JP3627784B2 - Discharge surface treatment method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a discharge surface treatment method for obtaining a good surface finish on a surface of a steel material or a cemented carbide, for example, a tungsten carbide-cobalt cauterized body, and forming a surface layer having tough wear resistance. Is.
[0002]
[Prior art]
The present inventors have already compression-molded a metal hydride such as titanium hydride (TiH ₂ ) and used it as an electrode (hereinafter referred to as a green compact electrode) as a steel material or cemented carbide, which is an object to be treated, for example, The sintered body of tungsten carbide-cobalt (WC-Co) is subjected to electric discharge machining in oil, and the surface of the object to be treated is coated with a carbide of electrode material [if titanium hydride (TiH ₂ ), titanium carbide (TiC)]. Proposing technology.
[0003]
Further, by nitriding the coated material as described above, titanium carbide (TiC) is chemically changed to nitrogenated titanium carbide (TiCN), and residual titanium (Ti 2) is chemically changed to titanium nitride (TiN), thereby carbonizing. A technique for forming a surface layer having higher wear resistance than titanium (TiC) has been proposed.
[0004]
[Problems to be solved by the invention]
By the way, when performing discharge surface treatment with a green compact electrode, if the surface treatment speed is increased, the finished surface roughness becomes rough. At present, the best finished surface roughness under conditions where the surface treatment speed is relatively high. The surface to be processed is about 6 μm Rz for cemented carbide and about 9 μm Rz for steel, and the surface finish before processing of the object to be processed is 1 μm Rz or less. Becomes rough.
[0005]
The reason is that the green compact electrode is uneven due to electrode consumption during the discharge surface treatment, and that particles such as titanium hydride (TiH ₂ ) forming the green compact electrode are extremely difficult to be micronized (fine powder) There is a risk of ignition and explosion during the pulverization process), and discharge is partially concentrated due to non-uniform electrical resistance of the green compact electrode.
[0006]
Compared with CVD (chemical vapor deposition), PVD (physical vapor deposition), plating, etc., the discharge surface treatment has a significantly higher adhesion because the coating components are projected and diffused in a molten state at a high temperature. However, as described above, it has a drawback that it is difficult to obtain a finished surface roughness of about 1 μm Rz. For this reason, even in the previously proposed technique for nitriding after the discharge surface treatment, the finished surface is roughened to a rough surface.
[0007]
As long as it is a normal surface treatment for wear-resistant parts, the above may be used as it is, but the application is a cutting tool, cold forging tool, die, or bearing used in harsh environments, civil engineering machinery, marine equipment, etc. When a considerably fine finished surface roughness (about 1 μm Rz) is required as in a machine part, this may not be sufficient, and the present invention has been made in view of the above problems.
[0008]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a discharge surface treatment method in which a carbon powder that is hardened by carbonization is compression-molded to form an electrode for electric discharge machining. The surface of the object to be processed is polished, and then the object to be processed is nitrided.
[0009]
According to a second aspect of the present invention, there is provided a discharge surface treatment method in which a metal powder that is carbonized and hardened is compression-molded to form an electrode for electric discharge machining. The surface of the object to be processed is subjected to electric discharge grinding, and then the object to be processed is nitrided.
[0010]
The discharge surface treatment method according to claim 3 is the discharge surface treatment method according to claim 1 or 2, wherein the metal powder having the property of carbonizing and hardening is hardened with carbide, nitride, boron. The object to be treated is subjected to discharge surface treatment with an electrode for electric discharge machining that is compression-molded by mixing at least one of the compounds.
[0011]
A discharge treatment method according to claim 4 is the discharge surface treatment method according to any one of claims 1 to 3, wherein the nitriding treatment is performed in a mixture of argon gas and nitrogen.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1 FIG.
The inventors of the present invention have been conducting an experiment in which an object to be treated is surface-treated by discharge in oil using a green compact electrode mainly composed of titanium hydride (TiH ₂ ). A high hardness and high adhesion coating layer containing a large amount of titanium carbide (TiC) by bonding of carbon and titanium (Ti) is realized. The surface roughness is 6 μm Rz on cemented carbide and 9 (μm Rz) on steel such as steel, and the surface processed with a compacted electrode of tungsten carbide-cobalt (WC-Co) or a known surface roughness. The finished surface is considerably better than the sprayed surface. However, the finished surface roughness of about 1 μm Rz required for the cutting tool surface or the cold forging tool surface has not been reached.
[0013]
Therefore, the present invention is a combination of the grinding technique and the nitriding technique, and an embodiment thereof will be described below with reference to the drawings.
[0014]
First, a green compact electrode is formed by compression-molding a metal having a property of carbonizing and hardening, for example, a hydrogenated metal such as titanium hydride (TiH ₂ ), and a steel material or tungsten carbide as a workpiece. A cemented carbide of a cauterized body of cobalt (WC-Co) or the like is subjected to electric discharge machining in a machining liquid in which carbon is decomposed by electric discharge, for example, oil, and the surface of the object to be processed is coated with a carbide of an electrode material. Thereafter, as illustrated in FIG. 1, the discharge treatment surface applied to the object to be processed is mechanically polished with, for example, a tool.
[0015]
That is, FIG. 1 illustrates a first embodiment of the present invention. In this figure, reference numeral 1 denotes a base material that is an object to be processed, 2 denotes a discharge treatment surface applied to the surface of the base material 1, 3 Indicates a round bar which is a tool for mechanical polishing. The surface of the round bar 3 is coated with a diamond paste having a particle size of about 1 to 3 μm kneaded with oil. The discharge treatment surface applied to the surface of the base material 1 by the round bar 3 is mechanically applied. Grind. In this embodiment, the polishing time was 10 minutes, and electric discharge machining in oil was performed under the following conditions.
Electrode: Green compact electrode polarity of titanium hydride (TiH ₂ ): Negative discharge current value Ip: 8A
Pulse width Ton: 2 μs
Pause time Toff: 255 μs
Processing time: 5 min
Object to be treated: Tungsten carbide-cobalt (WC-Co) and special tool steel (SKD-11)
[0016]
Next, the ground base material 1 is subjected to nitriding treatment. FIG. 2 is a diagram showing a schematic configuration of the nitriding apparatus, in which 20 is a casing, 1 is a base material stored in the casing 20, and 21 is a first of liquid nitrogen stored in the casing 20. A storage container, 22 is a heater for heating the base material 1, 23 is a second storage container of liquid nitrogen disposed outside the housing 20, and 24 is liquid nitrogen from the second storage container 23 into the housing 20. Shows the conduit that leads. In order to prevent oxidation of the base material 1, the liquid nitrogen is first put into the first storage container 21 and the housing 20 is sufficiently filled with nitrogen.
[0017]
Using the above apparatus, the base material was nitrided as follows. As a result of performing about 500 (° C.) and 10 (min) as the nitriding conditions, the finished surface roughness and the surface hardness were as shown in FIGS. In FIG. 3, the surface treated with the titanium hydride (TiH ₂ ) green compact as seen from the left side, the surface treated with nitriding, and the surface treated with titanium hydride (TiH ₂ ) green compact In addition, there is no change in roughness of the finished surface before and after the nitriding treatment. The base materials are tungsten carbide-cobalt (WC-Co) cemented carbide and steel (SKD11).
[0018]
As is clear from the hardness change before and after the nitriding treatment in FIG. 4, a titanium carbide (TiH ₂ ) treated (unpolished) nitriding treatment was used to coat the cemented carbide and Vickers It can be seen that the hardness Hv1450 is changed to Vickers hardness Hv1700, and the steel material is changed from Vickers hardness Hv1050 to Vickers hardness Hv1300. Moreover, the hardness increase by nitriding treatment is certain.
[0019]
The hardness of the titanium hydride (TiH ₂ ) treated surface was only polished and the cemented carbide was processed, the Vickers hardness Hv1450 was reduced to Vickers hardness Hv1300, and the Vickers hardness Hv1050 was reduced to Vickers hardness Hv500 for steel materials. . A material obtained by nitriding them is treated with a cemented carbide and has improved Vickers hardness Hv1450 and steel with Vickers hardness Hv950. This shows that the hardness is sufficiently higher than the base material hardness. However, the Vickers hardness is about Hv300 lower than that obtained by nitriding the coated and unpolished state. I imagined that this was because the part of the surface layer with a large amount of titanium (Ti) component and a small amount of titanium carbide (TiC) component was removed, but it was still covered with titanium hydride (TiH ₂ ). Compared with hardness, it is not inferior. Therefore, the finished surface roughness is clearly improved, and it is expected that the wear resistance is high due to the increase in hardness due to nitriding.
[0020]
Next, the wear test will be described. Also in the results of the Ogoshi type pin disk wear test, the wear amount is much smaller than the discharge-treated surface by the compacted electrode of titanium hydride (TiH ₂ ) and about 1/10 of that.
The conditions for the wear test are as follows.
Pin shape: 7.98 mmφ (0.5 cm ² )
Pressing force: 0.5 kgf, so pressing pressure 1 kgf / cm ²
Friction speed: 1 m / s: Disc material: SK-3
Atmosphere: Amount of wear in the air: Wear weight cemented carbide in 25 km running without discharge surface treatment: 2 mg
Discharge treatment of cemented carbide with titanium metal electrode: 0.7mg
Discharge treatment with powdered metal electrode of titanium hydride: 0.1mg
Compact powder electrode treatment of cemented carbide with titanium hydride → grinding → nitriding: about 0.01 mg that cannot be weighed finely.
In order to confirm whether the increase in hardness by the nitriding apparatus was due to the incorporation of nitrogen gas or simply due to heating, an in-air heat treatment was attempted under the same conditions (temperature: 500 ° C., atmospheric pressure) as the nitriding treatment. . As a result, it was confirmed that the hardness decreased. This is presumably because titanium carbide (TiC 3) or the like was oxidized and changed to spore titanium oxide (TiO), titanium oxide (TiO ₂ ), or the like. That is, the lower than the base metal hardness is that the titanium carbide (TiC) + titanium (Ti) layer coated on the surface is oxidized and changed to titanium oxide (TiO ₂ ), and there is no change in the base metal hardness. In both cases, a surface layer having a low hardness is formed.
[0022]
The first embodiment of the present invention is as described above. Next, the function and effect will be described.
First, the state of the surface generated when the discharge treatment surface is smoothed by mechanical polishing and then nitriding is performed will be described.
FIG. 5 shows a change in the hardness of a cross section from the surface of the base material to the inside of the coating layer when the base material is subjected to electric discharge machining in titanium with a compacted electrode of titanium (Ti 2). The surface was discharged in oil with a compacted electrode of titanium (Ti 2). In the figure, Vtic refers to titanium (Ti 2) combined with carbon (C) produced by oil decomposition to form titanium carbide (TiC 2), but titanium carbide (TiC) / titanium (Ti) discharge The volume ratio on the surface to be treated can be increased or decreased by controlling the discharge current pulse width, the discharge time, and the supply state of the oil as the working fluid. The Vickers hardness Hv is a value measured with a load of 0.01 kg (10 gr).
[0023]
Thus, the hardness of the base material surface is high, and the softening as it enters the inside means that titanium carbide (TiC) decreases and the proportion of titanium (Ti) increases as it enters the inside. . Therefore, polishing the surface of the base material with diamond abrasive grains smoothes the surface of the base material, but temporarily reduces the surface hardness.
[0024]
However, when nitriding is performed in that state, the remaining titanium (Ti 2) becomes titanium nitride (TiN), and titanium carbide (TiC) becomes nitrogenated titanium carbide (TiCN), as shown in FIG. The hardness increases again. As is clear from FIG. 3, the finished surface roughness is not changed by nitriding.
[0025]
Next, as shown in FIGS. 6A to 6D, the discharge electrical conditions (discharge current Ip = 7 A, pulse width Ton = 2 μs) are shown as cross-sectional profiles before and after polishing of the discharge treatment surface of the base material. A small discharge was performed for a short time.
In the case of this example (when it is desired to make the coating thin), the uneven portion of the treatment layer protrudes sufficiently more than the base material, but the valley portion penetrates inside the base material surface. There is. This is because the component titanium (Ti.sub.2) of the green compact electrode is indented into the base material because it has a working action when it hits the surface of the base material by electric discharge (therefore has high adhesion). This can also be seen from the fact that when the hardness of the base material is high (for example, cemented carbide), the depth of entering the base material is smaller than when the hardness is low (for example, steel material).
[0026]
Therefore, if the mechanical polishing is carried out to such an extent that it does not enter the base material from the surface of the discharge treatment, the discharge coating layer will remain, and as a proof of this, the mechanical polishing is performed to near the surface of the base material. As a result of the nitriding treatment, the surface hardness is sufficiently improved as can be seen from FIG.
[0027]
Next, the surface state by nitriding after the discharge surface treatment will be described. Nitriding the discharge treatment surface has the following important significance.
(1) It is widely known that tensile stress remains on the surface of the EDM surface due to melting and rapid cooling, and nitriding the base material after EDM not only increases the hardness. When the nitrogen enters, volume expansion is caused to reduce the tensile residual stress and, in some cases, shift to the compressive stress side. For this reason, wear resistance and the like increase.
(2) By nitriding cutting tools and plastic working molds that have been discharge-treated with a titanium (Ti 2) green compact electrode, the affinity with iron, which is the workpiece, is reduced, and wear due to adhesion is reduced. It has the effect of reducing and increasing the wear resistance.
(3) Further, as described above, since the surface roughness is not changed at all by nitriding, the finished surface roughness before nitriding is maintained. That is, the wear resistance can be improved under a good finished surface. (See Figure 3)
[0028]
Next, the structural concept of the surface of the base material nitrided after the discharge surface treatment and further nitrided will be described with reference to FIG.
The surface structure in the case where the discharge surface treatment layer cannot be sufficiently thick due to processing time restrictions, dimensional restrictions, etc., is not smooth as a whole, as shown in FIG. Smooth surface. This is not necessarily good if the finished surface roughness is measured, but if a low coefficient of friction is required or if wear resistance is required, it is a surface that can take a large load, and the recess is rather Since it acts as an oil groove for a lubricant such as oil, a good result is obtained.
[0029]
Next, when X-ray diffraction measurement and component analysis of the coating layer were tried, the surface of the nitrided surface after grinding the processed surface with a compacted electrode of titanium hydride (TiH ₂ ) by X-ray analysis measurement. As a result of X-ray diffraction analysis, it was confirmed that nitrogenated titanium carbide (TiCN) and titanium nitride (TiN) were present.
[0030]
Embodiment 2. FIG.
In the first embodiment, the example in which the discharge treated surface of the base material subjected to the discharge treatment with the green compact electrode of titanium hydride (TiH ₂ ) is polished with a round bar coated with diamond paste on the surface has been illustrated and described. Of course, any means may be used as long as it is a surface grinding combined with an electrochemical action such as mechanical polishing by manual motion, rotational motion, reciprocating motion, ultrasonic vibration or the like, or electrolytic grinding.
[0031]
Embodiment 3 FIG.
Next explained is the third embodiment of the invention. One of the uses of the present invention is a recoating process of an end mill or a drill coated with titanium nitride (TiN) or aluminum nitride [Ti (AlN)]. In that case, in order to remove the worn part, it is necessary to carry out a coating treatment after repolishing with a diamond wheel or the like. A discharge processing method that does not require re-polishing will be described.
[0032]
FIG. 8 shows the surface property of the base material when the surface coating layer by discharge is thickened. In the case of FIGS. 6a to 6d described above, the discharge current Ip = 7 A and the discharge pulse width Ton = 2 μs. However, in the case of FIG. 8, the discharge current Ip = 7 A and the discharge pulse width Ton = 16 μs. As can be seen from FIG. 8, since it can be easily coated to a thickness of 20 μm or more in about 10 minutes, a normal tool wear portion due to cutting can be repaired. If the discharge pulse width Ton is increased to about 32 μs, the thickness of about 100 μm is easily reached. In this case, the finished roughness becomes as rough as about 20 μm, but this is ground with a diamond wheel or the like to form a tool edge shape, and the finished surface roughness is also finished to about 1 μm Rmax necessary for a cutting tool surface. Thereafter, nitriding is performed.
[0033]
In this way, as long as the cutting tool is not significantly damaged, re-coating can be performed without causing re-polishing work and reducing the size of the cutting tool due to re-polishing. The reduction in the tool size due to re-polishing limits the number of re-polishing of the tool.
[0034]
FIG. 9 is a diagram showing a wear form of the cutting tool. In the case of re-polishing, it is necessary to remove even the part that becomes the base of the tool base material in order to remove the worn part, and the amount of grinding removal becomes remarkably large. If repair is made by embedding by discharge surface treatment, the amount of removal will be small and the number of times the tool will be used will increase significantly.
[0035]
In addition, as shown in FIG. 9, when the cutting tool is greatly worn, even if the discharge is performed with the green compact electrode from above, the discharge is performed only on the convex portion on the surface, and therefore only the convex portion is merely The shape correction may be difficult due to the high deposition. At that time, if the electrode is processed by applying a rotation or swing motion, the portion deposited on the convex portion is removed by discharging with the electrode moving from the lateral direction, and the concave portion is gradually buried. If the embedding is still inadequate, the green compact component is kneaded into an adhesive material such as Araldite and applied to the surface including the recesses, and then the green compact electrode or, in some cases, normal electrical discharge machining. If the discharge is performed with an electrode such as copper, graphite, or tungsten-silver used in the above, the finish surface roughness is not good, but embedding can be performed, and nitriding is performed thereon.
[0036]
This method can be used not only for correcting a damaged portion of a cutting tool but also for correcting a metal or a bearing portion, and can be applied to all industrial fields.
[0037]
Next, the blunting of the blade edge by the discharge process and the correction method will be described.
When performing discharge surface treatment on a sharp part such as a tool edge, the edge tends to be blunted. The reason is that it is processed with an electrode such as titanium hydride (TiH ₂ ) green compact. However, since the sharp blade edge has a high potential gradient, the discharge concentrates there, so that it tends to slow down.
[0038]
The method of correcting blunting is to perform discharge coating to a thickness sufficient to contain the cutting edge, and then finish the shape and finished surface into a shape preferable for the cutting operation by polishing means, and then nitriding I do.
[0039]
Embodiment 4 FIG.
Although the nitriding apparatus is shown in FIG. 2, the following can be cited as other embodiments.
Like a soldering iron heating device, a coil is wound with nichrome wire, and a heating part such as an end mill or a drill is placed in the coil. If this is placed in a nitrogen atmosphere and energized, it can easily reach about 500-600 ° C. Since nitriding is performed at about 300 ° C. or higher, heating in a nichrome wire coil is sufficient.
[0040]
Alternatively, partial nitridation may be performed by irradiating laser light (which may be either CO ₂ or YAG) while flowing nitrogen gas through the portion to be nitrided.
[0041]
Embodiment 5 FIG.
Next, a fifth embodiment will be described. This embodiment explains the production of titanium subnitride (Ti ₂ N) by adjusting the nitrogen atmosphere. As shown in FIG. 10, the amount of wear of the cutting tool is titanium subnitride (TiN) rather than titanium nitride (TiN). ₂ N) is known to be less. Therefore, even in the nitriding treatment, the use of a mixture of argon gas and nitrogen gas in order to reduce the partial pressure of nitrogen in the atmosphere resulted in the formation of titanium subnitride (Ti2N). Abrasion improved. The implementation condition was a volume ratio at atmospheric pressure, and argon gas: nitrogen gas = 70: 30.
[0042]
【The invention's effect】
As described above, according to the present invention, a metal powder that is carbonized and hardened is compression-molded to form an electrode for electric discharge machining. This is a combination of so-called cutting technology and nitriding technology, in which the surface of the treated body is polished or spark-ground, and then the object to be treated is nitrided. It is possible to obtain a finished surface roughness and to form a surface layer having tough wear resistance.
[Brief description of the drawings]
FIG. 1 is a schematic diagram for explaining a first embodiment of the present invention.
FIG. 2 is a schematic configuration diagram of a nitriding apparatus used in the first embodiment of the present invention.
FIG. 3 is a diagram showing the measurement results of the surface roughness of the object to be processed when nitriding is performed according to the first embodiment of the present invention.
FIG. 4 is a diagram showing the measurement results of the surface hardness of an object to be processed when nitriding is performed according to the first embodiment of the present invention.
FIG. 5 shows a change in hardness of a cross section extending from the surface of an object to be processed to the inside of a coating layer in the surface treatment according to the first embodiment of the present invention.
FIGS. 6A and 6B are diagrams showing cross-sectional profiles before and after polishing of the discharge-treated surface in the surface treatment according to the first embodiment of the present invention. FIGS.
FIG. 7 is a view for explaining the structural concept of the surface of an object to be treated which is subjected to polishing after the discharge surface treatment in the surface treatment according to the first embodiment of the present invention and is then nitrided.
FIG. 8 is a view showing a treated surface property when the surface coating layer of the object to be treated is thickly attached in the surface treatment according to the third embodiment of the present invention.
FIG. 9 is a diagram showing a wear form of a cutting tool in the surface treatment according to the third embodiment of the present invention.
FIG. 10 is a diagram showing the relationship between the hardness of a coating substance and the amount of wear in surface treatment according to a fifth embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 To-be-processed object, 2 Electric discharge process surface, 3 Tool, 20 Housing | casing, 21 1st storage container, 22 heater, 23 2nd heater, 24 pipe lines.

Claims (4)

An electric discharge machining electrode is formed by compression-molding a metal powder that is carbonized and hardened, and the surface to be treated is subjected to discharge surface treatment in a machining liquid in which carbon is decomposed by electric discharge, and then the surface of the workpiece is polished, Further thereafter, a discharge surface treatment method of nitriding the workpiece. A metal powder that is carbonized and hardened is compression-molded to form an electrode for electric discharge machining, and the object to be treated is subjected to discharge surface treatment in a machining fluid in which carbon is decomposed by electric discharge, and then the surface of the object to be treated is subjected to electric discharge grinding. And a discharge surface treatment method of nitriding the object to be treated. 2. The electric discharge machining electrode is formed by compression molding by mixing at least one of high hardness carbide, nitride and boride into a metal powder which is hardened by carbonization. 3. The discharge surface treatment method according to any one of 2 above. The discharge surface treatment method according to any one of claims 1 to 3, wherein the nitriding treatment is performed in a mixture of argon gas and nitrogen.

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