JPH11100655A - Gas soft-nitriding treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas soft-nitriding treatment process advantageous to the inhibition of excessive increase in the thickness of a compound layer formed on the surface of a material and also to the inhibition of the formation of pores. SOLUTION: The gas soft-nitriding process includes the following stages: a nitriding treatment stage where a material is heated up to a nitriding temp. of <570 deg.C in a nitriding atmosphere of <=200 Torr pressure containing ammonia gas to undergo nitriding; and a diffusion treatment stage where the material is held at 570-650 deg.C in a nitriding atmosphere having ammonia gas concentration lower than that in the above nitriding treatment stage or free from ammonia gas at <=200 Torr pressure. Further, the material can be subjected to a second- stage nitriding treatment stage where the material is heated up to a nitriding temp. of <570 deg.C in a nitriding atmosphere of <=200 Torr pressure containing ammonia gas to undergo nitriding.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a gas nitrocarburizing method.

[0002]

2. Description of the Related Art Induction hardening, carburizing, gas nitrocarburizing and the like are known as surface hardening methods. Above all, gas nitrocarburizing, in which ammonia gas decomposed and diffuses nitrogen into the material, can form a hardened layer with appropriate hardness and is a non-transformation treatment, so that the shape of the member can be made more precise. There are promising benefits.

In such a gas nitrocarburizing method, the following first to third methods are conventionally known. First
Is a method in which a material is heated to a nitriding temperature of 550 to 590 ° C. in a nitriding atmosphere containing an ammonia gas, a nitrogen gas, and a carbon dioxide gas to perform nitriding. The second method is
In a nitriding atmosphere containing ammonia gas,
The first stage of nitriding is performed by heating to a nitriding temperature of 530 ° C. for 4 to 6 hours, and then the temperature is raised to a nitriding temperature of 570 to 590 ° C. and maintained for 2 to 3 hours, followed by a second stage. Is a two-stage nitriding method for performing nitriding.

[0004] A third method is a low-temperature nitriding method in which a material is heated and held at a nitriding temperature of 500 to 530 ° C for 20 to 50 hours in a nitriding atmosphere containing ammonia gas. Furthermore, Japanese Patent Application Laid-Open No. 7-286256 discloses a method for manufacturing a nitrided steel member in which the nitriding temperature is continuously increased from the start to the end of the nitriding treatment. Further, the present applicant raises the temperature of the cleaned steel part in a vacuum or nitrogen atmosphere, and then nitrides the steel part in a nitriding atmosphere containing NH _3, N _{2 and} CO ₂ under a reduced pressure of 500 to 600 Torr. A method has been proposed (JP-A-4-364). According to the method disclosed in this publication, a nitriding atmosphere in a reduced pressure state is employed, which is advantageous for making the compound layer formed on the surface of the steel component thinner and less porous.

[0005]

According to the above-mentioned first method, the compound layer formed on the surface of the material tends to be thick, and the compound layer tends to have pores and become porous. According to the second and third methods described above, although the thickness of the compound layer formed on the surface of the material can be reduced, the compound layer tends to have pores and become porous as described above.

For this reason, there is a limit in applying a nitriding treatment to members requiring high quality. Particularly when applied to gears, it is difficult to obtain satisfactory surface pressure strength due to the influence of the thick compound layer. Further, in the technique according to JP-A-7-286256, although the nitriding temperature is increased with time, the amount of added ammonia in the nitriding atmosphere is the same from the start to the end of the nitriding treatment, thereby suppressing the thickness of the compound.
It is not always enough to make it non-porous.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a gas nitrocarburizing method which is advantageous for suppressing the thickness of the compound layer from becoming excessive and suppressing the formation of a porous layer.

[0008]

According to a first aspect of the present invention, there is provided a gas nitrocarburizing method, wherein a pressure including ammonia gas is 200 Torr.
r (≒ 26664 Pa ≒ 26 kPa) or less in a nitriding atmosphere to heat the material to a nitriding temperature of less than 570 ° C. to perform a nitriding treatment, and at a pressure of 200 Torr or less, the ammonia gas concentration is lower than that of the nitriding treatment process or ammonia. 570 ° C ~
And a diffusion treatment step of maintaining the temperature at 650 ° C.

In the gas nitrocarburizing method according to the second aspect, after the gas nitronitriding method according to the first aspect is performed, the material is further heated to a temperature of less than 570 ° C. in a nitriding atmosphere containing ammonia gas at a pressure of 200 Torr or less. A second stage nitriding treatment step of heating to a nitriding temperature to perform nitriding. In the gas nitrocarburizing method according to claim 3, the raw material is nitrided in a reduced-pressure nitriding atmosphere containing ammonia gas.
A first-stage nitriding process in which nitriding is performed by heating to a nitriding temperature of less than 570 ° C., and a nitriding atmosphere having a reduced ammonia gas concentration or containing no ammonia gas compared to the first-stage nitriding process. The material is heated at 570 ° C. in a nitriding atmosphere in a reduced pressure state containing ammonia gas, and a diffusion treatment step of further diffusing and penetrating the nitrogen diffused into the material in the first nitriding treatment step.
And performing a second nitriding treatment step in which nitriding is performed by heating to a nitriding temperature of less than 3. The ammonia gas concentration in the diffusion treatment step is 0% lower than the ammonia gas concentration in the first nitriding treatment step. ０．５0.5 times.

[0010]

According to a first aspect of the present invention, in the nitriding step, the pressure including ammonia gas is 200 To.
Since nitriding is performed in a nitriding atmosphere under a reduced pressure of rr or less,
Thickening and porous formation of the compound layer formed on the surface of the material are suppressed. Therefore, the thickness and the density of the compound layer can be reduced. Furthermore, since the nitriding temperature is lower than 570 ° C. and the temperature is low, that is, non-high, the increase in the thickness of the compound layer formed on the surface of the material is further suppressed.

Further, according to the method of the present invention, in the diffusion treatment step, the ammonia gas concentration is lower than that in the nitriding treatment step or in an atmosphere containing no ammonia gas.
Since the material is maintained at a heating temperature of 570 ° C. to 650 ° C., that is, in a high temperature region, diffusion and penetration of nitrogen into the material is promoted, and the nitrided hardened layer is deepened. That is, the depth of soft nitriding is increased.

Therefore, according to the method of the first aspect, a compound layer which is thin and has little or no porosity and a deep nitride hardened layer can be obtained. According to the method of the first aspect, in consideration of suppressing the thickness of the compound layer and suppressing the formation of the porous layer, the nitriding atmosphere is set at a pressure of 150 Torr (≒ 1).
A nitriding atmosphere of 9998 Pa 20 kPa) or less is preferable. More preferably, the pressure is 100 Torr (≒ 1333).
A nitriding atmosphere of 2 Pa ≒ 13 kPa) or less can be adopted.

The nitriding temperature according to the first aspect of the present invention may be set to a non-high temperature range, 57
It is set as less than 0 ° C, but 450-560 ° C,
Particularly, 500 to 540 ° C is preferable. If the temperature in the diffusion treatment step according to claim 1 is lower than 570 ° C., the time for nitrogen to diffuse and penetrate into the material becomes longer. If the temperature exceeds 650 ° C., a decrease in strength and a decrease in hardness at the core of the material are likely to be induced, which is not preferable from the viewpoint of securing the strength of the material. Therefore, 570-650 ° C is appropriate.

According to the method of the first aspect, the diffusion treatment step is performed in an atmosphere having a lower ammonia gas concentration than the nitridation treatment step or in an atmosphere containing no ammonia gas. Therefore, in the diffusion process, while further diffusing and infiltrating the nitrogen that has diffused into the material in the nitriding process,
An increase in the thickness of the compound layer is suppressed. Therefore, it is advantageous to obtain a deep nitride hardened layer while obtaining a thin and dense compound layer.

In the diffusion processing step according to the first aspect,
Depending on the material of the material, if the treatment time is too long, the nitrogen may be deepened due to excessive diffusion and infiltration of nitrogen, but denitrification may occur on the surface of the material. In this case, a decrease in hardness on the surface of the material is likely to be induced. According to the method of the second aspect, after performing the diffusion treatment step of the first aspect, the material is further reduced in a nitriding atmosphere containing ammonia gas at a pressure of 200 Torr or less.
A second-stage nitriding treatment step of performing nitriding by heating to a nitriding temperature of less than 0 ° C. is performed. Therefore, the surface of the material is nitrided again. Therefore, a decrease in hardness on the material surface due to excessive diffusion and infiltration of nitrogen, which is likely to occur in the diffusion treatment step according to claim 1, is suppressed or avoided.

According to the method of the second aspect, in consideration of suppression of thickening of the compound layer and suppression of porous formation, the nitriding atmosphere is preferably a nitriding atmosphere having a pressure of 150 Torr or less, more preferably a pressure of 150 Torr or less. A nitriding atmosphere of 100 Torr or less can be adopted. According to the method of claim 2, the concentration of the ammonia gas in the atmosphere is 30 to 50% by volume in the first stage nitridation process, 0 to 10% in the diffusion process stage, and Is 1 in nitriding
0% to 30%.

The nitriding temperature in the second nitriding step according to the second aspect is 450 to 560 ° C., particularly 500 to 540 in consideration of suppression of the thickness of the compound layer.
C is preferred. According to the method of the third aspect, since the nitriding is performed in a nitriding atmosphere under a reduced pressure, an increase in the thickness of the compound layer is suppressed, and the formation of a porous layer is also suppressed. Claim 3
According to the method according to the above, the ammonia gas concentration in the diffusion treatment step is 0 to 0.5 times the ammonia gas concentration in the first nitridation treatment step. The gas concentration is quite low. Therefore, while the compound layer is made thinner, the diffusion permeability of nitrogen into the material is secured, which is advantageous for making the nitrided hardened layer deeper.

The ammonia gas concentration in the diffusion treatment step according to claim 3 can be set to 0 as shown in a second embodiment described later. In other words, in the method according to claim 3, if the ammonia gas concentration in the diffusion treatment step is K1 and the ammonia gas concentration in the first nitridation treatment step is K2, (K1 / K2) = 0
0.5, that is, 0.5 or less. Note that (K1 / K
As 2), 0.4 or less, 0.35 or less, 0.3 or less, 0.25 or less, 0.2 or less, 0.1 or less can be adopted as necessary. In this case, the pressure of the atmosphere is 10-20.
0 Torr.

According to the third aspect of the present invention, the temperature of the diffusion process may be equal to the temperature of the first nitriding process, or the temperature of the first nitriding process may be equal to the temperature of the first nitriding process. The temperature may be higher than, for example, 450-
The temperature may be lower than 570 ° C, such as 560 ° C or 500 to 540 ° C, or may be 570 to 650 ° C. Generally, the higher the temperature, the shorter the time required for the diffusion process.

In the method of the present invention, the first nitridation step is, for example, 3 to 4 hours, and the diffusion step is, for example, 0.1 hour, although it differs depending on the material of the raw material, the heating temperature and the like.
5 to 3 hours, the second nitriding step is, for example, 0.5
１．５1.5 hours.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments will now be described with reference to the drawings, together with comparative examples. The test piece according to this example was prepared as follows. That is, a steel type having the following composition was melted in a vacuum of 2 tons, and the solidified body was formed into a steel bar having a diameter of 70 mm by hot rolling, and normalizing was performed at 880 ° C. for 1.5 hours to prepare a test piece for rolling. . The composition of the steel type is such that C is 0.1% by weight.
21%, Si was 0.25%, Mn was 0.46%, S was 0.020%, Cr was 1.48%, V was 0.22%, inevitable impurities, and the balance was Fe.

FIG. 1 shows a heating mode according to the main embodiment. As can be understood from FIG. 1, a first nitriding step (A), a diffusion step (B), and a second nitriding step (C).
Was performed in order. In the diffusion treatment step (B), the ammonia gas concentration is significantly reduced as compared with the first nitridation treatment step (A) and the second nitridation treatment step (C). In a nitriding atmosphere containing a pressure of 150 Torr or less, the iron-based test piece as a raw material was heated and held at a temperature of less than 570 ° C., that is, at a nitriding temperature of 500 to 540 ° C. to perform nitriding. That is, the first-stage nitriding treatment step was performed on the test piece.

Next, in the diffusion process, the pressure 15
The test piece was heated at 570 ° C. to 650 ° C. in an atmosphere having an ammonia gas concentration lower than that of the first nitriding treatment at 0 Torr or less or in an atmosphere containing no ammonia gas.
And kept at a temperature of. Next, in the second nitriding treatment step, the test specimen was heated and held at a nitriding temperature of less than 570 ° C. in a nitriding atmosphere containing ammonia gas at a pressure of 150 Torr or less to perform nitriding. That is, the second
The nitriding step of the stage was performed.

Thereafter, the test piece was put in oil at 130 ° C. and cooled with oil. Table 1 shows specific test conditions according to Examples 1 to 8. In Table 1, as the furnace pressure, the numbers without parentheses are shown in Torr units, and the numbers in parentheses are kP converted at 1 Torr = 133.322 Pa.
Indicates a number in a unit.

[0025]

[Table 1] As can be understood from Table 1, according to each example, the furnace pressure was basically 80 Torr in each of the first nitriding step, the diffusion step, and the second nitriding step. However, in Example 5, the furnace pressure was set to 130 Torr in each of the first nitriding process, the diffusion process, and the second nitriding process.

As can be understood from Table 1, Examples 1 to 8 (except for Example 6) are Claims 2 and 1.
According to these embodiments, the first
In the nitriding step and the second nitriding step, the pressure in the furnace was 80 Torr, and the ratio of each gas in the nitriding atmosphere was:
By volume ratio, that is, by flow ratio, NH ₃ : N ₂ : CO ₂ = 50:
Although it was set to 40:10, the ammonia gas concentration in the atmosphere in the diffusion treatment step was significantly reduced as shown in Table 1 than the ammonia gas concentration in the first nitridation treatment step.

That is, in Example 1, the proportion of each gas in the atmosphere of the diffusion treatment step is NH ₃ :
N ₂ : CO ₂ = 7: 80: 13, and in Example 2, NH 2 was used.
₃ : N ₂ : CO ₂ = 0: 100: 0, and in Example 3,
NH ₃ : N ₂ : CO ₂ = 4: 96: 0, in Examples 4 and 5, NH ₃ : N ₂ : CO ₂ = 10: 90: 0, and in Examples 6 to 8, As in Example 1, NH
_{3: N 2: CO 2 =} 7: 80: was 13.

In other words, if the ammonia gas concentration in the diffusion process is K1 and the ammonia gas concentration in the first nitridation process is K2 in volume ratio,
(K1 / K2) is (7/50) times according to the first embodiment.
= 0.14 times. According to the second embodiment, (0/5
0) = 0 times. According to Example 3, 4/50 times =
It was 0.08 times. According to Example 4 and Example 5,
(10/50) times = 0.2 times. According to Examples 6 to 8, (7/50) times = 0.14 times.

That is, according to Examples 1 to 8, (K1
/ K2) was from 0 to 0.14 to 0.2 times. Further, as can be understood from Table 1, in Example 6 (corresponding to claim 1), the first nitriding step and the diffusion step were performed, but the second nitriding step was not performed. FIG. 2 shows a heating mode in the sixth embodiment.

As shown in Table 1, Example 7 (Claim 3)
), The temperature in the first nitriding treatment step and the second nitriding treatment step was kept constant at 570 ° C. Example 8
In (corresponding to claim 3), the temperatures in the first nitriding treatment step, the diffusion treatment step, and the second nitriding treatment step were kept constant at 530 ° C. FIG. 3 shows a heating mode in the eighth embodiment. For the test pieces according to each example, the thickness of the compound layer, the depth of soft nitriding, the surface hardness, and the degree of porosity of the compound layer were measured.
The nitrocarburizing depth was defined as a depth having a hardness of Hv400 or more measured by a micro Vickers hardness meter from the surface including the compound layer. Surface hardness is 0.05 mm from the surface including the compound layer
And the hardness of the depth.

The results are shown in Table 2. Further, a wood-type rolling life test in which a three-point ball was slid on the surface of the test piece was performed, and the life (L ₁₀ : cumulative failure probability) was measured. The contact pressure was 3920 in the forest type rolling life test.
MPa, rotation speed was 1000 rpm, and turbine oil was used as a lubricant. The test piece according to the comparative example was measured in the same manner, and is shown in Table 2.

[0032]

[Table 2] As can be understood from Table 2, in Comparative Examples 1 and 2,
The compound layer was thick, the nitrocarburizing depth was not sufficient, the surface hardness was low, and the compound layer had a porous portion. The rolling life was not good.

It is presumed that in Comparative Example 1, the ammonia gas concentration was not reduced in the diffusion process.
In Comparative Example 2, since the ammonia gas concentration was not reduced in the diffusion treatment step,
It is presumed that it is as high as Torr. On the other hand, according to Examples 1 to 8, as can be understood from Table 2, it is advantageous for reducing the thickness of the compound layer, securing the depth of nitrocarburizing, securing the surface hardness, and making the layer nonporous. No porous portion was found, and the rolling life was good.

In Example 6 in which the first nitriding step and the diffusion step were performed, but the second nitriding step was not performed, the surface hardness was slightly reduced to Hv678, but ammonia gas was used in the diffusion step. The concentration is reduced, the compound layer is thin and non-porous,
The rolling life was also good. Although the first nitriding process and the diffusion process were performed isothermally, the rolling life was good in Examples 7 and 8 in which the ammonia gas concentration was reduced in the diffusion process.

(Application Example) The method of the present invention is applicable to general machine structural parts, for example, nitriding of gear teeth formed of a steel-based material such as alloy steel or carbon steel. An example of this is shown in FIG. Conventionally, a long nitriding time has been required to deepen the nitrided layer in order to increase the bending strength of the teeth. Therefore, the compound layer is caused to be thick and porous, and the surface pressure strength is not sufficient. However, if the method of the present invention is applied to the tooth portion 12 of the gear 10 shown in FIG. 4, it is advantageous for reducing the thickness of the compound layer, securing the depth of nitrocarburizing, securing the surface hardness, and suppressing the formation of porous material. Securing the bending strength of the teeth 12,
It is possible to secure both surface pressure strength. Further, the present invention can be widely applied to not only gears but also forged products such as hot forged products and cold forged products, and cut products.

In the method of the present invention, it is needless to say that the description in the claims can be defined by setting the numerical values in Tables 1 and 2 showing the nitriding conditions, atmosphere and measurement results as the upper and lower limits. In addition, the method of the present invention is not limited only to the above-described embodiments and application examples, and can be implemented with appropriate modifications without departing from the gist.

[0037]

According to the first aspect of the present invention, in the nitriding step, the material is heated to a nitriding temperature of less than 570 ° C. in a nitriding atmosphere in a reduced pressure state containing ammonia gas, that is, the nitriding is performed at a relatively low temperature. On the other hand, in the diffusion treatment step, the concentration of ammonia gas is lower than that of the nitridation treatment step or in an atmosphere containing no ammonia gas.
Is maintained at the heating temperature, that is, high temperature. Therefore, it is possible to promote the nitrogen that has diffused and penetrated into the material to diffuse and penetrate deeper. Therefore, the nitriding depth can be increased while suppressing an increase in the thickness of the compound layer generated on the surface of the material.

According to the method of claim 2, the material is further reduced in a nitriding atmosphere under reduced pressure containing ammonia gas.
A second nitriding step is performed by heating to a nitriding temperature of less than 70 ° C. Therefore, nitrogen diffuses and permeates the surface of the material. Therefore, even in a case where a decrease in hardness and a decrease in strength due to denitrification on the surface of the material due to the heating in the diffusion treatment step, the decrease in hardness and the decrease in strength can be suppressed. Therefore, it is advantageous for ensuring the surface hardness of the material after the treatment.

According to the method of the third aspect, the ammonia gas concentration in the first nitriding step is
The concentration of ammonia gas in the diffusion process is 0 to 0.
It is greatly reduced to five times. Therefore, similarly to the method according to the first aspect, it is advantageous in increasing the nitriding depth while suppressing an increase in the thickness of the compound layer formed on the surface of the material. According to the method according to each claim having the above-described effects, when applied to a gear, it is advantageous for achieving both the surface pressure strength and the bending strength at the tooth portion.

[Brief description of the drawings]

FIG. 1 is a graph showing a heating mode according to an example.

FIG. 2 is a graph showing a heating mode according to an example in which a second-stage nitriding process is not performed.

FIG. 3 is a graph showing a heating mode according to an example in which a first-stage nitriding process, a diffusion process, and a second-stage nitriding process are performed at substantially the same temperature.

FIG. 4 is a configuration diagram of a main part of a gear.

[Explanation of symbols]

 In the figure, reference numeral 12 denotes a tooth portion.

Claims (3)

[Claims] 1. A pressure containing ammonia gas of 200 Torr.
A nitriding process in which the material is nitrided by heating the material to a nitriding temperature of less than 570 ° C. in the following nitriding atmosphere; and in an atmosphere having a lower ammonia gas concentration or containing no ammonia gas than the nitriding process at a pressure of 200 Torr or less. And a diffusion treatment step of maintaining the material at a temperature of 570 ° C. to 650 ° C. 2. After the gas nitrocarburizing method according to claim 1, the material is further heated to a nitriding temperature of less than 570 ° C. in a nitriding atmosphere containing ammonia gas at a pressure of 200 Torr or less. A gas nitrocarburizing method comprising performing a second nitriding step. 3. A first-stage nitriding process in which a material is heated to a nitriding temperature of less than 570 ° C. in a nitriding atmosphere in a reduced-pressure state containing ammonia gas to perform nitriding; A diffusion treatment step of further diffusing and penetrating the nitrogen that has diffused and penetrated into the material in the first nitridation step by heating the material in a nitriding atmosphere in a reduced pressure state having a smaller ammonia gas concentration than the step; A second step of nitriding by heating the material to a nitriding temperature of less than 570 ° C. in a nitriding atmosphere in a reduced pressure state containing ammonia gas to perform nitriding; Wherein the gas nitrocarburizing method is 0 to 0.5 times the ammonia gas concentration in the first nitriding step.