WO2023036251A1

WO2023036251A1 - Vacuum carburizing method for obtaining dispersedly distributed fine carbide

Info

Publication number: WO2023036251A1
Application number: PCT/CN2022/117851
Authority: WO
Inventors: 丛培武; 徐跃明; 杜春辉; 陆文林; 陈旭阳; 何龙祥; 王赫; 薛丹若; 杨广文; 范雷; 凡占稳
Original assignee: 中国机械总院集团北京机电研究所有限公司
Priority date: 2021-09-09
Filing date: 2022-09-08
Publication date: 2023-03-16
Also published as: CN113737126A; CN113737126B

Abstract

Disclosed are a temperature-variable vacuum carburizing method for obtaining dispersedly distributed fine carbides, comprising a heating and insulating stage, a pulse carburizing stage, a temperature-variable stage and a quenching stage. The pulse carburizing stage comprises carrying out alternating carburizing periods and diffusion periods multiple times at a carburization temperature, wherein a carburizing gas is filled into a carburizing chamber in a pulsed manner during the carburizing periods. The temperature-variable stage comprises: introducing a cooling gas into the carburizing chamber after the pulse carburizing stage, so that a surface layer of a workpiece is rapidly cooled to a temperature below 700°C and stabilized for a period of time, so that carbides are uniformly precipitated and spheroidized; then pumping to vacuum, immediately heating to a quenching temperature of 750-980°C and maintaining the temperature, and quenching and cooling once the workpiece is completely austenitized. In the present method, the strength and toughness of the material are developed to the greatest extent, thereby improving the service performance of the workpiece.

Description

A Vacuum Carburizing Method for Obtaining Dispersed Fine Carbide

technical field

The invention relates to a vacuum carburizing heat treatment technology, in particular to a variable temperature vacuum carburizing method for obtaining dispersed fine carbides.

Background technique

Heat treatment is a process technology that gives or improves the service performance of the workpiece by changing the microstructure inside the workpiece, or changing the chemical composition of the workpiece surface, and fully exploits the potential of the material. Heat treatment is a key core technology in the machinery manufacturing industry. With the continuous emphasis on heat treatment technology, the heat treatment industry has gradually developed from extensive to refined, and more attention has been paid to new processes, new technologies, improving product intrinsic quality, saving energy, reducing consumption and prolonging life. Focus on economic benefits, etc. Vacuum low-pressure carburizing technology gradually replaces controllable atmosphere carburizing equipment with the advantages of less oxidation, energy saving and emission reduction, and clean heat treatment. It is more and more widely used in key parts such as gears and transmission shafts.

Heavy-duty gears, shafts and other key load-bearing parts are widely used in aerospace, wind power, high-speed locomotives, heavy-duty vehicles and other industries. It is generally required to be "hard on the outside and tough on the inside", and the surface layer is usually strengthened by carburizing and quenching. Studies have shown that increasing the carbon content of the carburized layer and obtaining fine, well-formed and well-distributed carbides is very beneficial to the strength and toughness of the heat-treated workpiece, and the surface hardness is as high as 800-1000HV1.

As shown in Figure 1 and Figure 3, the above requirements cannot be achieved by using the current vacuum carburizing method, and the effective control of the size and shape of carbides can only be achieved at a low carbon content (below 0.8%). Increasing the carbon content of the carburized layer often produces network carbides that are harmful to product performance, that is, carbon atoms are always precipitated at the austenite grain boundaries, but cannot be nucleated, grown and formed in the austenite grains. The formation of diffusely distributed carbides makes it difficult to maximize the strength and toughness of the material.

Contents of the invention

In view of the above-mentioned problems of the prior art, the present invention is hereby proposed.

One object of the present invention is to provide a variable temperature vacuum carburizing method for obtaining dispersed fine carbides, which can increase the carbon content of the carburized layer while avoiding harmful network carbides, so as to realize the external hardness of the material. The target characteristic of internal toughness. The method has the advantages of short technological process, high carburizing efficiency, cleanness and pollution-free, and strong practicability.

The purpose of the present invention is achieved through the following technical solutions:

The variable temperature vacuum carburizing method for obtaining dispersed fine carbides of the present invention includes a heating and heat preservation stage, a pulse carburizing stage, a variable temperature stage, and a quenching stage;

The pulse carburizing stage includes alternately performing multiple carburizing cycles and diffusion cycles at the carburizing temperature, wherein in the carburizing cycle, the carburizing gas is pulsed into the carburizing chamber, and the carbon concentration gradient of the surface layer of the workpiece and The effective hardened layer depth can be adjusted by the number of alternate carburizing cycles and the diffusion cycle and the duration of each time. The carbon content of the surface layer of the workpiece is close to the carbon in the austenite at the carburizing temperature. the maximum solubility;

The temperature changing stage includes: filling the carburizing chamber with cooling gas after the pulse carburizing stage to rapidly cool the surface layer of the workpiece to a temperature below 700°C and stabilize it for a period of time, so that carbides are uniformly precipitated and Spheroidizing, then evacuate to vacuum, immediately raise the temperature to the quenching temperature of 750°C to 980°C and hold it for a period of time, so that the workpiece is evenly heated and completely austenitized;

In the quenching stage, any one of the following quenching methods is selected: direct high-pressure gas quenching, direct oil quenching, and gas-oil composite quenching.

In a possible implementation of the present invention, at least one intermediate temperature change stage and at least one second pulse carburizing stage are included before the temperature change stage; the intermediate temperature change stage includes filling the carburizing chamber with cooling gas The surface layer of the workpiece is rapidly cooled to a temperature below 700°C and stabilized for a period of time, so that the carbides are uniformly precipitated and spheroidized, and then vacuumed, and the temperature is immediately raised to the temperature required for the second pulse carburizing stage.

In a possible implementation of the present invention, the alternating times and duration of each carburizing period and diffusion period in the second pulse carburizing stage and the respective carburizing periods and diffusion periods in the pulse carburizing stage The number and duration of the alternations are at least partially different.

In a possible implementation of the present invention, in the pulse carburizing stage, the carburizing temperature is 750 to 980°C, the carburizing pressure is 800 to 1500Pa, the total time of the carburizing cycle and the The ratio of the total time of the diffusion cycle is 1:2 to 1:7.

In a possible implementation of the present invention, in the temperature changing stage, the pressure of the cooling gas is ≥2×10 ⁵ Pa, and the holding time after cooling down to a temperature below 700°C is 5 to 60 minutes. The holding time after the temperature is 750°C to 980°C is 5 to 120 minutes.

In a possible implementation manner of the present invention, the carburizing gas is acetylene or propane, and the cooling gas is nitrogen, argon or helium.

In a possible implementation manner of the present invention, the duration of the pulse carburizing stage is ≥30 minutes.

In a possible implementation manner of the present invention, the duration of the pulse carburizing stage is ≥120 minutes.

Compared with the prior art, the variable temperature vacuum carburizing method for obtaining dispersed fine carbides provided by the present invention adopts alternate carburizing cycles and diffusion cycles in the pulse carburizing stage and adjusts their cycle numbers and Cycle time, to obtain the required carbon concentration gradient curve; in the variable temperature stage, the surface layer of the workpiece is rapidly cooled by pressurized air to uniformly precipitate carbides and refine the structure. Thus, while increasing the carbon content of the carburized layer of the treated member, avoiding the generation of harmful network carbides, the treated member can obtain dispersed fine carbides, so as to realize the principle of the treated member being hard on the outside and tough on the inside. target properties. The method has the advantages of short technological process, high carburizing efficiency, cleanness and no pollution, and strong practicability, so that the treated material can obtain better strength and toughness, and the service performance of the material is improved.

Description of drawings

Schematic representations are used in the drawings, in which some of the main features and effects are significantly enlarged to illustrate functions, principles of action, technical solutions and features. Therefore, the dimensions and proportions of the graphs shown in the accompanying drawings are not necessarily drawn to scale, nor represent specific numerical values. In many embodiments that can be implemented, the dimensions, proportions, etc. may be different from those shown here.

Fig. 1 is a schematic diagram of the traditional process curve of vacuum low-pressure carburizing in the prior art;

Fig. 2 is the schematic diagram of the curve of temperature and pressure changing with time of the variable temperature vacuum carburizing method of an embodiment of the present invention;

Fig. 3 is the topography diagram of the carbide structure on the surface of the shaft treated by the traditional vacuum low-pressure carburizing method in the prior art, with a magnification of 500;

Figure 4a is a topography diagram of carbides on the surface of the shaft treated by the variable temperature vacuum carburizing method according to an embodiment of the present invention, with a magnification of 500;

Fig. 4b is a SEM spherical carbide microstructure diagram of a shaft treated by a variable temperature vacuum carburizing method according to an embodiment of the present invention;

Fig. 5 is a schematic diagram of curves of temperature and pressure changing with time in a variable temperature vacuum carburizing method according to another embodiment of the present invention;

Fig. 6a is a fine carbide structure topography diagram of a gear treated by a variable temperature vacuum carburizing method according to another embodiment of the present invention, with a magnification of 500;

Fig. 6b is a fine carbide microstructure diagram of the gear treated by the vacuum pulse carburizing method without any variable temperature stage, with a magnification of 500;

Fig. 7 is the hardness-depth distribution curve of the carburized layer of the gear treated by the variable temperature vacuum carburizing method according to another embodiment of the present invention;

Fig. 8 is the contact fatigue damage accumulation curve of the gear treated by the variable temperature vacuum carburizing method according to another embodiment of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention; obviously, the described embodiments are only some of the embodiments of the present invention, not all of them, and this does not mean It does not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

First, the terms that may be used in this article are explained as follows:

The term "and/or" means that either or both can be realized at the same time, for example, X and/or Y means that both "X" or "Y" and "X and Y" are included .

The terms "comprising", "comprising", "containing", "having" or other descriptions with similar meanings shall be construed as non-exclusive inclusions. For example: including certain technical feature elements (such as raw materials, components, ingredients, carriers, dosage forms, materials, dimensions, parts, components, mechanisms, devices, steps, procedures, methods, reaction conditions, processing conditions, parameters, algorithms, signals, data, products or products, etc.), should be interpreted as including not only a certain technical feature element explicitly listed, but also other technical feature elements not explicitly listed in the art.

The term "consisting of" means excluding any technical characteristic elements not explicitly listed. If this term is used in a claim, the term will make the claim closed so that it does not contain technical characteristic elements other than those expressly listed, except for conventional impurities related to them. If the term only appears in a certain clause of a claim, it only limits the elements explicitly listed in the clause, and the elements stated in other clauses are not excluded from the entire claim.

Unless otherwise expressly stipulated or limited, terms such as "mounted", "connected", "connected" and "fixed" should be interpreted in a broad sense, for example: it can be a fixed connection or a detachable connection, or an integral Connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediary, and it can be the internal communication of two components. Those of ordinary skill in the art can understand the specific meanings of the above terms herein according to specific situations.

When the concentration, temperature, pressure, size or other parameters are expressed in the form of numerical ranges, the numerical ranges should be understood as specifically disclosing all the ranges formed by the pairing of any upper limit, lower limit and preferred value within the numerical range , regardless of whether the range is explicitly stated; for example, if the numerical range "2-8" is stated, then the numerical range should be interpreted as including "2-7", "2-6", "5-7" , "3~4 and 6~7", "3~5 and 7", "2 and 5~7" and other ranges. Unless otherwise indicated, numerical ranges recited herein include their endpoints and all integers and fractions subsumed within the numerical range.

The terms "center", "longitudinal", "transverse", "length", "width", "thickness", "top", "bottom", "front", "rear", "left", "right", " The orientation or positional relationship indicated by "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. is based on the orientation or positional relationship shown in the drawings , is only for convenience and simplification of description, but does not express or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation on this document.

The content not described in detail in the embodiments of the present invention belongs to the prior art known to those skilled in the art. In the embodiment of the present invention, if no specific conditions are indicated, it is carried out according to the conventional conditions in the art or the conditions suggested by the manufacturer. The reagents or instruments used in the examples of the present invention, whose manufacturers are not indicated, are all conventional products that can be purchased from the market.

The pulse carburizing stage includes alternate carburizing cycles and diffusion cycles, wherein carburizing gas is pulsed into the carburizing chamber during the carburizing cycle, and the carbon concentration gradient and effective hardened layer depth on the surface of the workpiece can pass through the carburizing cycle. The number of times the carburizing cycle and the diffusion cycle are alternated and the duration of each time are adjusted; in some embodiments, after the pulse carburizing stage, the carbon content on the surface of the material is close to the carbon at the carburizing temperature Maximum solubility in austenite;

The temperature changing stage includes: filling the carburizing chamber with cooling gas to quickly cool the surface of the workpiece to a temperature below 700°C, that is, the temperature below the Acl line, and stabilize it for a period of time, so that the carbides are uniformly precipitated and spheroidized. Then evacuate to a vacuum, immediately raise the temperature to the quenching temperature of 750°C-980°C, that is, 30-50°C above the Ac3 or Ac1 line, and keep it warm for a period of time, so that the workpiece is evenly heated and completely austenitized;

In the quenching stage, any one of the following quenching methods is selected: direct high-pressure gas quenching, direct oil quenching, and gas-oil quenching.

In the heating and heat preservation stage, it can be determined whether the heating process is segmented according to the material and size of the workpiece, and the heating temperature, heating time, and heat preservation time of each segment can be set. After the heating curve is compiled, the workpiece is placed in a vacuum furnace to evacuate and heated according to the heating curve.

In the pulse carburizing stage, the carburizing temperature is 750 to 980°C, and the carburizing pressure is 800 to 1500Pa. The number of carburizing cycles and diffusion cycles and the duration of each cycle are based on the required effective hardened layer depth. Or determined by the carbon concentration gradient curve, the ratio of the total time of the carburizing cycle to the total time of the diffusion cycle is 1:2 to 1:7, and the value of the surface carbon content can be set at 0.8% to 2.0%, preferably 0.8% to 1.3% .

In the pulse carburizing stage, acetylene or propane atmosphere is selected as the carburizing gas.

The duration of the pulse carburizing phase is ≥30 minutes. In some embodiments, the duration of the pulse carburizing phase is > 120 minutes.

In the temperature changing stage, the carburizing chamber is filled with cooling gas with a pressure ≥ 2×10 ⁵ Pa, the temperature is lowered to below 700°C and then kept for 5 to 60 minutes, and the temperature is raised to the quenching temperature of 750°C to 980°C and then kept for heat preservation 5 to 120 minutes.

In the temperature changing stage, nitrogen, argon or helium is selected as the gas filled into the carburizing chamber.

In the present invention:

In the pulse carburizing stage, an acetylene or propane atmosphere is introduced under vacuum conditions, and the high temperature causes active carbon atoms to diffuse in the solid material and form carbides, and the surface hardening of the material is achieved after cooling. The pulse carburizing stage is very critical. The duration and alternating times of the carburizing cycle and the diffusion cycle directly affect the carbon concentration gradient curve on the surface of the material, which in turn affects the microstructure and properties of the material, the carburizing temperature and the total carburizing The length of time determines whether the austenite grains will grow.

In the temperature-changing stage after the end of pulse carburizing, the workpiece remains in the vacuum carburizing chamber, the heating is stopped, and nitrogen, argon or helium is charged in a pulsed manner, and it can be assisted by a fan to extract air, so that The surface temperature of the workpiece can be quickly lowered to a temperature below 700°C and stabilized for a period of time, which promotes the precipitation and spheroidization of carbon atoms in the grains, and then nucleates and grows to form fine carbides in a dispersed distribution.

The method of the invention is especially suitable for surface layer strengthening of high-end high-performance carburized gear parts and shaft parts.

Compared with prior art, the beneficial effect of the present invention is:

In the pulse carburizing stage, the present invention designs the alternating times of carburizing cycle and diffusion cycle and the specific duration of each cycle according to the carbon concentration gradient curve required by the workpiece, and precisely controls the carbon concentration gradient curve of the carburizing layer; The forced circulation of pressurized air rapidly cools the surface of the workpiece to below 700°C, so that carbides are uniformly precipitated and the structure is refined. Martensite and retained austenite (grade 1-2) on the surface of the workpiece, with a hardness above 62HRC. The method of the invention has the advantages of short technological process, high carburizing efficiency, cleanness and pollution-free, and strong practicability.

In order to more clearly demonstrate the technical solutions provided by the present invention and the technical effects produced, the following will describe in detail the embodiments of the present invention provided by specific examples.

Example 1 - Shaft

Dimensions (mm): shaft diameter φ38, length 150mm

Material: 18CrNiMo7-6.

Heat treatment technical requirements: standard JB/T6141.3-1992 vacuum carburized carburized layer carbide ≤ 2 grades, surface martensite and retained austenite ≤ 2 grades, carburized layer 0.9 ~ 1.2mm, surface hardness above HRC62 .

Carburizing method: respectively adopt the traditional vacuum low-pressure carburizing method in the prior art and the variable temperature vacuum carburizing method of the present embodiment provided below.

Referring to Fig. 2, the present embodiment provides a variable temperature vacuum carburizing method for obtaining dispersed fine carbides, including the following stages:

(1) Heating and holding stage: heat the workpiece to 650°C within 30 minutes, hold it for 60 minutes, then heat it to 960°C required for the subsequent pulse carburizing stage within 30 minutes, and hold it at this temperature for 20 minutes.

(2) Pulse carburizing stage: At 960°C, 14 carburizing cycles and diffusion cycles were alternated for a total duration of 200 minutes. The carburizing time of the first carburizing cycle was 5.4 minutes and the number of pulses was 3. The second to fourteenth carburizing cycles are 1.8 minutes, and the number of pulses is 1. The second to fourteenth diffusion cycle durations were 2.8 minutes, 4.0 minutes, 5.1 minutes and 6.2 minutes, 7.3 minutes, 8.3 minutes, 9.4 minutes, 10.5 minutes, 11.6 minutes, 12.6 minutes, 13.7 minutes, 14.8 minutes, 15.9 minutes and 50 minutes. The carburizing gas is acetylene, the flow rate of acetylene is 40L/min, and the carburizing pressure is 1000Pa. After the carburizing cycle and the diffusion cycle are all over, the carbon content on the surface of the workpiece is close to the maximum solubility of carbon in austenite at the quenching temperature of the material.

(3) Temperature change stage: Fill the furnace with 3×10 ⁵ Pa nitrogen and use a fan to force circulation, so that the surface of the workpiece is rapidly cooled to 620°C, kept at this temperature for 10 minutes, then exhausted and vacuumed, and then immediately heated to The quenching temperature is 800°C, and the temperature is kept for 60 minutes for soaking. After changing the temperature, carbon atoms precipitate and spheroidize in the grains, which promotes the formation of fine carbides in a dispersed distribution.

(4) Quenching stage: fill the furnace with 0.8×10 ⁵ Pa nitrogen gas again and maintain the pressure, cool the workpiece with oil with an oil temperature of 60°C and continue stirring, and perform gas-oil composite quenching to solidify and further refine the grains .

As shown in Figure 3, in the microstructure of the specimen treated by the traditional vacuum low-pressure carburizing method in the prior art, the martensite is 4th grade, the retained austenite is 4th grade, and the surface carbide is 4th grade And was reticular. The surface hardness of the test piece is HRC58.5, the surface carbon content is 1.5%, and the effective hardened layer depth is 1.1mm. This shows that in the traditional vacuum low-pressure carburizing method in the prior art, the harmful structure of network carbide is often formed on the surface, and the phenomenon of excessive residual austenite and coarse martensite often appears in the near surface layer, which is difficult to meet the high-end requirements. Surface strengthening requirements for high-performance carburized gear parts and shaft parts.

As shown in Figure 4a and Figure 4b, in the microstructure of the specimen treated by the variable temperature vacuum carburizing method in Example 1, the martensite is grade 2 and is cryptocrystalline or fine needle-like structure, and the residual austenite The body is grade 1, the carbide is grade 1 and dispersedly distributed, the carbide size is 200nm to 500nm, and the shape is mainly spherical. The surface hardness of the test piece is HRC63.7, the surface carbon content is 1.2%, and the effective hardened layer depth is 1.1mm. This shows that the vacuum low-pressure carburizing method of the present invention can increase the carbon content of the carburized layer and obtain a fine, well-formed and well-distributed carbide structure, thereby enabling the processed material to obtain better strength and toughness, and improving the strength of the material. service performance.

Example 2 – Gears

Dimensions(mm):

169×75, module 6.5, number of teeth 24

Material: 18CrNiMo7-6.

Heat treatment technical requirements: standard JB/T6141.3-1992 vacuum carburized carburized layer carbide ≤ 2 grades, surface martensite and retained austenite ≤ 2 grades, carburized layer 1.9 ~ 2.2mm, surface hardness HRC62 or more .

Carburizing method: The variable temperature vacuum carburizing method and the vacuum pulse carburizing method without any variable temperature stage provided below are respectively used.

Referring to Fig. 5, the present embodiment provides a variable temperature vacuum carburizing method for obtaining dispersed fine carbides, including the following stages:

(1) Heating and holding stage: heat the workpiece to 650°C within 30 minutes, hold it for 100 minutes, then heat it to 930°C required for the subsequent pulse carburizing stage within 30 minutes, and hold it at this temperature for 45 minutes.

(2) Pulse carburizing stage: At 930°C, 18 carburizing cycles and diffusion cycles were alternated, and the total duration was 310 minutes. The duration of the first carburizing cycle in the pulse carburizing stage is 4.5 minutes, and the number of pulses is 3; the second to eighteenth carburizing cycles are 1.5 minutes, and the number of pulses is 1. The carburizing gas is acetylene, the flow rate of acetylene is 40L/min, and the carburizing pressure is 1000Pa. The durations of the first to eighteenth diffusion cycles in this pulse carburizing stage are 3.4 minutes, 5.0 minutes, 6.4 minutes, 7.8 minutes, 9.2 minutes, 10.6 minutes, 12.0 minutes, 13.4 minutes, 14.8 minutes, 16.2 minutes, 17.6 minutes , 19.1 minutes, 20.5 minutes, 21.9 minutes, 23.3 minutes, 24.8 minutes, 26.2 minutes and 27.7 minutes. After the carburizing cycle and the diffusion cycle are all over, the carbon content on the surface of the workpiece is close to the maximum solubility of carbon in austenite at the carburizing temperature.

(3) Intermediate temperature change stage: Fill the furnace with 3×10 ⁵ Pa nitrogen and use a fan to force circulation, so that the surface of the workpiece is rapidly cooled to 620°C, and kept at this temperature for 15 minutes, then exhausted and vacuumed, and then immediately in the After the temperature is raised to 930 °C required for the subsequent pulse carburizing stage within 15 minutes, carbon atoms are precipitated and spheroidized in the grains, which promotes the formation of fine carbides in a dispersed distribution.

(4) The second pulse carburizing stage: At 930°C, 15 carburizing cycles and diffusion cycles were alternated for a total duration of 615 minutes. The duration of the first carburizing cycle in the pulse carburizing stage is 3 minutes, and the number of pulses is 2; the second to fifteenth carburizing cycles are 1.5 minutes, and the number of pulses is 1. The carburizing gas is acetylene, the flow rate of acetylene is 40L/min, and the carburizing pressure is 1000Pa. The durations of the first to fifteenth diffusion cycles in the pulse carburizing stage are 5.8 minutes, 10.2 minutes, 14.6 minutes, 19.2 minutes, 23.9 minutes, 28.6 minutes, 33.5 minutes, 38.4 minutes, 43.3 minutes, 48.3 minutes, 53.4 minutes , 58.6 minutes, 64.1 minutes, 70.1 minutes and 79 minutes.

(5) Temperature change stage: Fill the furnace with 3×10 ⁵ Pa nitrogen and use a fan to force the circulation, so that the surface of the workpiece is rapidly cooled to 620°C within 5 minutes, kept at this temperature for 15 minutes, and then exhausted and evacuated. Then immediately raise the temperature to the quenching temperature of 850°C, and keep the temperature for 45 minutes to soak.

(6) Quenching stage: fill the furnace with 0.8×10 ⁵ Pa nitrogen again and maintain the pressure, cool the workpiece with oil at 60°C and continue to stir, and perform gas-oil composite quenching to solidify and further refine the grains .

In this embodiment, in order to increase the thickness of the carburized layer, an intermediate temperature change stage and a second pulse carburizing stage are added between the temperature change stage and the quenching stage.

Fig. 6a is a morphology diagram of the fine carbide structure of the gear treated by the variable temperature vacuum carburizing method in this embodiment. The martensite structure is grade 1, and the carbides are grade 1 and spherical in shape and fine in size and diffusely distributed.

Figure 6b shows the fine carbide structure morphology of the gear treated by the vacuum pulse carburizing method without any temperature change stage, and the carbide grade is grade 3 (grain block system).

Fig. 7 is a hardness-depth distribution curve of the carburized layer of the gear treated by the variable temperature vacuum carburizing method of this embodiment, wherein the carburized layer depth is 2.16mm, and the surface hardness is >820HV.

Fig. 8 is the detection result of the contact fatigue strength of the gear treated by the variable temperature vacuum carburizing method of this embodiment, the contact fatigue limit is 1921Mpa, which is 17.3% higher than the contact fatigue limit of the conventional carburizing and quenching gear which is 1638Mpa.

It will be apparent to those skilled in the art that the invention is not limited to the details of the above-described exemplary embodiments, but that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. For example, it can be determined whether the heating process is segmented according to the workpiece material, size and other factors. If segmented, the heating temperature, heating time, and holding time of each segment are determined. Similarly, the carburizing temperature of the pulse step, the number of carburizing cycles and diffusion cycles in the carburizing stage, the duration of each cycle, and the carburizing cycle can be determined according to the carbon concentration gradient of the workpiece surface and the effective hardened layer depth. The number of pulses; determine the gas pressure, temperature change temperature, temperature change duration, quenching temperature, and quenching duration in the variable temperature stage; and determine the specific quenching means in the quenching stage, such as oil temperature in oil quenching, gas-oil composite quenching in Inflation pressure, inflation time, oil temperature. Accordingly, the embodiments should be regarded in all points of view as exemplary and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and it is therefore intended that the scope of the invention be defined by the appended claims rather than by the foregoing description. All changes within the meaning and range of equivalents of the elements are embraced in the present invention.

In addition, it should be understood that although this specification is described according to implementation modes, not each implementation mode only contains an independent technical solution, and this description in the specification is only for clarity, and those skilled in the art should take the specification as a whole , the technical solutions in the various embodiments can also be properly combined to form other implementations that can be understood by those skilled in the art.

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person familiar with the technical field can easily conceive of changes or changes within the technical scope disclosed in the present invention. Replacement should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims. The information disclosed in this Background section is only intended to enhance the understanding of the general background of the present invention, and should not be considered as an acknowledgment or any form of suggestion that the information constitutes the prior art that is already known to those skilled in the art.

Claims

A variable temperature vacuum carburizing method for obtaining dispersed fine carbides, characterized in that it includes a heating stage, a pulse carburizing stage, a variable temperature stage, and a quenching stage;

The pulse carburizing stage includes alternately performing multiple carburizing cycles and diffusion cycles at the carburizing temperature, wherein in the carburizing cycle, the carburizing gas is pulsed into the carburizing chamber, and the carbon concentration gradient of the surface layer of the workpiece and The effective hardened layer depth can be adjusted by the number of alternate carburizing cycles and the diffusion cycle and the duration of each time. The carbon content of the surface layer of the workpiece is close to the carbon in the austenite at the carburizing temperature. the maximum solubility;

The temperature changing stage includes: filling the carburizing chamber with cooling gas after the pulse carburizing stage to rapidly cool the surface layer of the workpiece to a temperature below 700°C and stabilize it for a period of time, so that carbides are uniformly precipitated and Spheroidizing, then evacuate to vacuum, immediately raise the temperature to the quenching temperature of 750°C to 980°C and hold it for a period of time, so that the workpiece is evenly heated and completely austenitized;

In the quenching stage, any one of the following quenching methods is selected: direct high-pressure gas quenching, direct oil quenching, and gas-oil composite quenching.
The variable temperature vacuum carburizing method for obtaining dispersed fine carbides according to claim 1, further comprising at least one intermediate temperature variable stage and at least one second pulse carburizing stage before the variable temperature stage;

The intermediate temperature change stage includes filling the carburizing chamber with cooling gas to quickly cool the surface of the workpiece to a temperature below 700°C and stabilize it for a period of time, so that the carbides are uniformly precipitated and spheroidized, and then pumped to a vacuum to raise the temperature immediately to the temperature required for the second pulse carburizing stage.
The variable temperature vacuum carburizing method for obtaining dispersed fine carbides according to claim 2, characterized in that,

The number of alternations and durations of the respective carburization cycles and diffusion cycles in the second pulse carburizing stage are at least partially different from the number of alternations and durations of the respective carburizing cycles and diffusion cycles of the pulse carburizing stage.
The temperature-variable vacuum carburizing method for obtaining dispersed fine carbides according to any one of claims 1 to 3, characterized in that, in the pulse carburizing stage, the carburizing temperature is 750 to 980°C, The carburizing pressure is 800 to 1500 Pa, and the ratio of the total time of the carburizing cycle to the total time of the diffusion cycle is 1:2 to 1:7.
The variable temperature vacuum carburizing method for obtaining fine carbides dispersedly distributed according to any one of claims 1 to 3, characterized in that, in the variable temperature stage, the pressure of the cooling gas is ≥2×10 5 Pa, The heat preservation time after cooling down to a temperature below 700°C is 5 to 60 minutes, and the heat preservation time after the temperature is raised to a quenching temperature of 750°C to 980°C is 5 to 120 minutes.
The temperature-variable vacuum carburizing method for obtaining fine carbides dispersedly distributed according to any one of claims 1 to 5, wherein the carburizing gas is acetylene or propane, and the cooling gas is nitrogen, argon or helium.
The temperature-variable vacuum carburizing method for obtaining dispersed fine carbides according to any one of claims 1 to 5, characterized in that the duration of the pulse carburizing stage is ≥ 30 minutes.
The variable temperature vacuum carburizing method for obtaining fine carbides dispersedly distributed according to claim 7, characterized in that the duration of the pulse carburizing stage is ≥ 120 minutes.