JP3363284B2 - Electrode for electric discharge machining and metal surface treatment method by electric discharge - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric discharge machining electrode for performing electric discharge treatment on a material to be treated such as a mold, a tool, an internal combustion engine, a gas turbine, etc., which requires corrosion resistance and wear resistance, and a metal surface by electric discharge. It relates to a processing method.

[0002]

2. Description of the Related Art Conventionally, as disclosed in, for example, Japanese Patent Laid-Open No. 148615/1993, when a thick coating layer having a thickness of 10 μm or more is formed by surface treatment by electric discharge, a pressure of tungsten carbide powder and cobalt powder is used. Primary processing (deposition processing) is performed using a powder electrode, and then secondary processing (remelt processing) is performed by exchanging with an electrode with relatively little electrode wear such as a copper electrode (hereinafter referred to as non-consumable electrode). There is known a method for treating a metal surface by electric discharge, which comprises two steps such as

This method is an extremely excellent method for forming a fine ceramics layer having a high hardness and a large adhesive force to a thickness of several tens of μm, but it is necessary to replace it with a non-consumable electrode in the secondary processing. is there.

The conventional method will be further described. That is, fine ceramics (WC, TiC, TaC, Z
High melting point materials such as rC, SiC, TiB2, ZrB2, TiN, ZrN), tungsten W, molybdenum Mo, etc. may be difficult to diffuse sufficiently into the work material only by electrical discharge deposition. Many. As an example, an experimental example will be described in which tungsten carbide (hereinafter, referred to as WC) is deposited by electric discharge, and pulse electric discharge machining is applied thereto.

First, WC powder (average particle size 3 μm) is mixed with iron powder (hereinafter referred to as Fe powder) (average particle size 9.8 μm) at a ratio of 1: 1 and compression molding (compression pressure 4 t / square centimeter). ) Is applied to form a green compact, which is adhered to a copper round bar with a conductive adhesive to form a green compact electrode. Next, using carbon steel (S55C raw material) as the material to be treated, the electrical discharge conditions (pulse peak value Ip, pulse time τp, pulse dwell time τr were changed, and the ordinary die-sinking electric discharge machine shown in FIG. 13 was used. An experiment was conducted.

In FIG. 13, 1 is an electrode, 2 is a material to be processed, 3 is a processing tank, 4 is a working liquid, 5 is a servo mechanism of the electrode 1, 6 is a processing voltage between the electrode 1 and the material 2 to be processed. It shows the power supply to supply.

As a result, under machining conditions where the duty factor D is relatively large, arcs due to discharge are concentrated,
The green compact electrode 1 was destroyed, but under the condition that the duty factor D was 1.5% or less, the green compact electrode 1 was not worn out and was stably consumed and adhered to the surface of the material 2 to be treated. The processing conditions at that time are Ip = 20 A, τp = 16 μs, τ
r = 1024 μs (this is called primary processing). FIG. 14 shows a control circuit for the control circuit.
Reference numeral 11 is a resistor that limits the current flowing through each of the transistors 8 and 9, and 12 is a control circuit that controls the on / off operation of the transistors 8 and 9. FIG. 15 is a pulse waveform diagram showing the voltage waveform V and the current waveform Ip in the machining gap.

Next, the material 2 to be processed obtained by the above-mentioned electric discharge machining is subjected to pulse electric discharge machining in the following manner. First,
A tungsten carbide powder and a cobalt powder are mixed, and a compression-molded WC-Co sintered body (cemented carbide tool material) is bonded to a copper round bar with a conductive adhesive to form an electrode. Next, using this electrode, pulse electric discharge machining is carried out from above the WC and Fe deposition layers attached to the surface of the material to be treated 2. The processing conditions were such that the material 2 to be processed was not processed too much, the electrode polarity was negative, and the pulse peak value Ip, the pulse time τp, and the pulse rest time τr were changed. When the pulse time τp is short, the pulse peak value Ip is high, and the processing time is long, the WC-Fe deposit disappears, but under the condition that the pulse time τp is slightly long and the pulse peak value Ip is slightly low, WC-Fe It is possible to reduce the scattering of the deposits (this is called secondary processing).

[0009] WC-Fe is obtained only by the electrical discharge deposition of the primary processing.
However, it was confirmed that WC was diffused to the material to be treated when the pulse electric discharge machining of the secondary machining was performed on the WC. The hardness of a normal WC-Co sintered body is (WC7
0, Co30) Vickers hardness Hv850-
It is about 950. In the above experimental example, WC50, Fe50
And the hardness of the surface treatment layer having a hardness higher than that (Vickers hardness Hv1000
~ 1400) (The quenching hardness of S55C is Vickers hardness H
v800 +) was obtained. Further, in the above experimental example, the thickness that can obtain the Vickers hardness Hv of 1000 or more is about 60 μm, which is large.

[0010]

In the conventional electric discharge machining electrode, there is a possibility that the powder compact electrode may collapse when the discharge duty factor D is large (the ratio of the pulse pause time τr to the pulse time τp). There was one problem. The reason why the powder compact electrode collapses during processing is that the tissue adhered by compression is fragile, the thermal conductivity and electrical resistance are apparently high, and the discharge current causes heat to be generated near the discharge point. It is considered that the discharge is concentrated, the area near the discharge point is largely removed, and the electrode is partially melted and re-solidified (caused by the arc discharge) to cause chipping.

Further, in the conventional electric discharge machining electrode, the electrode is consumed a lot due to the powder compact electrode, so that it is not remelted and sintered.
There was a second problem that it was a deposition process.

Further, in the conventional metal surface treatment method by electric discharge, it is necessary to replace the non-consumable electrode by secondary processing,
If you switch to secondary processing conditions without replacing the electrodes,
There is a third problem that the electrode is broken and it becomes difficult to continue the processing.

Here, the term of the duty factor D and the phenomenon depending on its magnitude will be described. The duty factor D is, as shown in FIG. 3, D = pulse time τp / (pulse time τp + pulse rest time τr) (%) As shown in the above equation, one discharge cycle (pulse time τp
+ Pulse rest time τr), and the larger the duty factor D, the shorter the rest time and the higher the machining efficiency. However, the fact that the duty factor D is large means that the average value of the current per unit time becomes large,
Therefore, if the electric resistance of the electrode material is high and the thermal conductivity is low, not only the discharge point but also the temperature in the vicinity thereof becomes high. If the temperature is high, insulation recovery will not be sufficient, and therefore, electric discharges that occur one after another will occur at the same point and will be concentrated. This is so-called arc discharge (discharge without insulation recovery). Therefore, the specific discharge generation point collapses and the electrode shape becomes irregular. The powder compact electrode needs to have a small duty factor D (1.5% in the above example).

An object of the present invention is to solve the above-mentioned first to third problems, and to solve the easiness of collapsing of the powder compact electrode and to make the electrode even in the secondary processing condition. It is an object of the present invention to provide an electrode for electric discharge machining that enables secondary machining without changing the electric conditions of the electric discharge, and a method for treating a metal surface by electric discharge without changing the electric conditions.

[0015]

The electric discharge machining electrode according to the first invention is WC, TiC, TaC, ZrC, SiC,
Carbides such as VC, borides such as TiB2 and ZrB2, T
It is configured by compressing and molding a single substance of a nitride such as iN or ZrN or a mixture of two or more types, and pre-sintering at a temperature equal to or lower than the sintering temperature.

The electric discharge machining electrode according to the second invention is T
It is formed by temporarily sintering a green compact made of a material such as i, V, or Ta that is easily carbonized at a temperature equal to or lower than the sintering temperature.

The metal surface treatment method by electric discharge according to the third aspect of the present invention is WC, TiC, TaC, ZrC, SiC, VC.
Carbides such as TiB2, boride such as ZrB2, Ti
Nitride such as N and ZrN is compression-molded by adding a sintering aid to a single substance or a mixture of two or more kinds, and then pre-sintered at a temperature equal to or lower than the sintering temperature and used as a consumable electrode for electric discharge machining. The treatment material is subjected to an electric discharge treatment.

In the metal surface treatment method by electric discharge according to the fourth aspect of the present invention, a green compact made of a material such as Ti, V, or Ta which is easily carbonized is temporarily sintered at a temperature equal to or lower than the sintering temperature, and then this is sintered. As a consumable electrode for electric discharge machining, a material to be treated is subjected to electric discharge treatment in a machining fluid that produces carbon by thermal decomposition due to electric discharge.

In the method for treating a metal surface by electric discharge according to the fifth aspect of the present invention, the conditions for the electrode material to be deposited well in the first stage electric discharge treatment on the material to be treated are selected, and the second treatment on the material to be treated is selected. The electrode polarity and discharge electrical conditions are changed to conditions that increase hardness in the discharge treatment in stages.

[0020]

Function: WC, TiC, TaC, Zr according to the present invention
C, SiC, VC, etc. carbides, TiB2, ZrB2, etc. boride, TiN, ZrN, etc. nitrides alone or a mixture of two or more kinds are compression molded and pre-sintered at a temperature below the sintering temperature. The electric discharge machining electrode can be electric discharge machined without breaking even in electric discharge machining with a non-consumable polarity.

The electrode for electric discharge machining, which is formed by pre-sintering a green compact made of a material such as Ti, V, Ta, etc., which is easily carbonized, at a temperature below the sintering temperature is used in the electric discharge machining of non-consumable polarity. EDM can be performed without breaking.

WC, TiC, TaC, Z according to the present invention
Carbides such as rC, SiC, and VC, borides such as TiB2 and ZrB2, and nitrides such as TiN and ZrN are compression molded by adding a sintering aid to a single substance or a mixture of two or more types,
After that, the metal surface treatment method, in which the material to be treated is subjected to electrical discharge treatment as a consumable electrode for electrical discharge machining by pre-sintering at a temperature equal to or lower than the sintering temperature, is used for electrode conversion even under a wide range of electrical discharge conditions such as polarity conversion. EDM can be performed without breaking.

The green compact made of a material such as Ti, V, Ta, etc., which is easily carbonized, is pre-sintered at a temperature below the sintering temperature, and then is used as a consumable electrode for electric discharge machining by thermal decomposition by electric discharge. The metal surface treatment method in which a material to be treated is subjected to electric discharge treatment in a machining liquid that produces carbon can perform electric discharge machining without changing the polarity or breaking the electrode even under a wide range of electric discharge conditions.

In the discharge treatment in the first stage on the material to be treated according to the present invention, the conditions under which the electrode material is often deposited are selected,
The metal surface treatment method in which the electrode polarity and the discharge electrical condition are changed to the condition that the hardness is increased in the second stage electric discharge treatment on the material to be treated is such that the electrode is not broken and the deposition is performed.
Remelting and engraving processing can be performed.

[0025]

【Example】

Example 1. An embodiment of the present invention will be described below with reference to the drawings. First, the production of electrodes will be described. In the case of the conventional electrode formed only by powder compacting, the electrode may collapse under the secondary processing conditions.
Carbide powder and cobalt powder (WC-Co powder) were mixed, compression molded, and then pre-sintered. The respective conditions are as shown in Table 1.

[0026]

[Table 1]

Next, the material to be treated is SK-3, the surface is a ground surface, and the electrode is the pre-sintered green compact electrode shown in Table 1 above, and surface treatment is performed by electric discharge machining. Current value Ip is 5
˜25 A, the pulse time τp was 4 to 1024 μs, and the rest time τr was 1024 μs.

FIG. 1 shows the pulse peak value, the pulse dwell time, and the processing state when the electrode is switched between negative and positive, respectively. If the electrode is negative, there is a region to be deposited on the material to be treated. . In FIG. 1, the horizontal line represents the pulse peak value IP, the vertical line represents the pulse time τP,
"Engraving" means processing the material to be processed without depositing it. Such engraved areas can be removed by choosing discharge electrical conditions and lowering the sintering temperature. If the electrode is positive, the negative electrode deposited can be remelted. As in the above case, the engraved region can be removed by selecting discharge electrical conditions or by lowering the sintering temperature. Further, the hardness is increased when the electrode is processed by plus, as compared with the hardness of the surface layer when the electrode is processed by minus.

The reason for this is that if the electrode is positive, the material to be treated will be negative, and as shown in the assumed model diagram for the behavior of the arc column and the formation of discharge traces in FIG. It is considered that this is because the temperature becomes higher and the result is as if reheating was performed at a high sintering temperature. Note that reference numeral 13 in FIG. 2 shows a consumed portion of the electrode when the electrode 1 has a negative polarity. FIG. 3 shows the cross-sectional hardness distribution of this surface-treated layer.

4 to 7 show cross sections of the surface-treated material, FIG. 4 shows the cross section of the surface treatment for 30 minutes, and FIG. 5 shows the polarity of the electrode switched between negative and positive. 6 shows a cross section of the treatment layer subjected to electric discharge machining, FIG. 6 shows a cross section of the treatment layer of the workpiece having a deformed shape, and FIG. 7 shows a cross section of the treatment layer at the welded portion.

As shown in the above examples, it is apparent that the calcinated WC-Co electrode exhibits the following characteristics. Even if it is pre-sintered, if the electrode has a negative polarity, there is a region to be deposited on the material to be processed. If the polarity is positive, it is possible to increase the hardness by remelting the once-deposited material, although the material is not deposited.

It has been found that the electrode does not collapse even if the polarity is changed to plus or minus, and the discharge can be continued without breaking even if the polarity change is repeated or repeated frequently at high speed.

Further, even if the duty factor is set high, it is not easy to collapse or concentrate the discharge,
The processing efficiency can be improved.

From another experiment, it is also found that, unlike the one which is simply in contact like a green compact, the bonding in the powder is strengthened by the pre-sintering, the electric resistance is reduced, and the thermal conductivity is also increased. It is clear that Of course, it is natural that the electric resistance is higher and the thermal conductivity is lower than that of the completely sintered (high temperature treated).

To further explain the above-mentioned embodiment, when the green compact is heated to the sintering temperature as in the case of the sintering of ordinary sintered alloys or fine ceramics, a strong sintered body is formed. Becomes non-consumable, and deposition on the material to be treated cannot occur. Therefore, the means selected by the present inventor is (1) sintering at a temperature lower than the sintering temperature. (2) The contradictory characteristics of consumable and non-consumable are the conversion of the electrode polarity and the change of the discharge electrical condition at that time, which is based on the assumption of the arc model of discharge generation shown in FIG. It is an idea.

The above item (1) will be described below.
FIG. 8 shows the general tendency of sintering, the horizontal axis shows the sintering time, and the vertical axis shows the relative density. Sintering at a high temperature approaches the theoretical relative density, but sintering at a lower temperature lowers the low relative density, that is, strength. FIG. 9 shows the sintering temperature and the apparent density of the alumina ceramics, the horizontal axis shows the sintering time, and the vertical axis shows the apparent density. 160
If sintered at 0 ° C. or higher, the density will be extremely close to the theoretical density. In the present invention, about 50% to 90% of the theoretical density is in the range of use, which is sufficiently weaker than complete sintering and sufficiently stronger than that of a green compact, and has a low electric resistance. The thermal conductivity is also high.

Next, the above item (2) will be described.
The primary machining is performed under the condition that many electrodes are consumed, and the secondary machining is performed under the condition that the electrode is consumed less. As shown in the model model of the behavior of the arc column and the discharge mark formation in FIG. It is possible to control the consumption of the electrode by selecting That is, when the electrode is negative, the arc column due to one discharge is thin on the negative side and thick on the positive side as shown in the figure. Since the discharge current is constant, the discharge trace current density on the minus side becomes extremely high, and the consumption on the minus side increases.

On the contrary, since the discharge trace current density is relatively low on the positive side, the consumption is reduced. Therefore, when it is desired to significantly consume the electrodes, the discharge current value may be increased in order to make the polarity negative and further increase the current density of the discharge traces.

To make the electrodes non-consumable, the polarity of the electrodes should be positive and the current value of the discharge trace should be reduced. The longer the current pulse time τp, the lower the current density of one discharge mark.
Is long and has a positive polarity, and the current pulse time τp is short and has a negative polarity in order to make it a consumable type.

Example 2. Based on the above characteristics, the following new embodiments can be created. That is, the polarity of the electrode is repeated negative and positive at a frequency of several tens of times per minute. This processing method further increases the hardness of the processed surface. Also thick surface layers can be made. Alternatively,
It is possible to form a thick coating layer having a fine finished surface roughness.

After machining the provisional sintered electrode into a target shape by machining or ultrasonic processing, a cavity is formed (the shape is engraved) using this as a processing electrode. At this time, the polarity of the plus-polarity electrode, which is less consumed, improves the shape accuracy of the shape processing. Next, deposition processing is performed with the polarity of the electrode being negative (primary processing). Next, re-melting processing is performed with the polarity of the electrode being positive. In this way, the shape of the cavity can be processed without using the conventional copper electrode or graphite electrode, and then the surface can be hardened.

The material to be treated has a melting point of 1500, like steel.
If a material having a temperature higher than about ℃, for example, a material such as a super-hard alloy, is pre-sintered with an easily carbonizable material such as TiC and Ti or V or Ta, while processing the electrode polarity while changing it, For steel, deposition and remelting and sintering are possible even in the engraved area.

As shown in FIG. 10, if the provisional sintered electrode is formed into a wheel shape (disk shape) and is rotated, and at the same time the discharge surface treatment is performed while lubricating, machining can be performed while improving the circulation of the machining liquid. . In addition, the amount of wear of the pre-sintered electrode can be dispersed over the entire disk, which is useful for hardening cutting tools and parts. That is, it is synonymous that it is useful to process a cutting tool or a part with a grinder.

In FIG. 10, 20 is a material to be treated, 21 is a rotating wheel, 22 is a working fluid, 23 is a power source, 24 is an insulating spindle for rotating the electrode 21, 25 is a brush,
Reference numeral 26 denotes a rotating belt. Note that FIG. 11 is a cross-sectional view of the wheel 21, and 27 indicates an electrode mounted on the wheel 21.

Re-polishing of the cutting tool with the provisional sintered electrode,
When the surface is hardened, a structure integrated with a diamond grinding wheel can be adopted. That is, the outer peripheral portion of the diamond wheel is used for re-grinding, and the provisional sintered electrode is attached to the inner peripheral portion.

FIG. 12 is a sectional view thereof, and in FIG. 12, 30 is a wheel, 31 is a diamond attached to the wheel 30, and 32 is a pre-sintered electrode.

[0047]

As described above, the electric discharge machining electrode according to the first invention is composed of WC, TiC, TaC, ZrC and S.
It consists of carbide such as iC and VC, boride such as TiB2 and ZrB2, nitride such as TiN and ZrN, or a mixture of two or more kinds, and then pre-sintering at a temperature below the sintering temperature. Therefore, the electrodes do not collapse even when the polarity is changed or a wide range of electrical discharge conditions are met.

Further, since the electric discharge machining electrode according to the second aspect of the present invention is formed by temporarily sintering a green compact made of a material such as Ti, V, or Ta which is easily carbonized at a temperature equal to or lower than the sintering temperature, The electrode does not collapse even under the conversion of electricity and a wide range of electrical discharge conditions.

The metal surface treatment method by electric discharge according to the third aspect of the invention is WC, TiC, TaC, ZrC, Si.
A carbide such as C or VC, a boride such as TiB2 or ZrB2, or a nitride such as TiN or ZrN is compression-molded by adding a sintering aid to a single substance or a mixture of two or more types, and then,
Since the coating layer is formed on the surface of the material to be treated by performing the electric discharge treatment on the material to be treated by using the consumable electrode for the electric discharge machining as a consumable electrode for electric discharge machining, the coating layer is formed on the surface of the material to be changed in polarity or in a wide range. The electrode does not collapse even under electrical discharge conditions, and if the electrode is continuously used, the surface of the electrode is sintered and the hardness increases.

In the metal surface treatment method by electric discharge according to the fourth aspect of the present invention, a green compact made of a material such as Ti, V, or Ta which is easily carbonized is pre-sintered at a temperature lower than the sintering temperature, and then Is used as a consumable electrode for electrical discharge machining, and the material to be treated is subjected to electrical discharge in a machining fluid that produces carbon by thermal decomposition due to electrical discharge. After sufficient action, the electrode does not collapse, and if the electrode is continuously used, the surface of the electrode is sintered and the hardness is increased.

Further, in the method for treating a metal surface by electric discharge according to the fifth aspect of the present invention, the conditions for the electrode material to be deposited well in the first stage electric discharge treatment on the material to be treated are selected, and In the two-stage electric discharge treatment, the electrode polarity and the electric discharge condition are changed so as to increase the hardness, so that the electrode can be maintained and the deposition, remelting, and engraving processing can be performed.

[Brief description of drawings]

FIG. 1 is a diagram showing a pulse peak value, a pulse quiescent time, and a processing state when an electrode according to an embodiment of the present invention is switched between a negative electrode and a positive electrode.

FIG. 2 is a diagram showing a behavior of an arc column and an assumed model for forming discharge traces, which explains an embodiment of the present invention.

FIG. 3 is a diagram showing a cross-sectional hardness distribution of a surface treatment layer obtained according to an example of the present invention.

FIG. 4 Processing time 3 obtained by the embodiment of the present invention
It is a surface treatment sectional photograph of a to-be-processed material for 0 minutes.

FIG. 5 is a photograph of a cross section of a treatment layer of a material to be treated which has been subjected to electric discharge machining by changing the polarity of the electrode obtained by the embodiment of the present invention to negative or positive.

FIG. 6 is a cross-sectional photograph of a treatment layer of a material to be treated having an irregular shape obtained according to an example of the present invention.

FIG. 7 is a photograph of a cross section of a treatment layer of a welded portion obtained according to an example of the present invention.

FIG. 8 is a diagram showing a general relationship between a change in density of a sintered body sintered at a constant temperature and a sintering temperature.

FIG. 9 is a diagram showing the relationship between the sintering temperature and the apparent density of alumina ceramics.

FIG. 10 is a configuration diagram illustrating another embodiment of the present invention.

11 is a cross-sectional view of the wheel shown in FIG.

FIG. 12 is a sectional view of a diamond wheel illustrating still another embodiment of the present invention.

FIG. 13 is a general configuration diagram of an electric discharge machine.

FIG. 14 is a general configuration diagram showing a control circuit of an electric discharge machine.

FIG. 15 is a diagram showing a pulse voltage waveform and a pulse current waveform in a machining gap.

[Explanation of symbols]

1 electrode, 2 processed material, 3 processing tank, 4 processing liquid, 5
Servo mechanism 6 power supply, 8 transistors, 9 transistors,
10 resistance 11 resistance, 12 control circuit, 15 electrode consumption part,
20 processed material 21 rotating wheel, 22 working fluid, 23 power supply, 24
Insulation spindle 25 Brush, 26 rotating belt, 27 electrodes, 3
0 Wheel 31 Diamond, 32 Temporary sintered electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (73) Patent holder 591135853 Naotake Mori 1-50-3, Chuo 1-chome, Nakano-ku, Tokyo (74) One agent above 100102439 Patent attorney Kaneo Miyata (72) Inventor Nagao Saito 12 (72) Inventor 9-12 Iwanaridai, Kasugai City, Aichi Prefecture (72) Inventor Naotake Mori 661-51 (72) Yakashi Ishizaka, Tenshiro-ku, Nagoya City Katsushi Furuya 3837-3-23 Shimada Kuroishi, Tenshiro-cho, Tenshiro-ku, Nagoya City (72) ) Inventor Teruo Ishiguro 671 Kefu Town, Kasugai City, Aichi Prefecture (72) Inventor Toshiro Oizumi 5-14 Yada Minami 5-chome, Higashi-ku, Nagoya-shi Sanryo Electric Co., Ltd.Nagoya Works (72) Inventor Maji Takushi, Yada-minami, Higashi-ku, Nagoya-shi 5 chome 1-14 Sanryo Electric Co., Ltd. Nagoya Works (56) Reference JP 62-52182 (JP, A) JP 64-5731 (JP, A) JP 63-1702 63 (JP, A) JP-A-5-148615 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B23H 1/04 B23H 9/00 C25D 11/00

Claims (5)

(57) [Claims] 1. WC, TiC, TaC, ZrC, Si
C, VC, etc. Carbides, TiB2, ZrB2, etc. boride, TiN, ZrN, etc. nitrides alone or a mixture of two or more kinds are compression-molded and pre-sintered at a temperature below the sintering temperature. An electric discharge machining electrode characterized by. 2. An electrode for electric discharge machining, comprising a green compact made of a material such as Ti, V, or Ta which is easily carbonized, pre-sintered at a temperature equal to or lower than a sintering temperature. 3. WC, TiC, TaC, ZrC, Si
C, VC, etc. carbides, TiB2, ZrB2, etc. boride, TiN, ZrN, etc. nitrides alone or a mixture of two or more of them are compression-molded, and then pre-sintered at a temperature below the sintering temperature. Is used as a consumable electrode for electric discharge machining, and a coating layer is formed on the surface of the material to be treated by performing an electric discharge treatment on the material to be treated. 4. A powder compact made of a material such as Ti, V, and Ta that is easily carbonized is pre-sintered at a temperature equal to or lower than a sintering temperature, and then is used as a consumable electrode for electric discharge machining to cause carbon decomposition by electric discharge. A method for treating a metal surface by electric discharge, which comprises subjecting a material to be treated to an electric discharge in a working fluid that causes a lump. 5. A condition for increasing hardness in the second-stage electric discharge treatment on the material to be treated by selecting a condition for depositing an electrode material in the first-stage electric discharge treatment on the material to be treated. 5. The method of treating a metal surface by electric discharge according to claim 3 or 4, wherein the electrode polarity and the electric discharge condition are changed.