JP3315734B2 - Plasma carburizing and quenching method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for plasma carburizing and quenching.

[0002]

2. Description of the Related Art A plasma carburizing and quenching method is disclosed in
As disclosed in Japanese Patent No. 5857, this method is already known, and if this method is used, carburization is performed in a vacuum, so that chromium, manganese, silicon, etc. Is no longer internally oxidized, and the formation of an abnormal surface layer on the metal material can be prevented.

[0003]

However, in the above-mentioned plasma carburizing and quenching method, a quenching treatment is performed after the plasma carburizing treatment, and this quenching treatment causes heat treatment distortion of the entire metal material. ing. For this reason, in a material that requires accuracy, for example, a gear material, the distortion of the material body affects even the tooth portion, and a highly accurate material cannot be obtained. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a plasma carburizing and quenching method capable of obtaining a product having high strength and high accuracy.

[0004]

According to the present invention, in order to achieve the above object, the surface carbon concentration of a metal material reaches at least a saturation concentration at a first predetermined temperature. After performing the first plasma carburizing process as described above, the diffusion process is performed while maintaining the first predetermined temperature. Then, the metal material subjected to the diffusion process is subjected to the first predetermined temperature from the first predetermined temperature. The first slow cooling process was performed until the temperature reached a second predetermined temperature that was lower than the temperature, higher than the transformation point and near the transformation point, and then the first slow cooling process was performed. For a metal material, under the second predetermined temperature,
The second plasma carburizing treatment is performed so that the surface carbon concentration of the metal material reaches at least the saturation concentration.
For the metal material subjected to the plasma carburizing treatment, the second
Performing a second slow cooling process from a predetermined temperature to a third predetermined temperature lower than the second predetermined temperature, and then performing local quenching on the metal material that has been subjected to the second slow cooling process. The plasma carburizing and quenching method is a feature.

[0005]

According to the present invention, since the gradual cooling process (first and second gradual cooling processes) is performed instead of the quenching process, heat treatment distortion does not occur in the entire metal material. On the other hand, since the local quenching is performed after the above-described slow cooling process, it is possible to accurately secure the strength of the place where the strength is required. Therefore, it is possible to provide a plasma carburizing and quenching method capable of securing strength at a necessary portion in a product and obtaining high accuracy. Also, for metal materials,
After performing the first plasma carburizing process at the first predetermined temperature so that the surface carbon concentration reaches at least the saturation concentration, the diffusion process is performed while maintaining the first predetermined temperature. (Temperature, surface carbon concentration) can be determined while clearly grasping, and the first slow cooling process in which the slow cooling time is shortened is defined as the second slow cooling time using the position after the clear diffusion process as a starting point (reference point). By performing the process up to the predetermined temperature, it is possible to suppress the variation in the slow cooling time during that period, and to accurately shift the surface carbon concentration to the predetermined second plasma carburizing region. On the other hand, after this, the metal material subjected to the first slow cooling process is subjected to a second predetermined temperature at a second predetermined temperature.
The second plasma carburizing treatment is performed so that the surface carbon concentration of the metal material reaches at least the saturation concentration, the position after the second plasma carburizing treatment is clarified, and the clear position is defined as a starting point (reference point). The second slow cooling process in which the slow cooling time is shortened is performed up to the third predetermined temperature, and the variation of the slow cooling time during the second cooling process is suppressed, and the surface carbon concentration at the third predetermined temperature is reduced by the second plasma carburizing process. It can be set as desired according to the conditions and the second slow cooling processing conditions. For this reason, according to the method, the first plasma carburizing treatment, the diffusion treatment, and the second plasma carburizing treatment condition and the second slow cooling treatment condition are set in advance so as to have a desired surface carbon concentration. After the first slow cooling process, the second plasma carburizing process, and the second slow cooling process, the surface carbon concentration at the end point of the slow cooling process can be set to a desired concentration with high reproducibility.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. As a premise, the method according to the present embodiment is applied to 0.2% C steel as a metal material, and the surface carbon concentration after the completion of the slow cooling is set to 0.8 ± 0.02 (wt%). . (1) First, as shown in FIG. 1 and FIG.
The temperature is raised to 00 ° C. (first predetermined temperature) (step: end point P1). This step is performed to make austenite, and is a known step prior to the next plasma carburizing treatment. The reason why the temperature is raised to 1000 ° C. is to continuously perform the next plasma carburizing process at the same temperature. For this reason, this heating process is performed in a vacuum atmosphere in a furnace for performing the plasma carburizing process.

(2) Next, plasma carburizing treatment (first plasma carburizing treatment) is performed at 1000 ° C. (step: start point P 1, end point P 2). This step is for carburizing the metal material as in the other carburizing treatments. However, as described above, this step is performed to prevent the metal material from forming an abnormal surface layer. They focused on the fact that the plasma carburizing process was performed in a vacuum atmosphere.

[0008] Specifically, the plasma carburizing treatment is performed by an apparatus shown in FIG. That is, in FIG. 3, in a state where the metal material (object to be processed) 3 is placed on the cathode 2 in the furnace 1, the inside of the furnace 1 is evacuated by the vacuum pump 4, After the metal material 3 is heated to a certain temperature by the heater 6 applied through the power supply 5 (via the heating control system 11) (the above steps),
A hydrocarbon gas (C3 H8) 8 is supplied from a gas supply means 9 between the heater 6 serving also as a discharge electrode (anode) and the cathode 2, and thereafter, the two electrodes 2, A glow discharge is generated by applying a high DC voltage to 6.
As a result, activated carbon ions 10 are generated, and the activated carbon ions 10 collide with the surface of the metal material 3 and bind to iron. In FIG. 3, the solid arrow indicates the ion current,
Dashed arrows indicate carbon diffusion. Reference numeral 12 denotes a temperature measuring element, 13 denotes a temperature adjustment system, and 14 denotes a vacuum seal. The reason why the above-mentioned operation is performed at 1000 ° C. is to increase the carburizing speed and to shorten the carburizing time for obtaining a predetermined effective carburizing depth. However, the treatment at 1000 ° C. leads to coarsening of the crystal grains, but the re-heating by high frequency described below can make the crystal grains finer.

In this embodiment, at 1000 ° C.,
Carburizing is performed so that the surface carbon concentration of the metal material reaches at least the Acm line. This is done to clarify the end point P2 (surface carbon concentration (1.55 wt%), temperature) of the carburizing process and use it as a reference point for the next step. In this case, the end point P2 of the carburizing process
In order to clarify, under a constant temperature (1000 ° C.), A
It is utilized that when the carbon line reaches the cm line, the surface carbon concentration does not change beyond the saturation concentration even if the carburizing treatment is continued.

(3) Next, in this embodiment, prior to the slow cooling process described later, a diffusion process is performed at 1000 ° C. (step: start point P2, end point P3). This is performed to adjust the surface carbon concentration at the later-described slow-cooling processing start point (diffusion processing end point) P3. In this case, the surface carbon concentration at the point P2 is determined in a clear state, and the diffusion process is performed at a constant temperature to keep the diffusion rate of carbon constant. If only the treatment time is controlled, the surface carbon concentration at point P3 will be determined by itself. Therefore, the point P3 can be determined in a clear state, and the point P3 can be used as a reference point in the next process.

(4) Next, a first slow cooling process is performed as a slow cooling process (step: start point P3, end point P4). This is performed in order to prevent heat treatment distortion of the entire metal material as in the case of the rapid cooling process.

(5) Next, when the metal material reaches 750 ° C. (second predetermined temperature) by the first slow cooling process, a new plasma carburizing process (second plasma carburizing process) is performed again, and the plasma Carburizing treatment is performed under the condition of Ac
The process is performed so as to reach the m-line (step: start point P4, end point P5). This is done for two reasons. The first is that, by interposing a new plasma carburizing process during the slow cooling process, the slow cooling process is divided, and the temperature difference to be lowered by one slow cooling process is narrowed to one slow cooling process. This is for shortening the required slow cooling time. Part 2
When the plasma carburizing treatment is performed so as to reach the Acm line at a certain temperature, the fact that the surface carbon concentration does not rise above the saturation concentration at the certain temperature is utilized, and thus the first slow cooling treatment is performed. This is because a new end point (surface carbon concentration, temperature) of the new plasma carburizing treatment can be obtained in a clear state at a temperature lower than the temperature at the start (1000 ° C.). For these two reasons, it is possible to reduce the variation of the slow cooling time (the slow cooling time of the second slow cooling process described later) due to the degree of filling of the metal material and the like, and the slow cooling process (the second slow cooling process described later) In order to obtain a desired surface carbon concentration after completion, the end point of a new plasma carburizing process can be accurately and easily determined. In this embodiment, the temperature at the time of new plasma carburizing treatment is set to 750 ° C.
However, considering that the desired surface carbon concentration after the completion of the slow cooling treatment is 0.8 ± 0.02 (wt%), the surface carbon concentration at the new plasma carburizing treatment end point P5 is set to the above value. It shall be within the allowable range of the surface carbon concentration (0.8
2 wt%), and the second slow cooling process described later attempts to achieve a more desired surface carbon concentration.

(6) Next, as a slow cooling process, a second slow cooling process is performed (step: start point P5, end point is not shown in FIG. 2). This is performed for the purpose of lowering the temperature to a third predetermined temperature (650 ° C. in this embodiment) without causing heat treatment distortion.

In this case, at the end point of the second slow cooling process, the surface carbon concentration needs to be a desired value (0.8 ± 0.02 wt% in the present embodiment). A new plasma carburizing process is provided before the process,
As described above, a new plasma carburizing process is performed in order to reduce the variation in the slow cooling time of the second slow cooling process depending on the degree of filling of the metal material and to obtain a desired surface carbon concentration after the second slow cooling process. Can be determined accurately and easily, so that the desired surface carbon concentration can be reliably obtained.

(7) Next, the metal material is subjected to induction hardening as local hardening after gas cooling.
This induction hardening is performed in order to properly secure the strength where metal material requires strength. Therefore, strength can be secured at necessary places, but high accuracy can be obtained because, in the preceding step, annealing is performed instead of quenching to prevent heat treatment distortion from occurring in the entire metal material. become.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a process according to an example.

FIG. 2 is a diagram illustrating a method according to an embodiment.

FIG. 3 is a diagram conceptually showing a plasma carburizing apparatus.

[Explanation of symbols]

 1 furnace 3 metal material

Continuation of front page (56) References JP-A-58-96865 (JP, A) JP-A-4-52265 (JP, A) JP-A-2-232354 (JP, A) JP-A-2-145759 (JP) JP-A-4-76878 (JP, A) JP-A-5-148535 (JP, A) Patent 2890422 (JP, B2) (58) Fields studied (Int. Cl. ⁷ , DB name) C21D 1 / 06 C23C 8/38 C23C 8/80

Claims (1)

(57) [Claims] 1. A method according to claim 1, wherein the metal material is under a first predetermined temperature.
In this way, the surface carbon concentration of the metal material is at least a saturated concentration.
After performing the first plasma carburizing treatment so as to reach the first
Performs spreading processing while maintaining a predetermined temperature, then, to the diffusion process metallic material, the first plant
From the constant temperature to a temperature lower than the first predetermined temperature and
A second place where the temperature is higher than the transformation point and near the transformation point
The first slow cooling process is performed until a constant temperature is reached, and then the first slow cooling process is performed on the metal material.
Under the second predetermined temperature, the surface carbon concentration of the metallic material is low.
2nd plasma carburizing treatment to reach saturation concentration at least
Was carried out, then, against the metal material was subjected to the second plasma carburizing
The second predetermined temperature is lower than the second predetermined temperature.
The second slow cooling process is performed until the temperature reaches the third predetermined temperature, and thereafter, the metal material subjected to the second slow cooling process is
A plasma carburizing and quenching method characterized by performing local quenching.