Surface strengthening method for third-generation aviation gear steel

Abstract

The invention discloses a surface strengthening method for third-generation aviation gear steel, which comprises the following steps: carrying out heat treatment on initial-state gear steel, so that the structure of the gear steel is transformed into martensite, and the matrix hardness of the gear steel is preliminarily improved; the gear steel subjected to heat treatment is subjected to ultrasonic shot blasting treatment, so that grains of the gear steel are subjected to gradient refining, and the matrix hardness is further improved; and the gear steel subjected to ultrasonic shot peening treatment is put into an inert gas shielding vacuum heat treatment furnace to be subjected to internal energy homogenizing, tempering and annealing treatment, and the hardness of the base body is improved again. According to the invention, the original martensite phase size of the gear steel is changed through the ultrasonic processing and low-temperature post-treatment method and through severe plastic deformation, and a gradient layer and a matrix have no bonding surface, so that stress concentration, carburizing cracking and non-uniformity of a carburizing layer caused by the bonding surface can be avoided, and certain plasticity is retained while the hardness is improved; the wear resistance and impact toughness are better, the service life of the gear steel is greatly prolonged, the treatment time is shortened, and power consumption and environmental pollution are reduced.

Description

Technical Field

[0001] The invention relates to the technical field of gear steel surface strengthening, and in particular to a surface strengthening method for third-generation aviation gear steel. Background Art

[0002] In the 1990s, the American Latrobe Specialty Steel Company developed the carburized high-temperature resistant gear steel CSS-42L. After carburization, the structure has excellent high-temperature red hardness, wear resistance and corrosion resistance. At the same time, the core martensite structure still maintains extremely high strength and fracture toughness, and the service temperature is raised to 450℃ compared with the second-generation carburized gear steel M50NiL. CSS-42L is the first third-generation carburized gear steel grade to be applied in practice. The common grades of third-generation carburized gear steel known so far are all developed by the United States. The industry generally believes that the service temperature of the third-generation carburized gear steel is ≤500℃. The alloying content exceeds 30%, which is a low-carbon ultra-high alloy steel. Compared with the traditional second-generation carburizing gear steel, the third-generation carburizing gear steel has greatly improved the corrosion resistance of the matrix by significantly increasing the total amount of [Cr+Ni] alloying elements. After solid solution quenching and (deep) cold treatment, the matrix is ​​transformed from conventional martensitic structural steel to martensitic stainless steel with better corrosion resistance. The introduction and significant increase of Co alloying elements make the third-generation carburizing gear steel have excellent high-temperature thermal stability above 400°C, ensuring that the gear still exhibits good high-temperature red hardness in the oil-free dry running state.

[0003] At present, the known CSS-42L derivative domestic material grades are mainly 15Cr14Co12Mo4Ni2VNb and 15Cr14Co12Mo5Ni. At present, the methods to improve the surface damage resistance of domestic third-generation steel mainly include carburizing strengthening, adding coatings, improving heat treatment, etc., but the carburizing process requires a certain time and temperature, and the selection of carburizing agents, treatment time and temperature need to be strictly controlled, which increases the process complexity and cost. For example, Chinese patent CN109338274A discloses a 15Cr14Co12Mo5Ni2WA steel secondary low-pressure vacuum carburizing method, which includes the following steps: first, through a low-pressure pulse carburizing + high-pressure gas quenching combined furnace, a 1:1 mixture of acetylene and hydrogen is continuously introduced for 12 hours at a pressure of 1100pa and a temperature of 1140°C, and then quenched at 1100°C for 1 hour, and then subjected to two -196°C deep cooling for 2 hours and 500°C for 2 hours of tempering heat treatment. After the sample has gone through the whole process, the maximum surface hardness is 796HV, the core hardness is 520HV, and the effective carburized layer depth is 0.77mm. After carburizing treatment, although the surface hardness and wear resistance of metal materials are improved, the toughness and toughening ability of the material will be reduced. Under special working conditions such as high strength and high toughness, problems such as fracture may occur. There are also problems such as carburizing cracking, carbide regulation and uneven carburized layer that need to be solved. High-temperature vacuum carburizing and the addition of heavy metal coatings are also accompanied by high energy consumption, long time consumption, heavy metal pollution and other problems. Therefore, there is an urgent need for an efficient and environmentally friendly surface modification technology to avoid surface defects of carburized third-generation steel and improve fatigue life. Summary of the invention

[0004] In view of the shortcomings of the high-temperature vacuum low-pressure carburizing pre-treatment process used in the existing third-generation steel processing, which is time-consuming and energy-intensive, the present invention proposes a surface strengthening method for the third-generation aviation gear steel based on the concept of low carbon and environmental protection. The initial gear steel does not undergo carburizing, but is directly heat-treated and then subjected to ultrasonic processing and low-temperature treatment for post-treatment instead of carburizing pre-treatment. The treatment process is environmentally friendly and controllable, which greatly reduces the processing time and reduces power consumption and environmental pollution.

[0005] The technical solution adopted by the present invention is:

[0006] A surface strengthening method for third-generation aviation gear steel comprises the following steps:

[0007] S1. Heat treat the initial gear steel to transform the gear steel structure into martensite and preliminarily improve the matrix hardness of the gear steel;

[0008] S2. Perform ultrasonic shot peening on the heat-treated gear steel to make the grains of the gear steel gradient refined and further improve the matrix hardness;

[0009] S3. The gear steel after ultrasonic shot peening is placed in an inert gas protected vacuum heat treatment furnace for internal energy homogenization tempering annealing to further improve the matrix hardness.

[0010] In the above scheme, in step S1, the heat treatment includes:

[0011] The gear steel is placed in a heat treatment furnace and heated to a temperature within the range of 1000°C to 1100°C, and kept at this temperature for 1 to 1.5 hours;

[0012] Quickly put the gear steel into the quenching oil, and the gear steel temperature quickly drops to room temperature;

[0013] Clean the gear steel and place it in liquid nitrogen for deep freezing. After standing in liquid nitrogen for 1.5 to 2 hours, take it out and place it in the air to wait for the gear steel temperature to return to room temperature.

[0014] Place the gear steel in a heat treatment furnace and heat it to 450℃~550℃ and keep it warm for 1.5~2 hours. After the heat preservation is completed, take out the gear steel and let it stand in the air until the temperature drops to room temperature.

[0015] Clean the gear steel again and perform a round of liquid nitrogen cryogenic treatment. After standing in liquid nitrogen for 1.5 to 2 hours, take it out and place it in the air to wait for the gear steel temperature to return to room temperature.

[0016] The gear steel is placed in the heat treatment furnace again and heated to 450°C to 550°C and kept warm for 1.5 to 2 hours. After the insulation is completed, the sample is taken out and placed in the air to wait for the temperature to drop to room temperature.

[0017] In the above scheme, in step S2, the parameters of ultrasonic shot peening include: ultrasonic frequency of 20KHz to 10000KHz, diameter of the steel ball medium used for shot peening of 0.1mm to 10mm, number of steel ball media of 10 to 1000, shot peening distance of 5 to 15cm, and ultrasonic shot peening time of 1s to 1200s.

[0018] In the above scheme, in step S2, the ultrasonic shot peening is performed at room temperature.

[0019] In the above scheme, in step S3, the temperature range of the annealing treatment is 200° C. to 500° C., and the treatment time is 10 to 100 minutes.

[0020] In the above scheme, after the heat treatment in step S1, the black carbides precipitated on the surface of the gear steel are removed to restore the metallic color of the gear steel, and the surface to be loaded is processed to be bright before the ultrasonic shot peening treatment in step S2 is performed.

[0021] In the above scheme, the third generation aviation gear steel is 15Cr14Co12Mo4Ni2VNb or 15Cr14Co12Mo5Ni.

[0022] The beneficial effects produced by the present invention are:

[0023] Compared with the method of increasing the surface hardness of the metal by carburizing pre-treatment in the prior art, the present invention can avoid the joint surface between the carbide and the matrix caused by carburization through ultrasonic processing and low-temperature post-treatment methods. The post-treatment method of the present invention changes the original martensite phase size of the gear steel through large plastic deformation. The gradient layer and the matrix have no bonding surface, which can not only avoid the shortcomings of stress concentration, carburizing cracking and uneven carburized layer caused by the bonding surface, but also retain a certain plasticity while increasing the hardness, and has better wear resistance and impact toughness, greatly improving the service life of the gear steel.

[0024] In the prior art, high-temperature vacuum low-pressure carburizing pre-treatment requires at least 12 hours, while the ultrasonic machining of the present invention only takes about 15 minutes, and the low-temperature post-treatment only takes about 30 minutes. The post-treatment strengthening process of gear steel can be completed in no more than 1 hour in total, and the whole process is shortened from 21 hours to less than 10 hours, which greatly reduces the processing time and reduces power consumption and environmental pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0026] Figure 1 is a flow chart of a surface strengthening method for third-generation aviation gear steel in an embodiment of the present invention;

[0027] Figure 2 This is an EBSD scanning image of 15Cr14Co12Mo5Ni gear steel after heat treatment in an embodiment of the present invention;

[0028] Figure 3 is an EBSD scanning image of a gradient cross section of 15Cr14Co12Mo5Ni gear steel after ultrasonic shot peening in an embodiment of the present invention;

[0029] Figure 4 This is an EBSD scanning image of a gradient cross section of 15Cr14Co12Mo5Ni gear steel after annealing in an embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] In this embodiment, the average grain size of the initial state 15Cr14Co12Mo5Ni is about 23.445 μm, and the material is composed of ferrite and carbide, with M23C6 as the main carbide. Figure 1 As shown, a surface strengthening method for third-generation aviation gear steel proposed in an embodiment of the present invention includes the following steps:

[0032] S1. Heat treat the initial gear steel to transform the gear steel structure into martensite and initially improve the matrix hardness of the gear steel.

[0033] In this embodiment, the specific steps of heat treatment include: first, placing the gear steel in a heat treatment furnace and heating it to a temperature within a range of 1000°C to 1100°C, and keeping it at this temperature for 1 hour. Argon protection is not required at this stage; after the insulation is completed, the gear steel is quickly placed in quenching oil, and the temperature of the gear steel quickly drops to room temperature. The gear steel will catch fire instantly when placed in the quenching oil, and a layer of dark blue skin will be produced on the surface; after waiting for the temperature to drop to room temperature, the gear steel is cleaned and placed in liquid nitrogen for deep cooling treatment, and after standing in liquid nitrogen for 2 hours, it is taken out and placed in the air to wait for the temperature of the gear steel to return to room temperature. When the sample is taken out from liquid nitrogen, a layer of white frost will be covered on the surface; after returning to room temperature, the gear steel is placed in a heat treatment furnace again and heated to 500°C for 2 hours. After the insulation is completed, the gear steel is taken out and placed in the air to wait for the temperature to drop to room temperature; after waiting for the temperature to drop to room temperature, the gear steel is cleaned again for a round of liquid nitrogen deep cooling treatment and 500°C insulation for 2 hours of tempering treatment. After the insulation is completed, the sample is taken out and placed in the air to wait for the temperature to drop to room temperature. After the above heat treatment, a layer of black carbide will precipitate on the surface of the gear steel. Before the next processing, the black carbide on the surface needs to be removed to restore the sample to its normal metallic color.

[0034] The initial state of 15Cr14Co12Mo5Ni gear steel shows a typical austenite microstructure, and the original austenite grain size is about 23.445μm. The heat treatment of 15Cr14Co12Mo5Ni gear steel completely dissolves carbides in the temperature range of 1000℃ to 1100℃, and the martensite recovers at the tempering temperature of 500℃, which plays a role of precipitation strengthening. The -196℃ liquid nitrogen cryogenic treatment makes the untransformed austenite further transform into martensite, reducing the content of residual austenite in the organization. Figure 2The EBSD scan results of the 15Cr14Co12Mo5Ni gear steel after the above heat treatment are shown. The final tempered 15Cr14Co12Mo5Ni gear steel structure is composed of martensite, carbides disappear, and the average lath martensite structure size is 7.995 microns. After testing, the matrix hardness is increased from 350HV to 500HV.

[0035] S2. Perform ultrasonic shot peening on the heat-treated gear steel to make the grains of the gear steel gradient refined and further improve the matrix hardness.

[0036] Specifically, after the surface of the gear steel that needs to be loaded is processed and brightened, ultrasonic shot peening is performed at room temperature. The ultrasonic shot peening device uses high-frequency ultrasonic waves with a frequency of 20KHz to 10000KHz; the diameter of the steel ball medium used for shot peening is 0.1mm to 10mm, the shot peening distance is 5 to 15cm, and the number of steel ball media is 10 to 1000; the ultrasonic shot peening treatment time is 1s to 1200s.

[0037] After room temperature ultrasonic machining, the martensitic matrix near the surface of the gear steel is refined to the nanometer level by ultrasonic shot peening. Ultrasonic shot peening causes the lath martensite to transform into equiaxed crystals, and the average phase size on the workpiece surface is measured to be less than 100 nanometers. Figure 3 As shown in the figure, after ultrasonic shot peening, the surface of the gear steel undergoes severe plastic deformation, and the surface grains are refined to nanometers. A gradient heterogeneous nanostructure is formed on the surface of the workpiece. Due to the large plastic deformation, the lath martensite on the surface transforms into austenite (red is the austenite phase, and light pink is the martensite phase), but the surface hardness is still greatly improved due to fine grain strengthening. After testing, the microhardness of the gradient heterogeneous nanostructured gear steel is further improved compared to that after heat treatment, from 500HV to 695HV, an increase of about 40%, and the thickness of the surface gradient structure is about 800μm.

[0038] S3. The gear steel after ultrasonic shot peening is placed in an inert gas protected vacuum heat treatment furnace for internal energy homogenization tempering annealing to further improve the matrix hardness.

[0039] According to the test, after ultrasonic shot peening, the material itself is significantly improved through grain refinement. During the refinement process, the martensite will undergo a partial reverse phase transformation to produce part of the austenite. Figure 3It can be seen that in any selected section from the surface to 200μm deep and 50μm wide, after measurement, the ferrite content is 88.1%, the austenite content is 11.9%, and a large amount of austenite is concentrated on the surface. Common sense shows that the hardness of austenite is lower than that of ferrite. The austenite content can be reduced by keeping the heat at a certain temperature, so that the surface hardness can be increased again. Therefore, the gear steel after ultrasonic shot peening can be further tempered to improve the surface hardness. Specifically, after ultrasonic shot peening, the gear steel is placed in an argon protection (to avoid nanocrystal coarsening) vacuum heat treatment furnace for internal energy homogenization tempering annealing treatment. The temperature range is 200℃~500℃, and the treatment time is 10~100min. Figure 4 It can be clearly found that after annealing and tempering treatment, the austenite produced by the large plastic deformation on the surface of the gear steel transforms into martensite, and the content is reduced from 11.9% to less than 5%, and is evenly scattered in the cross section, which not only retains a certain toughness to ensure the impact toughness required during use, but also improves the hardness of the third-generation gear steel again, and can deepen the gradient layer while increasing the hardness more evenly. Through microhardness testing, it was found that after the annealing temperature was less than 500°C for half an hour, the hardness of the top layer increased again compared to that after ultrasonic processing, from 695HV to 800HV, and the thickness of the gradient influence layer increased from 800μm to 1100μm.

[0040] It should be pointed out that, according to the needs of implementation, the various steps / components described in this application can be split into more steps / components, and two or more steps / components or partial operations of steps / components can be combined into new steps / components to achieve the purpose of the present invention.

[0041] The order of execution of each step in the above embodiment does not mean the order of execution. The order of execution of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of the present application.

[0042] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A surface strengthening method for third-generation aviation gear steel, characterized in that: The following steps are involved: S1. Heat treat the initial gear steel to transform the gear steel structure into martensite and preliminarily improve the matrix hardness of the gear steel; S2. Perform ultrasonic shot peening on the heat-treated gear steel to make the grains of the gear steel gradient refined and further improve the matrix hardness; S3. The gear steel after ultrasonic shot peening is placed in an inert gas protected vacuum heat treatment furnace for internal energy homogenization tempering annealing to further improve the matrix hardness.

2. The surface strengthening method for third generation aviation gear steel according to claim 1, characterized in that: In step S1, the heat treatment includes: The gear steel is placed in a heat treatment furnace and heated to a temperature within the range of 1000°C to 1100°C, and kept at this temperature for 1 to 1.5 hours; Quickly put the gear steel into the quenching oil, and the gear steel temperature quickly drops to room temperature; Clean the gear steel and place it in liquid nitrogen for deep freezing. After standing in liquid nitrogen for 1.5 to 2 hours, take it out and place it in the air to wait for the gear steel temperature to return to room temperature. Place the gear steel in a heat treatment furnace and heat it to 450℃~550℃ and keep it warm for 1.5~2 hours. After the heat preservation is completed, take out the gear steel and let it stand in the air until the temperature drops to room temperature; Clean the gear steel again and perform a round of liquid nitrogen cryogenic treatment. After standing in liquid nitrogen for 1.5 to 2 hours, take it out and place it in the air to wait for the gear steel temperature to return to room temperature. The gear steel is placed in the heat treatment furnace again and heated to 450°C to 550°C and kept warm for 1.5 to 2 hours. After the insulation is completed, the sample is taken out and placed in the air to wait for the temperature to drop to room temperature.

3. The surface strengthening method for third generation aviation gear steel according to claim 1, characterized in that: In step S2, the parameters of the ultrasonic shot peening treatment include: the ultrasonic frequency is 20KHz to 10000KHz, the diameter of the steel ball medium used for shot peening is 0.1mm to 10mm, the number of steel ball media is 10 to 1000, the shot peening distance is 5 to 15cm, and the ultrasonic shot peening treatment time is 1s to 1200s.

4. The surface strengthening method for third generation aviation gear steel according to claim 1, characterized in that: In step S2, the ultrasonic shot peening is performed at room temperature.

5. The surface strengthening method for third generation aviation gear steel according to claim 1, characterized in that: In step S3, the temperature range of the annealing treatment is 200° C. to 500° C., and the treatment time is 10 to 100 minutes.

6. The surface strengthening method for third generation aviation gear steel according to claim 1, characterized in that: After the heat treatment in step S1, the black carbides precipitated on the surface of the gear steel are removed to restore the metallic color of the gear steel, and the surface to be loaded is processed to be bright before the ultrasonic shot peening treatment in step S2 is performed.

7. The surface strengthening method for third generation aviation gear steel according to claim 1, characterized in that: The third generation aviation gear steel is 15Cr14Co12Mo4Ni2VNb or 15Cr14Co12Mo5Ni.

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