WO2019091222A1

WO2019091222A1 - Heat treatment method for controlling nitride in 31crmov9 gear material

Info

Publication number: WO2019091222A1
Application number: PCT/CN2018/106070
Authority: WO
Inventors: 顾晓明; 牟杏华; 许仁伟
Original assignee: 常州天山重工机械有限公司
Priority date: 2017-11-13
Filing date: 2018-09-18
Publication date: 2019-05-16
Also published as: CN108220871A; DE112018000499T5

Abstract

Disclosed is a heat treatment method for controlling a nitride in a 31CrMoV9 gear material. The process thereof comprises subjecting the 31CrMoV9 gear material to procedures such as pretreatment, exhausting, oxidation, strong permeation, diffusion and cooling. The method effectively solves the problem of net-shaped or vein-shaped nitrides caused by an original nitridation process, thereby effectively improving the fatigue resistance and abrasion resistance of a nitride layer in a 31CrMoV9 gear material.

Description

Heat treatment method for controlling nitride of 31CrMoV9 gear material

Technical field

The invention belongs to the technical field of metal material heat treatment, and in particular relates to a heat treatment method for controlling a nitride of a 31CrMoV9 gear material.

Background technique

In order to improve the hardness and wear resistance of the surface of 31CrMoV9 gears, gas nitriding is usually used in China as an effective heat treatment method to improve the surface performance of 31CrMoV9 gears. In industrial production, the layer thickness of the nitride layer of 31CrMoV9 gear material is ≥0.6mm. Since 31CrMoV9 contains V element, the penetration rate of nitrogen atoms during nitriding is low, and alloying elements such as V hinder the entry of nitrogen atoms. If nitrides are formed, nitrogen atoms are hard to migrate and diffuse, so that materials are not easy to form white bright layers and Loose, and nitride is difficult to control. At present, the following methods are often used in gas nitriding: 1. Strictly control the nitriding process, especially to control the nitrogen potential of the furnace gas, to reduce or prevent the formation of veins or reticular nitrides; 2. Strictly control the moisture in the ammonia gas, Use first-stage liquid ammonia or use high-efficiency moisture absorbent to reduce its moisture content; 3. When designing the workpiece, avoid sharp angle of sharp angle and control the roughness of the workpiece surface.

Due to improper gas nitriding process, the nitride layer often has network and vein nitrides, which seriously affects the quality of nitriding, increases the brittleness of the layer, and reduces the fatigue strength and wear resistance of the nitride layer. The layer is easily peeled off. The nitride in the metallographic structure of the 31CrMoV9 gear material after the nitriding process is more serious. According to the GB/T11354 metallographic evaluation standard, the nitride grade is 4 grade.

Summary of the invention

The invention mainly provides a heat treatment method for controlling the nitride of the 31CrMoV9 gear material, effectively solving the problem of the mesh or vein nitride generated by the original nitriding process, thereby effectively improving the fatigue resistance of the nitride layer of the 31CrMoV9 gear material. Strength and wear resistance. Its technical solutions are as follows:

A heat treatment method for controlling nitride of a 31CrMoV9 gear material, comprising the following steps:

(1) Pretreatment: The quenched and austenitizing temperature is 910-920 °C. High-temperature austenitization will avoid the formation of matrix ferrite and bainite, and eliminate the carrier of severe vein-like network nitride. The fire control quenching and tempering hardness is 330-350HB, the 31CrMoV9 gear is cleaned with organic solvent, the oil stain and the residue on the 31CrMoV9 gear are removed, the 31CrMoV9 gear is impervious, and the rack is hoisted into the gas nitriding furnace;

(2) Exhaust: After checking the cover of the furnace cover, adjust the voltage and current, and raise the temperature of the nitriding furnace and pass nitrogen;

(3) Oxidation: When the temperature of the furnace rises to 300-350 °C, the nitrogen gas is turned off, oxygen is introduced, and the workpiece is subjected to high-intensity oxidation treatment to break the damping of the nitrogen element by the V element. After the oxidation is completed, the oxygen is turned off, and the nitrogen gas is passed. Warming the furnace body;

(4) Strong seepage: When the temperature of the furnace rises to 440-460 °C, ammonia gas is introduced to replace the ammonia gas. When the temperature of the furnace rises to 550-570 °C, the nitrogen potential and the nitriding time are controlled to conduct strong permeation, which makes it fast. Nitriding to form a nitride layer body;

(5) Diffusion: After the end of strong osmosis, the diffusion is controlled by a variable temperature alternating potential energy control model with low temperature and high nitrogen potential and high temperature and low nitrogen potential. The first 510 ° C and the nitrogen potential are maintained for 10 h, then the temperature is raised to 530 ° C and the nitrogen potential is 2.18 for 10 h. Diffusion treatment is performed alternately 3 times, and the number of alternations can be increased when the depth of the layer is deeper;

(6) Cooling: After the end of the diffusion, the ammonia gas is turned off, and the nitrogen gas is continuously supplied to make the furnace temperature cool and then air-cooled, and the workpiece can be discharged.

Preferably, the organic solvent in the step (1) is alcohol.

Preferably, the flow rate of nitrogen gas introduced in the step (2) is 15-20 m ³ /h, and the furnace pressure is 300-400 Pa.

Preferably, the oxygen flow rate in step (3) is 0.8 m ³ /h, and the oxidation time is 60 min.

Preferably, in step (4), the strong osmosis is carried out under the conditions of a nitrogen potential of 16 and a nitriding time of 10 h.

Preferably, in step (6), when the furnace temperature is slowly cooled to 400-450 ° C, the cooling fan is air-cooled, and the air is cooled to 150-180 ° C to release the workpiece.

Preferably, the gas nitriding furnace is a well type gas nitriding furnace or a hood type gas nitriding furnace.

With the above scheme, the present invention has the following advantages:

The process design of the invention is reasonable, and the high temperature austenitizing and tempering at 910-920 °C before nitriding can provide an excellent matrix for nitriding, and abandon the conventional 890-900 °C austenitizing process, which is before the gas nitriding stage. Adding artificial oxidation intervention process, 31CrMoV9 requires higher oxygen flow rate and holding time to break the damping of nitrogen element by V element, and will quickly form a thin uniform oxide film on the surface of 31CrMoV9 gear material. The surface free energy has a strong adsorption to nitride, which significantly increases the nitrogen content on the surface of the workpiece, increases the nitrogen concentration gradient on the surface of the workpiece, forms a high concentration difference, and significantly increases the diffusion rate of nitrogen atoms to the substrate. The formation of veins and network nitrides is avoided, and the fatigue strength and wear resistance of the workpiece surface are improved. In the strong infiltration stage, the high-temperature and high-nitrogen potential model is used to break the damping of nitrogen atoms by alloying elements such as V, and the main body of the nitride layer is rapidly formed. The diffusion temperature energy is used to alternately control the model during diffusion, and the high-temperature and low-nitrogen potential diffusion can diffuse the nitride. Low-temperature and high-nitrogen potential diffusion can increase the depth of the layer, and the nitride generated by the strong seepage can be diffused through the process without forming severe vein nitride. At the same time, the invention does not need to add any auxiliary equipment in the oxidation stage, the oxidizing medium is the bottled industrial oxygen, and the flow meter is connected into the furnace through the pipeline, which is more uniform and more efficient than the simple oxidation in the air, and the oxidation and nitridation are in the gas permeation. It is carried out in a nitrogen furnace and is easy to operate.

DRAWINGS

Figure 1 is a graph of a prior art nitriding process;

2 is a graph showing a nitriding process of the present invention;

3 is a metallographic structure of nitride in a prior art gas nitriding layer;

Figure 4 is a view showing the metallographic structure of nitride in the gas nitriding layer of the present invention.

Detailed ways

The experimental methods in the following examples are conventional methods unless otherwise specified, and the experimental reagents and materials involved are conventional biochemical reagents and materials unless otherwise specified.

Example 1

(1) Pretreatment: the quenched and austenitizing temperature is 920 ° C, the tempering control quenching and tempering hardness is 330HB, the 31CrMoV9 gear is cleaned with alcohol, the oil stains and debris on the 31CrMoV9 gear are removed, and the 31CrMoV9 gear is impervious. After loading, hoist into a well type gas nitriding furnace or a hood gas nitriding furnace;

(2) Exhaust gas: the nitriding furnace is heated and nitrogen is passed, the flow rate of nitrogen is 15 m ³ /h, and the furnace pressure is 400 Pa;

(3) Oxidation: When the temperature of the furnace rises to 300 °C, the nitrogen gas is turned off, oxygen is introduced, and the workpiece is subjected to high-intensity oxidation treatment. The oxygen flow rate is 0.8 m ³ /h, the oxidation time is 60 min, and the oxygen is turned off after the oxidation is completed. Nitrogen, continue to heat the furnace;

(4) Strong seepage: When the temperature of the furnace rises to 460 °C, ammonia gas is introduced to replace the ammonia gas. When the temperature of the furnace rises to 550 °C, the strong osmosis is carried out under the conditions of nitrogen potential 16 and nitriding time 10 h. Rapid nitridation to form a nitride layer body;

(5) Diffusion: After the end of strong permeation, first 510 ° C and nitrogen potential 0.5 for 10 h, then warm to 530 ° C and nitrogen potential 2.18 for 10 h, alternating diffusion treatment 3 times;

(6) Cooling: After the end of the diffusion, the ammonia gas is turned off, and the nitrogen gas is continuously introduced. When the furnace temperature is slowly cooled to 450 ° C, the cooling fan is air-cooled, and the air is cooled to 150 ° C to release the workpiece.

The above process flow is shown in Figure 2.

Example 2

(1) Pretreatment: Tempering and austenitizing temperature is 910 °C, tempering control and quenching hardness is 350HB, using alcohol to clean 31CrMoV9 gears, removing oil stains and debris on 31CrMoV9 gears, and impervious to 31CrMoV9 gears. After loading, hoist into a well type gas nitriding furnace or a hood gas nitriding furnace;

(2) Exhaust gas: the nitriding furnace is heated and nitrogen is passed, the flow rate of nitrogen is 20 m ³ /h, and the furnace pressure is 300 Pa;

(3) Oxidation: When the temperature of the furnace rises to 350 °C, the nitrogen gas is turned off, oxygen is introduced, and the workpiece is subjected to high-intensity oxidation treatment. The oxygen flow rate is 0.8 m ³ /h, the oxidation time is 60 min, and the oxygen is turned off after the oxidation is completed. Nitrogen, continue to heat the furnace;

(4) Strong seepage: When the temperature of the furnace rises to 440 °C, ammonia gas is introduced to replace the ammonia gas. When the temperature of the furnace rises to 570 °C, the infiltration is carried out under the conditions of nitrogen potential 16 and nitriding time 10 h. Rapid nitridation to form a nitride layer body;

(6) Cooling: After the end of the diffusion, the ammonia gas is turned off, and the nitrogen gas is continuously supplied. When the furnace temperature is slowly cooled to 400 ° C, the cooling fan is air-cooled, and the air is cooled to 180 ° C to release the workpiece.

Comparative example 1

The 31CrMoV9 gear is heated in a nitriding furnace and protected by nitrogen gas, and then subjected to strong permeation under the condition of a nitrogen potential of 6, and then subjected to constant temperature diffusion and cooling. The conventional process flow is shown in FIG.

Result detection

The workpieces prepared in the processes of Example 1 and Comparative Example 1 were respectively observed for the metallographic structure of the nitride in the nitrided layer. According to the GB/T11354 metallographic evaluation criterion, only a small amount of nitride distributed in the nitride diffusion layer formed in the process of the first embodiment has a grade of grade 1, the depth of the nitride layer can reach 0.65 mm, and the surface hardness can reach 850HV1, the metallographic structure of nitride in the specific nitrided layer is shown in Figure 4. The existing process, that is, the metallurgical structure of the 31CrMoV9 gear material after the process nitridation in the first embodiment has a relatively veiny and a small amount of intermittent network-like nitride. According to the GB/T11354 metallographic evaluation standard, the nitride level is Level 4, metallographic organization is shown in Figure 3.

According to the analysis, the high-temperature austenitizing and tempering of 910-920 °C before nitriding can provide an excellent matrix for nitriding. In the present invention, the kiln gas nitriding furnace is heated, and when the temperature reaches a certain temperature, the nitrogen is turned off. The valve opens the oxygen flow meter and performs high-intensity oxidation treatment on the workpiece through artificial oxygen flow intervention to break the damping of nitrogen element by V element. The pre-oxidation effect of bottled high-purity oxygen is more uniform, purer and more efficient than conventional air oxidation. Higher, after this oxidation treatment, a very thin uniform oxide film is formed on the surface of the 31CrMoV9 gear material. The oxide film is loose, has many defects, has high surface free energy, and has strong adsorption to nitride, which is remarkably improved. The nitrogen content on the surface of the workpiece increases the nitrogen concentration gradient on the surface of the material and accelerates the diffusion of nitrogen atoms into the matrix. In the strong infiltration stage, the high-temperature and high-nitrogen potential model is used to break the damping of the nitrogen atoms of the alloy elements, and the main body of the nitride layer is rapidly formed. The model is alternately controlled by the variable temperature potential energy during diffusion, and the high-temperature low-nitrogen potential diffusion can diffuse the nitride, and the low temperature is high. Nitrogen potential diffusion can increase the depth of the layer, and the nitride produced by the strong permeation can be diffused through the process without forming severe vein nitride. Compared with the gas nitriding in the non-oxidation stage, after the workpiece is treated by the same treatment temperature, time and the same nitrogen potential, there is no vein or network nitride in the nitrided layer after the nitriding process of the present invention, so that the nitride is from 4 The grade is reduced to level 1, which improves the surface fatigue strength and wear resistance of the 31CrMoV9 gear material. At the same time, by the oxidation treatment before nitriding, the residual residue and oil stain on the surface of the workpiece will be burned off, which will clean the workpiece and remove the water stains remaining in the furnace and on the workpiece.

The invention does not need to add any auxiliary equipment, the bottled industrial oxygen in the oxidation stage, the pipeline connection, and the flow meter with precise control, the control is precise and simple.

Various other changes and modifications may be made by those skilled in the art in light of the above-described technical solutions and concepts, and all such changes and modifications are intended to fall within the scope of the appended claims.

Claims

A heat treatment method for controlling a nitride of a 31CrMoV9 gear material, comprising: the following steps:

(1) Pretreatment: Tempering austenitizing temperature is 910-920 °C, tempering control quenching hardness is 330-350HB, using 31cmMoV9 gear cleaning with organic solvent, removing oil stains and debris on 31CrMoV9 gear, to 31CrMoV9 The gear is impervious, and is hoisted into the gas nitriding furnace after being assembled;

(2) Exhaust: heating the nitriding furnace and passing nitrogen;

(3) Oxidation: When the temperature of the furnace rises to 300-350 °C, the nitrogen gas is turned off, oxygen is introduced, and the workpiece is subjected to high-intensity oxidation treatment. After the oxidation is completed, the oxygen is turned off, and the nitrogen gas is passed to continue to heat the furnace body;

(4) Strong seepage: When the temperature of the furnace rises to 440-460 °C, ammonia gas is introduced to replace the ammonia gas. When the temperature of the furnace rises to 550-570 °C, the nitrogen potential and the nitriding time are controlled to conduct strong permeation, which makes it fast. Nitriding to form a nitride layer body;

(5) Diffusion: After the end of the strong permeation, the first 510 ° C and the nitrogen potential 0.5 for 10 h, and then the temperature is raised to 530 ° C and the nitrogen potential is 2.18 for 10 h, alternating diffusion treatment;

(6) Cooling: After the end of the diffusion, the ammonia gas is turned off, and the nitrogen gas is continuously supplied to make the furnace temperature cool and then air-cooled, and the workpiece can be discharged.
The method for heat-treating a nitride of a 31CrMoV9 gear material according to claim 1, wherein the organic solvent in the step (1) is alcohol.
The method for heat-treating a nitride of a 31CrMoV9 gear material according to claim 1, wherein the flow rate of nitrogen gas in the step (2) is 15-20 m 3 /h, and the furnace pressure is 300-400 Pa.
The method for heat-treating a nitride of a 31CrMoV9 gear material according to claim 1, wherein the oxygen flow rate in step (3) is 0.8 m 3 /h, and the oxidation time is 60 min.
The method for heat-treating a nitride of a 31CrMoV9 gear material according to claim 1, wherein in step (4), the osmosis is performed under the conditions of a nitrogen potential of 16 and a nitridation time of 10 hours.
The method of heat-treating a nitride of a 31CrMoV9 gear material according to claim 1, wherein the number of times of the diffusion treatment alternately in the step (5) is three.
The method for heat-treating a nitride of a 31CrMoV9 gear material according to claim 1, wherein in the step (6), when the furnace temperature is slowly cooled to 400-450 ° C, the cooling fan is air-cooled, and air-cooled to 150-180. °C workpiece is released.
The method for heat-treating a nitride of a 31CrMoV9 gear material according to claim 1, wherein the gas nitriding furnace is a well gas nitriding furnace or a hood gas nitriding furnace.