KR100327741B1 - Method for surface treatment of forging die - Google Patents

Abstract

The present invention relates to a surface treatment method for improving the life of the forging die for automobile parts.

The method of the present invention comprises a sputtering process of forming a plasma around a forging die charged in a vacuum furnace and washing and heating the mold surface using hydrogen gas and argon gas, and using a bipolar micropulse plasma. Nitriding process to allow nitrogen atoms to diffuse into the mold, Glowing process to decompose and diffuse nitrogen compounds formed on the surface of the mold, and steam inside the furnace after the diffusion process. It is composed of a series of stepwise processes in which an oxidation process of supplying and forming an Fe ₃ O ₄ oxide film on the surface of the mold is additionally performed.

In the surface treatment method of the forging die of the present invention, by uniformly curing the forging die having a complex shape and a non-uniform surface through a micropulse plasma treatment, the overall life of the mold is improved by overcoming the limitation of the shape and improving the wear resistance. In addition, the plasma heat treatment is performed after the nitriding process to decompose and diffuse nitrogen compounds on the surface of the mold, thereby lowering the hardness of the outermost surface and increasing the toughness. There is an advantage to improve the impact resistance.

Description

Method for surface treatment of forging die

The present invention relates to a surface treatment method of a forging die, and more particularly, by using a micropulse plasma to form a hardened layer uniformly on the surface of the mold of a complex shape to improve the wear resistance while following the plasma nitridation treatment The present invention relates to a forging mold surface treatment method which can improve corrosion resistance and impact resistance by forming an oxide layer as a subsequent step.

In general, forging molds used for the production of various automotive parts including connecting rods and spindles are hardened to the surface of the mold for improved wear resistance. The chromium plating method and gas nitriding are mainly performed.

However, in the conventional hard chromium plating, the chromium plating layer formed on the surface of the forging die has a softening property at a temperature of 400 ° C. or higher. It is pointed out that the deformation occurs in the base material due to the stress of. In addition, the hard chromium plating method has a problem that harmful pollutants are discharged during plating.

On the other hand, gas nitriding, one of the nitriding methods, is used to harden various tools such as cutters and drills, as well as forging dies, by forming nitride on the surface of the base material to improve hardness. In some cases, nitride is rapidly formed at grain boundaries, and in this case, there is a problem that peeling of the surface nitride layer occurs due to a nitride needle.

In particular, when nitriding a forging die using gas nitriding, a white layer is formed on the outermost surface and a needle is formed at the grain boundary. Energy is concentrated on the needle, which in turn can cause cracks.

As forging die-hardening treatment methods other than the above-mentioned methods, the so-called T.D (Toyoda Diffusion Process) treatment and the plasma nitriding treatment which harden the surface using chemical | medical agent in 900 degreeC or more salt bath are known.

In the case of the T.D treatment, since a severe deformation occurs after the treatment, there is a problem that requires a post-processing process.

The plasma nitridation treatment has attracted a lot of attention in terms of environmental friendliness with the advantage that it does not involve deformation even after the treatment, but it is difficult to produce a uniform hardened layer for complex forging molds. It has problems to be solved such as adhesion.

The present invention was devised in view of various problems pointed out in the surface treatment method of the conventional forging mold, and irregular and complicated by controlling nitrogen atoms and electrons in a plasma state using a bipolar micropulse plasma. It is an object of the present invention to provide a forging die surface treatment method aimed at improving abrasion resistance through overcoming the limitation of the shape by performing uniform surface treatment over the entire area of the mold having the surface shape.

Another object of the present invention is to forge a mold to improve the impact resistance by lowering the outermost surface hardness of the mold by decomposing nitrogen compounds formed on the surface of the mold through the diffusion process following plasma nitridation and diffusing into the metal. It is to provide a surface treatment method of.

It is still another object of the present invention to provide a forging die surface treatment method in which an oxide film is formed after completion of a plasma nitriding treatment followed by a diffusion process to improve corrosion resistance and high temperature lubrication of a mold.

The technical background and basic principles and characteristics of the micropulse plasma surface treatment method as the most important technical matters for achieving the above object of the present invention will be briefly described.

In general, in order to improve the life through the plasma surface treatment on the surface of the iron-based material, the surface reaction must be activated and proceed in the plasma state, and the plasma must be uniformly formed throughout the product regardless of the limitation of the shape.

In order to satisfy the above two conditions, the number of iron atoms separated from the base material must be large, and the number of neutral nitrogen atoms present in the plasma state must be large.

The probability that these two types of atoms will react with each other increases as the power of the plasma increases, so it would be advantageous to use a higher voltage, but the voltage that can be used for plasma nitridation is transferred to the area before moving to the arc region. It is limited.

In addition, the more complicated the shape of the material to be processed, the thinner the CFT (Cathode Fall Thickness) should be formed at a higher pressure and voltage. The increase in pressure in the presence of the plasma means the breakdown of the vacuum state. The use of voltage is inevitable.

Particularly, in case of surface treatment using a conventional plasma nitridation method for a product having a complex surface shape such as a forging die for automobile parts, it is localized by ions causing impact energy and electrons generating HCD (Hollow Cathode Discharge). Phosphorus overheating occurs and the adverse effect of the softening of the material appears.

Accordingly, in the method of the present invention, bipolar micropulse plasma is used to control ions and electrons that cause impact energy when the shape is complex, such as forging dies, so that local overheating of the material is suppressed, thereby making it uniform regardless of the shape. Surface reactions occur.

That is, in the present invention, the anode is instantaneously applied to the mold to dissipate the electrons and remove the shield generated during the plasma surface treatment to maintain a uniform plasma state so that the surface treatment can be performed without overheating the mold. In addition, to increase the stability of the material, the applied voltage is stopped in μs when the arc is generated.

1 is a longitudinal cross-sectional view of an embodiment specimen of the present invention.

2 is a hardness profile from the surface to the interior of an example specimen treated by the method of the present invention.

The forging die surface treatment method includes a sputtering step of forming a plasma around a forging die charged in a vacuum furnace, and washing and heating the surface using hydrogen gas and argon gas, and a micropulse. Nitriding process that allows nitrogen atoms to diffuse into the mold by using plasma, and Glowing, which allows decomposition and diffusion of nitrogen compounds formed on the surface to improve impact resistance. By forming an oxide film on the surface of the mold to further increase the corrosion resistance and high temperature lubrication (Oxidizing) process may be performed.

The washing step is a kind of pretreatment step, which facilitates diffusion of nitrogen atoms in the subsequent nitriding step. In order to improve the wear resistance of the forging mold, how much nitrogen atom diffuses to the surface of the mold and reacts with the iron system is important. In the washing process, plasma is formed around the mold and hydrogen and argon gas are applied. It is used to remove the impurities and oxide film adsorbed on the mold surface. In the washing process, the temperature of the vacuum furnace is maintained at 400 to 530 ° C. in order to shorten the overall processing time so that the mold is heated to the temperature required for nitriding.

Next, the nitriding process is the most important step to actually form the nitride and diffusion layers on the surface of the mold. Nitrogen atoms having a size similar to that of carbon atoms are diffused into the metal by bipolar micropulse plasma nitriding. When the reaction proceeds rapidly, nitrogen compounds are formed at grain boundaries, which adversely affects impact resistance. On the other hand, if the reaction proceeds too slowly, the depth of the diffusion layer is reduced so that the wear resistance cannot be sufficiently improved.

Therefore, in order to satisfy both impact resistance and abrasion resistance characteristics of the forging mold, the reaction speed must be appropriately controlled.The reaction speed of such forging mold is changed according to the shape of the forging mold, the total loading amount, etc., so pay attention to the adjustment of these variables. in need.

Meanwhile, in the nitriding process, gases such as hydrogen, nitrogen, and methane are used, and some argon gas is also used.

Next, the diffusion process is a step of decomposing the nitrogen compound formed on the surface of the mold through the nitriding process and at the same time to diffuse into the metal to improve the impact resistance. In the mold after the diffusion process, the white layer and the intergranular nitrogen compounds should not remain. The white layer generated on the outermost surface during nitriding is mainly Fe _2-3 N or Fe ₄ N, and has a very brittle characteristic of hardness of Hv1000 or more. Therefore, since the peeling phenomenon occurs during the forging process and the peeled compound causes abrasion ware, the white layer controls the atmosphere to suppress the generation of the white layer as much as possible during the nitriding of the present invention.

In addition, during the diffusion process, the surface hardness should be lowered and activated at the same time. As a result of the diffusion process, the outermost surface of the mold is lower in hardness than the inside thereof, but the toughness is increased.

Hydrogen and some argon are used as the gas used in the diffusion process, and the uniform distribution of plasma plays a very important role.

The forging die surface treatment method of the present invention can be completed by a three-step process leading to the above washing → nitriding → diffusion, but an oxidation process is additionally performed when high temperature wear resistance and surface lubricity and corrosion resistance are required.

The oxidation process maintains the furnace atmosphere in a vacuum state (about 0.2-0.3 mbar) to supply water vapor into the furnace to form Fe ₃ O ₄ as an oxide film on the surface of the hardened mold. In general, during hot forging, the surface temperature of the forging die rises to about 800 ° C. or higher due to the mass flow of the forged product. In view of the fact that the pyrolysis starts at a temperature of 600 ° C. or higher, the mold It is expected that the shortening of the service life will be sufficient. At this time, the oxide film of the present invention serves to inhibit heat transfer from the forged product to the nitride layer, thereby preventing thermal decomposition of the nitride layer, and the oxide film acts as a passive layer to further oxidize the mold surface. To prevent it.

Since the oxide film formed through the oxidation process is dense and uses a chemical reaction, it shows superior bonding strength than the film layer formed by the physical method.

The present invention is described in more detail with reference to examples.

Example

A specimen made of SKD61 material having a cross-sectional shape as shown in FIG. 1 was prepared. The depth d of the groove formed in the specimen is 17.5 mm, and the width w is 3.5 mm.

First, the specimen was charged in a vacuum furnace and hydrogen and argon gas was injected into the furnace while plasma was formed around the specimen to remove impurities and oxide films remaining on the surface of the specimen. This washing process was performed under the conditions of voltage 780V, pressure 0.5-2 mbar, temperature 400-530 degreeC, holding time about 0.7 hours, and bipolar ratio 50: 1.

Next, a nitriding process was performed to form a nitride and a diffusion layer on the surface of the specimen. At this time, hydrogen, nitrogen, methane gas was used, and the furnace conditions were maintained at a voltage of 500 to 560 V, a pressure of 4 to 6 mbar, a temperature of about 530 ° C., a holding time of about 10 hours, and a bipolar ratio of 20 to 25: 1. During the nitriding process, the reaction rate was properly adjusted to suppress the generation of the white layer and the formation of the grain boundary nitrogen compound as much as possible.

Next, the nitrogen compound formed on the surface of the nitrided specimen was decomposed by the nitriding process and diffused into the interior. At this time, hydrogen and some argon gas were used to uniformly distribute the plasma. The conditions of the diffusion process were voltage 530-610V, pressure 5-7 mbar, temperature 530 degreeC, holding time about 3 hours, and bipolar ratio 15: 1.

As a final process, the inside of the furnace was simply maintained in a vacuum of 0.2 to 0.3 mbar while steam was added to form a 3 to 4 μm thick Fe ₃ O ₄ oxide film on the surface of the specimen.

As a result of comparing the characteristics of the specimens of the present invention surface-treated by the above-described method and the comparative specimens surface-treated by the conventional method and examining whether these characteristics are within a predetermined evaluation criteria, the results were as follows. Is shown in Table 1 below.

Evaluation Item Evaluation standard Example Comparative Example Material Hardness Distribution HRc 42 ± 2 HRc 42-43 HRc 42-48 Uniformity of Surface Treatment Hs ± 3 Hs 90-95 Hs 85-105 Hardening depth ＞ 180μm 200-250 μm 150-250 μm White layer <2 μm 0-1 μm 2-5 μm Uniformity of Hardening Depth ± 30μm for each part 220-250 μm 130-240 μm Color after treatment Uniformity of color Uniformity Heterogeneity Surface hardness Hv _0.1 800-950 Hv _0.1 850-920 Hv _0.1 1110-1150 Hardness Hv _0.1 > 1000 Hv _0.1 1070-1200 Hv _0.1 850-920 Needle nitride No none existence Material hardness after treatment HRc 42 ± 2 HRc 42-43 2-3 points down Coarse particle after treatment No change in particle size No change in particle size Particle size change

As shown in Table 1, the sample of the present invention satisfies the evaluation criteria in all evaluation items including hardness distribution and uniformity of surface treatment, and showed superior characteristics compared to the comparative sample.

In addition, the specimen surface-treated in accordance with the method of the present invention exhibits a characteristic that the hardness of the outermost surface portion is lower than that inside as the nitrogen compound of the surface portion is decomposed by the diffusion process and diffused into the interior. The hardness profile of the specimen of the present invention is shown.

As described above, in the surface treatment method of the forging mold of the present invention, by forging a uniform curing treatment to the forging mold having a complex shape and a non-uniform surface through a micropulse plasma treatment to overcome the limitation of the shape and to improve the wear resistance Through this, it is possible to increase the overall life of the mold.

In addition, in the present invention, after performing the nitriding process, plasma heat treatment is performed to decompose the nitrogen compound on the surface of the mold and to diffuse it into the inside so that the impact resistance is improved by lowering the hardness of the outermost surface and increasing the toughness. There is an advantage.

In addition, in the method of the present invention, an oxide film is formed on the surface of the surface-treated mold to prevent heat from the forged product to be transferred to the inside of the mold during hot forging, thereby suppressing thermal decomposition of the nitride hardened layer and consequently the life of the mold. In addition to the extension, the oxide film acts as a passivation layer to suppress further oxidation, thereby improving high temperature corrosion resistance.

In addition, the method of the present invention has the effect of eliminating pollution by adopting an environmentally friendly plasma nitriding treatment method because the whole process is performed in a single apparatus.

Therefore, the method of the present invention can be usefully used for forging various automotive parts including ring gears, spindles, cranks, connecting rods, etc., as well as being applied to extrusion molds, injection molds, and die casting molds. It is expected to be able.

Claims (3)

A sputtering process that forms a plasma around a forging mold charged in a vacuum furnace, and washes and heats the surface of the mold using hydrogen gas and argon gas, and nitrogen atom inside the mold by using a bipolar micropulse plasma. Nitriding process to diffuse into the furnace, Glowing process to dissolve and diffuse into the nitrogen compound formed on the surface of the nitrided mold, and to supply the steam to the furnace after the diffusion process Surface treatment method of forging die, characterized in that the oxidation (Oxidizing) process to form an oxide film of Fe ₃ O _{4 on} the surface. The process of claim 1, wherein the washing process is performed under hydrogen and argon gas under a voltage of 780 V, a pressure of 0.5 to 2 mbar, a temperature of 400 to 530 DEG C, a holding time of about 0.7 hours, and a bipolar ratio of 50: 1. Under nitrogen, methane and methane gas at a voltage of 500 to 560 V, a pressure of 4 to 6 mbar, a temperature of about 530 ° C., a holding time of about 10 hours, and a bipolar ratio of 20 to 25: 1, and the diffusion process is performed at a hydrogen atmosphere of 530 to 610 V. The surface treatment method of the forging die, characterized in that the pressure is carried out under the conditions of 5-7mbar, temperature 530 ℃, holding time about 3 hours and bipolar ratio 15: 1. delete