Welding equipment for metallic materials and method for welding metallic materials

Abstract

A welding equipment for metallic materials capable of performing heat treatment such as tempering based on partial heating in spot welding is provided. The welding equipment sandwiches metallic materials with a pair of electrodes, and heats different regions of the metallic materials by energization, with the pair of electrodes maintained at the same position with respect to the metallic materials. The welding equipment includes a first heating means connected to the pair of electrodes for heating and welding the internal region of the circle defined by projecting the cross-sectional area of the axis of the electrodes on the metallic materials by applying power having a low first frequency, a second heating means for heating a ring-shaped region along the circle by applying power having a second frequency that is higher than the first frequency, and an energization control unit for independently controlling the first and the second heating means.

Claims

1. A method for spot-welding metallic materials by sandwiching the metallic materials with respective tips of a pair of electrodes opposing to each other and energizing the metallic materials, wherein: a spot-welding power source is connected to the pair of electrodes for supplying a spot-welding power to the pair of electrodes to heat and spot-weld a prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes, the spot-welding power being either AC electric power having a first frequency or DC electric power, and a high-frequency power source is connected to the pair of electrodes for supplying AC electric power having a second frequency to the pair of electrodes to heat a region of the metallic materials different from the prescribed region, the method comprising: a first step for heating and spot-welding the prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes by a first energization applying, via said spot-welding power source, the AC electric power having the first frequency or the DC electric power to the pair of electrodes; and a second step for heating said region different from the prescribed region in the first step by a second energization applying, via the high-frequency power source, the AC electric power having the second frequency to the pair of electrodes that are maintained at the same position as in the first step, wherein the spot-welding power source and the high-frequency power source are respectively connected to the pair of electrodes in parallel, wherein the first frequency is lower than the second frequency, and wherein the heating time in the first step and that in the second step are independently controlled.

2. The spot-welding method for metallic materials according to claim 1, wherein the inside of the prescribed region of the metallic materials is heated by the AC electric power having the first frequency or the DC electric power, a proximity of the prescribed region of the metallic materials is heated by the AC electric power having the second frequency, and the heating by the spot-welding power source in the first step and that by the high-frequency power source in the second step are independently controlled.

3. The spot-welding method for metallic materials according to claim 2, wherein the prescribed region heated by the AC electric power having the first frequency or the DC electric power is an internal area of a circle defined by projecting contacting area of the opposing tips of the pair of electrodes on the metallic materials, and the different region heated by the AC electric power having the second frequency is a ring-shaped area along a circumference of said prescribed region of the metallic materials.

4. The spot-welding method for metallic materials according to claim 3, wherein the ring-shaped are undergoes either resistance heating induction heating or both resistance heating and induction heating by the AC electric power having the second frequency.

5. A method for spot-welding metallic materials using a pair of electrodes having respective tips opposing to each other, wherein: a spot-welding power source is connected to the pair of electrodes for supplying a spot-welding power to the pair of electrodes to heat and spot-weld a prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes, the spot-welding power being either AC electric power having a first frequency or DC electric power, and a high-frequency power source is connected to the pair of electrodes for supplying AC electric power having a second frequency to the pair of electrodes, the method comprising: a first step for sandwiching the metallic materials with the tips of the pair of electrodes; a second step for heating and spot-welding the prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes by applying, via said spot-welding power source, the AC electric power having the first frequency or the DC electric power to the pair of electrodes; and a third step for heating the welded prescribed region of the metallic materials or a region different from the prescribed region by applying, via the high-frequency power source, the AC electric power having the second frequency to the pair of electrodes, wherein the spot-welding power source and the high-frequency power source are respectively connected to the pair of electrodes in parallel, wherein the first frequency is lower than the second frequency, and wherein the heating time in the second step and that in the third step are independently controlled.

6. A method for spot-welding metallic materials using a pair of electrodes having respective tips opposing to each other, wherein a spot-welding power source is connected to the pair of electrodes for supplying a spot-welding power to the pair of electrodes to heat and spot-weld a prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes, the spot-welding power being either AC electric power having a first frequency or DC electric power, and a high-frequency power source is connected to the pair of electrodes for supplying AC electric power having a second frequency to the pair of electrodes, the method comprising; sandwiching the metallic materials with the tips of the pair of electrodes; a welding step for heating and spot-welding the prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes by applying, via said spot-welding power source, the AC electric power having the first frequency or the DC electric power to the pair of electrodes; and a heat-treating step for heating the welded prescribed region of the metallic materials or a region different from the prescribed region by applying, via the high-frequency power source, the AC electric power having the second frequency to the pair of electrodes, wherein the spot-welding power source and the high-frequency power source are respectively connected to the pair of electrodes in parallel, wherein the first frequency is lower than the second frequency, wherein the heating time in the welding step and that in the heat treating step are independently controlled, and wherein a power of the AC electric power having the second frequency is controlled.

7. The method for spot-welding metallic materials according to claim 6, wherein before ending applying the AC electric power having the first frequency or the DC electric power in the welding step, applying the AC electric power having the second frequency to the pair of electrodes in the heat-treating step starts.

8. A method for spot-welding metallic materials using a pair of electrodes having respective tips opposing to each other, wherein: a spot-welding power source is connected to the pair of electrodes for supplying a spot-welding power to the pair of electrodes to heat and spot-weld a prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes, the spot-welding power being either AC electric power having a first frequency or DC electric power, and a high-frequency power source is connected to the pair of electrodes for supplying AC electric power having a second frequency to the pair of electrodes to heat a region of the metallic materials different from the prescribed region, the method comprising: sandwiching the metallic materials with the tips of the pair of electrodes; a welding step for heating the prescribed region of the metallic materials sandwiched by the tips of the pair of electrodes by applying, via said spot-welding power source, the AC electric power having the first frequency or the DC electric power to the pair of electrodes; and a heating step for heating a region different from the prescribed region of the metallic materials by applying, via the high-frequency power source, the AC electric power having the second frequency to the pair of electrodes, wherein the spot-welding power source and the high-frequency power source are respectively connected to the pair of electrodes in parallel, wherein the first frequency is lower than the second frequency, wherein a heating time in the welding step and that in the heating step are independently controlled, and wherein the welding step and the heating step at least partially overlap with each other such that the AC electric power having the second frequency in the heating step is superimposed to the AC electric power having the first frequency or the DC electric power applied to the pair of electrodes in the heating step by controlling the heating time and a power of the AC electric power having the second frequency in the heating step.

9. The spot-welding method for metallic materials according to claim 1, wherein an inductance for blocking current is connected between the spot-welding power source and the pair of electrodes, wherein a capacitor for blocking current is connected between the high-frequency power source and the pair of electrodes, wherein the inductance blocks high-frequency current supplied from the high-frequency power source to the pair of electrodes so as to prevent the high-frequency current from flowing into the spot-welding power source, and wherein the capacitor blocks current supplied from the spot-welding power source to the pair of electrodes so as to prevent the current from the spot-welding power source from flowing into the high-frequency power source.

10. The spot-welding method for metallic materials according to claim 5, wherein an inductance for blocking current is connected between the spot-welding power source and the pair of electrodes, wherein a capacitor for blocking current is connected between the high-frequency power source and the pair of electrodes, wherein the inductance blocks high-frequency current supplied from the high-frequency power source to the pair of electrodes so as to prevent the high-frequency current from flowing into the spot-welding power source, and wherein the capacitor blocks current supplied from the spot-welding power source to the pair of electrodes so as to prevent the current from the spot-welding power source from flowing into the high-frequency power source.

11. The spot-welding method for metallic materials according to claim 6, wherein an inductance for blocking current is connected between the spot-welding power source and the pair of electrodes, wherein a capacitor for blocking current is connected between the high-frequency power source and the pair of electrodes, wherein the inductance blocks high-frequency current supplied from the high-frequency power source to the pair of electrodes so as to prevent the high-frequency current from flowing into the spot-welding power source, and wherein the capacitor blocks current supplied from the spot-welding power source to the pair of electrodes so as to prevent the current from the spot-welding power source from flowing into the high-frequency power source.

12. The spot-welding method for metallic materials according to claim 8, wherein an inductance for blocking current is connected between the spot-welding power source and the pair of electrodes, wherein a capacitor for blocking current is connected between the high-frequency power source and the pair of electrodes, wherein the inductance blocks high-frequency current supplied from the high-frequency power source to the pair of electrodes so as to prevent the high-frequency current from flowing into the spot-welding power source, and wherein the capacitor blocks current supplied from the spot-welding power source to the pair of electrodes so as to prevent the current from the spot-welding power source from flowing into the high-frequency power source.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 illustrates a typical configuration of the welding equipment for metallic materials related to an embodiment of the present invention.

(2) FIG. 2 is an electric circuit diagram of the welding equipment for metallic materials shown in FIG. 1.

(3) FIG. 3 is an electric circuit diagram of modification 1 of the welding equipment for metallic materials.

(4) FIG. 4 is an electric circuit diagram of modification 2 of the welding equipment for metallic materials.

(5) FIG. 5A is a cross-sectional view illustrating a current distribution formed on two steel plates contacting each other when electric power is applied to the steel plates simultaneously from a low-frequency power source and a high-frequency power source. FIG. 5B is a cross-sectional view illustrating the heating status of three steel plates laid on top of one another by high-frequency current.

(6) FIGS. 6A-6F illustrate the heating status of steel plates.

(7) FIG. 7 shows heating waveforms obtained when spot welding and heating treatment are performed simultaneously using the power from a low-frequency power source and that from a high-frequency power source.

(8) FIG. 8 shows a heating waveform obtained when the power from a high-frequency power source is applied after the power from a low-frequency power source is applied.

(9) FIG. 9 shows a heating waveform obtained when preheating is performed using a high-frequency power source before the power from a low-frequency power source is applied.

(10) FIG. 10 shows a heating waveform obtained when preheating using a high-frequency power source, heating using a low-frequency power source, and post-heating using the high-frequency power source are performed consecutively.

(11) FIG. 11 shows a heating waveform obtained when preheating using a high-frequency power source and partial heating using the high-frequency power source and a low frequency power source are performed simultaneously.

(12) FIG. 12 shows a heating waveform obtained when heating using a low-frequency power source and post-heating using a high-frequency power source are performed, and furthermore simultaneous partial heating is performed using the low-frequency power source and the high-frequency power source.

(13) FIG. 13 is an electric circuit diagram of modification 3 of the welding equipment for metallic materials.

(14) FIG. 14 is an electric circuit diagram of modification 4 of the welding equipment for metallic materials.

(15) FIG. 15 shows heating waveforms obtained when simultaneous heating is performed using a DC power source and a high-frequency power source.

(16) FIG. 16 shows heating waveforms obtained when a high-frequency power source is used for post-heating.

(17) FIG. 17 shows heating waveforms obtained when a high-frequency power source is used for preheating.

(18) FIG. 18 shows heating waveforms obtained when preheating using a high-frequency power source, heating using a DC power source, and post-heating using the high-frequency power source are performed consecutively.

(19) FIG. 19 shows heating waveforms obtained when preheating is performed using a high-frequency power source, and partial and simultaneous heating are also performed using the high-frequency power source and a DC power source.

(20) FIG. 20 shows heating waveforms obtained when partial and simultaneous heating is performed using a high-frequency power source and a DC power source, and then post-heating is performed using the high-frequency power source.

(21) FIG. 21 is a chart illustrating the application of electric power from a low-frequency and high-frequency power supplies.

(22) FIG. 22 is a chart illustrating the hardness distribution on the surface of chromium molybdenum steel (SCM435) having undergone quenching in embodiment 4.

(23) FIG. 23 is a chart illustrating the hardness distribution on the surface of chromium molybdenum steel (SCM435) having undergone tempering in embodiment 5.

(24) FIG. 24 is a cross-sectional view illustrating the spot welding of steel plates.

(25) FIGS. 25A and 25B are plan views of the samples used for tensile test for measuring the strength of spot welding of high-tension steel plates. FIG. 25A is a superposed joint sample, and FIG. 25B is a cross joint sample.

REFERENCE SIGNS LIST

(26) 1, 25, 30, 35, 40: Welding equipment for metallic materials 1A, 25A, 30A, 35A, 40A: Welding circuit unit of welding equipment 1B, 25B, 30B, 35B, 40B: Welding unit of welding equipment 2: Gun arm 2A: Top portion of gun arm 2B: Top portion of gun arm 3: Electrode support 4: Electrode 5: Floating inductance 6: Low-frequency power source 7: Matching capacitor 8: High-frequency power source 9: Work 9A: Inside the circle 9B: Ring-shaped region 10: Energization control unit 11: Bypass capacitor 12: Commercial power source 13: Inductance for blocking high-frequency current 14: Low-frequency power control unit 16: Welding transformer 18: Oscillator 20: Matching transformer 22: High-frequency current 24: Low-frequency current 26: DC current 36: DC power source

DESCRIPTION OF EMBODIMENTS

(27) The embodiments of the present invention will hereafter be described by referring to the drawings.

(28) (Welding Equipment for Metallic Materials)

(29) FIG. 1 illustrates a typical configuration of welding equipment 1 for metallic materials according to the embodiment of the present invention. The welding equipment for metallic materials 1 includes an electrode arm 2, electrode supports 3 whose one end and the other end are respectively connected to the top portion 2A and the bottom portion 2B of the electrode arm 2, a pair of electrodes 4 connected to the other end of each of the electrode supports 3, a welding power source 6 connected to the electrode arm 2 via an inductance 5, a high-frequency power source 8 connected to the electrode arm 2 via a capacitor 7, and an energization control unit 10 for controlling each output of the welding power source 6 and the high-frequency power source 8.

(30) The welding equipment 1 for metallic materials further includes a fixed base for supporting the electrode arm 2, a drive mechanism for driving the electrode arm 2, and pressing mechanism for pushing out one of the electrodes 4 from the electrode supports 3 (none of them shown). The pressing mechanism is used when the metallic materials 9 to be welded, which will be described later, are energized with the electrodes 4, 4.

(31) The electrode arm 2 has a top portion 2A and a bottom portion 2B connected to the electrodes 4, 4 respectively via each electrode support 3. The electrode arm 2 is also called a gun arm. Since the gun arm 2 shown is in a shape of C, it is called C-type gun arm. In addition to C-type gun arm, X-type gun, etc. are used as well for portable- or robot-type welding equipment. The electrode arm 2 of any shape is applicable. The following description assumes C-type gun arm 2.

(32) The pair of electrodes 4, 4 are facing opposite to each other across a gap, into which two steel plates 9 are inserted as metallic materials 9. The electrodes 4 are made of copper, for example, and in a circular or elliptical shape or in a shape of a rod.

(33) FIG. 2 is the electric circuit diagram of the welding equipment 1 for metallic materials shown in FIG. 1. As shown in FIG. 2, the electric circuit of the welding equipment 1 for metallic materials includes a welding circuit 1A enclosed by dotted line and a welding unit 1B. The welding circuit unit 1A includes electric circuits such as welding power source 6, high-frequency power source 8, inductance 5, capacitor 7, and an energization control unit 10 for controlling the output from the welding power source 6 and the high-frequency power source 8. The welding unit 1B, or a circuit electrically connected to the welding circuit unit 1A, includes a gun arm 2, a pair of electrodes 4, 4 electrically connected to the gun arm 2, and metallic materials 9 sandwiched by the pair of electrodes 4, 4.

(34) The welding power source 6 is a low-frequency power source including a commercial power source 12 whose output frequency is 50 or 60 Hz, a low-frequency power source control unit 14 connected to one end of the commercial power source 12, and a welding transformer 16 connected to the other end of the commercial power source 12 and to the output end of the low-frequency power source control unit 14. Both ends of the secondary winding of the welding transformer 16 are connected to the left end of the top portion 2A, and to the left end of the bottom portion 2B, of the C-type gun arm 2 respectively. The low-frequency power source control unit 14 included of a power control semiconductor device, such as thyristor and gate drive circuit, controls the power supplied from the commercial power source 12 to the electrodes 4.

(35) A bypass capacitor 11 is connected to the secondary winding 16A, namely the side of the welding transformer 16 close to the gun arm 2, in parallel. The bypass capacitor 11 has low-capacitive impedance to the frequency of the high-frequency power source 8. Consequently, the high-frequency voltage applied from the high-frequency power source 8 to the secondary winding 16A can be minimized, and high-frequency inductive voltage to the primary side of the welding transformer 16 can be decreased.

(36) The high-frequency power source 8 includes an oscillator 18 and a matching transformer 20 connected to the output end of the oscillator 18. One end of the matching transformer 20 is connected to the top portion 2A of the C-type gun arm. The other end of the matching transformer 20 is connected to the bottom portion 2B of the C-type gun arm 2 via a capacitor 7. The capacitor 7 can also function as a matching capacitor of the DC resonance circuit, which will be described later. The capacity of the capacitor 7 depends on the oscillating frequency of the oscillator 18 and the floating inductance 5 of the C-type gun arm 2. The oscillator 18, which includes an inverter using various transistors, controls the power supplied from the high-frequency power source 8 to the electrodes 4.

(37) As shown in FIG. 2, the route from the C-type gun arm 2 connected to the secondary winding of the welding transformer 16 to the electrodes 4, 4 has inductances 5. As the inductance 5, a floating inductance formed in the C-type gun arm 2 can be used.

(38) If the capacitor 7 also functions as a matching capacitor, a DC resonance circuit may be configured with the matching capacitor 7 and the inductance 5.

(39) (Modification 1 of the Welding Equipment for Metallic Materials)

(40) FIG. 3 is an electric circuit diagram of modification 1 of the welding equipment 1 for metallic materials. With the electric circuit of the welding equipment 25 for metallic materials shown in FIG. 3, the high-frequency power source 8 is connected directly to a pair of electrodes 4, 4 not via a C-type gun arm 2, whereas with the electric circuit of the welding equipment 1 for metallic materials shown in FIG. 2, the high-frequency power source 8 is connected to the electrodes 4, 4 via the C-type gun arm 2. The high-frequency power source 8 may be connected to the base of the electrodes 4, 4 via the capacitor 7. Since other circuit configurations are the same as those shown in FIG. 2, description is omitted.

(41) (Modification 2 of the Welding Equipment for Metallic Materials)

(42) FIG. 4 is an electric circuit diagram of modification 2 of the welding equipment for metallic materials. The electric circuit of the welding equipment 30 for metallic materials shown in FIG. 4 differs from the welding equipment 1 for metallic materials shown in FIG. 2 in that a capacitor for parallel resonance 32 is connected between a pair of electrodes 4, 4 in parallel. Namely, the capacitor for parallel resonance 32 is connected to the top portion 2A and the bottom portion 2B of a C-type gun arm 2 in parallel. Consequently, the capacitor for parallel resonance 32 and the inductances 5 constitute a parallel resonance circuit. In this case, the capacitor 7 has the function of blocking the low-frequency current from the low-frequency power source 6. Since other circuit configurations are the same as those shown in FIG. 2, description is omitted.

(43) (Separation of Low-Frequency Power Source 6 and High-Frequency Power Source 8)

(44) The relation between the low-frequency power source 6 and the high-frequency power source 8 is described below.

(45) The inductances 5 and the capacitor 7 are connected between the low-frequency power source 6 and the high-frequency power source 8, and the inductive reactance X.sub.L (X.sub.L=2f.sub.LL, where f.sub.L is the frequency of the low-frequency power source 6, and L is the value of the inductance 5) of the inductance 5 (L) is small at low frequency. Meanwhile, the capacitive reactance X.sub.C (X.sub.C=1/(2f.sub.LC)) of capacitor 7 (C) is large at low frequency (f.sub.L). Consequently, leakage of current from the low-frequency power source 6 to the high-frequency power source 8 can be blocked with the large capacitive reactance X.sub.C of the capacitor 7 at low frequency (f.sub.L). Namely, the capacitor 7 functions as a capacitor for blocking low-frequency current.

(46) Of the impedances with the low-frequency power source 6 viewed from the high-frequency power source 8, the capacitive reactance X.sub.C (X.sub.L=1/(2f.sub.HC), where f.sub.L is the frequency of the high-frequency power source 8) is small at high frequency.

(47) Meanwhile, at high frequency, the inductive reactance X.sub.L (X.sub.L=2f.sub.HL, where f.sub.H is the frequency of the high-frequency power source 8) is large at high frequency. Consequently, the leakage of current from the high-frequency power source 8 to the low-frequency power source 6 is blocked by the large inductive reactance X.sub.L of the inductance 5 at high frequency (f.sub.H). Namely the inductance 5 functions as the inductance for blocking high-frequency current.

(48) In the welding equipment 1, 25, 30 for metallic materials, the capacitor 7 functions as a capacitor for blocking the flow of current from the low-frequency power source 6 to the high-frequency power source 8, and the inductance 5 functions as an inductance for blocking the flow of current from the high-frequency power source 8 to the low-frequency power source 6, namely functions as a choke coil.

(49) C-type gun arms 2 of various shapes are used depending on the size of steel plates 9 to be subject to spot welding. Therefore, if the floating inductance 5 of the C-type gun arm 2 is not large, an inductance 13 for blocking high-frequency current may be added so that a predetermined inductive reactance X.sub.L is obtained at high frequency in the welding equipment 1, 25, 30 for metallic materials. This external inductance 13 can be connected to the secondary winding of the welding transformer 16 on the side of the low-frequency power source 6, for example.

(50) The features of the welding equipment 1, 25, 30 for metallic materials according to the present invention include that the low-frequency power source 6 and the high-frequency power source 8 are separated from each other using the inductance 5 and the capacitor 7 and that power from the low-frequency power source 6 and the high-frequency power source 8, namely the power having two different frequencies, can be applied to the electrodes 4 simultaneously.

(51) (Current Distribution on Steel Plates)

(52) FIG. 5A is a cross-sectional view illustrating the current distribution seen on two steel plates 9 contacting each other when electric power is applied to the steel plates 9 simultaneously from a low-frequency power source 6 and a high-frequency power source 8. FIGS. 6A-6F illustrate the heated status of the steel plates 9.

(53) The solid line in FIG. 5A represents the flow of high-frequency current 22 from the high-frequency power source 8, and the broken line represents the flow of low-frequency current 24 from the low-frequency power source 6. The electrodes 4, which is made of copper, has the diameter of 6 mm, and the frequency of the low-frequency power source 6 is 50 Hz. Each steel plate is 2 mm thick, and the frequency of the high-frequency power source 8 is 40 kHz. The low-frequency current flows within the entire area of the electrodes 4, 4, and the steel plates 9 are energized within the width approximately the same as the cross-sectional area of the nugget.

(54) FIG. 6A is a plan view illustrating the region of the steel plates 9 heated by low-frequency current 24 only. The major heated region is the inside of the circle 9A defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9. FIG. 6B is a chart showing the temperature distribution seen along the line X-X in FIG. 6A. It is apparent that the inside of the circle 9A, namely the projection of the cross-sectional area of the axis of the electrodes 4, of the steel plates 9 are heated intensively.

(55) Meanwhile, the high-frequency current 22 flows mainly on the surface of the electrodes 4 and along the outer peripheral region of the nugget. The difference in distribution between the low-frequency current 24 and the high-frequency current 22 derives from so-called skin depth.

(56) FIG. 6C is a plan view illustrating the region of the steel plates 9 heated by high-frequency current 22 only. The major heated region is the outer periphery of the circle defined by projecting the cross-sectional area of the axis of the electrodes 4, and the proximity of the outer periphery of the circle, namely the outermost ring-shaped area 9B close to the circle. FIG. 6D is a chart illustrating the temperature distribution seen along the line X-X in FIG. 6C. It is apparent that the outer periphery of the circle, defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9, and the ring-shaped proximity 9B of the outer periphery undergo resistance heating. In this case, the region heated by high-frequency current 22 includes the area of the steel plates 9 close to the high-frequency current 22 flowing on the surface of the electrodes 4 and therefore subject to induction heating. This induction heating is different from general induction heating performed using an induction heating coil. Consequently, the circle defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9, and the ring-shaped region 9B in proximity the outer periphery can be heated by resistance heating by the high-frequency current 22, or by the above high-frequency induction heating superposed on the resistance heating.

(57) In FIG. 6D, by further changing the operating frequency of the high-frequency power source 8, the width of the ring-shaped region 9B can be changed. In actual spot welding performed by actually feeding low-frequency current 24, it was confirmed that with the change of the operating frequency of the high-frequency power source 8, the width of the high-temperature area in the outer peripheral region of the nugget also changed. Consequently, the circle defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9, and the ring-shaped region 9B in proximity to the periphery of the circle can be heated by resistance heating by the high-frequency current 22, or by the above high-frequency induction heating superposed on the resistance heating.

(58) In view of this, if electric power is applied to two steel plates 9 contacting each other from the low-frequency power source 6 and the high-frequency power source 8 simultaneously, the heated region on the steel plates 9 is the superposed region of the inside of the circle 9A, namely the region where the low-frequency current 24 flows, and the ring-shaped region 9B, namely the region where the high-frequency current 22 flows, as shown in FIG. 6E. Furthermore, as shown in FIG. 6F, the temperature distribution on the steel plates 9 obtained by feeding these currents 22, 24 is the superposition of the temperature distribution by the low-frequency current 24 (FIG. 6B) and that by the high-frequency current 22 (FIG. 6D). Namely, the inside of the circle 9A defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9, the periphery of the circle defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9, and the ring-shaped region 9B in proximity to the periphery of the circle of the steel plates 9 are heated.

(59) (Skin Depth)

(60) The skin depth () is expressed by formula (1) shown below:
=503.3(/(f)).sup.1/2 (m)(1)

(61) where, represents the resistivity of the material, represents the relative permeability of the material, and f represents frequency (Hz).

(62) Since the skin depth changes in proportion to the one-half power of the frequency, the lower the frequency, the thicker the skin depth, and the higher the frequency, the thinner the skin depth, on condition that the material is the same. Since the frequency of the power source used for spot welding is generally 50 Hz or 60 Hz, current is fed in the entire electrodes if the diameter of the electrodes is approximately 6 mm.

(63) Meanwhile, to heat the surface of the steel plates 9 only, the frequency of the high-frequency power source 8 can be set using the above formula (1) so that a predetermined skin depth can be obtained. Consequently, the heating width of the outer peripheral region of the nugget can be selected by setting frequency. Namely, by changing the frequency of the high-frequency current 22, the heating width of the outer peripheral region of the nugget can be changed, and the ring-shaped region 9B can undergo heat treatment such as tempering. Therefore, if relatively soft materials, such as S20C annealed material for example, are used as steel plates 9, the ring-shaped region 9B can be softened.

(64) The magnitude of the high-frequency current 22 at the skin depth of the material is expressed as 1/e (e: natural logarithm) of the value on the outermost surface, namely approximately . The skin depth of the steel plates 9 is approximately 9.3 mm when the frequency is 50 Hz, and 0.3 mm when the frequency is 40 kHz.

(65) (Selection of the Frequency of High-Frequency Power Source)

(66) The frequency of the high-frequency power source 8 is determined by the capacity of the inductance 5 connected to the secondary winding of the welding transformer 16, inductance 13 that is inserted as required, and matching capacitor 7. When the floating inductance of the gun arm 2 is used as inductance 5, the capacity of the inductance 5 is determined by the shape of the gun arm 2. As a result, the frequency is determined by the capacity of the matching capacitor 7. If the frequency increases, the heating width in the temperature increase pattern of the outer peripheral region becomes narrow and local due to skin effect. However, since the inductance 5 (L) of the gun arm 2 is proportional to the frequency, the voltage of the matching capacitor 7 also increases. The circuit with the electrodes 4, 4 viewed from the high-frequency power source 8 is a series resonance circuit. At the series resonance frequency, since the voltage of the inductance 5 and that of the matching capacitor 7 are identical, with the increase of the voltage of the matching capacitor 7, combination of dual frequencies, namely low and high frequencies, becomes difficult and an inductance 5 for blocking large current or inductance 13 must be provided. Since the inductances 5, 13 for blocking large current affect the low-frequency current 24, the secondary voltage of the conventional spot welding equipment must be increased significantly.

(67) On the contrary, if the series resonance frequency decreases, the heating width of the temperature increase pattern of the outer peripheral region of the nugget increases. However, since the voltage of the matching capacitor 7 decreases, combination of dual frequencies becomes easier. The gun arm 2 must be equipped with a welding transformer 16, bypass capacitor 11, and inductance 13 for blocking current as necessary. Of these, the weight of the welding transformer 16 is the heaviest, and it is inversely proportional to the frequency. In view of the above, the optimum operating frequency falls within 5 kHz to 40 kHz range, provided that a gun arm 2 is mounted to the welding equipment such as welding robot. It is desirable that the difference between the low frequency and the high frequency be 10 times higher than two-frequency synthesis circuit.

(68) (Heat Treatment Using Welding Equipment for Metallic Materials)

(69) Spot welding and heat treatment using the welding equipment 1, 25, 30 for metallic materials according to the present invention will hereafter be described.

(70) Metallic materials 9 are welded as follows: a pair of electrodes sandwiches the metallic materials 9 and power is applied to the materials to heat them. For example, it is sufficient that the spot welding has a first step of heating a predetermined region of metallic materials 9 by first energization to the pair of electrodes 4, 4, and a second step of heating a region different from that of the first step by second energization to the pair of electrodes 4, 4, with the pair of electrodes 4, 4 that sandwich the metallic materials 9 maintained at the same position. In this case, the heating time in the first step and that in the second step can be controlled independently from each other. If the first energization is performed with the low-frequency power source 6, the region of the metallic materials 9 heated by the first energization is the region within the circle 9A described above. If the second energization is performed with the high-frequency power source 8, the region of the metallic materials 9 heated by the first energization is the ring-shaped region 9B described above. The first and the second steps described above may be combined.

(71) FIGS. 7 to 9 are charts illustrating the waveform of the current flowing in the pair of electrodes 4, 4. In FIGS. 7 to 9, the horizontal axis represents time (arbitrary scale), and the vertical axis represents waveform (arbitrary scale) of the current 22, 24 fed from the high-frequency power source 8 and the low-frequency power source 6.

(72) FIG. 7 shows heating waveforms obtained when spot welding and heating treatment are performed simultaneously using the power from the low-frequency power source 6 and that from the high-frequency power source 8. As shown in FIG. 7, the entire nugget formed by welding is heated by the power from the low-frequency power source 6, and at the same time, the outer peripheral region of the nugget is also heated by the power from the high-frequency power source 8. The entire area of the nugget corresponds to the internal area 9A of the circle defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9. The outer peripheral region of the nugget corresponds to the circle defined by projecting the cross-sectional area of the axis of the electrodes 4 on the steel plates 9, and the ring-shaped region 9B in proximity to the periphery of the circle.

(73) The current distribution obtained when power is simultaneously applied from the low-frequency power source 6 and the high-frequency power source 8 indicates that according to the welding equipment 1, 25, 30 for metallic materials of the present invention, spot welding of steel plates 9 can be performed using the low-frequency power source 6, and that the surface of the two steel plates 9 surrounding but not contacting the electrodes 4 can be heated by the high-frequency power source 8.

(74) FIG. 8 shows a heating waveform obtained when the power from the low-frequency power source 6 is applied and then the power from the high-frequency power source 8 is applied. As shown in FIG. 8, when the power is applied from the low-frequency power source 6 first and then from the high-frequency power source 8 after the first power application is stopped, the steel plates 9 are spot-welded by the application of power from the low-frequency power source 6. By the power from the high-frequency power source 8, the region on the surface of the two steel plates 9, which is the outer peripheral region of the nugget and does not contact the electrodes 4, is heated.

(75) According to the welding equipment 1, 25, 30 of the present invention, by applying the power from the low-frequency power source 6 and then from the high-frequency power source 8, heating treatment (also called annealing) of the outer peripheral region of the nugget formed by spot welding can thus be performed. By adjusting the temperature and heating time, this treatment can be adopted to tempering, etc. of the steel plates 9 and those made of other materials.

(76) FIG. 9 shows a heating waveform obtained when preheating is performed using a high-frequency power source 8 before the power from a low-frequency power source 6 is applied. As shown in FIG. 9, if the power from the high-frequency power source 8 is applied and then the power from the low-frequency power source 6 is applied, the surface of the region of the steel plates 9 not spot-welded, namely the region in proximity to but not contacting the copper electrodes 4, is heated first. By applying the power from the low-frequency power source 6 after this preheating is performed, the two steel plates 9 are spot-welded.

(77) According to the welding equipment 1, 25, 30 for metallic materials of the present invention, by applying power from the high-frequency power source 8 and then from the low-frequency power source 6, the proximity of the region to be spot-welded can also be preheated before being welded. By adjusting the temperature and heating time of the preheating, hardening that may occur during spot welding can be prevented.

(78) The current distribution on two steel plates 9 contacting each other was described above. The current distribution on three or more steel plates 9 laid on top of one another will hereafter be described.

(79) FIG. 5B is a cross-sectional view illustrating the heating status of three steel plates 9 laid on top of one another by high-frequency current. As shown in FIG. 5B, when three steel plates 9 are laid on top of one another, ring-shaped regions B at two positions, edge portions C of the contacting faces of the steel plates 9 at two positions, namely ring-shaped regions at four positions in all, are heated by the high-frequency current 22.

(80) (Skin Depth)

(81) When power having low or high frequency is applied to steel plates 9, the skin depth changes in proportion to the minus one-half power of the frequency, Therefore, the lower the frequency, the thicker the skin depth, and the higher the frequency, the thinner the skin depth, on condition that the material is the same. Since the frequency of the power source used for spot welding is generally 50 Hz or 60 Hz, current is fed in the entire electrodes if the diameter of the electrodes is approximately 6 mm.

(82) Meanwhile, to heat the surface of the steel plates 9 only, the frequency of the high-frequency power source 28 can be set so that a given skin depth can be obtained. Consequently, the heating width of the outer peripheral region of the nugget can be selected by setting frequency. Namely, by changing the frequency of the high-frequency current 22, the heating width of the outer peripheral region of the nugget can be changed, allowing the ring-shaped regions B to be subject to heat treatment such as tempering to soften the ring-shaped regions 2B.

(83) The magnitude of the high-frequency current 22 at the depth of the skin depth of the material is expressed as 1/e (e: natural logarithm) of the value of the outermost surface, namely approximately . The skin depth of the steel plates 9 is approximately 9.3 mm when the frequency is 50 Hz, and 0.3 mm when the frequency is 40 kHz.

(84) (Modification of Heat Treatment Using the Welding Equipment for Metallic Materials)

(85) Yet another heating method by the welding equipment 1 for metallic materials will hereafter be described.

(86) FIGS. 10 to 12 show typical waveforms of the current fed to a pair of electrodes. The horizontal axis represents time (arbitrary scale), whereas the vertical axis represents the waveforms 22, 24 (arbitrary scale) of the voltage applied from the low-frequency power source 6 and the high-frequency power source 8 to the pair of electrodes.

(87) FIG. 10 shows a heating waveform obtained when preheating using a high-frequency power source 8, heating using a low-frequency power source 6, and post-heating using the high-frequency power source 8 are performed consecutively. The term post-heating is defined as the heating to be performed after preheating. Namely, post-heating is the heat treatment to be performed after the spot welding of the steel plates 9 is performed using the low-frequency power source 6.

(88) If power is applied from the high-frequency power source 8 and then from the low-frequency power source 6, the surface area of the steel plates 9 not spot-welded is heated first. By applying the power from the low-frequency power source 6 after this preheating, the two steel plates 9 are spot-welded. Furthermore, heat treatment of the outer peripheral region of the nugget, which is formed by spot-welding, can be performed by post-heating using the power from the high-frequency power source 8. By adjusting the temperature and heating time, the treatment is adaptable to the heat treatment of steel plates 9 such as tempering.

(89) FIG. 11 shows a heating waveform obtained when preheating using a high-frequency power source 8 and partial heating using the high-frequency power source 8 and a low frequency power source 6 are performed simultaneously. As shown in FIG. 11, the power from the high-frequency power source 8 is applied for the period of preheating, and for the predetermined initial period starting from the time when the application of power from the low-frequency power source 6 is started. Namely, for the initial period of application of power from the low-frequency power source 6 only, the power from the high-frequency power source 8 is superposed. The effect of preheating similar to that of the heating method shown in FIGS. 6A-6F can be expected. Furthermore, since the power from the low-frequency power source 6 and the high-frequency power source 8 are applied to the steel plates 9 partially superposed, spot-welding can be performed using the simultaneous heating method as shown in FIG. 7, and at the same time, the outer peripheral surface of the two steel plates 9 not contacting the electrodes 4 can also be heated using the power from the high-frequency power source 8.

(90) FIG. 12 shows a heating waveform obtained when heating using a low-frequency power source 6 and post-heating using a high-frequency power source 8 are performed, and then simultaneous partial heating is performed using the low-frequency power source 6 and the high-frequency power source 8. As shown in FIG. 12, the power from the high-frequency power source 8 is applied for a given period before the completion of application of power from the low-frequency power source 6 and for the period of post-heating performed immediately after that period. Since the power from the low-frequency power source 6 and the high-frequency power source 8 are superposed partially, spot-welding having the heating waveform as shown in FIG. 7 can be performed, and at the same time, the region on the surface of the two steel plates 9 that falls within the outer peripheral region of the electrodes 4 and does not contact the electrodes 4 can also be heated using the power from the high-frequency power source 8. The effect of the post-heating similar to the heating method shown in FIG. 8 can be expected.

(91) Since the heating time of the steel plates 9 by the high-frequency power source 8 described above can be controlled by the energization control unit 10, the temperature of the region of the steel plates 9 to be spot-welded only can be increased, and consequently the power consumption for the heating can be reduced.

(92) (Modification 3 of the Welding Equipment for Metallic Materials)

(93) Modification 3 of the welding equipment for metallic materials will be described below.

(94) FIG. 13 is the electric circuit diagram of modification 3 of the welding equipment for metallic materials. The welding equipment 35 for metallic materials shown in FIG. 13 is the same as the welding equipment 1 shown in FIG. 2 except that a DC power source 36 is used as a spot-welding power source 6 instead of low-frequency power source. The DC power source 36 uses an inverter, etc., and the magnitude of DC current and the energization time are controlled by the energization control unit 10. Since other configurations are the same as those of the welding equipment 1 for metallic materials, description is omitted.

(95) (Modification 4 of the Welding Equipment for Metallic Materials)

(96) Modification 4 of the welding equipment for metallic materials will be described below.

(97) FIG. 14 is the electric circuit diagram of modification 4 of the welding equipment for metallic materials. The welding equipment 40 for metallic materials shown in FIG. 14 is the same as the welding equipment 30 for metallic materials shown in FIG. 4 except that a DC power source 36 is used instead of the low-frequency power source 6. The DC power source 36 uses an inverter, etc. and the magnitude of direct current and energization time are controlled by the energization control unit 10. Since other configurations are the same as those of the welding equipment 30 for metallic materials, description is omitted.

(98) With the welding equipment 35, 40 for metallic materials also, the capacitor 7 functions as a capacitor for blocking current from the DC power source 36 to the high-frequency power source 8, and the inductance 5 functions as an inductance for blocking the flow of current from the high-frequency power source 8 to the low-frequency power source 6, namely functions as a choke coil.

(99) According to the welding equipment 35, 40 for metallic materials, since spot-welding is performed by feeding direct current to the electrodes 4, 4, no skin effect is expected unlike the case where the low-frequency power source 6 is used, and that is why the size of the electrodes 4, 4 can be selected depending on the work 9.

(100) (Heating Method to be Adopted when DC Power Source is Used as Welding Power Source)

(101) With the welding equipment 35, 40 for metallic materials using a DC power source 36 as a welding power source 6 also, the heating methods applied to the welding equipment 1, 25, 30 for metallic materials can be adopted.

(102) FIGS. 15 to 19 provide heating waveforms of the welding equipment 35, 40 for metallic materials. The horizontal axis of each figure represents time (arbitrary scale), whereas the vertical axis represents the magnitude of the waveforms 26, 22 of the current fed from the DC power source 36 and the high-frequency power source 8 (arbitrary scale).

(103) FIG. 15 shows heating waveforms obtained when simultaneous heating is performed using the DC power source 36 and the high-frequency power source 8. The effect of the simultaneous heating is the same as that of the simultaneous heating performed using the low-frequency power source 6 and the high-frequency power source 8 shown in FIG. 7.

(104) FIG. 16 shows heating waveforms obtained when the high-frequency power source 8 is used for post-heating. The effect of the post-heating using the high-frequency power source 8 is the same as that of post-heating performed using the high-frequency power source 8 shown in FIG. 8.

(105) FIG. 17 shows heating waveforms obtained when the high-frequency power source 8 is used for preheating. The effect of the preheating using the high-frequency power source 8 is the same as that of the preheating performed using the high-frequency power source 8 shown in FIG. 9.

(106) FIG. 18 shows heating waveforms obtained when preheating using the high-frequency power source 8, heating using the DC power source 36, and post-heating using the high-frequency power source 8 are performed sequentially. The effect of the heating in this case is the same as that of the heating method shown in FIG. 10.

(107) FIG. 19 shows heating waveforms obtained when preheating is performed using the high-frequency power source 8, and partial and simultaneous heating are also performed using the high-frequency power source 8 and the DC power source 36. In this case, the high-frequency power source 8 applies power for preheating, and keeps applying power for a given period immediately after the start of application of power from the DC power source 36. Namely, the power from the high-frequency power source 8 is superposed for the initial period of application of power from the low-frequency power source 6. The effect of the preheating is the same as that of the heating method shown in FIG. 11.

(108) FIG. 20 shows heating waveforms obtained when partial and simultaneous heating is performed using the high-frequency power source 8 and the DC power source 36, and then post-heating is performed using the high-frequency power source 8. In this case, the power from the high-frequency power source 8 is applied for a given period immediately after the completion of application of power from the DC power source 36. Namely, the power from the high-frequency power source 8 is superposed immediately before the completion of the application of the power from the low-frequency power source 6. The effect of the pre-heating is the same as that of the heating method shown in FIG. 12.

(109) Since the heating time of the steel plates 9 by the high-frequency power source 8 described above can be controlled by the energization control unit 10, the temperature of the region of the steel plates 9 to be spot-welded only can be increased, and consequently the power consumption for the heating can be reduced.

(110) According to the present invention, by connecting the high-frequency power source 8 to the work 9 via the electrodes 4 of the welding equipment 1, 25, 30, 35, 40 for metallic materials, partial heating of non-contacting region can be performed. The timing for performing high-frequency heating of the work 9 can be selected from before, after, and at the same time as the application of the power from the low-frequency power source 6 or the DC power source 36.

(111) With the welding equipment 1, 25, 30, 35, 40 for metallic materials, the steel plates 9 can be hardened as a result of quenching performed after welding. In this case, as the direction of quenching, heat dissipation in the horizontal direction of the steel plates 9 (FIG. 7) and the heat transfer from the electrodes 4, 4 in the vertical direction are assumed. The effect of heat transfer from the electrodes 4, 4 in the vertical direction is significant because the electrodes 4, 4 are water cooled. As a specific example of heat reservoir, high-frequency energization is performed after spot-welding to create a region where heat is reserved along the outer peripheral region of the nugget, and the nugget is then cooled based on the heat transfer in the vertical direction of the electrodes 4, 4. Consequently, since heat transfer occurs in the vertical direction only, unlike the case in which high-frequency energization is not performed and therefore heat transfer occurs both in the vertical and horizontal directions, formation of tissues due to solidification can be controlled.

(112) With the temperature increase profile of steel plates 9 of conventional spot welders, the temperature of the central area, where the electrodes 4, 4 and the steel plates contact each other, becomes the highest, and a nugget is formed in this high-temperature region. Namely, with the conventional spot welders, the region immediately under the electrodes 4, 4 is heated. However, when the high-frequency current 22 is fed to the electrodes 4, 4, the high-frequency current 22 concentrates on the surface of the electrodes 4, 4 due to skin effect, and if the electrodes 4, 4 contact with the steel plates 9, the high-frequency current 22 flows on the surface of the steel plates 9 due to skin effect. In this current circuit, the region where the temperature of the steel plates 9 becomes highest is the outer periphery of the electrodes 4, 4, namely the outer peripheral region of the nugget.

(113) In this way, by feeding high-frequency current 22 supplied from the high-frequency power source 8 to the electrodes 4, 4, partial heating of the outer peripheral region of the nugget only is ensured, and the temperature of this region becomes the highest. In addition, by narrowing this partial heating region, the heating method becomes more efficient compared to the heating of the entire region just below the electrodes 4, 4. Since high-frequency energization ensures the heating of outer peripheral region of the electrodes 4, 4, state of thermal well can be created. Consequently, melting and solidification are ensured in a state in which heat removal within the steel plates 9 is suppressed, and thus welding can be performed in a short time.

(114) By selectively heating the outer peripheral region of the nugget, which determines the strength of the welded region, using high-frequency energization, a spot-welded junction having sufficient strength can be formed in a short time even if the carbon content of the steel plates is high.

(115) With the welding equipment 1, 25, 30, 35, 40 for metallic materials, by performing two-frequency energization, the power from the spot-welding power source 6 can be used to form a melt-textured portion in the steel plates 9, whereas the power from the high-frequency power source 8 can be used to intensively perform heat treatment of the outer peripheral region of the nugget, which determines the strength. Consequently, since the area of the steel plates 9 to be welded can be heated intensively and independently, a desired spot-welding quality can be obtained in a period much shorter than that of the conventional spot welding.

(116) With the conventional spot welding adopting the thyristor phase control method, the current is interrupted, which is undesirable from the viewpoint of welding quality. Meanwhile, the welding equipment 1, 25, 30, 35, 40 for metallic materials improve the quality of the spot welding of the steel plates 9 because the magnitude of the high-frequency current 22 is controlled, and the high-frequency current 22 is not interrupted.

(117) (Work that can be Used for the Present Invention)

(118) The above description assumes that the metallic materials 9 to be spot-welded are steel plates 9, but any other metallic materials can be used. In addition, any shapes of the work 9 can be selected. The above description assumes that two steel plates 9 are spot-welded, but three or more plates can be welded.

(119) Furthermore, the metallic materials 9 to be spot-welded can be selected from those different from each other.

(120) (Electrodes that can be Used for the Present Invention)

(121) The above description assumes that the shape of the region defined by projecting the cross-sectional area of the electrodes 4 on the steel plates 9 is circular. However, any shapes other than the circular shape, such as ellipse, or polygonal shapes including square and triangle, can be selected.

Embodiment 1

(122) The specific example of spot-welding the steel plates 9 with the welding equipment 1 for metallic materials of the present invention will hereafter be detailed.

(123) Spot-welding of two steel plates 9 were performed. FIG. 21 illustrates the application of electric power from a low-frequency power source 6 and a high-frequency power source 8. The conditions of the steel plates 9, low-frequency power source 6, and high-frequency power source were as follows:

(124) Steel plates 9: Thickness; 2 mm, Size; 5 cm15 cm

(125) Low-frequency power source 6: 50 Hz (Material of electrodes 4: copper, diameter: 6 mm), capacity: 50 kVA

(126) Energization time of low-frequency power source 6: 0.3 to 0.5 sec.

(127) High-frequency power source 8: 30 kHz, 50 kW output

(128) Energization time of high-frequency power source 8: 0.3 to 0.6 sec.

(129) The steel plates 9 contain carbon (C) in 0.19 to 0.29 weight %, as a component other than iron.

(130) As shown in FIG. 21, preheating was performed for 0.3 sec. using the electric power from the high-frequency power source 8. The high-frequency power applied was changed from 4.9 kW to 37.0 kW.

(131) Welding was then performed by applying power from the low-frequency power source 6. As shown in FIG. 18, power from the low-frequency power source 6 was applied twice, namely as the first energization and as the second energization. With the startup of the first current regarded as one cycle, the first energization was performed for two cycles with the first current value set at 11 kA. After cooling was performed for one cycle, the second energization was performed for 16 cycles with the second current value set at 8 kA. The two-stage energization by the low-frequency power source 6 was performed for 20 cycles including cooling, etc., and the welding time was 0.4 sec.

Embodiment 2

(132) In embodiment 2, the power from the high-frequency power source 8 was applied together with the power from the low-frequency power source 6 for 0.3 sec. The high-frequency power was changed from 2.7 kW to 39.9 kW. The power from the low-frequency power source 6 was applied in the same manner as embodiment 1.

Embodiment 3

(133) In embodiment 3, the power from the high-frequency power source 8 was applied for 0.3 sec. immediately after the completion of application of the power from the low-frequency power source 6. The high-frequency power was changed from 2.7 kW to 39.9 kW. The power from the low-frequency power source 6 was applied in the same manner as Embodiment 1.

Comparative Example

(134) As a comparative example of Embodiments 1 to 3, welding was performed by applying power from the low-frequency power source 6, without applying power from the high-frequency power source 8. Namely, conventional spot-welding was performed.

(135) A cross-tension strength test of the welded samples of the embodiments and comparative example was conducted to find the breaking force. Table 1 summarizes the high-frequency energization patterns, applied high-frequency power, and the average breaking force of the welded samples of the embodiments and the comparative example.

(136) TABLE-US-00001 TABLE 1 High-frequency Average High-frequency energization High-frequency Number of Breaking breaking energization pattern power (kW) samples force force Embodi- Performed Preheating 4.9 3 19.54 19.5 Ment 1 1 18.46 1 20.28 8.6 1 21.26 20.9 1 19.59 28.5 1 17.98 37 1 19.58 Embodi- Simultaneous 2.7 to 3.8 1 15.97 18.8 Ment 2 heating 2.7 to 3.8 1 17.70 22.8 to 25 1 20.50 33.3 to 39.9 1 21.05 Embodi- Post-heating 4.2 1 18.70 18.7 Ment 3 8.6 1 18.35 30.8 1 17.94 39.9 1 19.73 Comparative Not 1 12.47 12.7 example 1 performed 1 12.88

(137) In embodiment 1, high-frequency power of 4.9 kW was applied to three welding samples for welding. The breaking force of each sample was 19.54 kN, 18.46 kN, and 20.28 kN respectively. When the high-frequency power was set to 8.6 kW, 20.9 kW, 28.5 kW, and 37.0 kW, the breaking force of respective samples was 21.26 kN, 19.59 kN, 17.98 kN, and 19.58 kN. From the above, it was found that the average breaking force of spot-welded samples in embodiment 1, in which spot-welding was performed using the low-frequency power source 6 after preheating was performed by high-frequency energization, was 19.5 kN.

(138) In embodiment 2, high-frequency power of 2.7 to 3.8 kW was applied to two samples. The breaking force of each sample was 15.97 kN and 17.70 kN respectively. When the high-frequency power was set to 22.8 to 25.0 kW and 33.3 to 39.9 kW, the breaking force of respective samples was 20.5 kN and 21.05 kN. From the above, it was found that the average breaking force of spot-welded samples in embodiment 2, in which spot-welding was performed using the low-frequency power source 6 while applying high-frequency power, was 18.8 kN.

(139) In embodiment 3, when the high-frequency power was set to 4.2 kW, 8.6 kW, 30.8 kW, and 39.9 kW, the breaking force of respective samples was 18.7 kN, 18.35 kN, 17.94 kN, and 19.73 kN. From the above, it was found that the average breaking force of the welded samples in embodiment 3, in which high-frequency power was applied after welding was performed using the low-frequency power source 6, was 18.7 kN.

(140) Two samples were used for comparative example, and the breaking force of the samples was 12.47 kN and 12.88 kN respectively. From the above, it was found that the average breaking force of the samples having undergone conventional spot-welding based on two-step energization in the comparative example was 12.7 kN.

(141) The average breaking force of the welding samples having undergone preheating in embodiment 1, simultaneous heating in embodiment 2, and post-heating in embodiment 3 was 1.54, 1.48, and 1.47 times as high as that of the comparative example respectively. More specifically, the average breaking force of the samples used for embodiments 1 to 3 was found to be approximately 50% higher than that of the samples having undergone spot-welding using the low-frequency power source 6 only. With any of the heating methods used for embodiments 1 to 3, the breaking force improved significantly compared to the spot-welding performed in comparative example using the low-frequency power source 6 only, despite the difference in the timing of high-frequency energization performed, namely as preheating, as simultaneous heating, or as post-heating.

(142) The breaking force was found to be much higher than that of the comparative example, on condition that the carbon content of the steel plates 9 falls within the 0.19 to 0.26 weight %.

Embodiment 4

(143) To check the heating effect of the high-frequency power source 8, chromium molybdenum steel 9 was quenched using the same welding equipment 1 for metallic materials in embodiment 1. The chromium molybdenum steel 9 made of SCM435 has the same size as those of the steel plates in embodiment 1. Power was applied from the high-frequency power source 8 for 0.3 sec. at the same frequency as embodiment 1 to perform quenching.

(144) FIG. 22 is a chart illustrating the hardness distribution on the surface of chromium molybdenum steel (SCM435) having undergone quenching in embodiment 4. The horizontal axis of the chart represents the position of the electrodes 4 in the direction of the cross-sectional area of the axis on the surface of the chromium molybdenum steel (SCM4359), on which the position of the electrodes 4 and its outside dimension are also shown. The vertical axis of the chart represents Vickers' hardness (HV) value.

(145) As shown in FIG. 22, the hardness of the chromium molybdenum steel (SCM435) in embodiment 4 corresponding to the outermost peripheral region of the electrodes 4 was the highest, approximately 670 HV, which is higher than 370 HV, the hardness of the region not having undergone quenching. It was thus found that the ring-shaped region, namely the outer peripheral region, of the electrodes 4 only, of the chromium molybdenum steel (SCM435), can be quenched by applying power from the high-frequency power source 8.

Embodiment 5

(146) Chromium molybdenum steel (SCM435) having the hardness of approximately 620 HV and having undergone quenching was heated using the same welding equipment 1 for metallic materials used in embodiment 1, and then was tempered. The energization by the high-frequency power source 8 was conducted for 0.3 sec. at the same frequency as embodiment 1 to perform tempering.

(147) FIG. 23 is a chart illustrating the hardness distribution on the surface of chromium molybdenum steel (SCM435) having undergone tempering in embodiment 5. The horizontal and vertical axes represent the same items as those in FIG. 23. As shown in FIG. 23, the hardness of the chromium molybdenum steel (SCM435) in embodiment 5 corresponding to the outermost peripheral region of the electrodes 4 was the lowest, approximately 550 HV, which is lower than the hardness observed before tempering was performed (approximately 620 HV). It was thus found that the ring-shaped region, namely the outer peripheral region, of the electrodes 4 only, of chromium molybdenum steel (SCM435), can be tempered by applying power from the high-frequency power source 8.

(148) In addition to the embodiments described above, various modifications of the present invention are possible within the scope of the claims of the invention. Needless to say, all of them are included in the scope of the present invention. The configuration of the gun arm 2 and the electrodes 4, and the capacity of the inductance 5 and the capacitor 7 can be set arbitrarily depending on the type and shape of the work 9.