US3806690A - Case hardening arrangement utilizing high q tuned circuit - Google Patents

Case hardening arrangement utilizing high q tuned circuit Download PDF

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US3806690A
US3806690A US00300287A US30028772A US3806690A US 3806690 A US3806690 A US 3806690A US 00300287 A US00300287 A US 00300287A US 30028772 A US30028772 A US 30028772A US 3806690 A US3806690 A US 3806690A
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arrangement
set forth
high frequency
frequency oscillator
heating
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F Frungel
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Impulsphysik GmbH
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F Frungel
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/02—Induction heating
    • H05B6/36—Coil arrangements
    • H05B6/42—Cooling of coils
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation
    • C21D1/10—Surface hardening by direct application of electrical or wave energy; by particle radiation by electric induction
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

Definitions

  • Striker 5 7 ABSTRACT Heating pulses are applied to body to be case hardened through a heating inductor coupled to a high Q tuned circuit through a low resistance, low inductance coupling loop. Tuned circuit is pulsed by high energy pulse. Short fall time of pulse causes increased selfcooling.
  • a photoelectric arrangement indicates when [56] References Cited body is properly positioned relative to heating coil and UNITED STAT S PATENTS initiates heating pulse.
  • the present invention relates to methods and arrangements for creating very hard surfaces having an extremely finely grained structure on metal bodies and particularly on steel.
  • the so-created surfaces are highly corrosion resistant. More particularly, it relates to methods and arrangements for case hardening steel structures or bodies by means of applied pulsed high frequency oscillations, wherein the pulses have a very steep trailing edge and the pulse duration is generally less than 0.1 second. As criteria for the steep, trailing edge a fall time of less than 1 percent of the pulse duration has proved to yield proper results.
  • the present invention constitutes an improvement of the known method and arrangement which improves said efficiency and results in an improved product of having increased hardness.
  • the present invention comprises an arrangement for case hardening of a metallic body.
  • the present arrangement comprises high frequency oscillator means having a high Q tuned circuit means and low internal impedence amplifier means connected to said high Q tuned circuit means. It comprises pulsing means for pulsing said high frequency oscillator means and terminating means for terminating said pulsing of said high frequency oscillator means after a predetermined heating time interval. Further comprised are heating inductor means forapplying heat to a predetermined portion of the surface of said body. Low resistance, low inductance coupling loop means couple said high frequency oscillator means to said heating inductor means.
  • the heating time interval is less than 0.1 second.
  • the pulse of less than 0.1 second created by said pulsing and terminating means has a F all time less than 1 percent of the pulse width. This results in a high utilization of the self-cooling effect in carbon steels.
  • FIG. 1 shows a circuit diagram of an induction heater in accordance with the present invention
  • FIG. 2 shows wave forms at selected points of the circuit of FIG. 1;
  • FIG. 3 shows an arrangement for initiating the pulses in the circuit of FIG. 1;
  • FIG. 4 shows the construction of a heating inductor suitable for use in the circuit of FIG. 1;
  • FIG. 5 shows a heating inductor partially surrounded by ferrite material
  • FIG. 6 shows an arrangement for terminating the pulses generated in the arrangement of FIG. I.
  • FIG. 7 shows a curve of heating current as a function of time using the arrangement of FIGS. 1 and 6.
  • FIG. 1 shows a circuit in accordance with the present invention.
  • a known high freuqency oscillator with a frequency for example in the region between 20 and 40 MHz is used.
  • the Q of the oscillator must be much higher than that used in conventional generators employed, for example, in the welding of plastic sheeting.
  • the Q of the circuit should be at least and may for example be in the range of between 100 and 300.
  • the high Q was found to be necessary afterexperimentation with conventional high frequency oscillators showed surprisingly poor results when used for the case hardening of a body in accordance with the present invention. This may'be explained as follows: the inductivity of the heating inductor is only of the order of several nH.
  • the present induction hardening apparatus must be of extremely large size and is commercially unfeasible.
  • Use of a tuned circuit with a very high Q, for example a Q of 100 and an oscillator power of IOkW can result in a l MVA oscillation in the tuned circuit.
  • a small coupling loop is introduced into the oscillating cavity of the high Q circuit,and the actual heating inductance is connected in parallel to this coupling loop.
  • a tuning capacitor of high capacity is connected in parallel to the two inductors in-order that an exact resonant frequency corresponding to the resonant frequency of the main oscillating circuit can be achieved.
  • Reference numeral 1 in FIG. I indicates a cavity resonator having a very high O.
  • the cavity may have a volume of one quarter cubic millimeter and may be constructed from copper sheeting with possibly some silver plating.
  • the cavity is tuned by means of an air condenser 2 to a convenient frequency, for example 27.l2 MHz.
  • the cavity resonator is supplied by an oscillator tube 3 herein also referred to as amplifier means in conventional fashion, the anode of tube 3 being connected to the circuit via a coupling capacitor 4. Feedback is achieved by means of a small loop 5, a conventional grid capacitor 6 and grid resistance 7.
  • a special differential pulse transformer may be used.
  • the primary winding 10, the secondary winding 9 and the iron core 11 are comprised in this pulse transformer.
  • a small, low inductance capacitor. 12 serves to short-circuit the high frequencies.
  • FIG. I Also shown in FIG. I is the source of electrical energy 15, and connected in parallel thereto a high capacitance capacitor 13 which is connected in series with the primary winding of the pulse transformer and also with electronic switch means (first switch means) 14.
  • Switch means 14 may be a large thyristor, an ignitron or even a conventional switch.
  • capacitor 14 discharges through primary winding 10, causing a current as shown in curve a of FIG. 2 to flow through primary winding 10.
  • This current in turn causes a voltage proportional to the first derivative thereof to appear across secondary winding 9.
  • This voltage is shown in curve b in FIG. 2. Since the rate of increase of current is substantially linear, a substantially rectangular induced voltage pulse b results. It thus becomes possible to operate with very high operating voltages, for example, l0,000 volts at the anode of tube 3. For such high operating voltages no semiconductor switching means such as thyristors are available at the current state of the art.
  • the circuit location in the primary circuit of the transformer as is shown for element 14 is very desirable.
  • the thyristor does not undergo potential differences of more than one kV, but is subjected to very high currents.
  • the thyristor need only have an operating voltage of 1,000 volts, while capacitor 13 under these conditions has 100 times the capacity but is charged to only l/lO of the voltage of capacitor 12.
  • the charging of capacitor '13 from the stabilized voltage source 15 must take place sufficiently rapidly so that the capacitor is charged in time for the next subsequent heating pulse. Since the so-required rate is approximately pulses per second, such circuits are well known and their constructions present no difficulty.
  • the oscillations generated in the tuned circuit 12 are supplied to the heating inductor with a high efficiency.
  • the coupling loop 16, which has a low resistance and a low inductivity, is inserted into the resonant cavity 1.
  • the Q of the tuned circuit and the geometry of the insertion of the coil are interrelated.
  • Heating inductor 17 is connected in parallel with coupling loop 17 and positioned very closely to the surface of the body.
  • the circuit including the heating inductor is tuned via a capacitor 18, which may for example be a ceramic capacitor, to approximately the oscillating frequency of the high frequency oscillator.
  • the exact frequency adjustment is carried out by means of a trimming capacitor 19 connected in parallel with capacitor 18.
  • Capacitor 19 may be a parallel plate air capacitor. Under these conditions reactive powers of up to l MVA or more can appear in the tuned circuit comprising elements 16, 17 and 18. Under such high reactive powers, voltage in the order of kilovolts can appear across the heating inductor loop 17 which may under some conditions comprise only a single small straight section of wire or a single small loop.
  • plain tap water has a very high breakdown characteristic for the short pulses employed in this invention.
  • the breakdown strength is approximately 200 killivolt per centimeter.
  • the surface of the body can be insulated relative to inductor 17 by use of streaming water and, in particular, plain tap water.
  • isolating foil 21 is not required.
  • the insulating effect of water is approximately the same as could be obtained by the use of air at a pressure of approximately 7 atmospheres between the body 20 and the inductor 17.
  • the cooling effect of the water is here a secondary effect, the primary purpose of the water being the insulation.
  • a cooling medium other than water can be used.
  • methanol which has strongly reducing characteristics and leaves a smooth metallic surface after hardening may be employed.
  • propane, hydrogen or methane as cooling medium applied in the direction of arrow 22 will also prevent tarnishing of the surface. It must be stressed that in no case can the self-cooling due to the thermoconductivity of the object itself be dispensed with.
  • carbon steel has a heat conductivity which is at least I000 times that of a good cooling gas.
  • the cooling by water or oil or methanol can further result in a slight product improvement when layers of 0.2 millimeter thickness are to be hardened.
  • pulses having a relatively long pulse width of approximately 10 millisecends are used.
  • the self-cooling effect is stronger for the layers which are more internal to the body, while the very external layers of the heated surface remain hot longer since the heated layers underneath prevent the rapid conduction of heat from the outer surface towards the interior of the object.
  • the outer layers are actually less hardened than the inner layers, so that the additional cooling of the outer surfaces under moving air, gas or liquid will aid in the hardening of these outer surfaces.
  • a uniform hardness can be reached for a thickness of from 0.2 to 0.3 millimeters.
  • hardnesses 950 kp/mm over a whole hardened section can be achieved in the production of band saws from carbon steel.
  • the method of the present invention does not require the subsequent heating that is usual in spontaneous quenching processes to prevent embrittlement, since the so-manufactured layers are very elastic and not subject to failure by brittleness. Thus, further thermal treatment is not required.
  • the case hardened structures manufactured in accordance with the present invention are so stable that microscopic observation shows that they decompose into the martensitic base material only after an application time of approximately six hours and at a temperature of 450C.
  • Reference numeral 30 in that Figure refers to a neon-helium laser of the conventional type which sends a fine beam 31 through the body 32 which may, for example, be the toothed profile of a saw.
  • the beam impinges upon a photodiode 33 j which transforms it into an electrical signal which in per second can be used and an average anode loss of only 2 kW results.
  • the peak anode voltages may range between 10,000 and 15,000 volts and the reactive power in the tuned circuit may reach 5 MVA.
  • FIG. 1 Further shown in FIG. 1 is a special circuit arrangement for permitting the initial tuning of the circuit.
  • a switch 24 is shown whereby the anode of tube 3 may be connected directly to the output of the relatively low voltage source through a resistance 25. Under such conditions the tuned circuit only contains about one one-hundredth of its normal power.
  • a small antenna 26 is positioned near the tuned circuit comprising elements 16, 19 and 17. This antenna may, for example, comprise a small length of wire 26 connected to a glow lamp 27 and having at the other side of the glow lamp an additional length of wire as a counterweight. The circuit is then tuned for maximum brightness of the glow lamp, thereby assuring that it is properly tuned when full power is applied.
  • the resonant frequency at the heating inductor is trimmed in such a way that the full resonance is only available when the Curie temperature and higher temperatures are reached on the surface of the body.
  • steel has a high permeability below the Curie temperature, while having only a permeability comparable to air above the Curie temperature.
  • the tuning described above may be carried out by using a body of copper or other substance having a low ferromagnetic permeability. In this way, the full power of the heating pulse is not applied to the body at the beginning of the pulse, that is in the first few microseconds thereof.
  • pulses having a longer width for example, 50 millisec onds, may be employed and the conductivity of the body causes a corresponding deeper layer for example 0.2 millimeters from the surface, to be heated also.
  • FIG. 4 shows a particular preferred embodiment of the heating inductor.
  • the inductor itself in this particular embodiment is a hollow tube with a very thin opening 41 which may have substantially conical terminations 42 which in turn are adjacent to the jaws of the contact clamp 43.
  • a high pressure pipe 44. which is situated within the jaws 43 can be used for supplying the high pressure water required for cooling.
  • Clamp 43 may comprise two flat conducting portions which are rigidly clamped together and whose inner surfaces 19a and 19b are insulated from each other by a thin dielectric 40.
  • the contact clamp can be so designed that capacities of an order of magnitude of pF are realized so that capacitors 19 or 18 may be smaller capacitors than would otherwise be required. Because of conical portions 42 the inductors, if damaged or destroyed, may be easily interchanged.
  • the water under high pressure flows in the direction of the arrow 45 through the bore 41 and flows out in the direction of the arrow 46.
  • Either distilled water or tap water which is free of lime may be used.
  • the pressure to be used here should be more than the pressure of three atmospheres, and preferably as high as 40 atmospheres, while the pipe used for inductor 17 may be very thin, as for example 2/10 mm.
  • a good material for the manufacture of the inductors is silver piping. Surprisingly, drawn steel tubing can also be used if it is copper plated and if possible also silver plated.
  • the higher the velocity of the cooling medium the higher the pulse rate that may be used and correspondingly, the higher the thermal requirements set for inductor 17.
  • the inductor 17 may be surrounded in part by a ferrite material 47 as shown in FIG. 5.
  • a ferrite material 47 Commercial high frequency ferrites are available which have permeabilities of the order of magnitude of 600 to 1500 and, therefore, represent a substantial magnetic shortcircuit of the air space which lies outside of the body.
  • the body 20 can be heated with very high field strength and at very low losses even through insulating foil 21. If the correct type of magnetic material 47 is chosen, a decrease in permeability for increasing currents in inductor l7, and a corresponding decrease of inductivity can be achieved, while for decreasing currents in inductor 17 the permeability can increase.
  • the circuit of FIG. 6 may be used.
  • the body to be casehardened hereis a saw having reference numeral 20.
  • Inductor I7 is to harden a predetermined portion of the surface of the sawteeth.
  • the loops of the inductor may enclose the teeth of the saw from each side, one loop on each side.
  • the arrangement of FIG. 6 is used to terminate the pulse.
  • the circuit is adjusted in such a manner that the pulse is merely initiated by the circuit of FIG. 3 and has a pulse width which is arbitrarily chosen to be longer than the maximum desired pulse width.
  • the pulse is to be terminated by electronic terminating means which, for example, may comprise means connected in parallel with the oscillator circuit for shortcircuiting the latter when the desired temperature is reached, thereby preventing any energy from reaching inductor 17.
  • the switch means (herein referred to as second switch means) may, for example, be an ignitron or a high voltage thyratron.
  • the heat waves which emanate from the body 20 are focused onto a photodiode 54 via a lens 52 and a filter 53.
  • the photocell 54 is connected in series with a resistance 56 and a battery 55 in the conventional fashion.
  • the pulses resulting when the sofocused heat waves strike photodiode 54 are amplified by an amplifier 57 which may for example be a threshold amplifier which furnishes a pulse on line 58 to the control grid of a thyratron 59 (FIG. 1), whenever the voltage at its input exceeds a predetermined threshold voltage.
  • thyratron 59 has an anode connected to the anode of the oscillator tube 3 via a choke 8 and a cathode connected to ground.
  • the pulse is thus actually shortcircuited by thyratron 59 prior to the natural termination thereof.
  • a hydrogen thyratron which has a current carrying capacity of more than 300 amps and can withstand peak voltages of to kV, it can readily be accomplished that the remaining current of the pulse is completely absorbed by the thyratron instead of the oscillator tube.
  • Filter 53 may, for example, be a light blue filter which emits light only after the predetermined portion of the surface of the body has reached a temperature of more tha 1200C.
  • the heating pulse is again shown in FIG. 7.
  • the pulse starts, for example, initiated by the circuit of FIG. 3 in conjunction with thyristor 14.
  • the pulse would normally have a trailing edge as pictured in curve b.
  • thyratron 59 is ignited at time c, so that the pulse decreases to zero vary rapidly at time c which of course preceeds the time at which it would normally collapse.
  • delay means following amplifier 57 so that thyratron 59 is not fired until a time d following the reception of the heat pulse at time c. In this case, a selected amount of time can elapse, so that the body (workpiece) is heated to a greater depth than would be the case if the pulse collapsed immediately at the time that the outermost surface reached the desired temperature.
  • Arrangement for casehardening of a metallic body comprising, in combination, high frequency oscillator means having a high Q tuned circuit means and low internal impedance amplifier means connected to said tuned circuit means; pulsing means for applying a substantially rectangular pulse to said high frequency oscillator means over a determined heating interval, said pulsing means comprising first switch means for initiating said pulse and second switch means for terminating said pulse; heating inductor means for applying heat to a predetermined portion of the surface of said body; and low resistance, low inductance coupling loop means for coupling said heating inductor means to sai high frequency oscillator means.
  • said predetermined heating interval is a time interval of less than 0.l second.
  • Arrangement for casehardening of a metallic body comprising, in combination, high frequency oscillator means having a high Q tuned circuit means including cavity resonator means and low internal impedance amplifier means connected to said tuned circuit means; pulsing means connected to said high frequency oscillator means for initiating the oscillations thereof and terminating said oscillations after a determined heating interval of less than 0.1 seconds; heating inductor means for applying heat to a predetermined portion of the surface of said body; and low resistance, low inductance coupling loop means extending into the cavity of said cavity resinator means for coupling said heating inductor means to said high frequency oscillator means.
  • said cavity resonator means comprise copper sheeting enclosing a volume of approximately one quarter cubic meter.
  • tuning capacitor means comprise variable capacitor means.
  • variable capacitor means comprise parallel plate air capacitor means.
  • Arrangement for casehardening of a metallic body comprising, in combination, high frequency oscillator means having a high Q tuned circuit means and low internal impedance amplifier means connected to said tuned circuit means; pulsing means coupled to said high frequency oscillator means for initiating the oscillations thereof and for terminating said oscillations after a determined heating interval, said pulsing means comprising electronic switch means having a control electrode and having a conductive state in response to a control signal at said control electrode, connected to said high frequency oscillator means in such a manner that said switch means shunt said high frequency oscillator means when in a conductive state, thereby terminating said oscillations; heating inductor means for applying heat to a predetermined portion of the surface of said body; low resistance, low inductance coupling loop means for coupling said heating inductor means to said high frequency oscillator means; and control signal furnishing means for furnishing said control signal to said switch means at the end of said heating interval.
  • control signal furnishing means comprise temperature measuring means measuring the temperature of said predetermined portion of said surface of said body and furnishing said control signal when said someasured temperature is a predetermined temperature.
  • Arrangement for casehardening of a metallic body comprising, in combination, high frequency oscillator means having a high Q tuned circuit means and low internal impedance amplifier means connected to said tuned circuit means; pulsing means coupled to said high frequency oscillator means for initiating the oscillations thereof and for terminating said oscillations after a determined heating interval; heating inductor means for applying heat to a predetermined portion of the surface of said body; low resistance, low inductance coupling loop means coupling said heating inductor means to said high frequency oscillator means; and ferrite material partially enclosing said heating inductor means, said ferrite material having a permeability exceeding and low high frequency losses at frequencies higher than l5MHz.
  • Arrangement for casehardening of a metallic body comprising, in combination, high frequency oscillator means having a high Q tuned circuit means and low internal impedance amplifier means connected to said tuned circuit means; pulsing means coupled to said high frequency oscillator means for initiating the oscillations thereof and for terminating said oscillations after a determined heating interval; heating inductor means for applying heat to a predetermined portion of the surface of said body, said heating inductor means comprising thin, hollow metal pipe means having a first and second conical termination; low resistance low inductance coupling loop means coupling said heating inductor means to said high frequency oscillator means; contact clamp means having conical jaws for receiving said conical terminations of said metal pipe means; and means for circulating high pressure liquid cooling means through said contact clamp means and said metal pipe means.
  • said contact clamp means comprise first and second contact clamp means respectively receiving said first and second conical termination, said first and second contact clamp means each having an inner surface closely spaced to the corresponding inner surface to the other of said contact clamp means; further comprising insulator plate means positioned between said sofacing inner surfaces, for effecting electrical insulation therebetween.
  • Arrangement for casehardening of a metallic body comprising, in combination, high frequency oscillator means having a high Q tuned circuit means and low internal impedance amplifier means connected to said tuned circuit means, said tuned circuit means comprising cavity resonator means; pulsing means coupled to said high frequency oscillator means for initiating the oscillations thereof and for terminating said oscillations after a determined heating interval; heating inductor means for applying heat to a predetermined por- 12 tion of the surface of said body; and low resistance low inductance coupling loop means for coupling said heating inductor means to said high frequency oscillator means, said coupling loop means extending into the cavity of said cavity resonator means.
  • Arrangement for casehardening of a metallic body comprising, in combination, high frequency 0scillator means having a high Q tunedcircuit means and low internal impedance amplifier means connected to said tuned circuit means; pulsing means coupled to said high frequency oscillator means for initiating the oscillations thereof and for terminating said oscillations after a determined heating interval, said pulsing means comprising energy storage means, pulse transformer means having a secondary winding connected to said high frequency oscillator means and a primary winding,
  • first switch means for connecting said primary winding of said pulse transformer means to said energy storage means when in a conductive state; heating conductor means for applying heat to a predetermined portion of the surface of said body; and low resistance low inductance coupling loop means for coupling said heating inductor means to said high frequency oscillator means.
  • photoelectric means comprise a light source; and photoelectric transducing means receiving light from said light source intermittently in dependence on the position of said body relative to said heating inductor means.

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  • Chemical & Material Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
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  • Crystallography & Structural Chemistry (AREA)
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US00300287A 1972-02-12 1972-10-24 Case hardening arrangement utilizing high q tuned circuit Expired - Lifetime US3806690A (en)

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DE2206816A DE2206816C3 (de) 1972-02-12 1972-02-12 Vorrichtung zum Erzeugen extrem feinkörniger metallischer Oberflächengefüge

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Cited By (4)

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US4109127A (en) * 1973-07-25 1978-08-22 Frank Frungel Apparatus and method for case hardening steel tools by application of heating pulses
US4859940A (en) * 1987-09-09 1989-08-22 Westinghouse Electric Corp. Apparatus for detecting onset of slag entrainment in a molten metal stream
US5444221A (en) * 1993-03-31 1995-08-22 Honda Giken Kogyo Kabushiki Kaisha High-frequency induction heating apparatus for rocker arms
US20090250440A1 (en) * 2008-04-04 2009-10-08 Yap Tze-Yee Ryan Out-of-phase electrical welder and process

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DE102009058232A1 (de) * 2009-12-14 2011-06-16 Mazac, Karel, Prof. Dr.-Ing. Verfahren und Vorrichtung zur ortsgebundenen Gefügeumwandlung an elektrisch, leitenden, flächigen oder längsgezogenen Strukturen hinsichtlich Festigkeitserhöhung

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US3375468A (en) * 1964-08-12 1968-03-26 Park Ohio Industries Inc Control device for an industrial heating oscillator
US3398444A (en) * 1966-01-18 1968-08-27 Trw Inc Hard tooth gears and method of making
US3403241A (en) * 1966-08-22 1968-09-24 Ohio Crankshaft Co Induction heating element
US3622138A (en) * 1969-09-16 1971-11-23 Park Ohio Industries Inc Control device for induction heating
US3637970A (en) * 1970-07-06 1972-01-25 Ronald J Cunningham Induction heating apparatus

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4109127A (en) * 1973-07-25 1978-08-22 Frank Frungel Apparatus and method for case hardening steel tools by application of heating pulses
US4859940A (en) * 1987-09-09 1989-08-22 Westinghouse Electric Corp. Apparatus for detecting onset of slag entrainment in a molten metal stream
US5444221A (en) * 1993-03-31 1995-08-22 Honda Giken Kogyo Kabushiki Kaisha High-frequency induction heating apparatus for rocker arms
US5528019A (en) * 1993-03-31 1996-06-18 Honda Giken Kogyo Kabushiki Kaisha High frequency induction heating method for rocker arms
US20090250440A1 (en) * 2008-04-04 2009-10-08 Yap Tze-Yee Ryan Out-of-phase electrical welder and process

Also Published As

Publication number Publication date
CH568394A5 (pl) 1975-10-31
DE2206816B2 (de) 1977-06-23
CA954193A (en) 1974-09-03
JPS4889811A (pl) 1973-11-24
DE2206816A1 (de) 1973-08-16
DE2206816C3 (de) 1981-12-10
JPS5517473B2 (pl) 1980-05-12

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