Method for improving processing performance of 12CrNi3A steel after vacuum carburization

Abstract

A method for improving the processing performance of 12CrNi3A steel after vacuum carburization comprises the steps that firstly, the preset time of'strong carburization + diffusion 'of the 12CrNi3A steel is input into VacCard vacuum carburization simulation software for simulation of the vacuum carburization stage of the 12CrNi3A steel, the time of'strong carburization + diffusion' is adjusted, and the better time of'strong carburization + diffusion 'vacuum carburization simulation is obtained; and then vacuum carburization of the 12CrNi3A steel is conducted by adopting the obtained better strong carburization and diffusion time, after the vacuum carburization stage is finished, when the 12CrNi3A steel is cooled to the temperature of 560-650 DEG C, heat preservation is conducted for 30-60 min, and then the 12CrNi3A steel is cooled to the temperature below 90 DEG C and discharged out of the furnace. According to the invention, the problems of high hardness and poor processability of the low-alloy carburizing steel 12CrNi3A after carburizing are effectively solved, so that pearlite is formed after the 12CrNi3A carburizing steel part is carburized, the surface hardness and the core hardness meet the requirements, the processing is convenient, and the production efficiency is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal material processing, and particularly relates to a method for improving the processing performance of 12CrNi3A steel after vacuum carburizing. BACKGROUND

[0002] 12CrNi3A steel is high-grade carburizing steel, which has the advantages of high surface hardness, good core mechanical properties, good impact and surface wear resistance, and is commonly used in the fields of aviation, aerospace and the like, such as transmission shafts, main shafts, camshafts, mandrels, bearings and pistons. The carburizing and subsequent heat treatment process of the carburizing steel is as follows: carburizing-high temperature tempering-machining-primary quenching-secondary quenching-cold treatment-tempering, and the purpose of high temperature tempering is to reduce the hardness of the carburized part, so as to facilitate processing. However, the holding time of high temperature tempering is generally 3h to 6h, which is long, resulting in long part turnover time and thus reducing the production efficiency of the part. SUMMARY

[0003] The technical problem solved by the present application is to provide a method for improving the processing performance of 12CrNi3A steel after vacuum carburizing, so as to solve the problems in the above background.

[0004] The technical problem solved by the present application is solved by adopting the following technical solution:

[0005] A method for improving the processing performance of 12CrNi3A steel after vacuum carburizing, wherein the preset "strong carburizing + diffusion" time of 12CrNi3A steel is input into a VacCard vacuum carburizing simulation software to simulate the vacuum carburizing stage of 12CrNi3A steel, and then the "strong carburizing + diffusion" time is continuously optimized and adjusted to make the simulation result meet the requirements, so as to obtain a better "strong carburizing + diffusion" vacuum carburizing simulation time. The obtained better "strong carburizing + diffusion" time is used for vacuum carburizing of 12CrNi3A steel. After the vacuum carburizing stage ends, the temperature is cooled to 560℃ to 650℃, and the temperature is kept for 30min to 60min. Then the temperature is cooled to below 90℃, and the furnace is discharged, so as to improve the processing performance of 12CrNi3A steel after vacuum carburizing. The specific steps are as follows:

[0006] 1) "Strong carburizing + diffusion" vacuum carburizing simulation

[0007] First, the preset "strong carburizing + diffusion" time of 12CrNi3A steel vacuum carburizing is input into a VacCard vacuum carburizing simulation software to simulate the vacuum carburizing of 12CrNi3A steel. Then, the "strong carburizing + diffusion" time is continuously optimized and adjusted to make the simulation result meet the requirements, so as to obtain a better "strong carburizing + diffusion" vacuum carburizing time;

[0008] 2) Vacuum carburizing stage

[0009] 12CrNi3A steel is loaded into a vacuum carburizing furnace, heated, and held at 650℃~700℃ for 30min~60min. The furnace temperature is then raised to 920℃~950℃ and held for 30min~60min. The optimal "strong carburizing + diffusion" vacuum carburizing time obtained in step 1) is used for "strong carburizing + diffusion" pulse carburizing. Each pulse includes one strong carburizing stage and one diffusion stage. Ethylene and acetylene are introduced during the strong carburizing process.

[0010] 3) Stepped insulation stage

[0011] After the vacuum carburizing stage is completed, nitrogen gas of 0.2 bar to 0.5 bar is introduced for cooling. When the temperature is cooled to 560°C to 650°C, it is held for 30 min to 60 min. Then, a vacuum is drawn to maintain the working pressure at 0.05 mbar to 0.06 mbar.

[0012] 4) Cooling stage

[0013] After the stepped heat preservation stage is completed, nitrogen gas of 0.2 bar to 0.5 bar is introduced to cool the furnace to below 90°C, and then the furnace is removed from the furnace.

[0014] In this invention, in step 2), before the vacuum carburizing furnace is heated, a vacuum is drawn to maintain the working pressure below 0.1 mbar.

[0015] In this invention, in step 2), the flow rate of ethylene and acetylene is 14-16 L / min, the ratio of ethylene to acetylene is 1:1, and the pressure is maintained at 4 mbar; during vacuum carburizing diffusion, the introduction of ethylene and acetylene is stopped, and nitrogen gas is introduced to maintain the working pressure at 0.1 mbar.

[0016] In this invention, in step 2), the "strong penetration + diffusion" pulsed carburizing is set with 6 pulses, and the diffusion time is longer than the strong penetration time. The strong penetration time decreases with the increase of the number of pulses, and the diffusion time increases with the increase of the number of pulses. The first stage of strong penetration is set to 5 minutes, and the first stage of diffusion is set to 10 minutes.

[0017] In this invention, in step 2), 12CrNi3A steel is held at 650°C for 35 minutes, then heated to 945°C in the furnace and held for 35 minutes.

[0018] Beneficial effects: This invention effectively solves the problems of high hardness and poor machinability of low-alloy carburized steel 12CrNi3A after carburizing. It enables 12CrNi3A carburized steel parts to form pearlite after carburizing, with a surface hardness of 20HRC~30HRC and a core hardness of 7HRC~18HRC, which facilitates processing, thereby reducing the high-temperature tempering heat treatment process and improving production efficiency. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the microstructure of 12CrNi3A steel after vacuum carburizing in a preferred embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of the microstructure of 12CrNi3A steel after vacuum carburizing and subsequent heat treatment in a preferred embodiment of the present invention.

[0021] Diagram caption: Figure 1 In the figure, (a) is the overall microstructure after vacuum carburizing (50X), (b) is the microstructure of the carburized layer after vacuum carburizing (500X), and (c) is the microstructure of the matrix after vacuum carburizing (500X). Figure 2 In the figures, (a) is the overall microstructure after vacuum carburizing and subsequent heat treatment (50X), (b) is the carburized layer microstructure after vacuum carburizing and subsequent heat treatment (500X), and (c) is the matrix microstructure after vacuum carburizing and subsequent heat treatment (500X). Detailed Implementation

[0022] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific illustrations.

[0023] A method for improving the processing performance of 12CrNi3A steel after vacuum carburizing involves inputting a pre-set "strong carburizing + diffusion" time for 12CrNi3A steel into VacCard vacuum carburizing simulation software to simulate the vacuum carburizing stage of 12CrNi3A steel. The "strong carburizing + diffusion" time is continuously optimized and adjusted to achieve the desired simulation results, obtaining an optimal "strong carburizing + diffusion" vacuum carburizing simulation time. This optimal "strong carburizing + diffusion" time is then used for vacuum carburizing of the 12CrNi3A steel. After the vacuum carburizing stage, the steel is cooled to 560℃~650℃ and held at that temperature for 30min~60min, then cooled to below 90℃ before being removed from the furnace. This method aims to improve the processing performance of 12CrNi3A steel after vacuum carburizing. The specific steps are as follows:

[0024] 1) Simulation of vacuum carburizing using "strong infiltration + diffusion"

[0025] First, the pre-set vacuum carburizing "strong carburizing + diffusion" time for 12CrNi3A steel is input into the VacCard vacuum carburizing simulation software to simulate the vacuum carburizing of 12CrNi3A steel. Then, the "strong carburizing + diffusion" time is continuously optimized and adjusted to make the simulation results meet the requirements and obtain the best "strong carburizing + diffusion" vacuum carburizing time.

[0026] 2) Vacuum carburizing stage

[0027] 12CrNi3A steel was loaded into a vacuum carburizing furnace. Before heating, a vacuum was evacuated to a level below 0.1 mbar. As the furnace temperature increased, the vacuum was evacuated again to maintain the working pressure below 0.1 mbar. The furnace was held at 650℃ for 35 minutes, then heated to 945℃ and held for 35 minutes. The optimal "strong carburizing + diffusion" vacuum carburizing time obtained in step 1) was used for "strong carburizing + diffusion" pulse carburizing. Each pulse included one strong carburizing stage and one diffusion stage, with the diffusion time being longer than the strong carburizing time. The strong infiltration time decreases with increasing pulse count, while the diffusion time increases with increasing pulse count. The first stage of strong infiltration is usually set to 5 minutes, and the first stage of diffusion is usually set to 10 minutes, for a total of 6 pulses, as shown in Table 1. Simultaneously, during vacuum carburizing and strong infiltration, ethylene and acetylene are introduced at a flow rate of 15 L / min, with an ethylene to acetylene ratio of 1:1, and the pressure is generally maintained at 4 mbar. During vacuum carburizing and diffusion, the introduction of ethylene and acetylene is stopped, and nitrogen is introduced to maintain the working pressure at 0.1 mbar.

[0028] Table 1

[0029]

[0030] 3) Stepped insulation stage

[0031] After the vacuum carburizing stage is completed, nitrogen gas at 0.4 bar is introduced for cooling to 600°C. Nitrogen gas is then introduced to maintain the working pressure at 0.1 mbar, and the temperature is maintained at 600°C for 30 minutes.

[0032] 4) Cooling stage

[0033] After the stepped heat preservation stage is completed, nitrogen gas at 0.4 bar is introduced to cool the temperature to below 90°C, and then the furnace is removed from the oven.

[0034] 5) Heat treatment

[0035] After the cooling stage, subsequent heat treatment is performed, which includes primary quenching, secondary quenching, cold treatment, and low-temperature tempering.

[0036] First quenching: After the cooling stage is completed, the 12CrNi3A steel is sent into the warm-entry furnace, held at 860℃ for 10 minutes, and then oil-cooled at 28℃.

[0037] Secondary quenching: After the first quenching, the 12CrNi3A steel is sent into a warm-entry furnace, held at 790℃ for 10 minutes, and then oil-cooled at 28℃.

[0038] Cold treatment: After the 12CrNi3A steel has been quenched twice, it is sent into a warm-entry furnace, held at -70℃ for 130 minutes, and then air-cooled.

[0039] Low-temperature tempering: After the cold treatment, the 12CrNi3A steel is sent into the warm-entry furnace, held at 160℃ for 180 minutes, and then air-cooled.

[0040] In this embodiment, the 12CrNi3A steel is required to have an effective carburized layer depth of 0.7mm to 1.0mm, a surface hardness of ≥58HRC after heat treatment, and a core hardness of 28-42HRC.

[0041] In this embodiment, Figure 1 The surface microstructure of the 12CrNi3A steel obtained after vacuum carburizing is a large amount of pearlite + a small amount of ferrite, and the core microstructure is ferrite + pearlite. The hardness test shows that the average surface hardness is 24.2 HRC and the average core hardness is 7.1 HRC, which is convenient for machining after carburizing. The carburized layer depth is 0.9 mm, which meets the requirements.

[0042] Figure 2 In the case of 12CrNi3A steel, the surface microstructure after vacuum carburizing and subsequent heat treatment consists of high-carbon martensite, a small amount of retained austenite, and a very small amount of carbides, while the core microstructure consists of low-carbon martensite. According to HB5492-2011, the microstructure of the carburized layer is rated as follows: retained austenite on the surface is grade 2, carbides are grade 2, and low-carbon martensite in the core is grade 1, which meets the requirements. Hardness testing was also performed, and the average surface hardness was 61.7 HRC, while the average core hardness was 41.0 HRC, which also meets the requirements.

[0043] The specific embodiments described herein are merely illustrative examples of the present invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or adopt similar methods to replace them, but without departing from the spirit of the present invention or exceeding the scope defined by the appended claims.

Claims

1. A method for improving the machinability of 12CrNi3A steel after vacuum carburizing, characterized in that, The preset "strong penetration + diffusion" time of 12CrNi3A steel is input into the VacCard vacuum carburizing simulation software to simulate the vacuum carburizing stage of 12CrNi3A steel, and then the "strong penetration + diffusion" time is continuously optimized and adjusted to make the simulation result meet the requirements, and the better "strong penetration + diffusion" vacuum carburizing simulation time is obtained. Then, the better "strong penetration + diffusion" time obtained is used for vacuum carburizing of 12CrNi3A steel. After the vacuum carburizing stage ends, cooling to 560-650℃ is carried out, and then the temperature is kept for 30-60min, and then cooling to below 90℃ is carried out.

2. The method for improving the machinability of 12CrNi3A steel after vacuum carburizing according to claim 1, characterized in that, The specific steps are as follows: 1) "Strong penetration + diffusion" vacuum carburizing simulation First, the preset "strong penetration + diffusion" time of 12CrNi3A steel vacuum carburizing is input into the VacCard vacuum carburizing simulation software to simulate the vacuum carburizing stage of 12CrNi3A steel, and then the "strong penetration + diffusion" time is continuously optimized and adjusted to make the simulation result meet the requirements, and the better "strong penetration + diffusion" vacuum carburizing time is obtained. 2) Vacuum carburizing stage 12CrNi3A steel is loaded into a vacuum carburizing furnace, heated and raised to 650-700℃, kept for 30-60min, raised to 920-950℃ with the furnace, and kept for 30-60min. And the better "strong penetration + diffusion" vacuum carburizing time obtained in step 1) is used for "strong penetration + diffusion" pulse carburizing, each pulse including one strong penetration stage and one diffusion stage, while the ethylene and acetylene are introduced during the strong penetration of vacuum carburizing. 3) Stepwise holding stage After the vacuum carburizing stage ends, 0.2-0.5bar of nitrogen is filled to cool, and then kept for 30-60min when the temperature is cooled to 560-650℃, and then vacuum is drawn to maintain the working pressure at 0.05-0.06mbar. 4) Cooling stage After the stepwise holding stage ends, 0.2-0.5bar of nitrogen is filled to cool to below 90℃, and then the furnace is discharged.

3. The method for improving the machinability of 12CrNi3A steel after vacuum carburizing according to claim 2, characterized in that, In step 2), the vacuum is drawn to maintain the working pressure below 0.1mbar when the vacuum carburizing furnace is raised.

4. The method for improving the machinability of 12CrNi3A steel after vacuum carburizing according to claim 2, characterized in that, In step 2), the flow rate of ethylene and acetylene introduced is 14-16L / min.

5. The method for improving the machinability of 12CrNi3A steel after vacuum carburizing according to claim 4, characterized in that, The ratio of ethylene to acetylene is 1:

1.

6. The method for improving the machinability of 12CrNi3A steel after vacuum carburizing according to claim 2, characterized in that, In step 2), the "strong penetration + diffusion" pulse carburizing has 6 pulses, and the diffusion time is greater than the strong penetration time, the strong penetration time decreases with the increase of pulse number, and the diffusion time increases with the increase of pulse number.

7. The method for improving the machinability of 12CrNi3A steel after vacuum carburizing according to claim 6, characterized in that, The first stage of strong penetration is set to 5min, and the first stage of diffusion is set to 10min.

8. The method for improving the machinability of 12CrNi3A steel after vacuum carburizing according to claim 2, characterized in that, In step 2), the 12CrNi3A steel is kept at 650℃ for 35min, and then raised to 945℃ with the furnace, and kept for 35min.

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