JP2004115836A - Nitriding treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitriding treatment method by which an oxide film on the surface of maraging steel is removed to activate the metal surface, and defects are not caused on the metal surface. <P>SOLUTION: Maraging steel W is heated in the presence of a halogen compound gas, and an oxide film on the surface of the maraging steel W is removed to form a halide film. The maraging steel W on which the halide film is formed is heated in a vacuum or in a pressure-reduced atmosphere to remove the halide film. The maraging steel W from which the halide film is removed is heated in the presence of gaseous ammonia, and is subjected to nitriding treatment. The maraging steel W on which the halide film is formed is heated in a vacuum or in a pressure-reduced atmosphere in the temperature range of 450 to 490°C for 5 to 10 min, so that the halide film is removed. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for nitriding a maraging steel used for a belt for a continuously variable transmission or the like.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in order to transmit power to a continuously variable transmission (CVT) of an automobile or the like, a CVT belt in which a plurality of metal rings are laminated to form a laminated ring, and the laminated ring is attached to an element having a predetermined shape and held. Are known.
[0003]
The metal ring forming the laminated ring is formed by cutting a thin plate of low carbon steel containing Ni and Mo, such as maraging steel, into a rectangular shape, rounding the thin plate along a long side, and welding the ends on the short side. After forming a cylindrical drum, the drum is cut into a predetermined width. The maraging steel is age-hardened in a martensitic state by being heated to an appropriate temperature, and is a super-strength steel having both high strength and high toughness.
[0004]
However, when a laminated ring formed by laminating the metal rings is used for power transmission of the CVT, it is desired that the metal ring be further provided with wear resistance and fatigue resistance. Therefore, a surface hardening treatment is performed on the age-hardened maraging steel.
[0005]
The surface hardening treatment is generally performed by performing a nitriding treatment on the metal ring to form a nitrided layer on the surface of the metal ring. Examples of the nitriding treatment include a gas nitriding treatment and a gas nitrocarburizing treatment.
[0006]
The gas nitriding treatment is performed by heating the metal ring in a heating furnace to, for example, 500 to 550 ° C., and then maintaining the temperature in the above range in an ammonia gas atmosphere for a predetermined time. Further, the gas nitrocarburizing treatment is performed in exactly the same manner as the gas nitriding except that a mixed gas atmosphere of ammonia gas and RX gas is used instead of the ammonia gas atmosphere in the gas nitriding treatment.
[0007]
According to the gas nitriding treatment or the gas soft nitriding treatment, nitrogen generated by decomposition of ammonia permeates into the metal structure of the maraging steel. As a result, a nitride layer is formed and hardened on the surface of the metal ring, and the wear resistance and fatigue resistance of the metal ring can be improved.
[0008]
However, in order for nitrogen to penetrate into the metal structure of the maraging steel, it is necessary that the surface of the metal ring be activated. Is not formed uniformly. In order to solve the problem, prior to the gas nitriding treatment or the gas nitrocarburizing treatment, a technique of replacing the oxide film with a fluoride film by heating the metal ring in an atmosphere containing a fluorine compound or fluorine is known. It has been proposed (for example, see Patent Document 1).
[0009]
According to the technique, when the gas nitriding treatment or gas nitrocarburizing treatment is performed on the metal ring on which the fluoride film is formed, the surface of the metal ring is activated instead of decomposing and disappearing the fluoride film, and the active state is increased. It is said that nitrogen penetrates into the converted metal surface to form a uniform nitrided layer.
[0010]
However, in the gas nitriding treatment or the gas nitrocarburizing treatment, ammonia is decomposed into hydrogen and nitrogen, and the hydrogen combines with fluorine generated by decomposition of the fluoride film to form hydrogen fluoride having strong oxidizing property. In addition, there is a disadvantage that the surface of the metal ring is eroded by the hydrogen fluoride to cause defects. If the surface of the metal ring has the defect, when the metal ring is stacked and used as a CVT belt, it may cause cracks or the like.
[0011]
[Patent Document 1]
Japanese Patent Publication No. Hei 8-9766 (page 2)
[0012]
[Problems to be solved by the invention]
An object of the present invention is to provide a nitriding treatment method which can eliminate such an inconvenience, activate an metal surface by removing an oxide film on the surface of a maraging steel, and can generate no defects on the metal surface. With the goal.
[0013]
[Means for Solving the Problems]
In order to achieve this object, the nitriding method of the present invention comprises the steps of: heating a maraging steel in the presence of a halogen compound gas to remove an oxide film on the surface of the maraging steel and form a halide film. And removing the halide coating by heating the maraging steel on which the halide coating is formed in a vacuum or reduced-pressure atmosphere, and converting the maraging steel from which the halide coating has been removed with ammonia gas. And performing a nitriding treatment by heating in the presence.
[0014]
In the nitriding method of the present invention, first, a maraging steel is heated in the presence of a halogen compound gas. By doing so, the oxide film on the surface of the maraging steel reacts with the halogen compound and is removed, and instead, a film of a halide is formed on the surface of the maraging steel. As the halogen compound gas, a gas such as a fluorine compound, a chlorine compound, and a bromine compound can be used.
[0015]
Next, the maraging steel on which the halide coating is formed is heated in a vacuum or reduced pressure atmosphere. In this way, the halide film is decomposed and removed. However, in the nitriding method of the present invention, ammonia does not exist at this time, and the halogen generated by the decomposition of the film is converted into halogen gas without generating acid. Removed. As a result, the surface of the maraging steel has neither the oxide film nor the halogenated film, and is in an extremely clean and activated state.
[0016]
Then, by heating the maraging steel from which the halide coating has been removed in the presence of ammonia gas and performing a nitriding treatment, a uniform nitride layer can be formed without causing defects on the surface of the maraging steel. Can be formed. The nitriding treatment performed in the presence of the ammonia gas may be a gas nitriding treatment or a gas nitrocarburizing treatment.
[0017]
In the nitriding treatment method of the present invention, the maraging steel on which the halide coating is formed is heated in a vacuum or reduced pressure atmosphere at a temperature in the range of 450 to 490 ° C. for 5 to 10 minutes to thereby form the halide coating. Is preferably removed.
[0018]
If the temperature is lower than 450 ° C., it may be difficult to perform a nitriding treatment subsequent to the removal of the halide film, and if it is higher than 490 ° C., the residual stress may be excessive. If the heating time is less than 5 minutes, it is difficult to discharge the generated halogen gas. If the heating time is more than 10 minutes, the residual stress is reduced, and it is difficult to obtain sufficient strength.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. FIG. 1 is a process explanatory view schematically showing a manufacturing process of a metal ring used as a CVT belt.
[0020]
When manufacturing the metal ring, first, as shown in FIG. 1, a thin sheet 1 of maraging steel is bent and looped, and then the ends are welded to form a cylindrical drum 2. The maraging steel is a low-carbon steel having 0.03% or less of C, 0.10% or less of Si, 0.10% or less of Mn, 0.01% or less of P, and 0.01% or less of S. Yes, 18-19% Ni, 4.7-5.2% Mo, 0.05-0.15% Al, 0.50-0.70% Ti, 8.5-9.5% This is a so-called 18% Ni steel containing Co. At this time, since the maraging steel exhibits age hardening due to the heat of welding, high hardness portions appear on both sides of the welded portion of the drum 2.
[0021]
Then, next, the drum 2 is accommodated in the vacuum furnace 3 and held at a temperature of 820 to 830 ° C. for 20 to 60 minutes to perform a first solution treatment to remove unevenness in hardness. When the first solution treatment is completed, the drum 2 is unloaded from the vacuum furnace 3 and cut into a predetermined width to form a metal ring W.
[0022]
The metal ring W formed as described above is then rolled at a reduction of 40 to 50%. A rolled structure is formed on the rolled metal ring W with a thickness of about 30 μm from the surface. Then, the rolled metal ring W is accommodated in the heating furnace 4 and subjected to a second solution treatment, whereby the rolled structure is eliminated and uniform metal crystal grains are formed.
[0023]
Next, the solution-annealed metal ring W is stored in the heating furnace 5 after the circumference correction, and is kept at a temperature in a range of 445 to 475 ° C. for 30 to 150 minutes to perform an aging treatment. When the aging treatment is completed, the metal ring W is cooled in the heating furnace 5 and transferred to a nitriding device to perform a nitriding treatment.
[0024]
In the nitriding treatment of the present embodiment, first, the metal ring W that has been subjected to the aging treatment is housed in the heating furnace 6 and in the presence of a chloride gas such as methylene chloride at a temperature in the range of 440 to 490 ° C. Heat for 1-10 minutes. An oxide film is often formed on the surface of the metal ring W, but when heated in the presence of the chloride gas, the oxide film reacts with the chloride and is removed, and instead, the metal ring W A chloride film is formed on the surface.
[0025]
When the heating in the presence of the chloride gas is completed, the chloride gas is then discharged, and the inside of the heating furnace 6 is further depressurized to a vacuum or a reduced pressure atmosphere of less than 10 ⁻³ Pa. . Then, the metal ring W on which the chloride film is formed is heated at a temperature in the range of 450 to 490 ° C. for 5 to 10 minutes under the vacuum or reduced pressure atmosphere. Note that the heating furnace 6 is constantly evacuated in order to maintain the vacuum or reduced pressure atmosphere.
[0026]
In this case, the chloride film is decomposed to generate chlorine gas, and the chlorine gas is exhausted. On the other hand, the surface of the metal ring W is in a clean and activated state by the decomposition and removal of the chloride film.
[0027]
After the completion of the heating under the vacuum or reduced pressure atmosphere, an atmosphere containing at least ammonia gas is introduced into the heating furnace 6, and the atmosphere is heated at a temperature in the range of 440 to 490 ° C. for 30 to 120 minutes. The metal ring W is nitrided by heating. The atmosphere introduced into the heating furnace 6 may be pure ammonia gas, an ammonia gas atmosphere containing an inert gas or the like in addition to pure ammonia gas, or a mixed gas atmosphere of pure ammonia gas and RX gas. Is also good.
[0028]
In the present embodiment, when heating is performed in an atmosphere containing the ammonia gas, the surface of the metal ring W has no chloride film, and the surface is in a clean and activated state as described above. . Therefore, the hydrogen generated by the decomposition of ammonia does not react with the chloride film to generate hydrogen chloride, thereby preventing the surface of the metal ring W from being damaged by the hydrogen chloride and causing defects. be able to. Further, since the surface of the metal ring W is clean and activated, a uniform nitride layer can be formed.
[0029]
In the present embodiment, the case where the oxide film on the surface of the metal ring W is removed with a chloride gas is described. However, instead of the chloride gas, another halide gas such as a fluoride gas or a bromide gas is used. May be used.
[Brief description of the drawings]
FIG. 1 is a process explanatory view schematically showing a manufacturing process of a metal ring.
[Explanation of symbols]
1: Thin sheet of maraging steel, W: Metal ring of maraging steel.

Claims (2)

Heating the maraging steel in the presence of a halogen compound gas to remove an oxide film on the surface of the maraging steel and form a halide film;
Removing the halide coating by heating the maraging steel having the halide coating formed thereon under a vacuum or reduced pressure atmosphere;
Heating the maraging steel from which the halide coating has been removed in the presence of ammonia gas to perform a nitriding treatment. The maraging steel on which the halide coating is formed is characterized in that the halide coating is removed by heating at 450 to 490 ° C. for 5 to 10 minutes in a vacuum or reduced pressure atmosphere. The nitriding method according to claim 1.

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