KR20200069745A - The method of carburizing for improve durability - Google Patents

Abstract

The present invention relates to a carburizing heat-treatment method for increasing durability and, more specifically, to a carburizing heat-treatment method for increasing durability, which controls the shape of an oxide film formed on the surface of a target metal product during carburizing heat treatment by performing carburization while supplying carburizing gas containing an oxygen partial pressure (pO_2) of 10^-17 to 10^-23 atm, and having a value of pCO/pCO_2, which is the ratio of the partial pressure (pCO) of carbon monoxide to the partial pressure (pCO_2) of carbon dioxide ranging from 1 to 75 in the carburizing process. The method comprises a carburizing step and a carburizing spread step.

Description

Carburizing heat treatment method to improve durability{THE METHOD OF CARBURIZING FOR IMPROVE DURABILITY}

The present invention relates to a carburizing heat treatment method for improving durability, and more specifically, by optimizing the components of the carburizing gas of the gas carburizing heat treatment method, by controlling the shape of the oxide film formed on the surface of the target metal product during gas carburizing heat treatment. It relates to a carburizing heat treatment method that can improve the durability.

In general, the carburizing heat treatment method is a heat treatment method capable of dramatically improving the strength of parts by increasing strength and hardness by artificially adding carbon to a metal material containing 0.2% carbon.

The carburizing heat treatment method is largely divided into a gas carburizing heat treatment method (hereinafter, also referred to as'gas carburizing') and a vacuum carburizing heat treatment method (hereinafter, also referred to as'vacuum carburizing'). General gas carburization so far is air and methane or liquefied natural gas (LNG) and air and propane or liquefied petroleum gas (LPG) in a certain ratio, and a nickel catalyst is used to react and stabilize the gas at temperatures below 1070 degrees. It is used as an atmosphere gas. In the case of using such an atmosphere, it is impossible to prevent the formation of a grain boundary oxide layer that affects the fitting resistance and abrasion resistance of the product to be treated during the carburizing process.

Vacuum carburization may be applied to suppress the formation of such a grain boundary oxide layer, but this is a difficult situation for a wide range of applications due to equipment and cost problems for forming a vacuum.

Japanese Registered Patent No. 6082302 Japanese Patent Publication No. 2017-166035

In view of the above, the present invention suppresses the formation of a grain boundary oxide layer formed during a gas carburization process, so that the surface quality of the vacuum carburization level can be secured even with gas carburization, oxygen (O ₂ ) among components of the atmospheric gas, By optimizing the partial pressure of carbon monoxide (CO) and carbon dioxide (CO ₂ ), it is an object to provide a method of carburizing heat treatment for improving durability by controlling the shape and distribution of an oxide film formed on the surface of a metal product during carburizing heat treatment.

In order to achieve the above object, the carburizing heat treatment method for improving the durability of the present invention, the target metal product is introduced into a carburizing tank to raise the temperature to a set carburizing temperature, and the oxygen partial pressure (pO ₂ ) is 10 ^-17 to 10 ^-23 atm , Carburizing step of accelerating carburization by supplying a carburizing gas containing a value of pCO/pCO ₂ which is a ratio of a partial pressure of carbon monoxide (pCO) and a partial pressure of carbon dioxide (pCO ₂ ) of 1 to 75, and the carburizing temperature of the carburizing tank and A carburizing diffusion step of diffusing carbon to the target metal product while maintaining the process pressure for a predetermined time may be included.

The carbon monoxide partial pressure (pCO) is 5 to 22% and the carbon dioxide partial pressure (pCO ₂ ) is 0.3 to 5% so as to satisfy the ratio of the carbon monoxide partial pressure (pCO) and the carbon dioxide partial pressure (pCO ₂ ) indicated in the carburizing gas. desirable.

In the carburizing step, the carburizing temperature may be 850 to 1050°C, and more preferably 850 to 950°C.

In the carburizing gas, oxygen and hydrocarbon fuel may have a volume ratio of 2 to 6.5:1 or less, and an appropriate mixing ratio of the oxygen and hydrocarbon fuel may vary depending on the type of hydrocarbon fuel.

When the hydrocarbon fuel is methane or LNG, oxygen and methane or LNG are preferably in a volume ratio of 2:1 or less. When the hydrocarbon fuel is propane or LPG, oxygen and propane are in a volume ratio of 6.5:1 or less. It is preferred.

The carburizing step may be performed for 60 minutes to 90 minutes while maintaining the carburization temperature set in the carburizing tank, and more preferably for 90 minutes.

The carburization diffusion step may be performed for 30 minutes to 70 minutes, more preferably 50 minutes.

In the carburizing heat treatment method for improving the durability of the present invention, a pretreatment step such as washing to remove foreign substances remaining on the surface of the target metal product may be further included before the carburizing step.

It is preferable that the target metal product to which the charcoal heat treatment for improving the durability of the present invention is performed is a component for a transmission requiring durability improvement.

The carburizing heat treatment method of the present invention is a simple method of controlling the partial pressures of oxygen (O ₂ ), carbon monoxide (CO) and carbon dioxide (CO ₂ ) in carburizing gas, which is a grain boundary oxide layer that reduces the durability of products that were a problem in conventional gas carburizing heat treatment methods. By suppressing the formation of, and forming a dense surface oxide layer on the surface of the carburized layer, there is an effect of improving the durability.

In addition, the carburizing heat treatment method of the present invention is very effective in commercializing to be applied to various products because it is easy to reduce costs and handle since it does not require a separate equipment and can utilize an existing gas carburizing heat treatment device when compared with the vacuum carburizing heat treatment method.

1 is a schematic schematic diagram of a mechanism for forming a grain boundary oxide layer according to a conventional gas carburizing heat treatment method.
Figure 2 is a schematic schematic diagram of the surface oxide coating mechanism according to the gas carburizing heat treatment method of the present invention.
3 is a cross-sectional view of a target metal product subjected to a gas carburizing heat treatment method according to Embodiment 1 of the present invention.
4 is a cross-sectional view of a target metal product subjected to a gas carburizing heat treatment method according to Comparative Example 1 of the present invention.
FIG. 5 is a photograph of an enlarged cross-section of a target metal product when the value of pCO/pCO ₂ in carburizing gas is 76 or more.
FIG. 6 is an enlarged cross-section of a target metal product when 75>pCO/pCO ₂ >40 in carburizing gas.
FIG. 7 is an enlarged cross-sectional view of a target metal product when 40>pCO/pCO ₂ >1 in carburizing gas.
8 is a photograph of the surface observed after the endurance test of a carburized product in which a conventional gas carburization heat treatment method was performed.
Figure 9 is a photograph of the surface after the endurance test of the carburized product subjected to the gas carburizing heat treatment method of the present invention.
FIG. 10 is a scanning electron microscope (SEM) of the PCD surface before the endurance test of a carburized product in which a gas carburizing heat treatment method was performed according to the present invention.
11 is a scanning electron microscope (SEM) of the PCD surface after the endurance test of a carburized product in which a gas carburization heat treatment method was performed according to the present invention.
12 is an enlarged observation of a cross-section of the PCD surface before the endurance test of a carburized product in which a gas carburizing heat treatment method was performed according to the present invention.
13 is an enlarged observation of a cross-section of a PCD surface after an endurance test of a carburized product in which a gas carburization heat treatment method was performed according to the present invention.
14 is a scanning electron microscope (SEM) of the PCD surface before the endurance test of a carburized product in which a conventional gas carburization heat treatment method was performed.
15 is a scanning electron microscope (SEM) of the PCD surface after the endurance test of a carburized product in which a conventional gas carburization heat treatment method was performed.
16 is an enlarged observation of the cross-section of the PCD surface before the endurance test of the carburized product in which the conventional gas carburization heat treatment method was performed.
17 is an enlarged observation of a cross-section of the PCD surface after the endurance test of a carburized product in which a conventional gas carburization heat treatment method was performed.
18 is a cross-sectional view of a product subjected to a gas carburizing heat treatment method according to an embodiment of the present invention.
19 is an image of a result of oxygen (O) elemental analysis in the cross section of the product of FIG. 18.
20 is a cross-sectional view of a product in which a conventional gas carburization heat treatment method is performed.
FIG. 21 is a result image of oxygen (O) elemental analysis in the cross-section of the product of FIG. 20.

It should be noted that the technical terms used in the present invention are only used to describe specific examples, and are not intended to limit the present invention. In addition, the technical terms used in the present invention should be interpreted as meanings generally understood by a person having ordinary knowledge in the technical field to which the present invention pertains, unless defined otherwise. It should not be interpreted as a meaning or an excessively reduced meaning.

In addition, the singular expression used in the present invention includes a plural expression unless the context clearly indicates otherwise. In this specification, the terms "consisting of" or "comprising" should not be construed as including all of the various components, or various steps described in the specification, among which some components or some steps It may not be included, or it should be construed to further include additional components or steps.

Hereinafter, a carburizing heat treatment method for improving the durability of the present invention and a conventional gas carburizing heat treatment method using examples and comparative examples will be described in more detail with reference to the accompanying drawings. This is not limited to what is described herein, as one of ordinary skill in the art to which the present invention pertains may be implemented in various different forms as an example.

1 shows a mechanism for forming a grain boundary oxide layer according to a conventional gas carburizing heat treatment method, and FIG. 2 shows a surface oxidation film forming mechanism according to a gas carburizing heat treatment method of the present invention.

As shown in FIG. 1, in the conventional gas carburizing heat treatment method, carbon (C) is charged to the surface of the target metal product by carbon monoxide (CO) in the carburizing reaction during the carburizing reaction. Thus, an amorphous grain boundary oxide layer is formed on the surface.

However, unlike this, in the gas carburizing heat treatment method of the present invention, by reducing the partial pressure of carbon monoxide (CO) in the carburizing gas relatively and increasing the partial pressure of carbon dioxide (CO ₂ ), the carburization reaction by carbon monoxide is suppressed as much as possible and the carburizing layer as shown in FIG. 2. A surface oxide film is formed on the surface of the formed metal product.

Specifically, the carburizing heat treatment method for improving the durability of the present invention performs a pretreatment step (S100) of performing a washing process for removing foreign substances and scales on the surface of the target metal product to be subjected to the carburizing treatment, as described above, It includes a carburization step (S200) to put the target metal product subjected to the pre-treatment step into a carburizing tank, and carburizing the carburization diffusion step (S300) so that carburization is entirely performed on the target metal product.

In the present invention, the "metal product" includes a semi-finished product and/or a finished product, which is, for example, a metal part tool such as a bolt or nut, and a metal product such as an automobile part, the shape or material of which is not limited. It may be, for example, a high-strength metal material such as carbon steel, boron steel, alloy steel, and/or bearing steel.

In the carburizing step (S200), carburization is performed on the surface of the product after supplying the target metal product having the pre-treatment step to the carburizing tank.

Specifically, in the carburizing step, a target metal product is introduced into a carburizing tank, heated to a carburizing temperature of 850 to 950°C, and a value of pCO/pCO ₂ which is a ratio of partial pressure of carbon monoxide (pCO) and partial pressure of carbon dioxide (pCO ₂ ) is 1 to 75. The carburizing is accelerated by supplying a controlled carburizing gas.

Atmospheric gas used for carburizing is produced by a gas generating furnace (modification furnace), which is obtained by mixing propane, butane and natural gas with high temperature after mixing raw materials and air, and is also commonly called endothermic gas. Is introduced into a gas carburizing furnace to perform a carburizing step.

Tables 1 and 2 below show the results for the formation of the atmosphere gas, which is a modified gas used in the carburizing step (S200) of the present invention.

division 890℃ 1070℃ The present invention Composition of the present invention + existing temperature Existing composition + existing temperature CO 21.0 20.7 20.3 20.3 20.3 CO ₂ 0.540 0.345 0.260 0.220 0.190 CH ₄ 0.115 0.180 0.230 0.265 0.305 H ₂ O 1.06 0.689 0.541 0.47 0.41 H ₂ 37.3 37.8 38.1 38 38 Log(pO ₂ ) -19.84 -20.21 -20.46 -20.58 -20.70 pCO/pCO ₂ 38.9 60 78.1 92.3 106.8 Air: Fuel 1.81:1 2.00:1 1.89:1 1.91:1 2.45:1

890℃ 1070℃ The present invention Composition of the present invention + existing temperature Existing composition + existing temperature CO 21.9 21.6 21.3 21.3 21.0 20.8 21.0 20.7 20.8 20.8 CO ₂ 0.582 0.395 0.260 0.216 0.176 0.154 0.136 0.126 0.118 0.103 CH ₄ 0.030 0.045 0.070 0.080 0.100 0.110 0.135 0.130 0.150 0.165 H ₂ O 1.38 0.979 0.623 0.519 0.449 0.414 0.349 0.349 0.301 0.288 H ₂ 36.4 37.0 37.5 37.6 37.8 38.0 37.9 38.1 38.1 38.1 Log(pO ₂ ) -18.58 -18.90 -19.25 -19.42 -19.57 -19.69 -19.82 -19.85 -19.90 -20.02 pCO/pCO ₂ 37.6 54.7 81.9 98.6 119.3 135.1 154.4 164.3 176.3 201.9 Air: fuel 1.89:1 1.98:1 1.88:1 1.99:1 2.49:1 2.60:1 2.60:1 2.65:1 2.65:1 2.71:1

In the carburizing step (S200), in order to suppress the formation of a Fe oxide film by oxygen (O ₂ ) in the carburizing temperature range of 850 to 950° C., and at the same time to form a dense and homogeneous Cr oxide film, the oxygen partial pressure (pO ₂ ) condition is It is preferable to perform at an oxygen partial pressure (pO ₂ ) of 10 ^-17 to 10 ^-23 atm that satisfies the conditions for forming the Fe oxide film and the conditions for forming the Cr oxide film.

The carbon monoxide partial pressure (pCO) is 5 to 22% and the carbon dioxide partial pressure (pCO ₂ ) is 0.3 to 5% so as to satisfy the ratio of the carbon monoxide partial pressure (pCO) and the carbon dioxide partial pressure (pCO ₂ ) indicated in the carburizing gas. desirable.

In addition, in the carburizing gas, the ratio of oxygen and hydrocarbon fuels is when the hydrocarbon fuel is city gas, methane or liquid natural gas (LNG), and oxygen and city gas, methane or LNG is a volume ratio of 2:1 or less, and the hydrocarbon When the fuel is propane or LPG (liquid petroleum gas), it is preferable that oxygen and propane or LPG satisfy a volume ratio of 6.5:1 or less.

Table 3 below shows a comparison of the carburization performance conditions of the conventional gas carburization heat treatment method and the carburization performance conditions of the carburization heat treatment method of the present invention.

division reaction
Temperature
(℃) Air: Fuel Carburizing
Temperature
(℃) pO ₂
(ATM) pCO
(%) pCO ₂
(%) pCO/pCO ₂ Remark Conventional gas carburization 1050~
1100 2 to 3: 1 (methane)
7-8:1 (propane) 850~
950 10 ^-17 ~
10 ^-19 19~23 0.1~
0.25 76~230 Grain boundary: 5~10㎛
Surface: X Gas carburizing of the present invention 850~
1050 2:1 or less (methane)
6.5:1 or less (propane) 850~
950 10 ^-17 ~
10 ^-23 5~22 0.3~5 1~75 Grain boundary: 3㎛ or less
Surface: continuous

Carburization heat treatment method was performed under the conditions described in Table 4 below.

In Table 4, Examples 1 to 5 are performed by the carburizing heat treatment method of the present invention, and Comparative Examples 1 to 6 are the results of performing the conventional gas carburizing heat treatment method.

division pO ₂ (atm) pCO(%) pCO ₂ (%) pCO/pCO ₂ Grain boundary oxide layer
Formation Oxide film
Formation Example 1 10 ^-18 19 0.6 31.7 X ○ Example 2 10 ^-19 16 0.4 40 X ○ Example 3 10 ^-19 15 0.5 30 X ○ Example 4 10 ^-19 20 0.5 40 X ○ Example 5 10 ^-21 5 5 One X ○ Comparative Example 1 10 ^-18 21 0.2 105 ○ X Comparative Example 2 10 ^-18 20 0.17 117.6 ○ X Comparative Example 3 10 ^-19 21 0.2 105 ○ X Comparative Example 4 10 ^-19 19 0.2 95 ○ X Comparative Example 5 10 ^-17 19 0.2 95 ○ X Comparative Example 6 10 ^-20 19 0.2 95 ○ X

As shown in Tables 4 and 3, in Examples 1 to 5, a grain boundary oxide layer was not formed on the surface of the target metal product, and as shown in Tables 4 and 4, the target metals in Comparative Examples 1 to 6 It was confirmed that a grain boundary oxide layer was formed on the product surface and no oxide film was formed.

The value of pCO/pCO _{2 in} the carburizing gas is 80 to find out in detail whether intergranular oxidation and the formation of an oxide film are formed according to the value of pCO/pCO ₂ , which is the ratio of the partial pressure of carbon monoxide (pCO) and the partial pressure of carbon dioxide (pCO ₂ ) in the carburizing gas. (CO=20%, CO ₂ =0.25%), pCO/pCO ₂ value of 62.5 (CO=20%, CO ₂ =0.32%), and pCO/pCO ₂ value of 37.8 (CO= Carburization heat treatment was performed under the condition of 20%, CO ₂ =0.25%), and the results are shown in FIGS. 5 to 7, respectively.

As shown in FIG. 5, when the value of pCO/pCO ₂ in the carburized gas was 76 or more, it was confirmed that grain boundary oxidation occurred and the surface film was not formed when the cross section of the target metal product was observed.

As shown in FIG. 6, when 75>pCO/pCO ₂ >40 in the carburizing gas, intergranular oxidation did not occur when the cross-section of the target metal product was observed, and it was confirmed that the surface coating was not formed.

As shown in FIG. 7, when 40>pCO/pCO ₂ >1 in carburizing gas, it was confirmed that grain boundary oxidation did not occur and surface coating was formed when the cross-section of the target metal product was observed.

Through these results, the value of pCO/pCO ₂ , which is the ratio of the partial pressure of carbon monoxide (pCO) and the partial pressure of carbon dioxide (pCO ₂ ) in carburizing gas, is preferably 1 to 75 in order to suppress critical oxidation, but it is possible to stably coat the surface. It can be seen that it is more preferable that the value of pCO/pCO ₂ which is a condition to be formed is less than 1 to 40.

To examine the durability performance of the target metal product to which the carburizing heat treatment method of the present invention was applied, the specimens of Examples 6 and 7 subjected to carburizing heat treatment under the conditions of Table 5 were used for a power train gear durability tester (manufacturer: Space Creation). The product was tested for durability under conditions of 3000 rpm, torque: ±300 N·m, flow rate: 1 L/min, and 25° C., and the results are shown in FIGS. 10 to 17.

division Carburizing
Temperature
(℃) Air: Fuel Cp pCO
(%) pCO ₂
(%) pCO/pCO ₂ Carburizing time/
Diffusion time Example 6 900 1.7:1 One% 20.3 0.25 39.0 90 minutes/50 minutes Comparative Example 7 900 2.37:1 One% 21.5 0.26 82.7 90 minutes/50 minutes

As shown in FIGS. 10 to 13, the carburized products subjected to the carburizing heat treatment method according to the present invention were able to confirm that the Fe oxide film and the Cr oxide film were maintained when the endurance test was compared before and after the endurance test.

On the other hand, in the carburized products in which the existing gas carburization heat treatment method was performed, it was confirmed that a grain boundary oxide layer having an amorphous Si grain boundary layer was formed on the surface, and also a polishing phenomenon occurred on the surface after the endurance test to remove the formed oxide film.

8 is a photograph of the surface observed after the endurance test of the carburized product in which the conventional gas carburization heat treatment method was performed, and FIG. 9 is a photograph of the surface observed after the endurance test of the carburized product subjected to the carburization heat treatment method of the present invention.

As shown in FIG. 8, the carburized product in which the conventional gas carburization heat treatment method is performed is surface polished to deform the tooth shape of about 7 to 8 μm, and in contrast, in the case of a carburized product in which the carburization heat treatment method of the present invention is performed As shown in Fig. 9, the surface polishing phenomenon does not occur, and the tooth deformation is only about 2-3 µm.

18 is a cross-sectional view of a product subjected to a carburizing heat treatment method according to an embodiment of the present invention, and FIG. 19 is an image of a result of oxygen (O) element analysis in the product cross-section of FIG. 18, and the present invention as shown It can be seen that the carburized products performed in the carburizing heat treatment method according to the above have an oxide film formed on the carburizing layer and show a uniform oxygen distribution.

On the other hand, as shown in FIGS. 20 and 21, when looking at a product on which a conventional gas carburizing heat treatment method is performed, it can be confirmed that oxygen elements are not uniformly distributed and an oxide film is not formed on the carburizing layer.

As described above, the carburizing heat treatment method according to the present embodiment is a simple method of controlling the partial pressures of oxygen (O ₂ ), carbon monoxide (CO), and carbon dioxide (CO ₂ ) in carburizing gas. There is an effect of suppressing the formation of a grain boundary oxide layer that reduces durability and improving durability by forming a dense surface oxide layer on the surface of the carburized layer.

The above embodiment is only a preferred example to enable those skilled in the art to easily carry out the present invention, those skilled in the art to which the present invention pertains, and is not limited to the above-described embodiments and the accompanying drawings. The scope of the present invention is not limited. Therefore, it will be apparent to those skilled in the art that various substitutions, modifications, and changes are possible without departing from the technical spirit of the present invention, and it is obvious that a part easily changeable by a person skilled in the art is included in the scope of the present invention.

Claims (11)

The target metal product is introduced into a carburizing tank, heated to a set carburizing temperature, has an oxygen partial pressure (pO ₂ ) of 10 ^-17 to 10 ^-23 atm, and a ratio of carbon monoxide partial pressure (pCO) and carbon dioxide partial pressure (pCO ₂ ), pCO/pCO. the value of ₂ by supplying a carburizing gas containing 1 to 75 carburizing step of carburizing the acceleration; And
Carburizing heat treatment method for improving durability, comprising; carburizing diffusion step of diffusing carbon to the target metal product while maintaining the carburizing temperature and process pressure of the carburizing tank for a set time. According to claim 1,
Carbon monoxide partial pressure (pCO) in the carburizing gas is a carburizing heat treatment method for improving durability, characterized in that 5 to 22%. According to claim 1,
The carbon dioxide partial pressure (pCO ₂ ) in the carburizing gas is a carburizing heat treatment method for improving durability, characterized in that 0.3 to 5%. According to claim 1,
The carburization temperature is 850 to 1050 ℃ carburization heat treatment method for improving durability, characterized in that. According to claim 1,
Carburizing heat treatment method for improving durability, characterized in that the carburizing gas contains oxygen and hydrocarbon fuel in a volume ratio of 2 to 6.5:1 or less. The method of claim 5,
When the hydrocarbon fuel is methane, carburization heat treatment method for improving durability, characterized in that the volume ratio of oxygen and methane is 2: 1 or less. The method of claim 5,
When the hydrocarbon fuel is propane, oxygen and propane have a volume ratio of 6.5:1 or less. According to claim 1,
The carburization step is a carburization heat treatment method for improving durability, characterized in that performed for 80 minutes to 100 minutes. According to claim 1,
The carburization diffusion step is a carburization heat treatment method for improving durability, characterized in that performed for 40 minutes to 60 minutes. According to claim 1,
The carburizing heat treatment method for improving durability, including; a pre-treatment step of washing to remove foreign substances remaining on the surface of the target metal product before the carburizing step. According to claim 1,
The target metal product is a carburizing heat treatment method for improving durability, characterized in that the transmission parts.

Images