critical or transition temperature range. As it is generally understood, the transformation of the solid solution austenite into the aggregate pearlite implies first an allotropic transformation of gamma iron into alpha iron and secondly, the falling out of solution and crystallizing of the iron carbide. Since it appears that the allotropic transformation of iron must precede the crystallization of the carbide, then it follows that, before true pearlite may be formed, a condition must exist where the iron is in the alpha condition and the carbon stil-l in the solution. That condition is generally understood to be the martensitic condition. On further cooling, and during the necessary period of time, the carbide cornes out of solution in an extremely fine state of division (perhaps colloidal), and tro'ostite is formed. This is followed by further coagulation ofthe cementite (iron carbide) particles giving rise to the constituent sorbite. Finally, the cementite forms sharply defined lamellae embedded in a pearlite matrix, resulting in the formation of true pearlite.

Referring now to Figure l, it will be seen that the portion of the diagram designated Rate (1) plots temperature against composition; and rate (l) represents very slow lor substantially infinitely slow cooling, so that it will be seen that under such slow cooling conditions the following different constituent compositions are obtained in the order indicated:

". Ferrito(F)troos'ti tic(T) "f. 'Ferrito (F) -troostit`o(T) sorbitic(S) 4, g. Ferrito(F)sorbitic(S) h. Ferrito(F)`pe`arlito(P)lsorbitic(S)A i. Ferrito(F)-pearlitic(P), which in the final stable state contains about 65%ferrite and 35% pearlite.

The foregoing relates to a hypo-eutectoid (less than 0.8% carbon) steel; but,l if a eutectod (0.8% carbon) steel is used, the transition is essentially the sameexcept that no carbonless iron-appearsas ferrite; and if a hypereutectoid (more than 0.8% carbon) steel is used, a similar transition takes place except that a distinct cementite (FesC) phase appears instead of the ferrite.v

It will thus be seen from the chart yfor rate (v1 that during very slow cooling each of the so-called transition constituents becomes present in the composition vin an increasingly great amount, until-'a given maximum is reached, and then this particular transition constituent begins to decrease with further cooling, as the next lower transition constituent begins to increase in the amount thereof present in the composition. Finallypthev last transition constituent, Sorbite, disappears completely leaving only pearlite and ferrite; this is the stable pearlitic state. .lt will be noted that the final stable state is reached at about 700 C., which is generally understood to be the so-called transition temperature, whereas the transition is effectively complete upon cooling.

Referring'now to the table -designated rate (2), shown in Figure l, it will beseen that if the material is cooled at a slow rate, although slightly faster than that of rate l), the formation and subsequent disappearance of each of the transition constituents, which requires a certain amount of time, as well as the necessary temperature for each ot' such transition constituents, appears to be delayed somewhat and does not take place effectively until -a slightly lower Vtemperature is reached. As shown here, the iinal transition temperature, at which no further change takes place, is actually depressed about 25 C. below the 700 C. ordinarily lobtained at a very slow cooling rate. It will also be appreciated that the appearance and disappearance of each of the transition constituents in the composition requires a certain amount of plasticity or mobility within the composition. Once the material has cooled to a point at which this plasticity or mobility is no longer present, the material has then assumed final stable form, and any of the transition constituents which might be present in the material at that time will remain as such in the stable so-called pearlitic iron.

It will thus be seen that, at the slightly faster rate (2), the material cools to the non-plastic, stable state before all of the Sorbite has been transformed into pearlite, so that a definite proportion of Sorbite remains in the iinal stable product. Sorbite is substantially harder and stronger than the relatively soft ferrite, as well as the not so soft pearlite, so the resulting material has greater hardness and strength, but less softness and ductility than the material obtained using the very slow cooling rate (l).

lt will also be seen that at a slightly faster rate (3), the transition temperature appears to be depressed an additional 25 C., and the resulting stable material contains a very substantial proportion of Sorbite, so that it is harder and stronger and less ductile than the product obtained at the cooling rate (2). The diagram for still a faster cooling rate, rate (4), shows a depression in the transition temperature of still another 25 C., and also shows that a substantial proportion of troostite is present in the final stable product. Troostite is still harder and stronger than Sorbite, so this product has still greater hardness and strength and less ductility than any of the previous products.

Rate (5 as indicated in Figure 1, represents quenching or very rapid cooling. Such extremely rapid cool.

net result is a composition containing a substantial proportion of martensite, which imparts to the composition an extremely high degree of hardness. Martensite is understood to be the principal component in hardened tool 'steel andv the like material.

ln the instant invention, the cooling rates desired are those in the neighborhood of rates (2) through (4). lnlinitely or extremely slow cooling of the character of rate (l) is industrially impractical for obvious reasons. Mere cooling by exposure to air is much too fast because thefhigh convection and radiation losses greatly accelerate the cooling, and also such a procedure involves appreciable oxidation losses. Controlled cooling in furnaces having `a plurality of temperature zones involves the use of substantial amounts of space, equipment and power or fuel, as Well as the time of skilled labor to attend the furnace operation.

As shown in Figures 2 and 3, however, the instant apparatus, indicated generally by the reference numeral `10, avoids all these ditieulties and provides a unique and As can be seen from Fgure 3, each pair of cross beams, such as 16, 16, Yextends the full width of "the bin 11 in generally parallel closely spaced relation, and for each pair of beams, such as the beams 16, 16, there are `four depending tubes or steel pipes 18, 19, and 21 extending downwardly from between thev pair of beams 16, 16,

The pipes 18, 19, 2li and 21 are spaced from each other andfrom the wallsof the bin 11, ,and each is suitably welded, clamped or molded between the parallel beams 16, 16 and extends downwardly therefrom to the bin loor 11a. kSuch is generally the structure also assof ciated with the parallel beams 17, 17, although details of such structure are not shown in connection with the beams 17,17 for the sake of clarity in Figure 3.. Also, in Figure 3, thoserparallel beams which are positioned to the Vright of parallel beams 16, 16 and 17, 17 are not shown infFigure 3 for the sake of clarity and to avoid unnecessary confusion as tothe overall appearance of the ytop ofthe instant apparatus 10.

greatly simplilied arrangement for effecting the desired forced concrete, pit 11 which extendsv downwardly from the` ground level (GL) a substantial distance and which is closed at the bottom by a concrete slab or iloor 11a. This concrete pit or bin 11 is, of course, defined by a solid Wall of insulating material embedded in the earth or the like material which is also a reasonably good insulator. (As used herein, the terms insulator and conductor mean, respectively, heat-insulator and heat-conductor, unless otherwise specified.)

For reasons which will be explained hereinafter, one of the most important functions of the concrete bin 11 is that of providing a substantially dry container for the various other materials to be placed therein, so that any insulating material therein may be maintained in a substantially dry condition (for example, as contrasted to the relatively damp earth outside of the bin 11).

Accordingly, the floor 11a slopes gently toward one or more drain pits 12 which drop slightly below the level of the oor 11a. The drain pit 12 is provided with a sump tube 13, the bottom of which is provided with perforations 13a for communicating with the drain pit 12, and the sump tube extends upwardly the full height of the bin 11 to substantially the ground level (GL), at which it may be provided with the customary sump pump system (not shown) so as to remove any moisture which might have collected in the drain pit 12, whenever such is desired.

Near the top of the bin 11 there are provided angle iron support members 14 and 15 which are suitably xed to the side walls of the bin 11 and extend in a generally horizontal direction along opposite side walls of the bin 11. The supports 14 and 15 may be axed to the bin walls by any suitable means, such as bolts (not shown). Preferably, the sump tube 13 is suitably clamped or otherwise affixed to one of such side supports 14. A

plurality of pairs of cross beams or steel supports, as

'at 16, 16 and 17, 17 extend across the bin :11 and rest 16 and 17 .are mounted upon the concrete bin walls, l

which provide an insulator surrounding the instant' apparatus 10, so that the metal beams 14, 1S, 16 and 17 may not conduct heat from the instant system outwardly beyond the concrete walls of the bin 11.`

"As will be seen from Figure k2,` which is taken substantially along the line II-II of Figure 3, Ait will be seen that the pipes 18 and 19 extend downwardly from the cross beams 16, 16 to the bin lloor 11a. (As will be appreciated, the section line Il-v-II is drawn so as not to pass` through the center of the tubes 20 and 21, and dotted line reference to the tubes 20 and 21 is omitted from Figure 2 in order to avoid confusing this drawing.)

Actually, the overall structure of each of such tubes, such as the tube 18, is substantially the same, and the associated structural elements for each tube or pipe, such as the tube 18, are substantially the same, the only difference being the positioning of each of such tubes, and the general positional arrangement of each is brought out in Figure 3. Accordingly, certain details of the structure need be discussed only in connection with th pipe 18.

A steel spike 22 is suitably embedded or otherwise fixed in the lloor 11a directly beneath the pipe 18 and it extends upwardly through a suitable aperture in the closed bottom 18a of the pipe 18 so as to hold the bottom 18a of the pipe 18 in position and prevent the pipe 18 from swinging. Other suitable braces or structural members (not shown) may also be used if desired to support the tube 18 at one or more places between the cross beams 16, 16 andthe bin floor 11d, but in each case such structural members are mounted or fixed upon the concrete walls or floor of the bin 11, so that they may not conduct heat outside of the system- 10.

The top portion 1'8b of the pipe 18, which extends above the cross beams 16,116 is slightly ared, extend'- ing upwardly a short distance so as to define a funnellike structure. The funnel'or top 18!)v of the pipe 18 extends upwardly into an enlarged top chamber A which is an enlarged, but relatively shallow cylindrical chamber, as compared to the narrow elongated cylindrical chamberB defined by the tube 18 and extending concentrically downwardly from the enlarged chamber A. The chamber A ldefined by cylindrical metal walls 23, which rest upo'nthe cross beams 16, 16, and are suitably atiix'ed thereto such as by welding orvthe like and which extend upwardly from the cross beams 16, 16 toy approximately the ground level (GL). The cylinder orv sleeve 23k has' an open top that may expose thecham'- berv A to the atmosphere, but which is provided'withv a suitable closure'or lid 24, which will be described in detain hereinafter. As will be appreciated, the tube 19 has an arrangement substantially the same as that associated with the tube 18. The tube 19 extends upwardly from the bin floor 11a past thec'ross beams 16, 16 and flares slightly at 19b as it opens into the upper chamber A, which is constructedl as-described in connection with the'chamber A above the tube 18. This structurels VAcharacteristicY A- of the arrangement for each of the'other tubes20, 21,and thoseY others not specifically designated in the drawings,all of which are spaced -from each other.

:Weense The' bin 11, and the spaces in between the various structuralj elements just describedy are substantially lled with a relatively coarse refractory insulating material, preferably one-fourth inch mesh: slag, which iills the bin lflf substantially to the level (SL). The slag, designated generally by the reference numeral 25 completely snrrounds each of the tubeslS, 19, etc., extending from the binfloor 11a upwardly-past the parallel support beams 16, 16, etc., and upwardly around the outside of each of the sleeves 23 to approximately the top slag level (SL). The slag also extends upwardly past the cross beams 16, 16 within the sleeve member 23 a short distance, to approximately the top lip of the funnel 18h. Asis shown, slag consists of aplurality of metal oxides, which are resistent to high temperatures and which are eifectivenon-conductors or insulators. it will also be appreciated that the particular particle size of the slag particles will determine to a certain extent the amount of convection heat transmission which may take place by air'circulating through the interstices between the various particles'. In the instant embodiment it is desired to reduce the convection to a minimum, so a relatively small slag particle size has been employed. However, t will be appreciated that substantially larger slag particles might be` employed, if it were desired to obtain a predetermined slightlyfaster cooling rate, as will be explained hereinafter.

Also, the instant arrangement includes an intermediate top layer of highly heat resistant refractory material 26. Preferably, the layer 26 is composed of Castable, which is a well known commercial name for chrome ore base refractory bricks and the like articles used in high ternperature equipment. The Castable layer 26 is approximately twoinches thick, 1in the embodiment here shown, whichprovides a relatively thin, highly heat resistant or highly efficient insulator, which tends to prevent the escape of substantial amounts of heat upwardly from vthe slag bed 25, so as to prevent operators from moving about on top of the instant apparatus 1t), for reasons which will be described hereinafter.

Above the Castable layer 26, there is a very thin layer of foundry sand 27 (e. g., about one-half inch thick), which completes the bed of particulate refractory insulat ing materials which fill the bin 11. It will be seen that the sand layer 27 extends upwardly to almost the top of the sleeve 23 and a suitable angle iron 28 is embedded in the inside upper lip of the bin 11 as a curb or'protective iron running around the inner periphery of the top of the bin 11 from approximately the level of the sand layer 27 upwardly to the top. The purpose of the curb 28 is I to protect concrete during removal and replacement of the covers 24, for example.

Referring now to Figure 4, it will be seen that the cover 24 comprises a generally square top 29, having a short central groove 3G therein and a cross piece 31 secured thereto and extending laterally across the top of the groove 30. This arrangement .provides means for cooperating with a hook on ach'ain hoist or the like (not shown), so that the cover 24 maybe lifted and moved in and out of place. The side walls 32 of the cover 24 extend downwardly from each of the sides of the top 29 a substantial distance and terminate with a ange-like bottom member 33, which in cooperation with the side Walls 32 and the top 29 defines a large central chamber C within the cover 24. The chamber C is filled with lightweight Castable, so as to prevent a relatively thick layer of highly heat resistant and highly cicient insulating material. Also, the Castable layer in the chamber C forms substantially all of the bottom surface of the cover 24, generally in the plane of the bottom ange 33, so as to present an insulating top surface or cover for the chamber A, which will containr a'hot metal article as will he` described hereinafter. As will be .ap-

lpreiated, if the cover member 24 were to present a.

solid conductor, such as a metal surfaceof appreciable area to cover the top of the chamber A, this conductor would pickaup heat and would consequently heat the side walls 33 and the top 29 of the cover to such an extent that work above the cover Z4 would be greatly complicated'. It will thus be seen that the cover 24 provides essentially a metal (preferably steel) structure for supporting and positioning the solid non-conductor lid for the chamber A.

As has been mentioned, the opening 34 within the lower flange 33, wherein the Castable covering material is mounted, is preferably substantially the same size and contour as the top opening of the chamber A (or the sleeve 23). The opening 34 need not follow the exact contour, of course, but it has the general shape just mentioned. Extending downwardly from the opening 34 and suitably secured (as by welding) to the flange 33 is a ring 35 adapted to slip over and closely lit outside of the sleeve 23. The ring 35 has a bottom ange 35a extending outwardly, which is adapted to seat in the foundry sand layer 27, thus to effectively air seal the cover 24 upon the sleeve 23, as indicated in the lower right-hand side of Figure 4. provides means for sealingly closing the chamber A (and the chamber B communicating therewith). it will also 'oe appreciated that the sand seal is extremely effective for the purposes here involved, although it is somewhat crude in nature, in view of the necessity of using heat resistant materials of the type here involved.

Referring again to Figure 2, it will be seen that the article here involved is a steel forging indicated generally by the reference numerals 36, 36. This is a forging of a typical engine shaft having an integral hub (or wheel), the hub diameter It being substantially greater than the shaft diameter d. rl`he details of the formation of the forging 36 are not of particular consequence for the purpose of the instant invention, except that the article 36 has been heated to a very high temperature in order to carry out the required forging operations, and the hot forging 36 consists of austenitic iron, as hereinbefore described. As was mentioned hereinbefore, the details of the instant invention are here disclosed in most instances in connection with the use of ferrous metals, but the principles here involved are equally applicable to the cooling of non-ferrous metals and alloys. lt has been recognized by those skilled in the art that the mechanism of hardening or cooling of nonferrous metals is substantially similar to that involved in connection with the same operations with the ferrous metals. Hardening of such materials depends in almost every instance upon the fact that a certain base metal is capable of holding in solid solution, at high temperatures, a large proportion of an alloying element than it can hold at lower temperatures. Quenching brings down the temperature of the materials so fast that the so-called solid solution passes through a supersaturated condition and into the initial transformation and deposit of the alloying element. rlhe more time afforded for the deposit and coagulation of the alloying element (the more slowly the material being cooled), the softer and more ductile the material becomes.

It will also be understood that the particular shape of the forging 36, here disclosed, is not of critical importance for the purposes of the instant invention. The instant invention is adapted equally Well to articles of almost any particular shape. It is, of course, true that more or less standard shapes in the various articles will simplify the formation of the chambers, such as the chambers A and B, wherein the articles must be mounted in relatively closely -spaced relation to the walls of the chambers. It has been found that the instant invention is most suitable for use with articles having generally cylindrical surfaces, such as the surface of the shaft of the forging 36, here shown, since a particularly advantageous aspect of the instant invention involved the positioning of the chamber walls at a uniformly close distance from the `surface lt will thus be clear that the cover 24 of the hot'metal article 36, and temperature control and uniformity may bel `best achieved by the use 'of the particular generally concentric cylindrical article and cylindrical chamber arrangement here shown.

In the operation of the instant apparatus, an overhead crane or the like heavy duty lifting apparatus (not shown) is provided directly above the apparatus 10. One of the covers 24 is thus lifted from the top of one of the chambers A; and a hot forging 36 is removed-from the forge (not shown), suspended` by the top portion 36a thereof, positioned directly above one of the tubes, such as the tube 19,and then forging is carefully lowered, so that the elongated kshaft portion 36b1slides into and substantially fills the chamber B in the pipe 19 and the hub portion 36C comes to rest upon the mouth of the pipe funnel 19h. lt will be noted that a minimum of surface-tosurface contacts between the pipe 19 and the forging 36 (at the portionof the hub 36C contacting the pipe funnel 19a) is particularly desirable in order to control the heat losses by conduction from the forging 36 to the pipe 19. ln certain instances, it may be found to be more desirable to employ mounting means within the chambers A and B which presents solid insulator surfaces for contact with the hot metal article 36. On the other hand, the hub portion 36C of the instant article presents the greatest thickness of metal and, therefore, theportion which would ordinarily cool Vmost slowly. By providing even this limited surface-to-surface contact between the hub portion 36 and the solid conductor 19 it `is possible to effect slightly greater heat withdrawal from the hub'portion 36 and also to impart this heat to the pipe 19 which is positioned surroundingly of they elongated shaft portion 36b (so as to delay slightly heat losses from the shaft portion 3617).- It will thus be seen that, in the case of articles comprising an enlarged portion vand a relatively thinner elongated portion attached thereto, there is a distinct advantage in providing limited conducting surface contacts with the head portion, while also supporting the same, so that the headportion may be employed to transmit heat by conduction tothe chamber walls immediately surrounding theelongated thin portion.

After the vforging 36 has been lowered completely to the chambers A and Band suitably seated upon the metal portion 19a (substantially as is yshown in connection with the pipe arrangement 18), the cover 24.is lmoved over into sealing position upon the top of the sleeve 23, as shown in connection with the arrangement for the pipe i8.

Positioning of the cover 24 then effectively seals the chambers A and B, at least for the purposes of convection cooling'of the article'36 within the chambers. It will, of course, be appreciated that a perfect air-seal is not desired, since the vtemperature changes which will take place, even though slowly, within the chambers A and B will effectively reduce the air pressure therein, so `that a certain relatively small ,amount of air will seep into the chambers A and Bin order to obtain substantially atmospheric pressure therein. An important object of designing and positioning the chamber so that it may be in closely spaced relation to the hot article is the reduction of the overall volume of air entrapped therein, so as to minimize any convection effect within the chamber itself. Also, only a relatively small amount of oxygen is present in this small volume of air, and in the event that oxidation of the hot metal article is a problem, this relatively small amount of oxygen will be rapidly consumed, and the remaining atmosphere will be substantially inert with respect to the hot article, so that it is not necessary to rapidly cool the article to prevent further oxidation.

Also, since the instant invention provides for the cooling of the articles within a closed or sealed chamber, it will be readily appreciated that the atmosphere within such chamber may be a substantially inert atmosphere.

aol

For example,.in the instant apparatus `rIt), suitable'g'as inletmeans (not shown) Icould be provided adjacent or in conjunction withk the positioning bolts 22,/ so as to sweep the chambers A1 and B with, for example, nitrogen or some other inert gas during the insertion of the hot forging k36 therein, and this. inert gas source could be cut 'olf as' soon asv the cover v24 has been' placed upon the sleeve 23. 'f f i i v In any event, it will be appreciated that the inert gas forming the atmosphere or gaseous medium between the chamber walls and `th'ehot metal article constitutes a non-conducting'or' insulating gaseous atmosphere, through whichonly a minimum amount of heat may be transmitted by conduction or convection. Because ofthe very substantialheat differences between the hot metal article and the outside atmosphere, itk follows that there must be a. certain amount of conduction (and convection) taking place through the gaseous atmosphere :and also through the slag bed and concrete Walls which are, of

course, solid insulators known to havel very low heatconductivity.

It is believed that a very substantialamount of the heat transmitted from the hot article to the chamber wall is transmitted by radiation, which is the third known form of heat transmission. The amount ofradiation absorbed by gaseous materialsr is ordinarily: negligible, and the present gaseous atmosphere between .the-chamber`walls yand the hot metal article is also a non-absorbing atmosphere (i. e., an atmosphere that is non-absorbing with vrespect to heat radiation). f

f The chamber walls defined by the pipe 18a're, however, made of a solid conductor, capable of absorbing radiation andy also of reflecting or radiating heat energy. Preferably, the chamber Walls 18 are made of a suitable metal possessing good heat conductivity (i. e., at leasta thermallconductivity of about l0 B. t'. u. (hr.) (sq. ft.) F. per ft.) and preferably iron or steel withk the conductivity of 20-35). Also, the chamber walls are preferably made of a (radiation-absorbing) solid which is a reflector or radiator having an emissivity that is less than the 0.70 or emissivity of the known refractory materials,

but is at leastas high as about 0.1, preferably being within the range of 0.2-0.6, depending upon the temperature of the material itself. For each of the purposes just mentioned, steel or iron pipe or tubing has been found to be particularly desirable. L? One of Athe particularly advantageous aspects ofthe use of asolid conductor for the chamber walls resides inuthe fact that such conductorV tends to maintain itself at substantially the same temperature throughout its solid body, so that no hot spots or similar temperature lirregularitiesi may occur within the body of the solid conductor. By interposing the solid conductor between the hot metal article and the insulating bed of slag, .it is possible to effectively prevent the building up of hot spots at localized lpoints inthe insulator bed adjacent individual regions in the hot metal article, so as to effectively prevent-uniform cooling 'off-the article'throughout. As was previously mentioned, also, the conductor effectively withdraws heat, by conduction, from the hot portion 36e and then uniformly distributes this heat throughout the chamber walls, by virtue of its conductivity. It will thus be seen that by theuse of the instant insulator metal sleeve or pipe 18, it is possible to elect a uniform heat distribution (by virtue of the conductivity of the metal) intermediate the particulateinsulator bed and the small space between the chamber walls and the hot metal article; and this results in a substantially uniform temperature throughout the metal at any given time, so that the radiant heat emitted or reflected back toward the hot metal article by the lchamber Walls will be substantially the same per unit area throughout the surface of the het metal article. yThe amount of heat energy radiated is a function of the fourth power of the temperature (in degrees Rankine).

The instant apparatus does not involve the use of externalheat to maintain a given temperature within the closed chamber or. to effectively control a predetermined slow' reduction in that temperature, but instead, the instant invention employs a very4 effective insulating and sealing yarrangement to permit control of thek rela* tively low conduction and convection lieat transmission. In additiomthe instant invention provides a solid radiant heat absorbing medium in the form of the tube or pipe 18,; which by virtue of'its conductivityfrnaintains a sub stantially'unifonn temperature throughout itself-at any given time and which by virtue of its ability to radiatel or emit heat reflects back a certain 'amount'of the heat it absorbs, so that the heat loss of the hot metal article, in the form of radiant heat, is not purely a function of the emissivity of this rarticle times thefourth power of the temperature (in degrees Rankine). By the use of the slag-'insulator backingfor theinstant metal sleeve defining the chamber, it is possible to'maintain this metal sleeve at a relatively high temperature, comparable or proportionate to the temperature of the'hot metal article,

so that the amount of radiant heat returned to'the hot metal varticle will be appreciable. Inother words, the more effectively the back of thef'metal tube 4is insulated, so as to maintain the tube at ahigher temperature, the proportionately greater `amount of radiant heat'will be reected therefrom back to the hot metal article. In contrast, such a hot metalv article positioned in the normal atmosphere would receive almost no reflected radiant energy, since the various'radiant energy-absorbing media which might be located in the region of such article? are at substantially atmospheric or room temperatures and effectively emit almost no radiant energy; Moreover, such media are not backed by an insulator so-that theymight be caused to heat up as a result of absorbing the radiant energy.

It will, of course, be appreciated that the insulating effectivenessof the slag bedmight be changed so as to permit slightly more rapid or less rapid heat losses therethrough from the metal sleeve, Yso as to effectively permit a greater or ai smaller'temperature gradient between the hot' metal article and the'chamber walls, thereby effecting slightly faster or slower cooling of the hot metal article, as the case may be. The instant apparatus is-particularly well adapted for operations wherein the critical or austenite-pearlite transition temperature is depressed only about 25-75 C., and preferably 50 C. ,below the ideal or normal critical temperature obtained at infinitely slow cooling (i. e., about 700 C.). Thus the critical temperature is depressed to G25-675 C., and preferably 12 hub 36e and thickened top shaft 36a makes the shaping of thechamber A more diicult to define on the basis of ideal conditions; but, the generally closespacing of the walls 23 to the hub 36e` periphery is maintained.

The inside diameter of the tube 19 is thus preferably about l.1d-\/2d on the basis of the preferredchamber volumes. On the basis of emissivity, the surface are-a A, or the article 36 which has a relatively high emissivity e, of about 0.7-0.8 and the surface area Az of the exposed chamber walls which have a relatively lower emissivity ez of about 0.2-0.6 (depending upon their temperature to a certain extent) should be such that Ae=1/zA2e2 to about l1/2A2e2, and preferably about Azez. Under conditions of approximately Ae=A`2ez, the heat loss by radiation (which is critically important as the instant cooling arrangement) becomes solely a function of the difference between the fourth power of the temperature R.) of the article 36 and the sleeve 19; and the temperature of the sleeve 19 may be controlled by the control of the insulator backing therefor.

In general, also, the spaced arrangement of each of the tubes 18, 19, etc. makes possible the maintenance of articles 36 therein at different temperatures; but unless the insulation around one tube differs from that around another, the cooling rates, in each will be about the same. The conducting effects through the cross beams 16, 16 are substantially negligible, and can be made even more so by insulatingly mounting the tubes 18, 19 etc. thereon.

It will be understood that modification and variations may be effected without departing from the scope of the novell concepts of the present invention.

We claim as our invention:

A method of cooling a hot steel forging at 800- 1000 C. having surface area A, and emissivity e, which comprises enclosing the hot forging separate and apart from other articles to be cooled in a non-absorbing nonconducting atmosphere to prevent appreciable convection and conduction cooling thereof, exposing substantially all of the forging surface to a solid conductor-radiator of surface area A2 and emissivity e2 such that Ae=A2e2 while maintaining the conductor-radiator .spaced from the'forging and 'enclosing the conductor-radiator in an insulator in loose particulate form to controllably reduce heat transmission therethrough from said conductorradiator while cooling the forging to effect the austenitepearlite transition in the forging at 625-675 C.

References Cited in the file of this patent UNITED STATES PATENTS 83,605 Cochran Nov. 3, 1868 294,283 Snelus Feb. 26, 1884 604,195 Rogers May 17, 1898 2,696,378 Krischer Dec. 7, 1954 FOREIGN PATENTS 3,807 Great Britain 1882

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