JP3303741B2 - Gas nitrocarburizing method - Google Patents

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.

In a second method, a material is heated in a nitriding atmosphere containing ammonia gas to a nitriding temperature of 500 to 530 ° C. for 4 to 4 hours.
The first stage nitridation is performed by heating for 6 hours,
This is a two-stage nitriding method in which the temperature is raised to a nitriding temperature of 70 to 590 ° C., and is heated and held for 2 to 3 hours to perform the second nitriding.

The 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.

Further, Japanese Patent Application Laid-Open No. 7-286256 discloses a method for producing 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 a cleaned steel part in a vacuum or nitrogen atmosphere, and then sets NH ₃ , N 2,
500-600 steel parts in nitriding atmosphere containing CO ₂
A method of nitriding under a reduced pressure of Torr has been proposed (JP-A-4-364). According to the method according to this publication,
Since a nitriding atmosphere in a reduced pressure state is employed, it is advantageous for making the compound layer formed on the surface of the steel part thinner and nonporous.

[0008]

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 suppressed, the compound layer tends to have pores and become porous as described above.

[0010] Therefore, there is a limit in applying a nitriding treatment to a member 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.

In the technique according to Japanese Patent Application Laid-Open No. 7-286256, although the nitriding temperature is increased with time,
The amount of ammonia added in the nitriding atmosphere is the same from the start to the end of the nitriding treatment, and is not always sufficient for suppressing the thickness of the compound and making the compound nonporous.

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 compound layer from becoming excessively thick and suppressing the formation of a porous layer.

[0013]

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 ({26664Pa} 26 kPa) or less in a nitriding atmosphere, the material is heated to a nitriding temperature of 450 to 560 ° C. to perform a first nitriding process, and a pressure of 200 To.
a diffusion treatment step of holding the material at a temperature of 570 ° C. to 650 ° C. in an atmosphere having a lower ammonia gas concentration than the rr or less than the ammonia gas concentration or containing no ammonia gas, and then further removing the ammonia gas. the material in the following nitriding atmosphere pressure 200Torr including, 450
A second-stage nitridation process of performing nitriding by heating to a nitriding temperature of 560 ° C. is performed, and the temperature of the first-stage nitridation process and the second-stage nitridation process are higher than the temperature of the diffusion process. Is characterized by a low temperature.

According to a second aspect of the present invention, there is provided a gas nitrocarburizing method, comprising:
The first method of performing nitriding by heating to a nitriding temperature of 450 to 560 ° C.
By heating the material to 570 ° C. to 650 ° C. in a nitriding atmosphere in a reduced pressure state in which the ammonia gas concentration is lower than that in the first nitriding treatment step and the first nitriding treatment step,
A diffusion treatment step of further diffusing and penetrating the nitrogen diffused and infiltrated into the material in the first nitridation treatment step ;
A second nitridation step of performing nitridation by heating to a nitriding temperature of 60 ° C., wherein the ammonia gas concentration in the diffusion treatment step is higher than the ammonia gas concentration in the first nitridation step. 0 to 0.5 times, and the temperature of the first-stage nitriding process and the temperature of the second-stage nitriding process are lower than the temperature of the diffusion process.

[0015]

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 concentration of the ammonia gas is lower than that in the nitridation 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 porosification and a deep nitride hardened layer can be obtained.

According to the method of the first aspect, in consideration of suppression of thickening of the compound layer, suppression of formation of a porous layer, etc., the nitriding atmosphere has a pressure of 150 Torr (# 199998).
(Pa が 20 kPa) or less is preferable. More preferably, the pressure is 100 Torr ({13332 Pa}).
A nitriding atmosphere of 13 kPa) or less can be employed.

In addition, the nitriding temperature according to the first aspect may be set to a non-high temperature range, 57% in consideration of suppression of the thickness of the compound layer.
Is set as lower than 0 ℃, but the four hundred and fifty to five hundred and sixty ° C.
I do. Particularly, 500 to 540 ° C is preferable.

If the temperature in the diffusion step according to the first aspect is lower than 570 ° C., the time required 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 570-6
50 ° C. is suitable.

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.

By the way, in the diffusion processing step according to claim 1,
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 according to claim 1 In this respect, the
After performing the diffusion process step, the material is further heated in a nitriding atmosphere containing ammonia gas at a pressure of 200 Torr or less to a nitriding temperature of less than 570 ° C. (450 to 560 ° C.) to perform a second stage of nitriding. A nitriding process is performed.
Therefore, the surface of the material is nitrided again. The late, reduction in hardness in the material surface due to excessive diffusion penetration prone nitrogen generated in diffusion process is suppressed or avoided.

According to the method of the first 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 according to claim 1, the concentration of ammonia gas occupying the atmosphere, by volume, 30-50% in the first stage nitriding process, a diffusion process 0
10% and 10 to 30% in the second stage nitriding treatment.

The nitriding temperature in the second stage of the nitriding treatment according to claim 1 is set to 450 to 560 ° C. in consideration of the suppression of the thickness of the compound layer . Especially 500-
540 ° C. is preferred.

According to the second aspect of the present invention, 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. Further, according to the method according to claim 2 , 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. Such an atmosphere has a considerably low ammonia gas concentration. 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.

Ammonia gas concentration in [0028] a contact diffusion process, as shown in Example 2 below, can be zero.

[0029] In other words, the body volume ratio, the ammonia gas concentration in the diffusion treatment step and K1, if the ammonia gas concentration in the first stage nitriding process and K2,
(K1 / K2) = 0 to 0.5, that is, 0.5 or less. In addition, as (K1 / K2), if necessary,
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. in this case,
The pressure of the atmosphere can be 10 to 200 Torr.

[0030] The temperature of the diffusion process is generally the higher the temperature, the time required for the diffusion process can be shortened.

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.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments will now be described with reference to the drawings, together with comparative examples.

The test piece according to the present 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 first nitridation treatment step (A) and the second nitridation treatment step (C) have a significantly lower ammonia gas concentration than the first nitridation treatment step (C).
In the nitriding step, the pressure 1 containing ammonia gas is used.
In a nitriding atmosphere of 50 Torr or less, an iron-based test piece as a material was heated and held at a temperature of less than 570 ° C., that is, a nitriding temperature of 500 to 540 ° C., and nitriding was performed. That is, the first-stage nitriding treatment step was performed on the test piece.

Next, in the diffusion step, 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 step, the test specimen was heated and held at a nitriding temperature of less than 570 ° C. in a nitriding atmosphere containing an ammonia gas at a pressure of 150 Torr or less to perform nitriding. That is, the second stage nitriding process was performed on the test piece.

Thereafter, the test piece was poured into oil at 130 ° C. and oil-cooled .

Table 1 shows specific test conditions. In Table 1, as for the furnace pressure, the number without parenthesis is To.
rr unit, and the number in parentheses is 1 Torr = 13
Shows the number in kPa converted at 3.322 Pa.

[0039]

[Table 1]

As can be understood from Table 1, according to each embodiment, the pressure in the furnace was basically 80 Torr in each of the first nitriding treatment step, the diffusion treatment step, and the second nitriding treatment step.
However, in Example 5, the pressure in the furnace was changed to the first nitriding step,
130 Torr for both the diffusion process and the second nitriding process
And

[0041] As can be seen from Table 1, according to the actual施例, first nitriding treatment step, in the second nitriding process,
The furnace pressure was 80 Torr, and the ratio of each gas in the nitriding atmosphere was NH ₃ : N ₂ : CO _{2 in} volume ratio, that is, in flow ratio.
= 50: 40: 10, but 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 ratio of each gas in the atmosphere of the diffusion treatment step was 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 and then, in Examples 4 and _{5, NH 3: N 2:} CO 2 = 10: 90: 0 and then, in the comparative example A~ Comparative Example C , 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 nitriding 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. Comparative Example A to Comparative
According to Example C , (7/50) times = 0.14 times.

That is, according to Examples 1 to C , (K1
/ K2) was from 0 to 0.14 to 0.2 times.

As can be seen from Table 1, Comparative Example A
In this example, the first nitriding step and the diffusion step were performed, but the second nitriding step was not performed. Note that the comparison
The heating mode in Example A is shown in FIG.

As shown in Table 1, in Comparative Example B , the first
The temperature in the nitriding process and the second nitriding process is 57
It was kept constant at 0 ° C. In Comparative Example C , the temperature in the first nitriding step, the diffusion step, and the second nitriding step was 5
The temperature was kept constant at 30 ° C. FIG. 3 shows a heating mode in Comparative Example C.
Shown in

With respect to the test pieces according to each of the examples, 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. The surface hardness was a hardness of 0.05 mm from the surface including the compound layer.

Table 2 shows the results. 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 pieces according to the comparative examples were measured in the same manner, and the results are shown in Table 2.

[0050]

[Table 2]

As can be seen 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.

In Comparative Example 1, it is assumed that 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 5, as can be understood from Table 2, it is advantageous for reducing the thickness of the compound layer, securing the nitrocarburizing depth, and securing the surface hardness. It was made non-porous, there was no porous portion, and the rolling life was good .

[0054] (Suitable examples) The present invention method, machine structural parts generally, for example, can be applied to nitride the teeth of the gear formed by a steel 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 thickness of the compound layer is increased,
Porosity was induced and the surface pressure strength was 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. Ensuring the bending strength of the tooth portion 12 and ensuring the surface pressure strength can both be achieved. 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 comprises
The present invention is not limited only to the application examples, and can be appropriately changed and implemented without departing from the gist.

[0057]

According to the first aspect of the present invention, in the nitriding step, the material is heated to a nitriding temperature of 450 to 560 ° C. in a nitriding atmosphere under a reduced pressure containing ammonia gas, that is, the nitriding is performed at a relatively low temperature. On the other hand, in the diffusion process,
570 ° C. to 65 ° C. in an atmosphere containing less ammonia gas or containing no ammonia gas than the nitriding treatment step.
It is kept at a heating temperature of 0 ° C., that is, at a 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.

[0058] According to the method according to claim 1, further material in a nitriding atmosphere depressurized state containing ammonia gas 4
A second nitriding step is performed by heating to a nitriding temperature of 50 to 560 ° 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 second 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 (2)

(57) [Claims] 1. A pressure containing ammonia gas of 200 Torr.
In the following nitriding atmosphere, the material is nitrided at 450-560 ° C
A first stage nitridation step of nitriding by heating to a temperature of 570 ° C. to 650 ° C. in an atmosphere having a lower ammonia gas concentration or containing no ammonia gas at a pressure of 200 Torr or less than the nitridation step; A nitriding temperature of 450 to 560 ° C. in a nitriding atmosphere containing ammonia gas at a pressure of 200 Torr or less. Performing a second nitriding step, wherein the temperature of the first nitriding step and the temperature of the second nitriding step are lower than the temperature of the diffusion step. . 2. A first-stage nitridation process in which a material is heated to a nitriding temperature of 450 to 560 ° C. in a nitriding atmosphere under reduced pressure containing ammonia gas to perform nitriding; The above-mentioned material is 570 in a nitriding atmosphere under a reduced pressure in which the ammonia gas concentration is lower than that of the processing step.
A diffusion treatment step of further diffusing and penetrating the nitrogen that has diffused and penetrated into the material in the first nitridation treatment step by heating to about 650 ° C. to 650 ° C .; A nitriding temperature of 450 to 560 ° C. to perform nitriding, and a nitriding process in a second stage is performed. The gas concentration is 0 to 0.5 times the gas concentration, and the temperature of the first nitriding step and the second
A gas nitrocarburizing method, wherein the temperature of the second nitriding step is low.