Heat treatment process for gear parts

Abstract

The present invention discloses a heat treatment process for gear parts, comprising five carbon potential carburizing steps: S1, carburizing at a temperature of 930-940°C under specified carbon potential conditions for 40-50 hours; S2, carburizing at a temperature of 900-920°C under specified carbon potential conditions for X hours according to the relationship between the depth of the carburized layer of the part and the carburizing time; S3, carburizing at a temperature of 900-920°C under specified carbon potential conditions for 10-15 hours; S4, carburizing at a temperature of 900-920°C under specified carbon potential conditions for Y hours according to the relationship between the depth of the carburized layer of the part and the carburizing time; S5, carburizing at a temperature of 900-920°C under specified carbon potential conditions for Z hours; the depth of the carburized layer in S2, S4 and S5 is positively correlated with the carburizing time. The technical effect of the present invention is to reduce the depth of the grain boundary oxide layer of parts with a carburized layer depth of 4.00mm-5.00mm.

Description

Technical Field

[0001] The invention relates to heat treatment technology, in particular to a heat treatment process for a gear part. Background Art

[0002] Carburizing and quenching are key technologies for improving gear fatigue performance. However, grain boundary oxidation, which occurs on the surface during the carburizing and quenching process, has long been a difficult quality issue to resolve in the chemical heat treatment industry. Micro-area energy spectrum analysis of the grain boundary oxidation structure shows that the components of the grain boundary oxidation area are mainly oxides of Cr, Si, etc. Grain boundary oxidation forms because when oxygen invades the steel surface in a carburizing atmosphere, elements such as Cr and Si near the grain boundaries diffuse to the grain boundaries preferentially over other elements and combine with oxygen to form oxides. This depletes the alloying elements at and near the grain boundaries, resulting in reduced hardenability and the appearance of non-martensitic structure after quenching.

[0003] The depth of grain boundary oxidation directly affects the surface hardness, wear resistance, and fatigue strength of parts, especially bending fatigue strength. It reduces the beneficial residual compressive stress on the part surface, resulting in a shortened part life. Research has shown that when the grain boundary oxidation depth is less than 0.013mm, it has little effect on fatigue strength. When the grain boundary oxidation depth is greater than 0.016mm, the fatigue strength of the part can be reduced by 25%. When the grain boundary oxidation depth is above 0.04mm, the bending fatigue strength can be reduced by 50%. Therefore, controlling the grain boundary oxidation depth is a key technology in chemical heat treatment.

[0004] Statistics on the carburized layer and grain boundary oxidation depths of parts produced by the company show that for parts with a carburized layer depth greater than 3.00mm, the grain boundary oxidation depth indicator is prone to deviations, and the deeper the layer, the greater the deviation rate. For parts with a carburized layer depth between 4.00mm and 5.00mm, the pass rate according to ISO 6336-5ME grade requirements is only 43.4%, while the pass rate according to MQ grade requirements is 82.9%. Therefore, for parts with a carburized layer depth between 4.00mm and 5.00mm, it is necessary to identify the factors that affect the grain boundary oxidation depth and develop and adjust heat treatment processes to reduce the grain boundary oxidation depth. Summary of the Invention

[0005] The first object of the present invention is to provide a heat treatment process for gear parts, which can effectively reduce the depth of the grain boundary oxidation layer of parts with a carburized layer depth of 4.00mm to 5.00mm after carburizing and quenching.

[0006] To achieve the above-mentioned first purpose, the present invention adopts the following technical solution.

[0007] A heat treatment process for a gear part includes five-stage carbon potential carburizing, wherein the five-stage carbon potential carburizing includes the following steps:

[0008] S1, the first stage of strong carburizing, carburizing at a temperature of 930-940°C under the conditions of specified carbon potential, and the carburizing time is 40-50 hours;

[0009] S2, the second stage of strong carburizing, carburizing at a temperature of 900-920°C under the conditions of specified carbon potential, and the carburizing time is X hours according to the relationship between the depth of the carburized layer of the part and the carburizing time;

[0010] S3, the third stage of strong carburizing, carburizing at a temperature of 900-920°C under the conditions of specified carbon potential, and the carburizing time is 10-15 hours;

[0011] S4, the fourth stage of carbon potential carburizing in the diffusion period, carburizing at a temperature of 900-920°C under the conditions of a specified carbon potential, and the carburizing time is Y hours according to the relationship between the depth of the carburized layer of the part and the carburizing time;

[0012] S5, the fifth carbon potential carburizing in the diffusion period, adopts a temperature of 900-920°C carburizing under the conditions of specified carbon potential, and the carburizing time is Z hours according to the relationship between the depth of the carburized layer of the part and the carburizing time; the depth of the carburized layer in S2, S4 and S5 is positively correlated with the carburizing time.

[0013] The present invention adopts the above technical solution and is aimed at parts with a carburized layer depth of 4mm to 5.0mm. During the strong carburizing period, in order to shorten the carburizing strong carburizing period time and prevent grain growth, the process adopts a relatively high strong carburizing temperature of 930℃ to 940℃ for 40 hours to 50 hours, and then reduces the temperature to 900℃ to 920℃ to continue carburizing. The high temperature used in the early stage of this process is conducive to enhancing the activity coefficient of carbon atoms, thereby increasing the carburizing speed and shortening the carburizing time. At this time, the degree of oxide formation of elements such as Cr and Si is relatively low. After a certain period of carburizing, the temperature is reduced to prevent abnormal grain size growth. In this way, the purpose of reducing the Cr oxidation range can be achieved, ensuring the depth of the carburized layer, while preventing the growth of austenite grain size and shortening the formation depth of the grain oxidation layer.

[0014] Preferably, before the five-stage carbon potential carburizing step, the part is subjected to two stages of temperature equalization, the two ends of the temperature equalization, the two stages of temperature equalization include the first stage of temperature equalization and the second stage of temperature equalization, the temperature of the first stage of temperature equalization is 650°C, and the temperature equalization time is 2 hours.

[0015] By performing a temperature equalization treatment for 2 hours before the five-stage carbon potential carburizing, the material properties of the parts to be carburized are improved, the strength and toughness of the materials are increased, and the performance is better.

[0016] Preferably, the temperature of the second stage of temperature averaging is 850° C., and the temperature averaging time is 2 hours.

[0017] In this way, the parts are subjected to a second heat treatment at 850°C for 2 hours, which further increases the temperature of the uniform temperature and gradually heats up the material, making it closer to the temperature of the carburizing treatment, thereby preventing the temperature from rising sharply during the carburizing treatment and affecting the material properties.

[0018] Preferably, the depth of the carburized layer of the part in S2 and the carburizing time have the following relationship: 15e=X, where e is the depth of the carburized layer of the part, X is the carburizing time, the unit of e is mm, and the unit of X is hour.

[0019] By designing a corresponding relationship between the time of the second carbon potential carburizing and the depth of the carburizing time layer of the carburized part, the depth of grain boundary oxidation can be reduced while meeting the carburizing quality.

[0020] Preferably, the depth of the carburized layer of the part in S4 and the carburizing time have the following relationship: 10e=Y, where e is the depth of the carburized layer of the part, Y is the carburizing time, the unit of e is mm, and the unit of Y is hour.

[0021] In this way, a corresponding relationship is designed between the time of the fourth carbon potential carburizing diffusion stage and the depth of the carburizing time layer of the carburized part. The corresponding carburizing time is set according to the depth of the carburizing time layer of the carburized part, and the grain boundary oxidation depth is reduced while meeting the carburizing quality.

[0022] Preferably, the depth of the carburized layer of the part in S5 and the carburizing time have the following relationship: 5e=Z, where e is the depth of the carburized layer of the part, Z is the carburizing time, the unit of e is mm, and the unit of Z is hour.

[0023] In this way, a corresponding relationship is designed between the time of the fifth carbon potential carburizing diffusion stage and the depth of the carburizing time layer of the carburized part. The corresponding carburizing time is set according to the depth of the carburizing time layer of the carburized part, and the grain boundary oxidation depth is reduced while meeting the carburizing quality.

[0024] Preferably, the specified carbon potentials in S1 to S5 are 1.25% carbon to 1.3% carbon, 1.2±0.05% carbon, 1.0±0.05% carbon, 0.7±0.05% carbon and 0.8±0.05% carbon, respectively.

[0025] Thus, during the five-stage carbon potential carburizing process, the carbon potential can be controlled at 1.25% to 1.3% carbon during the first stage of the intense carburizing phase and last for 40 to 50 hours. During the first stage of carbon potential carburizing, the carbon potential is reduced to 1.20±0.05% carbon and lasts for 40 to 50 hours. This helps shorten the carburizing time and reduce the oxygen partial pressure in the furnace, thereby reducing the depth of grain boundary oxidation. Furthermore, due to the long-term high carbon potential carburizing, a large amount of carbon black is present in the furnace, making it difficult to reduce the carbon potential to the required range in a short period of time during the subsequent diffusion process. Therefore, the carbon potential is appropriately reduced in the later stage of intense carburizing to create conditions for diffusion. That is, during the third stage of carbon potential carburizing before entering diffusion, the carbon potential is reduced to 1.0±0.05% carbon and lasts for 10 to 15 hours to prepare for subsequent diffusion. During diffusion, two carbon potentials are also used. During carburizing in the fourth carbon potential, the carbon potential is reduced to 0.7%±0.05% carbon to diffuse the formed carbides. During carburizing in the fifth carbon potential, the carbon potential is increased to 0.8%±0.05% carbon to ensure the necessary oxygen partial pressure and surface carbon concentration in the furnace and avoid deepening the depth of grain boundary oxidation.

[0026] Preferably, after the five-stage carbon potential carburizing, the parts are subjected to quenching cooling, and the quenching cooling adopts nitrate bath quenching, and the water content of nitrate during the nitrate bath quenching is controlled at 0.5%-3.0%.

[0027] In this way, the water content of nitrate during nitrate bath quenching is controlled at 0.5%-3.0%, so as to utilize the cooling effect of high nitrate.

[0028] Preferably, the nitrate bath is stirred with compressed air during quenching in the nitrate bath, and the pressure of the compressed air is 0.3 MPa to 0.5 MPa.

[0029] The beneficial effect of the present invention is that it takes into account the prevention of coarsening of the grain size of parts and the reduction of deformation, while starting from the control purpose of reducing the depth of grain boundary oxidation of deep parts. During the carburizing and intensive carburizing period, in order to shorten the carburizing and intensive carburizing period time and at the same time prevent the grains from growing, the process adopts a higher intensive carburizing temperature of 930°C to 940°C for 40 hours to 50 hours, and then reduces the temperature to 900°C to 920°C to continue carburizing. The process adopts a higher temperature in the early stage, which is conducive to enhancing the activity coefficient of carbon atoms, thereby increasing the carburizing rate and shortening the time. At this time, the degree of oxide formation of elements such as Cr and Si is low. After a certain period of carburizing, the temperature is reduced to prevent abnormal growth of grain size. In this way, the purpose of narrowing the Cr oxidation range can be achieved, the depth of the carburized layer can be guaranteed, and the growth of the austenite grain size can be avoided, and the formation depth of the grain oxidation layer can be shortened. Thus, during the five-stage carbon potential carburizing process, the carbon potential can be controlled at 1.25% to 1.3% carbon during the first stage of the intense carburizing phase and last for 40 to 50 hours. During the first stage of carbon potential carburizing, the carbon potential is reduced to 1.20±0.05% carbon and lasts for 40 to 50 hours. This helps shorten the carburizing time and reduce the oxygen partial pressure in the furnace, thereby reducing the depth of grain boundary oxidation. Furthermore, due to the long-term high carbon potential carburizing, a large amount of carbon black is generated in the furnace, making it difficult to reduce the carbon potential to the required range in a short period of time during the subsequent diffusion process. Therefore, the carbon potential is appropriately reduced in the later stage of intense carburizing to create conditions for diffusion. That is, during the third stage of carbon potential carburizing before entering diffusion, the carbon potential is reduced to 1.0±0.05% carbon and lasts for 10 to 15 hours to prepare for subsequent diffusion. During diffusion, two carbon potentials are also used. During carburizing in the fourth carbon potential, the carbon potential is reduced to 0.7%±0.05% carbon to diffuse the formed carbides. During carburizing in the fifth carbon potential, the carbon potential is increased to 0.8%±0.05% carbon to ensure the necessary oxygen partial pressure and surface carbon concentration in the furnace and avoid deepening the depth of grain boundary oxidation. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a process curve diagram of the present invention. DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to the accompanying drawings, but the present invention is not limited to the scope of the embodiments described.

[0032] Example 1, see Figure 1 A heat treatment process for a gear part includes five-stage carbon potential carburizing, wherein the five-stage carbon potential carburizing includes the following steps:

[0033] S1, the first stage of strong carburizing, carburizing at a temperature of 930-940°C under the conditions of specified carbon potential, and the carburizing time is 40-50 hours;

[0034] S2, the second stage of strong carburizing, carburizing at a temperature of 900-920°C under the conditions of specified carbon potential, and the carburizing time is X hours according to the relationship between the depth of the carburized layer of the part and the carburizing time;

[0035] S3, the third stage of strong carburizing, carburizing at a temperature of 900-920°C under the conditions of specified carbon potential, and the carburizing time is 10-15 hours;

[0036] S4, the fourth stage of carbon potential carburizing in the diffusion period, carburizing at a temperature of 900-920°C under the conditions of a specified carbon potential, and the carburizing time is Y hours according to the relationship between the depth of the carburized layer of the part and the carburizing time;

[0037] S5, the fifth carbon potential carburizing in the diffusion period, uses a temperature of 900-920°C to carburize under the conditions of specified carbon potential, and the carburizing time is Z hours according to the relationship between the depth of the carburized layer of the part and the carburizing time. The depth of the carburized layer in S2, S4 and S5 is positively correlated with the carburizing time.

[0038] Among them, see Figure 1 Before the five-stage carbon potential carburizing step, the parts are subjected to two stages of temperature equalization. The temperature equalization at both ends includes the first stage of temperature equalization and the second stage of temperature equalization. The temperature of the first stage of temperature equalization is 650°C and the temperature equalization time is 2 hours.

[0039] See also Figure 1 The carburized layer depth of the part in S2 and the carburizing time are related by the following equation: 15e = X, where e is the carburized layer depth of the part and X is the carburizing time. The units of e are mm and X are hours. For parts with a carburized layer depth of 4 mm to 5.0 mm, the corresponding carburizing time is 60 h to 75 h. As the carburized layer depth increases, the corresponding carburizing time also increases.

[0040] See also Figure 1 The carburized layer depth and carburizing time in S4 are related by the following equation: 10e = Y, where e is the carburized layer depth and Y is the carburizing time. The units of e are mm and Y are hours. For parts with a carburized layer depth of 4 mm to 5.0 mm, the corresponding carburizing time is 40 h to 50 h. As the carburized layer depth increases, the corresponding carburizing time also increases.

[0041] See also Figure 1The relationship between the carburized layer depth and carburizing time in S5 is as follows: 5e = Z, where e is the carburized layer depth and Z is the carburizing time. The units of e are mm and Z are hours. For parts with a carburized layer depth of 4 mm to 5.0 mm, the corresponding carburizing time is 20 hours to 25 hours. As the carburized layer depth increases, the corresponding carburizing time also increases.

[0042] See also Figure 1 The specified carbon potentials in S1 to S5 are 1.25% carbon to 1.3% carbon, 1.2±0.05% carbon, 1.0±0.05% carbon, 0.7±0.05% carbon and 0.8±0.05% carbon, respectively.

[0043] See also Figure 1 After the five-stage carbon potential carburizing, the part is subjected to quenching cooling, wherein the quenching cooling adopts nitrate bath quenching, and the water content of the nitrate during the nitrate bath quenching is controlled to be 0.5%-3.0%. During the nitrate bath quenching, the nitrate bath is stirred with compressed air, and the pressure of the compressed air is 0.3MPa-0.5MPa.

[0044] Thus, during the five-stage carbon potential carburizing process, the carbon potential can be controlled at 1.25% to 1.3% carbon during the first stage of the intense carburizing phase and last for 40 to 50 hours. During the first stage of carbon potential carburizing, the carbon potential is reduced to 1.20±0.05% carbon and lasts for 40 to 50 hours. This helps shorten the carburizing time and reduce the oxygen partial pressure in the furnace, thereby reducing the depth of grain boundary oxidation. Furthermore, due to the long-term high carbon potential carburizing, a large amount of carbon black is generated in the furnace, making it difficult to reduce the carbon potential to the required range in a short period of time during the subsequent diffusion process. Therefore, the carbon potential is appropriately reduced in the later stage of intense carburizing to create conditions for diffusion. That is, during the third stage of carbon potential carburizing before entering diffusion, the carbon potential is reduced to 1.0±0.05% carbon and lasts for 10 to 15 hours to prepare for subsequent diffusion. During diffusion, two carbon potentials are also used. During carburizing in the fourth carbon potential, the carbon potential is reduced to 0.7%±0.05% carbon to diffuse the formed carbides. During carburizing in the fifth carbon potential, the carbon potential is increased to 0.8%±0.05% carbon to ensure the necessary oxygen partial pressure and surface carbon concentration in the furnace and avoid deepening the depth of grain boundary oxidation.

[0045] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A heat treatment process for a gear part, characterized in that: The method comprises performing five-stage carbon potential carburizing on a part having a carburized layer depth of 4 to 5 mm, wherein the five-stage carbon potential carburizing comprises the following steps: S1, the first stage of the strong carburizing period, carburizing at a temperature of 930-940°C under the conditions of a specified carbon potential of 1.25% carbon to 1.3% carbon, and a carburizing time of 40 hours to 50 hours; S2, the second stage of the strong carburizing period, carburizing at a temperature of 900-920°C under the condition of a specified carbon potential of 1.2±0.05% carbon, and the carburizing time is X hours according to the relationship between the depth of the carburized layer of the part and the carburizing time; the depth of the carburized layer of the part in S2 and the carburizing time have the following relationship: 15e=X, where e is the depth of the carburized layer of the part, X is the carburizing time, the unit of e is mm, and the unit of X is hours; S3, the third stage of strong carburizing, carburizing at a temperature of 900-920 °C under the condition of a specified carbon potential of 1.0 ± 0.05%, and the carburizing time is 10 hours to 15 hours; S4, carburizing at the fourth carbon potential stage of the diffusion period, carburizing at a temperature of 900-920°C under a specified carbon potential of 0.7±0.05%, and carburizing for Y hours according to the relationship between the depth of the carburized layer of the part and the carburizing time; the depth of the carburized layer of the part in S4 and the carburizing time have the following relationship: 10e=Y, where e is the depth of the carburized layer of the part, Y is the carburizing time, the unit of e is mm, and the unit of Y is hours; S5, the fifth carbon potential carburizing in the diffusion period, carburizing at a temperature of 900-920°C under the condition of a specified carbon potential of 0.8±0.05% carbon, and the carburizing time is Z hours according to the relationship between the depth of the carburized layer of the part and the carburizing time; the depth of the carburized layer of the part in S5 and the carburizing time have the following relationship: 5e=Z, where e is the depth of the carburized layer of the part, Z is the carburizing time, the unit of e is mm, and the unit of Z is hours; The depth of the carburized layer in S2, S4 and S5 is positively correlated with the carburizing time.

2. The heat treatment process for a gear part according to claim 1, characterized in that: Before the five-stage carbon potential carburizing step, the parts are subjected to two stages of temperature equalization, the two stages of temperature equalization including a first stage of temperature equalization and a second stage of temperature equalization, the temperature of the first stage of temperature equalization is 650° C., and the temperature equalization time is 2 hours.

3. The heat treatment process for a gear part according to claim 2, characterized in that: The temperature of the second stage is 850° C., and the temperature averaging time is 2 hours.

4. The heat treatment process for a gear part according to any one of claims 1 to 3, characterized in that: After the five-stage carbon potential carburizing, the parts are quenched and cooled. The quenching and cooling adopts nitrate bath quenching, and the water content of nitrate during the nitrate bath quenching is controlled at 0.5%-3.0%.

5. The heat treatment process for a gear part according to claim 4, characterized in that: During the nitrate bath quenching, the nitrate bath is stirred with compressed air, and the pressure of the compressed air is 0.3 MPa to 0.5 MPa.

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