A heat treatment method for a low-carbon low-alloy steel

Abstract

This application relates to the field of cycloid pinwheel precision reducers. More specifically, it relates to a heat treatment method for low-carbon low-alloy steel. A heat treatment method for low-carbon low-alloy steel successively includes the following steps: carburizing, spheroidizing annealing, quenching, carbon partitioning, and tempering at the M S critical point temperature. During the tempering at the M S critical point temperature, the tempering temperature is 245 - 255 °C, and the tempering time is 100 - 120 min. The surface structure is fine needle-like martensite + lower bainite + retained austenite + globular cementite. The heat treatment method of this application has the effect of improving the wear resistance of the input gear shaft, planet gear, crankshaft, and cycloid pinwheel made of low-carbon low-alloy steel.

Description

Technical Field

[0001] The present application relates to the field of cycloid pinwheel precision reducers, and more specifically, to a heat treatment method for low-carbon low-alloy steel. Background Art

[0002] Cycloid pinwheel precision reducers are widely used in the joints of industrial robots to decelerate and steer the robot arms through the joints. In each joint, the servo motor sequentially transmits the input torque through the input gear shaft, planetary gear, crankshaft, and cycloid pinwheel in the reducer, and finally outputs through the pin gear housing to achieve the purpose of deceleration and at the same time achieve steering.

[0003] During the transmission process, the above-mentioned parts not only have to bear a large torque, but also have to bear high friction and wear performance. For example, for the crankshaft, its eccentric wheel and the needle roller bearing and the cycloid wheel hole form a sliding friction pair. The surface layer of the eccentric wheel not only has to have high wear resistance, but also its core has to have high strength and toughness. The input gear shaft, planetary gear, and cycloid pinwheel also must have the above comprehensive mechanical properties so that the transmission efficiency, transmission accuracy, and service life of the reducer can be further improved.

[0004] In the field of cycloid pinwheel precision reducers, people have achieved fruitful research results in aspects such as the structural design, machining, overall assembly, and performance testing of the reducer, but there are few research results on the mechanical properties of the above-mentioned components, especially the wear resistance.

[0005] For the crankshaft and planetary gear, currently, 20CrMnTi, 20CrMo, 20CrNiMo, 20CrMnSi, 30CrMnSi and other low-carbon low-alloy steels are basically selected. Some enterprises use such low-carbon low-alloy steels for the cycloid pinwheel, and some enterprises use bearing steel. The raw materials are generally in the hot-rolled and annealed state.

[0006] Currently, for these parts, most domestic related enterprises generally first rough-machine the raw materials, and then adopt the traditional carburizing and quenching process, that is, carburizing, quenching, and low-temperature tempering heat treatment for manufacturing. However, after being assembled into the whole machine and serving for a period of time, the applicant found that the surface layer of these parts wears quickly, thus affecting the transmission efficiency, transmission accuracy, and service life. Summary of the Invention

[0007] In order to improve the defect that the surface layer of the above-mentioned parts wears quickly, the present application provides a heat treatment method for low-carbon low-alloy steel.

[0008] A heat treatment method for low-carbon low-alloy steel provided by the present application adopts the following technical solution:

[0009] A heat treatment method for low-carbon low-alloy steel sequentially includes the following steps: carburizing, quenching, MS (Martensite start transformation temperature) critical point temperature tempering; the M S In the critical point temperature tempering, the tempering temperature is 245 - 255 °C and the tempering time is 100 - 120 min.

[0010] The M of the surface layer structure of the low-carbon low-alloy steel after carburizing S The critical point temperature is 240 °C, and the tempering temperature of this application, 245 - 255 °C, is slightly higher than the M S critical point temperature. When tempering at the temperature of this application, the quenched martensite will gradually transform into lower bainite, and some retained austenite will also transform into lower bainite.

[0011] The ferrite of the lower bainite is located on the martensite plates, with small size and a fine needle-like morphology. Nanoscale M7C3 and M 23 carbides of C6 precipitate at the ferrite edge, with an average size of 30 - 50 nm. The transformation from martensite to lower bainite also requires a nucleation and growth process. Therefore, when the tempering temperature is not much higher than the M S transformation temperature, from the perspectives of phase transformation thermodynamics and kinetics, although the conditions for martensite to transform into lower bainite are met, due to insufficient energy, not all martensite can be transformed into lower bainite, nor can all retained austenite be transformed into lower bainite.

[0012] That is to say, after tempering at 245 - 255 °C, the surface layer structure is fine needle-like martensite + lower bainite + retained austenite + spherical cementite. The size of the fine needle-like martensite is 0.5 - 1 μm, the original austenite grain size of the carburized layer is 10.5 grades, the surface hardness reaches 60 - 61 HRC, and the carburizing depth reaches 0.8 mm. The core structure is still lath-shaped low-carbon martensite, but the length dimension of the laths has decreased, ensuring both high strength and sufficient toughness.

[0013] This surface layer tissue structure and grain size have high wear resistance. Among them, the fine needle-like martensite has high hardness but poor toughness; the lower bainite has high strength and toughness, with a high strength-toughness combination; the retained austenite further improves toughness and enhances the TRIP effect; a certain amount of spherical cementite improves the function of absorbing internal friction and improves the wear resistance; the higher grain size on the surface further improves the wear resistance, that is, the combination of these microstructures is very beneficial to the improvement of friction and wear performance.

[0014] For the traditional tempering process after carburizing and quenching, the tempering temperature is generally below the M S critical point temperature, that is, the commonly used tempering temperature is 170 - 200 °C. When tempering in the temperature range of 170 - 200 °C, the matrix structure is only tempered martensite and retained austenite, and lower bainite cannot be produced.

[0015] Preferably, there is carbon partitioning between the quenching and the tempering at the M S critical point temperature. The partitioning temperature is 290 - 310 °C and the partitioning time is 160 - 200 s.

[0016] The carbon partitioning heat treatment can cause the retained austenite to obtain a certain amount of carbon from the martensite, improving the stability of the retained austenite. When the surface layer of the matrix structure is subjected to frictional force, the TRIP effect of the retained austenite turning into martensite is enhanced, thereby further enhancing the toughness and strength of the matrix structure.

[0017] Preferably, spheroidizing annealing is also provided between the carburizing and the quenching. The spheroidizing annealing adopts a continuous annealing process. The austenitizing temperature is 780 - 820 °C, the austenitizing holding time is 25 - 35 min, and then it is furnace-cooled to 700 - 730 °C and held at this temperature for 160 - 200 min for spheroidizing.

[0018] In the traditional carburizing and quenching process, there is no spheroidizing annealing process, but direct quenching after carburizing. During the furnace-cooling process in the spheroidizing annealing process, the undissolved lamellar cementite begins to spheroidize, and the carbon dissolved in the austenite also precipitates, starting to form spherical cementite. The cementite after spheroidizing greatly improves the friction and wear performance of the matrix structure.

[0019] Moreover, for the crankshaft, through spheroidizing annealing, the pearlite matrix structure on the surface layer of the crankshaft will become equiaxed, and the pearlite structure in the core will also become equiaxed, thereby improving the performance of the crankshaft.

[0020] Preferably, the austenitizing temperature is 795 - 805 °C.

[0021] The austenitizing temperature affects the amount of lamellar cementite dissolved in the austenite in the surface layer structure and the size of the undissolved lamellar cementite, thereby affecting the quantity and morphology of the undissolved lamellar cementite after spheroidizing as heterogeneous nuclei.

[0022] The A c1 、A ccm 、A rcm and A r1 temperatures of the surface layer structure are 760 °C, 900 °C, 707 °C and 695 °C respectively. Therefore, at a determined austenitizing temperature, the quantity (mass percentage) of the undissolved lamellar cementite can be calculated according to the lever law. If the austenitizing temperature is too high, the quantity of the undissolved lamellar cementite is small and the size is small, and the quantity and size of the cementite after spheroidizing are small; if the austenitizing temperature is too low, the morphology of the cementite after spheroidizing is poor, the shape factor is low, and most of it is rod-shaped and massive.

[0023] Meanwhile, the austenitizing temperature affects the size of the original austenite grains. The austenitizing process is actually a recrystallization process. 20CrMnTi steel belongs to the essentially fine-grained steel. When the austenitizing temperature is 795 - 805 °C, the size of the original austenite grains is still small.

[0024] Preferably, the furnace cooling temperature is 710 - 720 °C.

[0025] The spheroidizing temperature mainly affects the quantity and morphology of cementite. When the spheroidizing holding temperature is slightly higher than the A r1 and A rcm temperature, carbon in the supercooled austenite will precipitate continuously. However, if the spheroidizing temperature is too high or too low, it will affect the quantity and morphology of globular cementite.

[0026] Preferably, the carburizing sequentially includes a strong carburizing stage and a diffusion stage. The temperature in the strong carburizing stage is 900 - 920 °C, the strong carburizing potential is 1.1% - 1.2%, and the strong carburizing diffusion time is 220 - 260 min; the temperature in the diffusion stage is 860 - 900 °C, the diffusion carburizing potential is 0.85 - 0.95%, and the diffusion time is 110 - 130 min.

[0027] In the traditional carburizing process, the carburizing potential in the strong carburizing stage is generally 0.85 - 1.05%, while the carburizing potential in the strong carburizing stage of this application is 1.1 - 1.2%. By using a higher carburizing potential, a higher carbon concentration can be obtained on the surface layer of the matrix structure, ensuring a sufficient carbon concentration gradient for the diffusion stage. At the same time, it ensures that the surface microstructure after furnace tapping is ledeburite (pearlite + lamellar cementite). A higher carburizing potential can obtain a larger number of lamellar cementite, and more globular cementite can also be obtained after spheroidization.

[0028] In addition, in the spheroidizing annealing stage, a higher carburizing potential can ensure that the diameter of the lamellar cementite reaches a certain size after spheroidizing annealing, and at the same time, a certain quantity should also be ensured.

[0029] Preferably, the temperature in the diffusion stage is 20 - 40 °C lower than that in the strong carburizing stage.

[0030] In the traditional carburizing process, the temperature in the diffusion stage is 0 - 20 °C lower than that in the strong carburizing stage, while the temperature in the diffusion stage of this application is 20 - 40 °C lower than that in the strong carburizing stage. This lower temperature can promote the retention of sufficient carbon concentration in the strong carburizing layer, so that carbon does not diffuse too much into the transition layer, and at the same time, it can also meet the gentle decline of the quenching hardness gradient in the transition layer.

[0031] Preferably, subcritical quenching is used for quenching, the quenching temperature is 820 - 830 °C, and the heating and holding time is 8 - 12 min.

[0032] Traditional carburizing and quenching processes include pre-cooling quenching, single quenching, and double quenching. The quenching temperature is generally 830 - 850 °C, and the heating and holding time is generally 30 - 60 min. In fact, the traditional carburizing and quenching temperature is too high, the carbon content dissolved in austenite is relatively high, and the carbon content of martensite formed after quenching and low-temperature tempering is also relatively high (the transformation from austenite to martensite is a non-diffusion phase transformation). In addition to high hardness, other mechanical properties such as strength, toughness, and wear resistance of the product are not optimal.

[0033] In addition, since the transformation from austenite to martensite is a non-diffusion phase transformation, when the quenching temperature is determined, according to the lever law, the staff can calculate the carbon content of austenite, and thus obtain the carbon content of martensite after quenching. Similarly, the number of spherical cementite can also be calculated.

[0034] When the quenching temperature is slightly higher than the austenitizing temperature of spheroidizing annealing, some very small-sized spherical cementite will redissolve and even completely disappear, and the larger-sized spherical cementite will also undergo partial redissolution, resulting in a smaller size and a rounder morphology. However, due to the short heating time before quenching, these changes are not very obvious. After quenching, the spherical cementite is dispersed, accounting for 3% (vol), with an average size of 0.1 - 0.2 μm and an average shape factor greater than 0.9, thereby improving the wear resistance of the matrix structure.

[0035] Preferably, the heat treatment method for low-carbon low-alloy steel sequentially includes the following steps: carburizing, spheroidizing annealing, quenching, carbon partitioning, M S critical point temperature tempering;

[0036] The carburizing sequentially includes a strong carburizing stage and a diffusion stage. The temperature of the strong carburizing stage is 900 - 920 °C, and the strong carburizing potential is 1.1% - 1.2%; the temperature of the diffusion stage is 860 - 880 °C, and the diffusion carbon potential is 0.85 - 0.95%.

[0037] The spheroidizing annealing adopts a continuous annealing process. The austenitizing temperature is 795 - 805 °C, the austenitizing holding time is 25 - 35 min, and then it is furnace-cooled to 710 - 720 °C, and spheroidizing is carried out at this temperature for 160 - 200 min.

[0038] The quenching adopts subcritical quenching, the quenching temperature is 820 - 830 °C, and the heating and holding time is 8 - 12 min.

[0039] In the carbon partitioning, the partitioning temperature is 290 - 310 °C, and the partitioning time is 160 - 200 s.

[0040] The M S In the critical point temperature tempering, the tempering temperature is 245 - 255 °C, and the tempering time is 100 - 120 min.

[0041] In summary, the present application has the following beneficial effects:

[0042] 1. After tempering at 245 - 255 °C, the surface layer structure is fine needle-like martensite + lower bainite + retained austenite + spherical cementite. This surface layer structure and grain size have high wear resistance. The core structure remains lath-shaped low-carbon martensite, but the lath length dimension is reduced, ensuring both high strength and sufficient toughness;

[0043] 2. After quenching, the spherical cementite is dispersedly distributed, with an average size of 0.1 - 0.2 μm and an average shape factor greater than 0.9; thus improving the wear resistance of the matrix structure;

[0044] 3. Carbon partitioning heat treatment can prompt the retained austenite to obtain a certain amount of carbon from the martensite, improving the stability of the retained austenite; after tempering at the critical temperature, the ferrite of the lower bainite is located on the martensite plates, with small size and a fine needle-like morphology. Nanoscale M7C3 and M 23 C6 carbides are precipitated at the ferrite edges, with an average size of 30 - 50 nm; the fine needle-like martensite has a size of 0.5 - 1 μm, the original austenite grain size of the carburized layer is 10.5 grades, the surface hardness reaches 60 - 61 HRC, and the carburizing depth reaches 0.8 mm;

[0045] 4. After being assembled into a complete machine and running for 20000 h under the rated speed and rated load, no obvious wear marks are found on the surface of the crankshaft. The wear resistance of the crankshaft is greatly improved, and the service life is also extended. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a flowchart of the heat treatment method for low-carbon low-alloy steel;

[0047] Figure 2 is a temperature-time curve graph of the heat treatment method for low-carbon low-alloy steel;

[0048] Figure 3 is a SEM metallographic diagram of the surface microstructure of the crankshaft after heat treatment according to Example 25;

[0049] Figure 4 is a surface grain diagram of the crankshaft after heat treatment according to Example 25 (corroded by saturated picric acid aqueous solution);

[0050] Figure 5 is a wear mark morphology diagram of the eccentric wheel surface after the crankshaft produced according to Example 25 is assembled into a complete machine and runs for 20000 h;

[0051] Figure 6It is a morphology diagram of the wear marks on the surface of the eccentric wheel after the crankshaft produced in Comparative Example 1 is assembled into a complete machine and runs for 7000 hours. Specific embodiments

[0052] The following further elaborates on the present application in conjunction with the appended Figure 1-6 , examples and comparative examples. It should be noted that the present application discusses the heat treatment method of low-carbon low-alloy steel by taking the crankshaft of a cycloid pinwheel precision reducer as an example, and the heat treatment method of the present application is equally applicable to the input gear shaft, planetary gear, and cycloid pinwheel in the reducer.

[0053] Examples

[0054] Example 1

[0055] A heat treatment method for the crankshaft of a cycloid pinwheel precision reducer includes the following steps:

[0056] Pre-preparation: Select 20CrMnTi steel (other low-carbon low-alloy steels such as 20CrMo, 20CrNiMo, 20CrMnSi, and 30CrMnSi can also be used). The raw material is round steel in a hot-rolled and annealed state, and is to be heat-treated after rough machining;

[0057] Carburizing: The temperature in the strong carburizing stage is 910 °C, the strong carburizing potential is 0.95%, the strong carburizing diffusion time is 210 min, the temperature in the diffusion stage is 900 °C, the diffusion carburizing potential is 0.95%, the diffusion time is 210 min. After carburizing, cool in the furnace to 300 °C and then take out and air-cool;

[0058] Quenching: The quenching temperature is controlled at 830 °C, the heating and holding time is controlled at 12 min, the quenching liquid uses a melt of KNO2 (50% wt) + NaNO2 (50% wt), and the temperature of the quenching liquid is 170 °C;

[0059] M S Critical point temperature tempering: The tempering temperature is 245 °C, and the tempering time is 100 min.

[0060] Examples 2 - 3

[0061] The difference from Example 1 lies in that in the M S Critical point temperature tempering, the tempering temperature and tempering time are different, as shown in Table 1 specifically.

[0062] Table 1 Tempering temperature and tempering time of Examples 1 - 3

[0063] Example 1 Example 2 Example 3 Tempering temperature / °C 245 250 255 Tempering time / min 100 110 120

[0064] Example 4

[0065] The difference from Example 2 lies in that the quenching and M SThere is also carbon partitioning set between the critical point temperature and tempering. The carbon partitioning temperature is 290 °C and the partitioning time is 160 s. After the carbon partitioning is completed, liquid nitrogen is introduced into the furnace to rapidly reduce the furnace temperature to the tempering temperature.

[0066] Examples 5 - 6

[0067] The difference from Example 4 is that the carbon partitioning temperature and the partitioning time are different, as shown in Table 2 specifically.

[0068] Table 2 Carbon partitioning temperature and carbon partitioning time schedule for Examples 4 - 5

[0069] Example 4 Example 5 Example 6 Carbon distribution temperature / °C 290 300 310 Distribution time / s 160 180 200

[0070] Example 7

[0071] The difference from Example 2 is that spheroidizing annealing is also set between carburizing and quenching. The continuous spheroidizing annealing process is adopted for spheroidizing annealing, with the austenitizing temperature of 780 °C, the austenitizing holding time of 25 min, then furnace cooling to 700 °C, holding for 160 min at this temperature for spheroidizing, and finally furnace cooling to 300 °C and taking out of the furnace for air cooling.

[0072] Examples 8 - 11

[0073] The difference from Example 7 is that the austenitizing temperature and its holding time are different, as shown in Table 3 specifically.

[0074] Table 3 Austenite temperature and austenitizing holding time schedule for Examples 7 - 11

[0075]

[0076] Examples 12 - 15

[0077] The difference from Example 7 is that the furnace cooling temperature and the holding time are different, as shown in Table 4 specifically.

[0078] Table 4 Furnace cooling temperature and its holding time schedule for Example 7 and Example 12

[0079]

[0080]

[0081] Example 16

[0082] The difference from Example 2 is that the temperature in the strong carburizing stage is 900 °C, the strong carburizing potential is 1.1%, the strong carburizing diffusion time is 220 min, the temperature in the diffusion stage is 860 °C, the diffusion carburizing potential is 0.85%, the diffusion time is 110 min, and after carburizing is completed, furnace cooling to 300 °C and taking out of the furnace for air cooling.

[0083] Example 17

[0084] It is different from Example 2 in that the temperature in the strong carburizing stage is 910 °C, the strong carburizing potential is 1.15%, the diffusion time in the strong carburizing stage is 240 min, the temperature in the diffusion stage is 870 °C, the diffusion carburizing potential is 0.90%, the diffusion time is 120 min, and after carburizing, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling.

[0085] Example 18

[0086] It is different from Example 2 in that the temperature in the strong carburizing stage is 920 °C, the strong carburizing potential is 1.2%, the diffusion time in the strong carburizing stage is 260 min, the temperature in the diffusion stage is 880 °C, the diffusion carburizing potential is 0.95%, the diffusion time is 130 min, and after carburizing, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling.

[0087] Examples 19 - 20

[0088] It is different from Example 17 in that the temperature in the diffusion stage is different, as specifically shown in Table 5.

[0089] Table 5 Temperature in the diffusion stage of Examples 17, 19 - 20

[0090] Example 17 Example 19 Example 20 Temperature in the strong carburizing stage / °C 910 910 910 Temperature in the diffusion stage / °C 870 880 890

[0091] Example 21

[0092] It is different from Example 2 in that subcritical quenching is adopted for quenching, the quenching temperature is 825 °C, and the heating and holding time is 10 min.

[0093] Examples 22 - 23

[0094] It is different from Example 21 in that the quenching temperature and the heating and holding time are different, as specifically shown in Table 6.

[0095] Table 6 Quenching temperature and heating and holding time of Examples 2, 21 - 23

[0096]

[0097] Refer to Figure 1 and Figure 2 , which are the flow chart and the temperature - time curve diagram of the heat treatment method for the low - carbon low - alloy steel of Examples 24 - 26. And the specific steps of the heat treatment method for the low - carbon low - alloy steel of Examples 24 - 26 are as follows:

[0098] Example 24

[0099] A heat treatment method for the crankshaft of a cycloid pinwheel precision speed reducer includes the following steps:

[0100] Pre-preparation: Select 20CrMnTi steel. The raw material is round steel in hot-rolled and annealed state, and it is to be heat-treated after rough machining;

[0101] Carburizing: The temperature in the strong carburizing stage is 900 °C, the strong carburizing potential is 1.1%, the strong carburizing diffusion time is 220 min, the temperature in the diffusion stage is 860 °C, the diffusion carburizing potential is 0.85%, the diffusion time is 110 min. After carburizing, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling;

[0102] Spheroidizing annealing: Adopt the continuous spheroidizing annealing process. The austenitizing temperature is 795 °C, the austenitizing holding time is 25 min, then it is cooled in the furnace to 710 °C, and spheroidizing is carried out at this temperature for 160 min. Finally, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling;

[0103] Quenching: The quenching temperature is controlled at 820 °C, the heating and holding time is controlled at 8 min, and the quenching liquid is KNO2(50% wt)+NaNO2(50% wt) melt, and the temperature of the quenching liquid is 170 °C;

[0104] Carbon partitioning: The partitioning temperature is 290 °C, the partitioning time is 160 s. After carbon partitioning, liquid nitrogen should be introduced into the furnace to quickly reduce the furnace temperature to the tempering temperature;

[0105] M S Critical point temperature tempering: The tempering temperature is 245 °C, and the tempering time is 100 min.

[0106] Example 25

[0107] A heat treatment method for the crankshaft of a cycloid pinwheel precision reducer, comprising the following steps:

[0108] Pre-preparation: Select 20CrMnTi steel. The raw material is round steel in hot-rolled and annealed state, and it is to be heat-treated after rough machining;

[0109] Carburizing: The temperature in the strong carburizing stage is 910 °C, the strong carburizing potential is 1.15%, the strong carburizing diffusion time is 240 min, the temperature in the diffusion stage is 870 °C, the diffusion carburizing potential is 0.90%, the diffusion time is 120 min. After carburizing, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling;

[0110] Spheroidizing annealing: Adopt the continuous spheroidizing annealing process. The austenitizing temperature is 800 °C, the austenitizing holding time is 30 min, then it is cooled in the furnace to 715 °C, and spheroidizing is carried out at this temperature for 180 min. Finally, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling;

[0111] Quenching: The quenching temperature is controlled at 825 °C, the heating and holding time is controlled at 10 min, and the quenching liquid is KNO2(50% wt)+NaNO2(50% wt) melt, and the temperature of the quenching liquid is 175 °C;

[0112] Carbon distribution: The distribution temperature is 300 °C, and the distribution time is 180 s. After carbon distribution, liquid nitrogen should be introduced into the furnace to quickly reduce the furnace temperature to the tempering temperature;

[0113] M S Critical point temperature tempering: The tempering temperature is 250 °C, and the tempering time is 110 min.

[0114] Example 26

[0115] A heat treatment method for the crankshaft of a cycloid pinwheel precision reducer, comprising the following steps:

[0116] Pre-preparation: Select 20CrMnTi steel. The raw material is round steel in a hot-rolled and annealed state. After rough machining, it is ready for heat treatment;

[0117] Carburizing: The temperature in the strong carburizing stage is 920 °C, the strong carburizing potential is 1.2%, the strong carburizing diffusion time is 260 min, the temperature in the diffusion stage is 880 °C, the diffusion carburizing potential is 0.95%, the diffusion time is 130 min. After carburizing, the furnace is cooled to 300 °C and then taken out of the furnace for air cooling;

[0118] Spheroidizing annealing: Adopt a continuous spheroidizing annealing process. The austenitizing temperature is 805 °C, the holding time is 35 min, then the furnace is cooled to 720 °C, and spheroidizing is carried out at this temperature for 200 min. Finally, the furnace is cooled to 300 °C and taken out of the furnace for air cooling;

[0119] Quenching: The quenching temperature is controlled at 830 °C, the heating and holding time is controlled at 12 min, and the quenching liquid is KNO2 (50% wt) + NaNO2 (50% wt) melt. The temperature of the quenching liquid is 180 °C;

[0120] Carbon distribution: The distribution temperature is 310 °C, and the distribution time is 200 s. After carbon distribution, liquid nitrogen should be introduced into the furnace to quickly reduce the furnace temperature to the tempering temperature;

[0121] M S Critical point temperature tempering: The tempering temperature is 255 °C, and the tempering time is 120 min.

[0122] Comparative example

[0123] Comparative example 1

[0124] A heat treatment method for the crankshaft of a cycloid pinwheel precision reducer, comprising the following steps:

[0125] Pre-preparation: Select 20CrMnTi steel. The raw material is round steel in a hot-rolled and annealed state. After rough machining, it is ready for heat treatment.

[0126] Carburizing: The temperature in the strong carburizing stage is 910 °C, the strong carburizing potential is 0.95%, the strong carburizing diffusion time is 210 min, the temperature in the diffusion stage is 900 °C, the diffusion carburizing potential is 0.95%, the diffusion time is 210 min, and after carburizing, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling.

[0127] Quenching: The quenching temperature is controlled at 830 °C, the heating and holding time is controlled at 30 min, the quenching liquid uses a KNO2(50% wt)+NaNO2(50% wt) melt, and the temperature of the quenching liquid is 170 °C.

[0128] Low-temperature tempering: The tempering temperature is 175 °C, and the tempering time is 120 min.

[0129] Comparative Example 2

[0130] A heat treatment method for the crankshaft of a cycloid pinwheel precision reducer, comprising the following steps:

[0131] Pre-preparation: Select 20CrMnTi steel, the raw material is round steel in the hot-rolled and annealed state, and it is to be heat-treated after rough machining.

[0132] Carburizing: The temperature in the strong carburizing stage is 910 °C, the strong carburizing potential is 1.05%, the strong carburizing diffusion time is 210 min, the temperature in the diffusion stage is 900 °C, the diffusion carburizing potential is 1.0%, the diffusion time is 210 min, and after carburizing, it is cooled in the furnace to 300 °C and then taken out of the furnace for air cooling.

[0133] Quenching: The quenching temperature is controlled at 835 °C, the heating and holding time is controlled at 30 min, the quenching liquid uses a KNO2(50% wt)+NaNO2(50% wt) melt, and the temperature of the quenching liquid is 175 °C.

[0134] Low-temperature tempering: The tempering temperature is 190 °C, and the tempering time is 150 min.

[0135] Performance detection test

[0136] Detection method

[0137] 1. A c1 、A ccm 、A rcm 、A r1 and M S The detection of temperature is carried out using a DTA103 type high-temperature differential thermal analyzer.

[0138] 2. The phase composition of the microstructure is analyzed using a D8 ADVANCE DAVINCI type X-ray powder diffractometer;

[0139] 3. The volume percentage content of retained austenite is measured using an X-ray diffractometer (ARL EQUINOX 100);

[0140] 4. Use a SU5000 scanning electron microscope to observe the microstructure of the specimen and measure the size of the microstructure;

[0141] 5. Use a ZEISS optical microscope to detect the grain size;

[0142] 6. Use a Wilson VH1102 microhardness tester to measure the hardness of the specimen surface;

[0143] 7. Use an SFT-2M pin-on-disk friction and wear tester to conduct a grease lubricated friction and wear test on the specimen, and measure the friction coefficient and wear rate of the specimen;

[0144] 8. Use a KH-8700 three-dimensional digital microscope to observe the morphology of the wear marks on the sample surface.

[0145] Table 7 Performance parameter tables of Examples 1-3 and Comparative Examples 1-2

[0146]

[0147] Referring to Examples 1-3 and Comparative Examples 1-2 and combining with Table 7, it can be seen that, compared with Comparative Example 1 and Comparative Example 2, the wear resistance of Examples 1-3 is significantly improved. The reason is that after tempering at a temperature slightly higher than the M S critical point, the quenched martensite will gradually transform into lower bainite, and part of the retained austenite will also transform into lower bainite, promoting the formation of fine needle-like martensite + lower bainite + retained austenite + spherical cementite in the surface layer structure. And this surface layer structure and grain size have high wear resistance, while low-temperature tempering at 170-200 °C cannot promote the generation of lower bainite.

[0148] Table 8 Performance parameter tables of Example 2 and Examples 4-6

[0149]

[0150]

[0151] Referring to Example 2 and Examples 4-6 and combining with Table 8, it can be seen that, compared with Example 2, the wear resistance of Examples 4-6 is further improved. The reason is that carbon partitioning can cause the retained austenite to obtain a certain amount of carbon from the martensite, improving the stability of the retained austenite, and enhancing the TRIP effect that the retained austenite transforms into martensite after the surface layer of the crankshaft eccentric wheel is subjected to frictional force.

[0152] Table 9 Performance parameter tables of Examples 7-11

[0153]

[0154] Referring to Example 2 and Examples 7 - 11 and combining with Tables 8 - 9, it can be seen that, compared with Example 2, the wear resistance of Examples 7 - 11 has been significantly improved. The reason is that the spheroidizing annealing process can promote the spheroidization of lamellar cementite, and the cementite after spheroidization improves the wear resistance of the matrix structure.

[0155] Among Examples 7 - 11, the wear resistance of Examples 8 - 10 is relatively better. The reason is that if the austenitizing temperature is too high, the number and size of undissolved lamellar cementite are small, and the number and size of cementite after spheroidization are also small; if the austenitizing temperature is too low, the morphology of cementite after spheroidization is poor, the shape factor is low, and most of them are blocky and rod-shaped.

[0156] Table 10 Performance parameter tables of Example 7 and Examples 12 - 15

[0157]

[0158]

[0159] Referring to Example 7 and Examples 12 - 15 and combining with Table 10, it can be seen that the wear resistance of Examples 12 - 14 is relatively better. The reason is that too high or too low furnace cooling temperature during spheroidizing annealing will affect the number and morphology of globular cementite, thus affecting the wear performance.

[0160] Table 11 Performance parameter tables of Example 2 and Examples 16 - 18

[0161]

[0162] Referring to Example 2 and Examples 16 - 18 and combining with Table 11, it can be seen that, compared with Example 2, the wear resistance of Examples 16 - 18 is relatively better. The reason is that using a higher carbon potential in the strong carburizing stage can make the surface layer obtain a higher carbon concentration, ensure a sufficient carbon concentration gradient for the diffusion stage, and at the same time ensure that the surface microstructure after furnace exit is ledeburite (pearlite + lamellar cementite). A higher carbon potential can obtain a larger number of lamellar cementite, and more globular cementite can also be obtained after spheroidization, further improving the wear resistance.

[0163] Table 12 Performance parameter tables of Example 17 and Examples 19 - 20

[0164]

[0165]

[0166] Referring to Example 17, Examples 19 - 20 and in combination with Table 12, it can be seen that, compared with Example 17, the wear resistance of Examples 19 - 20 is slightly reduced. The reason is that a lower temperature in the diffusion stage can promote the retention of sufficient carbon concentration in the strong carburized layer, preventing excessive carbon from diffusing into the transition layer, and at the same time can also meet the gentle decline of the quenching hardness gradient in the transition layer.

[0167] Table 13 Performance parameter table of Example 2 and Examples 21 - 23

[0168]

[0169] Referring to Example 2, Examples 21 - 23 and in combination with Table 13, it can be seen that, compared with Example 23, the wear resistance of Example 2 and Examples 21 - 22 is significantly improved. The reason is that quenching at too high a temperature will result in a higher carbon content dissolved in austenite, and the martensite formed after quenching and low-temperature tempering also has a higher carbon content (the transformation of austenite to martensite is a non-diffusion type phase transformation). In addition to having a higher hardness, other mechanical properties such as strength, toughness and wear resistance of the product are relatively poor.

[0170] Table 14 Performance parameter table of Examples 24 - 26 and Comparative Examples 1 - 2

[0171]

[0172]

[0173] Referring to Examples 24 - 26 and Comparative Examples 1 - 2 and in combination with Table 14, Figure 1-5 it can be seen that, compared with Comparative Examples 1 - 2, the wear resistance of Examples 24 - 26 is significantly improved, and compared with Examples 1 - 23, the wear resistance of Examples 24 - 26 is better, and the matrix structure on the surface layer is relatively better, specifically as Figure 3-6 shown.

[0174] This shows that high-carbon carburizing, spheroidizing annealing, subcritical quenching, carbon partitioning and M S critical point temperature tempering heat treatment are carried out synergistically, and there is a synergistic effect among the five, thereby further improving the wear resistance of the matrix structure.

[0175] This specific embodiment is only an interpretation of the present application and not a limitation thereof. Those skilled in the art can make modifications to this embodiment without creative contributions according to needs after reading this specification, but as long as it is within the scope of the claims of the present application, it is protected by the patent law.

Claims

1. A heat treatment method for a low-carbon low-alloy steel, characterized in that, The following steps are included in sequence: Carburizing, spheroidizing annealing, quenching, tempering at the critical point temperature of M S ; tempering The carburizing process includes a strong carburizing stage and a diffusion stage. The temperature of the strong carburizing stage is 900-920°C, the strong carburizing potential is 1.1%-1.2%, and the strong carburizing diffusion time is 220-260min. The temperature of the diffusion stage is 860-900°C, the diffusion carbon potential is 0.85-0.95%, and the diffusion time is 110-130min. The spheroidizing annealing adopts a continuous spheroidizing annealing process, the austenitizing temperature is 780-820°C, the austenitizing holding time is 25-35min, then the furnace is cooled to 700-730°C, and the temperature is kept at this temperature for 160-200min for spheroidizing; The said M S During tempering at the critical point temperature, the tempering temperature is 245 - 255 °C and the tempering time is 100 - 120 min.

2. The heat treatment method of the low-carbon low-alloy steel according to claim 1, wherein: The quenching and the tempering at the M S There is carbon partitioning between the critical point temperature tempering, the partitioning temperature is 290 - 310 °C, and the partitioning time is 160 - 200 s.

3. The heat treatment method of the low-carbon low-alloy steel according to claim 1, characterized in that: The austenitizing temperature is 795-805℃.

4. The heat treatment method of the low-carbon low-alloy steel according to claim 1, characterized in that: The furnace cooling temperature is 710-720℃.

5. The heat treatment method of the low-carbon low-alloy steel according to claim 1, characterized in that: The temperature in the diffusion stage is 20-40°C lower than that in the strong penetration stage.

6. The heat treatment method of the low-carbon low-alloy steel according to claim 1, wherein: The quenching adopts sub-temperature quenching, the quenching temperature is 820-830° C., and the heating and heat preservation is 8-12 minutes.

7. The heat treatment method of the low-carbon low-alloy steel according to claim 1, characterized in that: The following steps are included in sequence: Carburizing, spheroidizing annealing, quenching, carbon partitioning, M S Tempering at the critical point temperature; The carburizing process includes a strong carburizing stage and a diffusion stage. The temperature of the strong carburizing stage is 900-920°C, and the strong carburizing potential is 1.1%-1.2%; the temperature of the diffusion stage is 860-880°C, and the diffusion carbon potential is 0.85-0.95%. The spheroidizing annealing adopts a continuous spheroidizing annealing process, the austenitizing temperature is 795-805°C, the austenitizing holding time is 25-35min, then the furnace is cooled to 710-720°C, and the temperature is kept at this temperature for 160-200min for spheroidizing; The quenching adopts sub-temperature quenching, the quenching temperature is 820-830°C, and the heating and heat preservation is 8-12min; In the carbon distribution, the distribution temperature is 290-310°C and the distribution time is 160-200s; The said M S During tempering at the critical point temperature, the tempering temperature is 245 - 255 °C and the tempering time is 100 - 120 min.

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