JPH04364A - Gas soft nitriding method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for gas nitrocarburizing steel parts.

(Prior Art) Nitrocarburizing treatment is performed mainly for the purpose of improving wear resistance and fatigue resistance, and has conventionally been generally performed in a salt bath. However, since this salt bath contains harmful cyanide, there is a risk of causing a pollution problem.Therefore, recently, gas soft nitriding, which is a treatment in a nitriding gas, has been frequently used. This gas soft-nitriding treatment requires a soft-nitriding temperature (
Ammonia gas (
There is a method of introducing NH3) and endothermic metamorphic gas, but recently, in order to avoid the hassle of producing endothermic metamorphic gas and to speed up the nitriding time, NH3 gas and C02 gas are used as the nitriding gas. A method of directly introducing N2 gas into the furnace has been put into practical use (Japanese Patent Laid-Open No. 62-27
No. 0761, JP-A No. 83-255355, etc.).

By the way, the above-mentioned mixed gas of NH3, CO2, and N2 reacts in the furnace at the nitrocarburizing temperature as follows.

2 NH3=:2 (N) + 3 H2...(1
)002 + H2: CO+ N2 0
...(2)2 CO:: (C)+ CO2...
(3) That is, NH3 decomposes according to equation (1) to produce nascent (N), which invades the surface of the workpiece W and causes a nitriding reaction. Also, C02 is N generated by equation (1)
2 to generate CO and N20 (water vapor), and this CO further generates nascent (C) according to equation (3), which penetrates into the work surface and causes so-called carburization.

(Problem to be Solved by the Invention) However, according to the soft nitriding method using the above mixed gas, the CO2 gas contained in the mixed gas and the N20 generated in the above reaction cause an oxidation reaction on the work surface, and this causes However, there is a problem in that porous parts are generated in the nitride layer, leading to a decrease in surface hardness and wear resistance, or a decrease in sliding characteristics and rigidity, resulting in unstable performance of nitride parts.

The present invention was made to solve the above-mentioned conventional problems, and its purpose is to suppress the porous phenomenon of the nitride layer and thereby obtain a nitrided component with stable performance. The object of the present invention is to provide a gas nitrocarburizing method that can be used.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present invention provides that after heating cleaned steel parts in a vacuum or nitrogen atmosphere, NH3,
in a supply atmosphere of CO2 and N2 and between 500 and 800
It is characterized in that it is heated to the nitrocarburizing temperature for a predetermined time under reduced pressure of Torr, and then cooled.

To carry out the method of the present invention, a vacuum pump is connected to the nitriding chamber so that the chamber can be evacuated as desired, and the nitriding chamber is provided with inlets for NH3, CO2 and N2 gases. To enable gas to be introduced into a room with a predetermined composition. In this case, it is desirable that the inlets for the N)13, CO2, and N2 gases be provided separately and independently in order to prevent clogging of the gas inlets due to reactions between the gases.

During nitrocarburizing treatment, it is necessary to remove impurities such as cutting oil and antirust oil from the surface of the workpiece in advance, but the present invention
The method for removing these impurities is not limited, and cleaning with organic solvents or alkaline solvents, heating vaporization using combustion gas from a direct-fired burner, or heating vaporization using indirect heating may be used. Since forming an oxide film is effective in promoting soft nitriding treatment, it is desirable to employ heating vaporization cleaning using indirect heating.In addition, in soft nitriding treatment, in order to obtain a stable nitride layer divided by 1 sieve, Although it is necessary to cool the workpiece after the nitriding treatment, the method of this cooling is not limited in the present invention, and gas cooling or submerged cooling can be used.

(Function) In the gas nitrocarburizing method having the above configuration, 500 to E1
By nitriding under reduced pressure of 00 Torr, C0
2. Oxidation reactions caused by oxidizing gases such as N20 are suppressed,
Porous phenomenon of the nitride layer is suppressed.

(Example) Hereinafter, an example of the present invention will be described based on the accompanying drawings.

WS1, Figure 2 shows a gas soft nitriding treatment apparatus for carrying out the gas soft nitriding method according to the present invention. In these figures, a gas nitrocarburizing treatment apparatus 1 includes a nitriding chamber 2 lined with a heat insulating material and a charging/cooling chamber 3 having a water-cooled jacket structure, and the two chambers are arranged in series. The nitriding chamber 2 is provided with an open lower portion on the side facing the charging/cooling chamber 3 through which the workpiece passes. Further, the charging/cooling chamber 3 is provided with openings 9.10 through which the workpiece passes before and after the charging/cooling chamber 3.

The opening lower of the nitriding chamber 2 and the opening 8 of the charging/cooling chamber 3,
10 has a charging door 11. ! 2.13 are attached to each chamber, and each chamber 2.3 is divided as a sealed chamber.

The door 11 of the nitriding chamber 2 and the door 13 of the charging/cooling chamber 3 are as follows.
They are integrated via a connecting body 14 to form an intermediate door 15. Further, a door hood 16 is disposed between the nitriding chamber 2 and the charging/cooling chamber 3, and both chambers 2.3 are connected in a sealed state. A cylinder 17 is fixed to the upper end of the door hood 1B, and the output shaft 1? a has the above-mentioned connecting body 1
4 are connected. That is, the intermediate door 15 is configured to move up and down by the operation of the cylinder 17. on the other hand,
The door 12 of the charging/cooling chamber 3 is connected to an output shaft 19a of a cylinder 19 fixed to a support frame 18.
2 moves up and down by the operation of the cylinder 18 to open and close the opening 12.

A heater 20 and a fan 21 are provided in the nitriding chamber 2 and the charging/cooling chamber 3. Further, a roller conveyor 25 for conveying the workpieces is disposed within the nitriding chamber 2 and the charging/cooling chamber 3. As shown in FIG. 2, each roller 26 constituting the roller conveyor 25 is pulled out in the horizontal direction with both ends passing through the side wall of the nitriding chamber 2 (the same applies to the charging/cooling chamber 3). There is. Each roller 2
A seal box 27 is fixed to the outer wall of the nitriding chamber 2 from which the roller 6 is pulled out, and the seal box 27 allows the roller 26 to be inserted therethrough in an airtight manner to restrict air from flowing into the interior. The roller conveyor 25 is rotatably driven by a chain (as shown) that is hung around a sprocket 28 fixed to one end of each roller 26.

However, there is an NH3 gas inlet 3 on the ceiling of the nitriding chamber 2.
The NH3 gas inlet 30 has one end of a pipe 32 leading to the NH3 gas generation source (as shown), and the CO2 gas inlet 31 has one end of a pipe 32 leading to the C02 gas generation source (as shown). ) (Fig. 2), and one end of a pipe 34 leading to an N2 gas generation source (as shown) is connected to the seal box 27. 34 is introduced into the nitriding chamber 2 through the insertion gap of the roller 28. Further, an exhaust port 35 is provided on the side wall of the nitriding chamber 2, and one end of a pipe 37 leading to a vacuum pump 36 is connected to the exhaust port 35.

In addition, the above piping is equipped with a valve (electromagnetic valve) 38.38
... are interposed, and the flow of gas into and out of the nitriding chamber 2 is controlled by operating these valves. In addition, the charging/cooling chamber 3 has an N2 gas inlet 39.
and an exhaust port (as shown), and an N2 gas inlet 39
One end of a pipe (as shown) leading to an N2 gas generation source is connected to the exhaust port, and one end of a pipe (as shown) leading to a vacuum pump is connected to the exhaust port. Further, in front of the charging/cooling chamber 3, a charging table 40 having rollers 41 is arranged facing the opening 9 thereof.

Hereinafter, a gas soft nitriding method performed using the gas soft nitriding apparatus configured as described above will be described with reference also to FIG. 3.4. Incidentally, Fig. 3.4 shows the thermal cycle of the workpiece and the pressure cycle of the gas nitrocarburizing apparatus during treatment.

During the nitriding process, the intermediate door 15. is closed, the heater 20 is energized to maintain the nitriding chamber 2 at the soft nitriding temperature (550 to 650° C.), and the vacuum pump 36 is operated to exhaust the inside of the nitriding chamber 2 to 0.5 Torr or less. First, the door 12 of the charging/cooling chamber 3 is opened by the operation of the cylinder 19.
The cleaned workpiece W on the charging table 40 is opened.
is charged into the charging/preheating chamber 3 through the opening 9, and then the door 12 is closed by reactivating the cylinder 18.Next, the vacuum pump (as shown) is operated to move the inside of the charging/preheating chamber 3. 0.5T
When the pressure in the charging/preheating chamber 3 becomes the same as that in the nitriding chamber 2, the cylinder 17 is actuated to open the intermediate door 15, and the roller conveyor 25 transports the workpiece to the nitriding chamber 2. Transport to.

After carrying the workpiece W into the nitriding chamber 2, the intermediate door 15 is closed and the pipe 34 leading to the N2 gas generation source is closed.
Open the valve 38 (Fig. 2) and supply N2 gas to 650~7
00 Tarr and rotate the fan 23. The workpiece is heated by convection in an N2 atmosphere by the rotation of the fan 23, and quickly reaches the nitriding temperature (550 to 650°C).
heated up to.

Once the temperature of the workpiece W has risen to the nitriding temperature, the vacuum pump 36 is operated again to evacuate the inside of the nitriding chamber 2 to a high vacuum. , the valve 38 (second
) is also opened, and NH3 gas, CO2 gas, and N2 gas are continuously introduced into the nitriding chamber 2 at a predetermined ratio. At this time,
A valve 38 connected to a vacuum pump 36 is opened and closed to adjust the pressure inside the nitriding chamber 2 to 500 to 600↑orr.

The above-mentioned mixed gas is applied to the above-mentioned (1) to (3) in the nitriding chamber 2.
The reaction occurs as shown in the formula, and as a result, nitriding and carburization occur simultaneously on the surface of the workpiece W. At the same time, the CO2 gas introduced into the nitriding chamber 2, the N20 generated in the reaction, and the residual O2 cause an oxidation reaction on the surface of the workpiece, thereby creating a porous portion in the nitride layer. However, in the present invention, nitrocarburizing is performed under a reduced pressure of 500 to 600 Torr, so the oxygen partial pressure in the chamber 2 is lowered, the oxidation reaction is suppressed, and the formation of porous portions is suppressed.

After the above nitriding process is completed, the introduction paths for NH3, 002 and N2 gas are closed, and the inside of the nitriding process chamber 2 is evacuated to 0.5 Torr or less by operating the vacuum pump 36. Immediately thereafter, the intermediate door 15 is closed by operating the cylinder 17. The work W is opened and carried into the charging/cooling chamber 3 by the roller conveyor 25. At this time, the charging/cooling chamber 3 is at the same pressure as the nitriding chamber 2. Then, at the same time as closing the intermediate door 15, open the valve of the pipe leading to the N2 gas generation source, introduce N2 gas into the charging/cooling chamber 3 through the N2 gas inlet 38, and open and close the valve leading to the vacuum pump. The pressure inside the charging/cooling chamber 3 is adjusted to 650 to 700 Torr, and the fan 23 is rotated.

The workpiece W is forcedly cooled in an N2 gas atmosphere to stabilize the nitride layer. After cooling is completed, the charging/cooling chamber 3 is returned to the atmosphere, the charging door 12 is opened, and the work W is carried out onto the charging table 40, thereby completing a series of surface treatments.

As shown in FIG. 4, a dense nitride layer A is formed below one surface B in the surface layer of the work W subjected to the soft nitriding treatment as described above. On the other hand, in the conventional case where gas soft nitriding is simply performed without reducing the pressure, a porous portion C is formed in the nitride layer A as shown in FIG.

In the above embodiment, NH3,
Nitriding was carried out by continuously introducing a mixed gas of CO2 and N2, but in the present invention, as shown in FIG. A mixed gas may be supplied; in this case, the oxidation reaction caused by the oxidizing gas can be suppressed more reliably.
Porous phenomenon of the nitride layer can be more reliably prevented.

(Effects of the Invention) As explained above in detail, according to the gas soft nitriding method of the present invention, the porous phenomenon of the nitride layer caused by the oxidation reaction can be suppressed, and the surface hardness and wear resistance can be improved. It is possible to prevent the deterioration of sliding characteristics and rigidity,
This has an effect that greatly contributes to improving the performance of soft-nitrided products.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the structure of a gas soft-nitriding treatment apparatus for performing gas soft-nitriding according to the present invention, FIG. 3 and 6 are diagrams showing the thermal cycle and pressure cycle in this method,
FIG. 4 is a microphotograph showing the metallographic structure of the surface layer of a product obtained by this method, and FIG. 5 is a photomicrograph showing the metallographic structure of the surface layer of a conventional gas soft-nitrided product. 2...Nitriding chamber, 3...Charging and cooling chamber 11
．． 12.13...Door, 15...Intermediate door 20
...Heater, 21...Fan 25...
Roller conveyor, 3o...N2 gas inlet 31...
C02 gas inlet, 36...Vacuum pump W...Work Patent applicant Toyota Motor Corporation (and 2 others) Fig. 1 Intermediate door Fig. 2 Fig. 4:::4 ζ) I 5 ゛

Claims (1)

[Claims] (1) After heating the cleaned steel parts in a vacuum or nitrogen atmosphere, heating them to soft nitriding temperature for a predetermined time in an atmosphere supplied with NH_3, CO_2 and N_2 and under a reduced pressure of 500 to 800 Torr, and then cooling them. A gas nitrocarburizing method characterized by the following.