US 2326126 A
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Description (OCR text may contain errors)
Aug. 10, 1943. R. B. DIMMICK 2,326,126
METHOD OF HEAT TREATING CAR WHEELS AND THE LIKE Filed May 4, 1940 NVENTOR. PH 5- MMICK.
Patented Aug. 10, 143
PAT NT F 2,320,126 Mormon or? mar-resume can WHEELS AND 'rnn LIKE Ralph E. Diim'niclr, Butler, Pa., assignor to The p v American Rolling Mill Ohio, a corporation of Company, Middletown,
ilon'tinuation of application Serial No. 239,391, I November 9, 1938. This application May 4,
1940, Serial No. 333,355
i1 oiaima- (ci. 148-43) ously bring about the following conditions in the article:
(1) The tread of the wheel has suflicient hard- Iiess to successfully withstand difficult service conditions.
therefore remain soft, while the rim is hardened.
, While this procedure also results in a usable '(2) The hub is soft enough to permit easy machinlng.
(3) The'tensile and fatigue strength and the ductility of the plate of the wheel are very high;
adding to its factor of safety.
(4) The wheel is free-of dangerously high internal stresses.
The last two of these considerations are, of course, the most important since above all the wheels must be safe. In my process the desiderata above are coincidently achieved; and this, so far as I know, is a wholly new aspect of utility which my product possesses.
In one prior process it is the practiceto heat the wheels to above the critical point and then to quench the entire wheel in a liquid, usually oil, and then uniformly to draw the temper of theientire wheel to relieve the dangerously high quenching stresses. While this process has resulated in a usable wheel, it is not all that could be desired in several respects. The chief undesirable feature is thatwhen the wheel has been heat treated to produce a degree of hardness in the rim which will satisfactorily resist diflicult service.
conditions, the hub is so hard as to be very difficult to machine; and the finishing of the bore becomes a very difiicult and expensive operation.
Such wheels also have a higher degree of stress in their plates than is desirable, probably due wheel, several new disadvantages are introduced which were not present in the .unii'ormly quenched wheel previouslydescribed.- Since only the rim section of the wheel has been quenched, the steel in the other parts of the-wheel does not get the benefit of the refining action on themicrostructure which is afforded by a quench and a draw, and consequently the strength and dimtility are inferior. Also as can readily be appre--, elated, this unequal quenching sets up exceptionally high stresses in the wheel. 'Iheequipment for carrying out this process is, moreover, quitelarge and complicated and adds greathr to the inconvenience and'expense of the process. Following. this differential quenching operation it is customary to employ a uniform draw to relieve the stresses somewhat and to improve the physical properties of the rim. The rest of the wheel is little affected by this draw since it has not been quenched.
In my new process I avoid these undesirable features by providing- (following an over-all quench) a differential draw or tempering in which the temperature diflerence between the hub and the rim is carefully controlled'by .a diiferential to the unequal cooling rate or the various parts of the wheel, resulting from their very irregular cross section.
In attempting to avoid the first of these "diflle culties some manufacturers have resorted to a differential quench. That is, the hot wheel is.
held by the hub while a quenching medium, usually water, is caused to come into contact with the rim section-only. The hub and plate sections are protected from the quenching action andapplication of heat so that the hub is .heated to a high enough temperature to effectively soften.
it while the rim is held at sufllciently low temperature to prevent unduesoitening, followed byan equalization of temperatures and a slow cooling. .For the best combination or properties this differential draw ispreceded by a uniform quench.
Byfollowing this procedure I produce a wheel having 'a'combination or maximum rim hardness to resist, shelling and wear, softness in the hub to give good machining properties, and controlled stresses to provide the maximum of safety in service.
The figure'is a sectional view through a type I of draw furnace which I may employ, showing also an exemplary heating means.
In practicing my preferred process the wheels to be heat treated are first heated slowly in another iu'rn'acanot shown, to above the critical temperature, for example to around 1500 F.,
and are held there for a sumcien't length of time for all parts of the wheel to attain a uniform temperature.
to the quench is well understood in the art and the particular heating schedule is not critical for the achievement of my result.
The heated wheels .are. then quenched preierably in 011, although water, brine, or other This procedure of heating prior fluids are effective. It is preferred that this quench be uniform over the entire wheel, that is, that no part of the wheel be protected from the rapid cooling action of the cooling medium. The principles and practice of quenching are also well understood in the art, andmy invention does not contemplate the use of any particular method other than preferring a uniform quench. It is of course understood that the quench should be carried out in a manner consistent with generally understood metallurgical principles.
To accomplish the desired changes in microstructure it is only necessary to cool rapidly to below the critical temperature; but additional desirable hardness is imparted to the wheel the lower the temperature to which the wheel is quench, and there is a saving in both heat and time if the wheel is placed into the draw furnace while still quite hot from the quench. .The
erably of stainless steel, placed between the plates of the wheels, as shown at 6. The. purpose of these will hereinafter be described. .They may, however, be made of tallic or non-metallic. composition will serve. A third wheel is placed upon the second in the way described, and so on until the stack is completed. A furnace charge usually comprises six or seven wheels in my practice, Thermocouples are inserted between the rims so that their temperature may be accurately determined during the drawing period. It is also advantageous to locate thermocouples in a, the hub temperatures. ple cbhnections are shown at I and 8. The cables may be thought of as connected to visual or recording instruments in a position of convenient access to the operator.
When the stack of wheels has been completed in the draw furnace a heating element is positioned in the space formed by the concentric h bores of the wheels. A convenient form of heatquench must be continued to a sufficiently low temperature, however, to give the desired hardness of the rims, and to make possible, during the draw, the temperature differential between the hubs and the rims necessary to achieve my novel result, as will now be described. I,
The wheels now have a microstructure refined throughout by the uniform quench. The entire wheel is hard and stressed. In my process I soften the hubs without unduly softening the rims and at the same time modify internal stresses by a differential draw which in its simplest formconsists in heating the hubs hotter than the rims. In my preferred furnace, which is illustrated in the drawing, this is accomplished by the use of an internal heating means inserted within the bores of a number of stacked wheels. An exemplary arrangement for my draw furnace is illustrated in the figure. I prefer to employ a furnace which is cylindrical in form and of a diameter not much greater than the diameter of the wheels which are to be treated in it. On a suitable floor I, I mount a cylindrical casing 2 of insulating character. The casing may, if desired, be of such character as to be removable by a crane; and the arrangement of wheels may first be made and then the casing lifted over it. Or if desired, the casing may be fixed in position and closed by means of -a movable to'p member 3, also of insulating character. The car wheels are arranged in a vertical stack. The first wheel is placed 1n the furnace in a horizontal position resting on a suitable support. The second wheel is placed on top of the first, care being taken to keep the bores of the wheels in alignment. Whenever the hubs are thicker than the rims, as they usually are, I may employ spacers 5 between the rims to hold the wheels in a horizontal position. These spacers mayconveniently be made of cut sections of relatively heavy 4" or 6" diameter pipe, but may also be made of other substance, metallic or non-metallic. A number of these, usually three, are placed on the top of the rim of each whee1 and equally spaced around the periphery thereof, before the next wheel is placed in position.
I also commonly employ circular collars, prefing element, and one which I illustrate is an electrical resistance heater. In the figure, 9 represents the resistance element of such a heater, held by suitable heat resistive and insulating supports Ill. The terminal portions of the resistance element are brought out through a top brick spacer II, which forms a closure for a hole in the cover 3, of the furnace. If desired, a bottom brick spacer l2may be' employed, the two spacers being held together by a connecting rod l3, or other suitable means which passes through the several heater supports Ill.
The heating element in this form is a unitary structure to be inserted and withdrawn through the hole I l in the furnace top. and adapted to close the said hole when in position.
While an electrical heating element is most convenient and most easily controlled, I am not confined to the use of such meanssince other types of heating devices can also be employed.
- A radiant tube type of heater can be used with slight modification in the furnace to provide for the burner at one end and exhaust at the other. Also I find it advantageous to preheat the furnace, and sometimes to apply heat to the furnace generally, if that should appear necessary. As a consequence the furnace may be provided with electrical heating elements or burners for suitable fuel and with a, flue (not shown). Or the interior of thefurnace may be arranged to be heated by circulation through it of hot gases derived from a burner or burners or' electrical heating elements located in a chamber extennally of the casing 2; 1 In order tojllustrate the method of operation or my process'I will describe the treatment or a lot of railway wheels of the composition specified by the Association of American Railroads, Specification M-107 which calls for carbon, .67 to .82 per cent, and manganese, 0.65 to 0.85 per cent. These wheels are first heated and uniformly quenched as above described.
In order to obtain the optimum combination of rim hardness, hub softness, good p ysical rim draw temperature as follows:
This temperature depends on the severity of the quench, the chemical composition of the steel,
the desired hardness, the extent to which the other substance, 'me-- For example, asbestos position to measure Exemplary thermocouquenching stresses are to be relieved, and the time the steel is to be held at the draw temperature. For example, the more severe the quenching treatment the higher the draw temperature should be to relieve the internal tresses resulting from the quenching; the higher the carbon and manganese content the higher the draw temperature may be without appreciable loss of rim hardness; and the greater the desired relief from internal stresses the higher the draw temperature should be. In the case of a wheel of, say, 0.70 per cent carbon and 0.70 per cent manganese which has been uniformly quenched in oil and which is to have a Brinell hardness of at least 321 (a customary rim hardness for wheels of this analysis) and internal stresses not exceeding 20,000 psi, as measured in the plate of the wheel by a method to be described later, I would use a draw temperature for the rim of 800 to 900 F. and hold this for about 12 hours. However, it is possible to use a higher draw tem-- perature for a shorter length of time, or a lower temperature for a longer time, within limits.
In order to obtain satisfactory machining properties in the hubs of such wheels I have found that it is necessary to keep the Brinell hardness thereof below 2'70. This can be secured by drawing the hub for at least two hours at a temperature of 1050 F. or higher. There is a practical limit to how high this temperature can go because the carbides begin to go into solution as the temperature is raised, and an undesirable coarse lamellar structure might result from the subsequent slow cooling. Preferably the draw temperature should be such that spheroidization of the carbides occurs, thus leading to the optimum in machining properties. For the accomplishment of these objectives the hub should be heated to not over 1250 F. and I find that satisfactory results are obtained if it is held for three or four hours at 1100 to 1150 F.
My invention comprises supplying such means and devising the proper combination of temperatures and times as will achieve the fourfold object already recited, namely, sufllcient rim hardness to resist shelling and wear, satisfactory machining properties in the hub, and goodphysical properties throughout the entire wheel together with low internal stresses.
Having described the selection of the draw temperatures for the rim and hub portions of the wheels, one method of carrying out the invention will now be explained according to my process and with my preferred apparatus. The heated and uniformly quenched wheels are charged into the draw furnaces in the manner described, and the. thermocouples placed for indication and regulation of the temperature of the hubs and rims. The electric element 9, where this type of heating means is employed, is then inserted in the bore of the wheels and the power is turned on to bring up the hub temperature as rapidly as possible to the hub draw temperature that has been selected according to the method described.
This rapid heating of the hubs is the special feature which makes possible the temperature differential between the hubs and rims when the wheels are heated as described in a heat treating mechanism such as that shown in the drawing. More specifically, my object is attained by heating the hub up to the softening temperature and holding it there long enough to obtain the desired softening, before the rim is heated high enough to be unduly softened.
This rapid heating is assisted by high wattage or fuel input into the heating means, and by adequate enclosure of the hubs to restrict radi ation from the hubs and exposed parts of the .heating means between the hub faces, to the other parts of the wheels. In order to secure the difference in temperature between rims and hubs according to the preferred process already described, it is necessary of course that the wheels at the start of the drawing operation be lower in temperature than the selected drawing temperature for the rims, and the lower this initial temperature at the time the heat is first applied to the hubs, the more readily can the desired temperature difference be attained.
In the course of regular production, the draw furnace will naturally be heated to the selected temperature for drawing the rims of the wheels when the preceding lot of wheels is removed following the completion of the preceding. cycle. The furnace will cool somewhat during the charging of the next lot of Wheels but it is unnecessary to cool it to room temperature before starting the drawing operation on the next lot of wheels. As I ordinarilycarry out my process, the wheels, after uniform quenching until they are at a temperature of about 450 to 700 F., are immediately placed in the draw furnace and the draw furnace after closing the cover will usually level off at a temperature of about 600 to 700 F. Under these conditions I have found that with one form of apparatus in which I have been carrying out my process commercially, it is possible to heat the hubs to their selected softening temperature without raising the rims above their selected drawing temperature, provided that the heating means is capable of dissipating at least 16 kilowatts of electrical energy, or its equivalent if other forms of heating means are used.v To provide a margin of safety I prefer to use as much as 20 kilowatts or more.
These values are for average conditions with wheels of say 36" diameter and standard hubs of say 7" bore and with the use of radiation collars around the hubs as the wheels are charged in the draw furnace. If the wheels are hotter when the application of heat to the hubs is started it will be necessary to apply this heat at a greater rate in order to soften the hubs without unduly softening the rims.
After the hubs of the wheels in the charge of six or seven wheels have been heated to at least 1050" F. and held there for at least two hours as measured by the thermocouples on the hubs of the wheels, the power is shut off on the heating element and the temperature of the hubs allowed to cool and preferably allowed to equalize with the other, parts of the wheels at the selected drawing temperature for the rims.
get above their proper draw temperature at any time during the drawing operation and for this purpose I employ -a pyrometer controller attached to the rim thermocouple to shut off the power to the hub heating element if this should be necessary.
With my preferred apparatus it is advantageous to use the equalization period and external heat is applied to heat the recirculated furnace atmosphere when necessary to maintain the selected draw temperature for the rim of the wheels. Although the time may vary with other forms of apparatus I find that, in general, it is advisable to allow at least four hours after the In my preferred practice the rims are never permitted to hub heating element is tion to proceed.
At the conclusion of the drawing treatment the wheels are cooled slowly in accordance with usual practice in order to avoid introduction of internal stresses by the unequal cooling rates in different portions of the wheels which would result if the wheels were cooled more rapidly. It is customary to remove the wheels from the draw furnace at the conclusion of the draw period and place them in preheated brick lined pits or tunnels where they are closely piled or stacked and require from 24 to 48 hours to cool to about 200 or 300 F.
The same result can be obtained by allowing the wheels to remain in the draw furnace and shutting off the heat while they cool to 200 or 300 F. This delays the use of the draw furnace for treating subsequent lots of wheels and is a practice followed only rarely in actual plant operation, as when the entire heat treating unit may be shut down at the end of a run.
I find that wheels of the above composition treated according to my preferred process as described, will have a rimhardness of at least 321 Brinell and a hub hardness not over 270. Furthermore, when the internal stresses are determined by the so-called dyadic-circle or rosette method as described in the Bureau of Standards Journal of Research, volume 10, 1933, R. P. 559, and volume 15, December 1935, R. P. 851, and volume 19, October 1937, R. P. 1034, the highest stress measured in the plate portion is found to be not over 20,000 psi and in most cases will be under 15,000 psi. This is a much lower stress than that obtainable by any other known methshut off for the equalizaads of heat treatment which will produce hard rim and soft hubs.
Another distinguishing characteristic of wheels heat treated according to my new process is the improved microstructure in the hub portion. This consists of a partially or completely spheroidized condition of the cementite particles as contrasted with the lamellar pearlite found in wheels heat treated by other methods. Hubs of wheels that are uniforml quenched and uniformly drawn will tend to have a microstructure of finely spaced pearlite lamellae because the cooling rate is generally not great enough to produce martensite in wheels of the compositions in common use. The hubs of wheels that are nonuniformly quenched by protecting the hubs and sometimes also the plates from contact with the quenching medium, and are thereafter uniformly drawn, will have fairly coarse pearlite lamellae because of the slow cooling rate through the critical temperature range.
Thus wheels heat treated by my process are not only readily distinguishable by the spheroidized structure of the hubs but they possess the desirable properties that accompany such a structure among which are good ductility and impact resistance, high tensile and fatigue strength, and very satisfactory machinability.
My method thus produces a wheel with a rim section of sufiicient strength and hardness to withstand severe service conditions, which at the same time has a hub soft enough to be readily machinable, has a desirable microstructure and physical properties, and is very low in internal stresses.
It will be understood that the showing of the heat treating apparatus is only diagrammatic and process is only exemplary and that the invention is capable of many refinements which will readily occur to those skilled in the art. For example, instead of inserting the heating means through the top of the draw furnace, it may be fixed to the base so that the wheels are charged by slipping the bores over the heating means. The diameter of the heating means may be varied according to the size and mass of the hubs. Special precautions such as dampers in the recirculating system, if such is used, enclosures at top and bottom of the chimney formed by the stack of hubs to restrict convection currents, and extra tight sealing of removable furnace covers if such are used, may be necessary in order to get the desired temperature difierential between the hubs and the rims of some wheels. Although it will generally be found advantageous to use the circular collars 6, in some cases the required difierence in temperature between the hubs and the rims can be obtained by rough machining the faces of the hubs if they are not already smooth enough from the rough forging so that they can be piled to rest hub to hub and thus prevent the radiation of heat from the heating means through any open space between the hubs and adjacent wheels, as would occur if the wheels were supported by spacers placed on the rims as shown in the drawing. If the hubs are tightly enclosed, by means of circular collars or by being placed hub to hub and by closing the top and bottom of the chimney formed by the stack of hubs, it is possible and may be desirable to extract heat from the rims during the hub softening part of the cycle by providing for the circulation of the furnace atmosphere around them and cooling it if necessary by the use of cooling coils brother means, either in the furnace walls or externally.
Ordinarily the rims are allowed to come up to a certain temperature during the drawing opera-.
tion and then. are maintained at this temperature during the equalization period; but for some wheels of special design it may be necessary to allow the rims to heat for a short time to a temperature above their desired draw temperature so that the hubs may be heated sufiiciently to soften them. Then after the hub treatingmeans is shut off the rims are brought back to their desired draw temperature as quickly as possible by the recirculation of the furnace atmosphere. Such minor variations of my process can be made without departing from the spirit of the invention, and I intend, therefore, to be limited only as indicated by the scope of the following claims.
Having now fully described my invention, what I claim as new, and desire to secure by Letters Patent, is:
1. A process of treating forged steel car wheels which comprises heating said wheels to a temperature above the critical range, uniformly quenching said wheels, placing said wheels in an that the description which has been given of the 7 insulated space and inserting in the bores of said wheels a heating element, whereby to raise the hub portions of said wheels to a temperature suflicient to soften them and to relieve internal stresses throughout the wheels, and to SpheroidiZe the cementite therein, to that degree at least which would result from heating said hub portion for two hours at 1050 F. and whereby to heat the rim portions of said wheels by conduction primarily from said hubs to a temperature for a draw to give to said rims a hardness suitable for service.
2. A process as claimed in claim 1, in which the initial heating prior to quenching is of the order of 1500 F. and in which during the drawing stage the hub temperature rises to the order of at least 1000 F. and the rim temperature rises to the order of at least 700 F., but not so high as to impair unduly the physical properties imparted thereto by said quenching.
3. A process as claimed in claim 1, in which the initial heating prior to quenching is of the order of 1500 F. and in which during the drawing stage the hub temperatin-e rises to the order of at least 1000 F. and the rim temperature rises to the order of at least 700 F., but not so high as to impair unduly the physical properties imparted thereto by said quenching, and in which subsequent to the draw the wheels are cooled slowly to a temperature approaching room temperature.
4. A process of treating forged steel car wheels which comprises heating said wheels to a temperature above the critical range, uniformly quenching said wheels, and heating the hubs of said.
wheels to a temperature sufllci'ent to soften the and to spheroidize the cementite therein, to that degree at least which would result from heating said hub portion for two hours at 1050 F., said heating step being carried out with such rapidity that said hubs are softened before the rims of said wheels are heated sumciently to soften them unduly.
5. A process of treating forged steel car wheels which comprises heating said wheels to a temperto a temperature approaching room temperature, whereby to bring about gradual equalization of temperatures between said rim andhub portions ature above the critical range, uniformly quenching said wheels, and then heat treating said wheels to produce in the hub portion thereof a spheroidized cementite structure, by raising the temperature thereof to between 1000 F. and 1250 F. for a suiiicient length of time to produce at least that degree of spheroidization which would result from heating for two hours at 1050 F. without changing the metallographic structure in other parts of the wheel produced by said quench.
6. A process of treating forged steel car wheels which comprises heating said wheels to a temperature above the critical range, uniformly quenching said wheels, and then subjecting said wheels to a differential drawing operation in which the hub portions are heated to a temperature between 1000 F. and 1250 F., while the temperature of the rim portions is maintained between 700 F.
and 900 F. I
'7. A process of treating forged steel car wheels which comprises heating said wheels to a temper.- ature above the critical range, uniformly quenching said wheels, subjecting said wheels to a differential drawing operation in which the hub portions are heated to a temperature between 1000 F. and 1250 F., while the temperature of the rim portions is maintained between-700 F.
and 900 F., and then slowly cooling said wheels in order torelieve thermal stresses.
' 8. A process of treating forged steel car wheels which comprises heating said wheels to a temperature above the critical range, uniformly quenching said wheels, and then subjecting said wheels to a differential drawing operation in which the hub portions are heated to a temperature between 1100 F. and 1150 F. for three to four hours, while the temperature of the rim portions is maintained between 700 F. and 900 F.
' 9. A process of treating forged steel car wheels I. which comprises heating said wheels to a temperature above the critical range, uniformly quenching said wheels, subjecting said wheels to a differential drawing operation in which the hub portions are heated to a temperature. between 1100 F. and 11 0" F. for three to four hours, while the temperature of the rim portions is maintained between 700 F. and 900 F., and then permitting said hub portions to cool to the temperature of said rim portions and holding said wheels at said last mentioned temperature until said rim portions have been subjected thereto for about four hours.
10. A process of treating forged steel car wheels which comprises heating said wheels to a temperature above the critical range, uniformly quenching the said wheels, subjecting the wheels to a differential drawing operation in which the hub portion is heated to a temperature between 1000 and 1 250 degrees F. and maintained at this temperature for a suflicient length of time to transform carbon in the hub portion to the form of spheroidized cementite, while the temperature of the rim portion is being brought up to and maintained between 700 and 900 degrees F., and then permitting said hub portion to cool to the temperature of said rim portion, and holding the said last mentioned temperature for a period of approximately four hours, measured from the time the hub portion starts to cool.
11,." A process of treating forged steel car wheels which comprises heating said wheels to a temperature above the critical -range, uniformly quenchingsaid wheels, subjecting said wheels to a diflerential drawing operation in which the hub portion is heated to a temperature between 1000 and 1250 degrees F. and maintained at said temperature for a period or substantially one and one-half hours, while the temperature of the rim portion is being brought up to and maintained between 700 and 900 degrees F., and then permitting said hub portion to cool to the temperature of said rim portion, and holding said last mentioned temperature for a period of at least four hours measured from the time the hub starts to cool.
RALPH B. DIMMICK.