US2446202A - Induction heat-treatment - Google Patents

Description

A. VANG INDUCTION HEAT TREATMENT Aug. 3, 1948.

2 Sheets-Sheet 1 Filed Sept. 24, 1941 (WWW w .w a 2 INVENTOR fllfred any W0 ATTORNEY Aug. 3, 1948. VANG 4 2,446,202

INDUCTION HEAT TREATMENT Filed Sept. 24, 1941 2 Sheets-Sheet 2 @121, 4%, mATToRN f Patented Aug. 3, 1948 UNITED STATES PATENT OFFICE 3 Claims. I

The invention relates to means for induction heat treating metals or articles of metal, and particularly to induction hardening.

Induction hardening, as customarily practiced,

involves heating of the metal to or above the critical temperature, followed by rapid chilling or quenching. The latter step is performed by fluid quenching, as by immersion of the article in water or oil, or by spraying the surfaces of the article with a suitable cooling fluid. I have discovered that if the article which is to be treated is first subjected to extremely rapid heatin of its surface (or of selected portions of its surface) by induction from a high frequency oscillatory circuit of the type which will be described herein, so as to produce an extremely high temperature gradient between the heated surface and the metal adjacent thereto, it is possible to eliminate entirely the step of fluid quenching. This constitutes one of the principal objects of my invention.

It is also an object of my invention to provide a means for induction hardening which gives a more accurate control of the depth to which the hardening treatment is efiective. This is of advantage in the design of parts which must have hardened surface portions and tougher, more ductile portions immediately adjacent thereto. Thus, in the case of a gear wheel, it is essential to have a hardened outer circumference so that the surfaces of the gear teeth will be able to withstand wear, but the felloe and spokes should be softer so as to avoid brittleness. If the depth of hardening can be controlled with sufficient acwithout hardening the ends of the spokes or other adjacent portions, the design of the gear can be lighter than would otherwise be the case, or a gear of a given weight will be stronger than otherwise. Moreover, the hardening can be calized to a very thin skin of surface metal, leaving the interior metal undisturbed. This is of advantage in preventing or minimizing the warping of castings or other articles during heat treatment, and in many cases does away with the necessity of grinding operations which wouldbe required after heat treatment by methods known heretofore.

Another object of the invention is to decrease the length of time required for the hardening treatment, and thereby to increase the rate of output of the heat treating unit.

I A further object is to lower the energy consumed during the heat treating period. Other objects and advantages will appear as the description proceeds. In the drawings, Fig. 1 is a diagram illustrating a preferred 5 general arrangement of the electrical heat treating circuit in accordance with my invention.

Figs. 2 and 3 illustrate one form of induction unit forming a part of the circuit shown in Fig. 1; Fig. 2 being a plan view, and Fig. 3 a side eleval," tional view, partly in section.

Fig. 4 is a plan view similar to Fig. 2, but showing another form of induction unit. I

Referring first to Fig.1, alternating current from a suitable source H), such as the usual 110 or 220 volt A. C. line and controlled by a switch 39, is transformed to a high voltage-preferably oi the order of 10,000 volts, although other voltages can be employed-by the transformer II, and rectified by the tubes l2, which are protected by 'a choke l3, providing a convenient source H of high voltage direct current. It should be understood that this source is indicated merely by way of example, and that any other suitable source of high voltage direct current might be employed in its place.

. 0 densers connected in parallel, or in series-parallel, in any desired manner. The condenser discharge circuit l'l leads through an induction coil l8 which is so designed as to surround the metal article which is to be treated. In my preferred curacy to create the desired hardness in the teeth embodiment there are also provided auxiliary *variable inductance coils l9 connected in series with the coil I8, providing means for regulating the frequency and thus timing the rate of discharge in accordance with the composition or 40 size of the article being treated.

The switching means employed for closing the discharge circuit l1 should be of such a type and construction as to close the circuit substantially instantaneously and without dissipating the 5 charge, or any substantial proportion thereof, in the form of heat. An ordinary switch cannot be employed, because the heat generated upon percussion of the switch points would weld them together and dissipate a substantial proportion of the charge in this way, the discharging current being very high in order to create an electric field powerful enough to induce the requisite degree of heating within a small fraction of a second, as explained hereinbelow. The switching means should be capable of closing the discharge cir- (mil; in such a manner that the full charge, or as much of the charge as possible, will be trans-- lated into an oscillatory discharge without being dissipated as heat in the switching means, A spark gap might be employed as the switching means, but has the objection of creating a, high loss because of its relatively high resistance. This loss may run as high 'as 40% or more of the available charge. I have found that igniter tubes can be emplbyed, advantageously iii closing the discharge 'circuit"; and, by way 'ofexample, have shown a circuit employing two gaseous electron ic discharge tubes, 20 and 2|, connected in parallel,

" denser [5. This creates an extremely sharp tembut with the tube reversed in direction to the tube 2|. This arrangement permits the current" to flow through the circuit in either direction.

mere-tyre, gradient so that the depth of heat treatment can 'be accurately controlled through regu- "lation of the rate of discharge of the condenser.

The discharge is set off byasuitableionization controller 22 through which current is supplied Thus. 01: examp the heat treatment can be localized to a very'thin skin of surface metal, leaving the interior metal undisturbed, with the reto the igniter electrodes 23 0f the tubes 20, 21, 1".

The controller 22 maybe connected to any suitable, source of current, such as the source It as by means of the leads 2 1. A switch 24, may be provided for manual actuation of v the ionization controller, although, it desired, the controller may embody a relay which isautomatically actuated when; the condenser [5, has been chargedto a predetermined voltage-esay, 3,000. volts. Control- -lers;ofthlis typaare wellknown in the art, so it .i's. -believedunnecessary to include a spe ifi description thereof this application. The principle-.otoperation ofthe-controller resides ininitiating ionization in the tubes 20.,and 2i to therer ur nt. to ow thro h h hes thu close thepcondenser discharge circuit, [1. The; arrangement I have disclosed provides a switching. means which is characterized by ra- .piclity ofoperation, a positively controlled point of break-down oithe. initial high resistance, and n a il y o onduct u re tw h equ l ity in 9 h reet ns- M r o e e esistan of the w tchin me n is e i ibl ov t tu l all -.of. the available condenser charge is translated into useful oscillatory discharge. Iidesired, other types ormanually-controlled switching mean-s than thatdescribed specifically h e n igh e. m lcr dl hav d. ever, to the desirable characteristics enumerated above. liorexampleyone'of the ignitertubes 20, .21 might; beemployed alone, but with, some sacrifi ce. in the life. of the single tube. Alternative.- ly, the igniter tube 20, 2| might be replaced5by the switching means for high,- frequency circuits described and claimed in, my 'copendingapplication for patent,- Serial No; 364,701, filed November .7, .1 9 l0 (new Patent No. 2,287,541; vdated June 23, 9 2)- 1 A single, oscillatory condenser discharge provides the energy for heat treatment-in the preferredembodiment of my invention. In order to provide a single charge and ai single, oscillatory discharge, in place of multiple, "or repeated charges and discharges commonly produced by "high frequency generatorsof the condenser-discharge type, various selectiveswitching. methods may be employed, In the embodiment shown in vFig. l, the condenser I5.;may be chargedfby closing the switch 39, whereupon theiswitch 3.91 may be reopenedbefore closing theswitch '24. Thus theensuing. discharge will represent a single 'oscillatory discharge of the condenser, and thereaiter thecircuits will remain deadun-til the man. vu alv cycle is repeated, An alternative method is to adjust the constants of the charging, circuit fona slQW lfi/te of charging, and to adjiist the ionization controller, to be sensitive to the 'p'oten'e sultant advantages noted hereinabove. The completion e a m n within hetimeci. gle discharge of thecondenser provides ly accurate method of regulating the. de h treatment or of confining it to, a thi metal at the surface of the articl'egbe g: Moreover, no auxiliary timing'farrangemen needed to regulate .the length; of the; treatment. The treatment is determined by thecondenser charge, frequency, andthe magnetic ccupllngqf the coil with the worlcall of which are suscep 'ble of extremely delicate regulation,-m lill fielabo rate timing devices unnecessary; The rate of discharge can be accurately regulated by;- t,he.;var,- iable inductance coils l9, and by, this means the depth ofenergy penetration into,.tl1 e;.worl ch is subject to skin efiect. at high'ir-equency, can be controlled, Thus, wheregreater;depthofihtmt treatment, is desired, the, inductance ,oflthecoils H) can be adjustedgto a. higher val-ue whichwill lower the fundamental. frequencysofthc cilf Hit ll, which, in turn, will produce greaterpenetrag tion of the critical-heat treating temperature,

In accordance with my inventioirirequencies on the order of millions ofxcycle persecondmay be found useful, although frequencies 0115:; 10,000 cycles per secondm ay be desirable practical working range, depending upon the-com, position of, the metal being. treated and the ,de th to which the metal is to be. hardened. he.- tual time required-for. the condenser di sc h ,ge will-ordinarily be on the order of air-action of}; a second, and may, he on the order or /1000 oijwa second, extending, over a period w hjich may/rep;- resent, say, 8 cycles ofoscillation.

With particular reference to Figs-,2 211101;;3;,-I shall now describe my preferred-formof the induction heatingunit which is shown; diagram;- matic'ally at l8.,in Fi 1. This unit consists oi a hollow tube 25 of high; electrical-conductivity, such as a tube of copper or'aluminum or their alloys, wound in the form of .a 'c'oil designedto lie in proximity to those portions of, ,thefarticle which areto'be heat-treated. Thuswheiih idening the teeth 26 of gear wheels 21,1thefcoiiwill be in the form of a simple helix 28 the turigis of which lie closely along the circumference 'o'f the gears. It WiILof-cour'se, be understoodthat the coil can be lengthened in order to acet 'mt'jdate the number of gears which it maybe desired to treat simultaneously, or, preferably, a number of such coils may be arrangedihparallel. The-gear wheels 27 or other articlesto be treated may conveniently be supported on a'mo'vable platform arranged to bring the gears into the fieldof the coil 28' for treatment. Thusin the embodiment illustrated, the gear Wheels 21 are carried by a vertically reciprocable table 29 mounted on a plunger 30 connected to the piston of an hydraulic or pneumatic cylinder. In Fi 3 the platform 29 is shown in an intermediate position representing either the introduction or removal of the gear wheels to or from the field of induction of the coil. The ends of the hollow tube 25, in the embodiment illustrated, are connected respectively to a source of cooling fluid 3| and to an outlet 32. Cooling fiuid may be circulated through the coil 28 during the heattreating cycle so as to avoid melting of the coil under the high temperatures created in the adjacent portions of the gear wheels 21. The flow of cooling fluid may be regulated by a suitable valve 33.

In order to provide for efficient heat treatment of articles of different sizes and shapes, the mechanical and electrical connections of the coil are so designed as to permit its ready removal from the machine so that other coils can be interchanged therewith. For this purpose, I have shown the tube ends 25 brazed or welded into terminal blocks 34 arranged for electrical connection in the circuit l1, and insulated in any suitable manner from the mechanical connections for the cooling fluid supply and discharge. The blocks 34 have laterally extending conduits 35 to each of which is secured one element of a pipe union 30. In order to remove the coil 28 from the machine, the electrical connections from the terminal blocks 34 are disconnected, and the pipe unions 36 backed off, which leaves the coil free for renewal.

Fig. 4 illustrates another form of coil which is interchangeable with coil 28 previously described to accommodate articles which are of rectangular shape instead of circular. This coil is in the form of a rectangular loop 31 designed to receive within its field an article of corresponding shape. The terminal blocks 34 of this coil are provided with pipe union elements 38 for engagement with the union elements 36 shown in Fig. 2.

It will be appreciated that the coils shown in Figs. 2, 3 and 4 are merely illustrative of two of the many forms which these induction heating elements may assume, depending upon the size and shape of the article or articles which are to be treated, or upon other variables. The terms and expressions which I have employed are used in a descriptive and not a limiting sense, and I have no intention of excluding such equivalents of the invention described, or of portions thereof, as fall within the purview of the claims.

I claim:

1. Induction apparatus for the heat treatment of metals and metal articles subject to improvement upon hardening treatment and the like for surface-hardening metallic articles, comprising a condenser, means for charging the condenser, an induction element constructed and arranged to accommodate within its field an article to be treated, and a variable inductance element, said induction and inductance elements being connected in series to the condenser through a gaseous electronic discharge means capable of passing current in both directions to provide a single discharge in an oscillatory circuit of variable frequency for regulation of the depth to which heat treating is effective, the characteristics of the condenser circuit being such that the discharge takes place at a sufficiently high frequency to restrict the induced currents to the skin region of the article being heated, while the complete discharge occurs over so small a fraction of a second as to restrict efiectively the amount of heat flowing from the skin region to the interior of the article, whereby upon cessation of said discharge a steep temperature gradient exists between the skin and interior regions and rapid self-quenching begins immediately upon the termination of the discharge.

2. Induction apparatus as defined in claim 1 wherein the magnitudes of the elements of the condenser discharge circuit are such that the time of discharge is about one-thousandth of a second.

3. Induction apparatus as defined in claim 1 wherein the magnitudes of the elements of the condenser discharge circuit are such that the frequency is at least 8000 cycles per second.

ALFRED VANG.

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

UNITED STATES PATENTS Number Name Date 1,168,346 Thomson Jan. 18, 1916 1,171,832 Bishop Feb. 15, 1916 1,377,574 Frary 1 May 10, 1921 1,378,187 Northrup May 1'7, 1921 2,024,906 Bennett Dec. 17, 1935 2,175,607 Hinkead Oct. 10, 1939 2,178,201 Dake Oct. 31, 1939 2,179,105 Sidney Nov. 7, 1939 2,206,839 George July 2, 1940 2,236,810 Watson Apr. 1, 1941 2,287,540 Vang June 23, 1942 2,287,542 Vang June 23, 1942 2,288,033 Somes June 30, 1942 FOREIGN PATENTS Number Country Date 467,308 Great Britain June 15, 1937 OTHER REFERENCES Babat: High voltage condenser welding, The Welding Journal, August, 1935. Pages 6, 7 and 8.

Somes: The development of high speed induction heating for the hardening of internal diameters, Transactions of the Electrochemical Society, vol. 79, 1941, pages 56 and 58.

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