Cryogenic heat treatment method for reducing elastic hysteresis of 40CrNiMo alloy

Abstract

The invention discloses a cryogenic heat treatment method for reducing elastic hysteresis of 40CrNiMo alloy, which comprises the following steps of: (1) quenching the 40CrNiMo alloy at the quenching temperature of 850 DEG C, preserving heat for 1 hour, and cooling to room temperature by adopting a 10% PAG solution; (2) the quenched 40CrNiMo alloy is subjected to tempering treatment, the tempering temperature ranges from 400 DEG C to 600 DEG C, heat preservation is conducted for 3 h, and after heat preservation is completed, air cooling is conducted to the room temperature; and (3) the tempered 40CrNiMo alloy is subjected to subzero treatment, the subzero temperature ranges from-180 DEG C to-196 DEG C, the heat preservation time ranges from 12 h to 24 h, and then the temperature is naturally increased to the room temperature. According to the method, conventional quenching and tempering processes are combined with subzero treatment, so that the elastic lag of the 40CrNiMo alloy steel is effectively reduced.

Description

Technical Field

[0001] This invention belongs to the technical field of heat treatment of metallic materials and elastic hysteresis, specifically relating to a deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy. Background Technology

[0002] As an alloy structural steel, 40CrNiMo steel, in addition to its basic mechanical properties, is particularly important for its elastic hysteresis in precision engineering applications, such as in the elastomer materials required for weighing sensors.

[0003] Hysteresis refers to the phenomenon that the strain of a material lags behind the stress during loading and unloading. It directly affects the accuracy, stability, and energy dissipation characteristics of elastic elements. Therefore, the feedback results of sensors are closely related to the elastic hysteresis of the elastomer material.

[0004] Obtaining 40CrNiMo steel with excellent elastic hysteresis requires appropriate heat treatment processes. Traditional heat treatment processes involve quenching and tempering. However, during conventional quenching, due to incomplete martensite transformation, a considerable amount of retained austenite is often retained in 40CrNiMo alloy steel. Retained austenite is a metastable phase that gradually transforms into martensite during service, leading to changes in part dimensions and unstable performance. For high-precision elastic components, this structural instability can seriously affect component life and accuracy reliability. Although subsequent tempering can stabilize the retained austenite to some extent, it is difficult to completely eliminate its adverse effects.

[0005] Cryogenic treatment can effectively reduce the content of residual austenite and stabilize the microstructure and size. Therefore, it is particularly important to study the effect of cryogenic treatment on the elastic hysteresis of 40CrNiMo alloy steel.

[0006] Existing related technologies, such as patent document (CN107779566A), disclose a "heat treatment method for high-strength and high-toughness 40CrNiMo forgings," which aims to improve strength and impact toughness by increasing the austenitizing temperature to 900±10℃, using a water + organic liquid composite quenching, and tempering at 600℃. However, this scheme does not involve cryogenic treatment and does not focus on elastic hysteresis performance; its goal is mechanical strength and toughness, rather than microstructural stability and elastic response accuracy. Although patent document (CN106399653A) introduces cryogenic treatment (holding at -140℃ to -196℃ for more than 24 hours), it is applied to 1Ni9 low-temperature steel, and the cryogenic treatment is placed after quenching in the two-phase region and before tempering, with the aim of promoting the transformation of retained austenite to improve impact toughness. This process is not suitable for 40CrNiMo steel, and since it does not undergo tempering before cryogenic treatment, the microstructural stress is high, which easily leads to cracking; more importantly, it does not consider the key indicator of elastic hysteresis. Therefore, a deep cryogenic heat treatment method is needed to reduce the elastic hysteresis of 40CrNiMo alloy. Summary of the Invention

[0007] The purpose of this invention is to provide a deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy. By performing deep cryogenic treatment after conventional quenching and tempering heat treatment processes, the hysteresis of 40CrNiMo alloy steel can be effectively reduced, thereby improving its precision and stability.

[0008] To achieve the above objectives, the technical solution of the present invention is: a deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy, comprising the following steps: (1) The 40CrNiMo alloy was quenched at 850℃ for 1 hour and then cooled to room temperature using 10% PAG solution. (2) Temper the quenched 40CrNiMo alloy at a temperature of 400℃~600℃ for 3 hours, and then air cool it to room temperature after the tempering is completed. (3) The tempered 40CrNiMo alloy is subjected to deep cryogenic treatment at a temperature of -180℃ to -196℃ and a holding time of 12h to 24h, and then naturally rises to room temperature. Furthermore, the chemical composition of the 40CrNiMo alloy, by mass percentage, is: C 0.37~0.44%, Si 0.17~0.37%, Mn 0.5~0.8%, Cr 0.6~0.9%, Mo 0.15~0.25%, Ni 1.25~1.65%, with the balance being Fe. Furthermore, in step (1), the quenching process involves heating the steel to 850°C in a resistance furnace and stabilizing the furnace temperature. The forged 40CrNiMo alloy steel block is then placed into the furnace using the method of entering the furnace at the set temperature.

[0009] Furthermore, in step (2), the tempering process is carried out by heating the steel to 400℃~600℃ in a resistance box heat treatment furnace and stabilizing the furnace temperature. The quenched 40CrNiMo alloy steel block is placed into the furnace by entering the furnace at the set temperature.

[0010] Furthermore, in step (3), during cryogenic treatment, the tempered steel block is processed into a sample, then suspended on a wire and completely immersed in a cryogenic chamber.

[0011] Furthermore, the cryogenic treatment uses liquid nitrogen as the cooling medium, and the sample is completely immersed in liquid nitrogen at a cryogenic temperature of -190°C. Furthermore, when the tempering temperature is 400℃, the cryogenic holding time is 12h; when the tempering temperature is 600℃, the cryogenic holding time is 24h. Furthermore, the half-range hysteresis parameter of the 40CrNiMo alloy after the heat treatment is ≤0.02750%FS, and the zero-homing hysteresis parameter is ≤0.01030%FS. A method for testing the elastic hysteresis of 40CrNiMo alloy, used for 40CrNiMo alloy samples after cryogenic heat treatment based on the above-mentioned method for reducing the elastic hysteresis of 40CrNiMo alloy, includes the following steps: (1) Loading and unloading test of the sample: The sample is loaded and unloaded within one cycle. The loading and unloading time is set to 1s. The sample is installed using a torque wrench to ensure the accuracy of the sample installation. The two ends of the sample are parallel to the clamp and located in the center. The same sample is cycled 7 times with a cycle interval of 120s. (2) Calculation and processing of test data: The static torque measurement method in the national standard "JJG 924-2010 Verification Procedure for Torque and Speed ​​Measuring Devices" is adopted, and the zero-return lag calculation formula is as follows:

[0012] The formula for calculating half-range lag error is as follows:

[0013] in: 10mm; The average value after the zero-return lag; This is the average value of the half-time lag; This is the half-range lag calibration value, calibrated to 0; Finally, the zero-lag parameter and the half-lag parameter are obtained.

[0014] Further, the specific method of step (1) is as follows: First, the sample load is applied to 125 MPa, then held for 20s, and then the sample is pressurized to 250 MPa and held for 20s; then the unloading process is carried out, the sample is unloaded to 125 MPa and held for 20s, and finally unloaded to the initial preload force of 0.5N and held for 20s.

[0015] The beneficial effects of this invention are: by combining the conventional heat treatment process of forged 40CrNiMo alloy with deep cryogenic treatment, the content of residual austenite and elastic hysteresis in the steel are effectively reduced, thereby improving its precision and stability as an elastic element. Specifically: 1. Elastic hysteresis is significantly reduced: half-range hysteresis is reduced from 0.03018%FS to 0.02562%FS (400℃ tempering) or from 0.03820%FS to 0.02750%FS (600℃ tempering); zero-return hysteresis can be reduced to as low as 0.00103%FS; 2. High structural stability: The content of residual austenite is greatly reduced, resulting in extremely low risk of dimensional drift during service; 3. Good process safety: Cryogenic treatment is carried out after tempering to avoid deep cryogenic cracking under high stress in the quenched state; 4. Strong compatibility: A deep cryogenic step can be added to the existing tempering process, making it easy to implement in industrial applications. Attached Figure Description

[0016] Figure 1 This is a diagram showing the dimensions of the dumbbell-shaped sample used for the elastic hysteresis test of this invention. Figure 2 This is a schematic diagram of the hysteresis of the displacement difference under the same stress conditions during loading and unloading in an elastic hysteresis test. Figure 3 This is a comparison chart of the elastic hysteresis test results of 40CrNiMo alloy material samples after quenching at 850℃ and tempering at 400℃ without cryogenic treatment and after cryogenic treatment for 12 hours. Figure 4 This is a comparison chart of the elastic hysteresis test results of 40CrNiMo alloy material samples after quenching at 850℃ and tempering at 600℃ without cryogenic treatment and after cryogenic treatment for 24 hours. Specific implementation methods The embodiments of the present invention are described clearly and completely below. These embodiments are only some examples of the present invention, enabling those skilled in the art to more easily understand the features of the present invention. The present invention is a deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy. The 40CrNiMo alloy steel comprises the following chemical elements and their weight percentages: carbon 0.37~0.44%, silicon 0.17~0.37%, manganese 0.5~0.8%, chromium 0.6~0.9%, molybdenum 0.15~0.25%, nickel 1.25~1.65%, with the remainder being iron. The embodiments of the present invention include: Example 1: (1) Heat the resistance furnace to 850°C and stabilize the furnace temperature. Place the forged 40CrNiMo alloy steel block into the furnace using the method of entering the furnace at the specified temperature and keep it at the specified temperature for 1 hour. After the holding time is completed, cool it with 10% PAG solution. (2) Heat the resistance box heat treatment furnace to 400℃ and stabilize the furnace temperature. Place the quenched 40CrNiMo alloy steel block into the furnace by the method of entering the furnace at the temperature, keep it at the temperature for 3 hours, and then air cool it after the holding time is completed. (3) Process the steel block after tempering in step (2) into the following shape: Figure 1 The sample was then suspended on a wire and completely immersed in liquid nitrogen in a cryogenic chamber for 12 hours at a cryogenic temperature of -190°C.

[0017] Example 2: (1) Heat the resistance furnace to 850°C and stabilize the furnace temperature. Place the forged 40CrNiMo alloy steel block into the furnace using the method of entering the furnace at the specified temperature and keep it at the specified temperature for 1 hour. After the holding time is completed, cool it with 10% PAG solution. (2) Heat the resistance box heat treatment furnace to 600℃ and stabilize the furnace temperature. Place the quenched 40CrNiMo alloy steel block into the furnace by the method of entering the furnace at the temperature, keep it at the temperature for 3 hours, and then air cool it after the holding time is completed. (3) Process the steel block after tempering in step (2) into the following shape: Figure 1 The sample was then suspended on a wire and completely immersed in liquid nitrogen in a cryogenic chamber for 24 hours at a cryogenic temperature of -190°C.

[0018] like Figure 1 As shown in Figure 2, the elastic hysteresis testing equipment used in the above embodiments is the DMAGABO EPLEXOR 500N ultra-high temperature dynamic thermomechanical analyzer from Netzsch GmbH, Germany; the elastic hysteresis testing method used is to test the sample ( Figure 1 The loading and unloading process is performed as one cycle, with each loading and unloading time set to 1 second. The specific steps are as follows: first, the sample load is applied to 125 MPa, then held for 20 seconds, and then the sample is pressurized to 250 MPa and held for 20 seconds. Next is the unloading process, where the sample is unloaded to 125 MPa and held for 20 seconds, and finally unloaded to the initial preload of 0.5 N and held for 20 seconds. A torque wrench is used to install the sample to ensure the accuracy of the installation. Both ends of the sample are parallel to the clamp and located in the center. The same sample is cycled 7 times, with a cycle interval of 120 seconds.

[0019] The calculation formula for the test data is based on the static torque measurement method proposed in the national standard "JJG 924-2010 Verification Procedure for Torque and Speed ​​Measuring Devices". Due to the instability of the data in the first cycle of the 7 cycles, the half-stroke lag of each cycle is calculated based on the last data point after holding pressure at 125MPa for 20s, and the zero-return lag of each cycle is calculated based on the last data point after unloading and holding pressure for 20s. The lag value of each cycle is obtained, and the elasticity data of the material is calculated for the last 6 cycles according to formulas (1) and (2). Finally, the zero-return lag parameter and the half-stroke lag parameter are obtained. The zero-return lag calculation formula is as follows:

[0020] The formula for calculating half-range lag error is as follows:

[0021] in: 10mm; The average value after the zero-return lag; This is the average value of the half-time lag; This is the half-range lag calibration value, calibrated to 0.

[0022] The above embodiments, by employing the above-described equipment and testing methods, achieve the following beneficial results: Figure 3 As shown, compared with the conventional heat treatment process, the half-process hysteresis parameter of the heat treatment process of the present invention is reduced from 0.03018% (i.e., 0.03018%FS) at full scale of 10mm to 0.02562%FS, and the zero-return hysteresis parameter is reduced from 0.01288%FS to 0.01040%FS; Figure 4 As shown, compared with the conventional heat treatment process, the half-process hysteresis parameter of the heat treatment process of the present invention is reduced from 0.03820%FS to 0.02750%FS, and the zero-return hysteresis parameter is reduced from 0.01305%FS to 0.00103%FS. The results show that the heat treatment process of the present invention can effectively reduce the elastic hysteresis of 40CrNiMo alloy material and achieve the purpose of improving its precision stability.

Claims

1. A deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy, characterized in that, Includes the following steps: (1) The 40CrNiMo alloy was quenched at 850℃ for 1 hour and then cooled to room temperature using 10% PAG solution. (2) Temper the quenched 40CrNiMo alloy at a temperature of 400℃~600℃ for 3 hours, and then air cool it to room temperature after the tempering is completed. (3) The tempered 40CrNiMo alloy is subjected to deep cryogenic treatment at a temperature of -180℃ to -196℃ and a holding time of 12h to 24h, and then naturally rises to room temperature.

2. The deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy according to claim 1, characterized in that, The chemical composition of the 40CrNiMo alloy, by mass percentage, is: C 0.37~0.44%, Si 0.17~0.37%, Mn 0.5~0.8%, Cr 0.6~0.9%, Mo 0.15~0.25%, Ni 1.25~1.65%, with the balance being Fe.

3. The deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy according to claim 1, characterized in that, In step (1), the quenching process is carried out by heating the resistance furnace to 850°C and stabilizing the furnace temperature, and the forged 40CrNiMo alloy steel block is placed into the furnace by the method of entering the furnace at the temperature.

4. The deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy according to claim 1, characterized in that, In step (2), the tempering process is carried out by heating the steel to 400℃~600℃ in a resistance box heat treatment furnace and stabilizing the furnace temperature. The quenched 40CrNiMo alloy steel block is placed into the furnace by entering the furnace at the set temperature.

5. The deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy according to claim 1, characterized in that, In step (3), during cryogenic treatment, the tempered steel block is processed into a sample, then suspended on a wire and completely immersed in the cryogenic chamber.

6. The deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy according to claim 5, characterized in that, The cryogenic treatment uses liquid nitrogen as the cooling medium, and the sample is completely immersed in liquid nitrogen at a cryogenic temperature of -190°C.

7. The deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy according to claim 1, characterized in that, When the tempering temperature is 400℃, the cryogenic holding time is 12h; when the tempering temperature is 600℃, the cryogenic holding time is 24h.

8. The deep cryogenic heat treatment method for reducing the elastic hysteresis of 40CrNiMo alloy according to claim 1, characterized in that, The half-range hysteresis parameter of the 40CrNiMo alloy after the heat treatment is ≤0.02750%FS, and the zero-homing hysteresis parameter is ≤0.01030%FS.

9. A method for testing the elastic hysteresis of a 40CrNiMo alloy, used for 40CrNiMo alloy samples after cryogenic heat treatment according to any one of claims 1-8 to reduce the elastic hysteresis of the 40CrNiMo alloy, characterized in that, Includes the following steps: (1) Loading and unloading test of the sample: The sample is loaded and unloaded within one cycle. The loading and unloading time is set to 1s. The sample is installed using a torque wrench to ensure the accuracy of the sample installation. The two ends of the sample are parallel to the clamp and located in the center. The same sample is cycled 7 times with a cycle interval of 120s. (2) Calculation and processing of test data: The static torque measurement method in the national standard "JJG 924-2010 Verification Procedure for Torque and Speed ​​Measuring Devices" is adopted, and the zero-return lag calculation formula is as follows: The formula for calculating half-range lag error is as follows: in: 10mm; The average value after the zero-return lag; This is the average value of the half-time lag; This is the half-range lag calibration value, calibrated to 0; Finally, the zero-lag parameter and the half-lag parameter are obtained.

10. The method for testing the elastic hysteresis of the 40CrNiMo alloy according to claim 9, characterized in that, The specific method of step (1) is as follows: First, load the sample to 125 MPa, then hold the pressure for 20s, then continue to pressurize the sample to 250 MPa, and hold the pressure for 20s; then the unloading process is to unload the sample to 125 MPa and hold the pressure for 20s, and finally unload it to the initial preload force of 0.5N and hold the pressure for 20s.

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