A 40crni moa steel composite structure, a heat treatment method and application thereof

Abstract

This invention discloses a composite microstructure of 40CrNiMoA steel, its heat treatment method, and its application. The 40CrNiMoA steel composite microstructure comprises martensite, lower bainite, austenite, and bainite, with the bainite comprising 30%-50% by volume. The martensite comprises 80%-95% lath martensite and 5-20% acicular martensite by volume. This invention exhibits high hardness, good toughness, and wear resistance, and also possesses a high elastic modulus, meeting the requirements for use in harmonic reducer flexsplines.

Description

Technical Field

[0001] This invention relates to steel composite microstructure, and more particularly to a 40CrNiMoA steel composite microstructure, its heat treatment method, and its application. Background Technology

[0002] The flexure of a harmonic reducer has a circular curved cross-section. 40CrNiMoA steel, a commonly used medium-carbon alloy high-strength steel, possesses high strength, excellent toughness, and good fatigue resistance, making it widely used in the core component of harmonic reducers—the flexure. The flexure of a harmonic reducer is typically made of materials with high elastic modulus, such as heat-treated steel, stainless steel, or amorphous alloys. Traditional bainitic processes result in composite microstructures with lower matrix hardness and strength.

[0003] Chinese patent CN113564317A discloses a heat treatment method for controlling the microstructure and properties of high-temperature bearing steel. The method includes spheroidizing annealing of 8Cr4Mo4V steel, vacuum isothermal quenching of the spheroidized annealed 8Cr4Mo4V steel, and three tempering treatments of the vacuum isothermal quenched 8Cr4Mo4V steel. This invention patent utilizes vacuum isothermal quenching to control the multiphase microstructure under microthermal (MB) conditions in 8Cr4Mo4V bearing steel, ultimately improving its strength and impact toughness.

[0004] Chinese patent CN102392124B discloses a heat treatment process for improving the strength and toughness of high-speed steel. The process includes pre-treatment of high-speed steel parts by isothermal spheroidizing annealing, followed by isothermal quenching, graded quenching, cryogenic treatment, and tempering as a final heat treatment. The isothermal spheroidizing annealing preheating treatment facilitates machining and prepares the microstructure for subsequent quenching treatment. After the preheating treatment, the final heat treatment is performed, which involves isothermal oil quenching at the lower bainite transformation temperature, followed by cryogenic treatment for 24 hours or more, and then a second cycle treatment: graded quenching at 520-600℃ + cryogenic treatment, and finally tempering at 520-600℃ for 1-2 hours.

[0005] Chinese Patent Publication No. CN115896404A discloses an isothermal bainitic quenching heat treatment process for high-carbon chromium bearing steel bearings, comprising the following steps: Step 1, preheating: preheating the high-carbon chromium bearing steel bearing workpiece with a spheroidizing annealed structure; Step 2, austenitization: performing an austenitizing process on the preheated high-carbon chromium bearing steel bearing workpiece; Step 3, salt bath quenching: performing a salt bath quenching on the austenitized high-carbon chromium bearing steel bearing workpiece, causing it to undergo isothermal transformation in the intermediate temperature range after passing through the nose of the C-curve, thus obtaining a bainitic structure. This invention features the ability to form a bainitic structure on the surface of the bearing inner ring, improving the bearing's fracture strength and impact resistance, and is mainly used for the heat treatment of guide bearings in commercial vehicle axles.

[0006] As can be seen from the above, the heat treatment process of 40CrNiMoA steel has a significant impact on its properties. Optimizing the heat treatment process to achieve a Marbe composite microstructure can improve the overall mechanical properties of 40CrNiMoA steel. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a composite microstructure of 40CrNiMoA steel, its heat treatment method and application, and to improve the comprehensive mechanical properties of 40CrNiMoA steel, in order to address the shortcomings of the existing technology.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a 40CrNiMoA steel composite microstructure, comprising martensite, lower bainite, austenite and bainite, wherein the volume fraction of bainite is 30%-50%, and the martensite comprises 80%-95% lath martensite and 5-20% acicular martensite.

[0009] This invention improves the overall mechanical properties of 40CrNiMoA steel by controlling the volume fraction of bainite and the form in which martensite exists.

[0010] This invention also discloses a heat treatment method for a 40CrNiMoA steel composite microstructure, comprising the following steps:

[0011] S1. Heat treatment is performed on 40CrNiMoA steel;

[0012] S2. Perform an isothermal treatment on the 40CrNiMoA steel after heat treatment in S1.

[0013] S3. Perform a second isothermal treatment on the 40CrNiMoA steel after the first isothermal treatment in S2.

[0014] S4. Tempering treatment is performed on the 40CrNiMoA steel after the secondary isothermal treatment in S3.

[0015] The temperature of the first isotherm is 150-300℃, and the time of the first isotherm is 15-60 seconds.

[0016] This invention employs a two-stage isothermal process, with the temperature of the first isothermal stage being 150-300℃ and the duration of the first isothermal stage being 15-60 seconds. This process controls the volume fraction of bainite and the form in which martensite exists, thereby improving the overall mechanical properties of 40CrNiMoA steel.

[0017] More preferably, the volume fraction of the bainite is 35%-45%.

[0018] The total content of martensite, lower bainite and austenite in this invention is 50%-70%.

[0019] The primary and secondary isothermal treatments refer to isothermal quenching. Isothermal quenching involves maintaining the workpiece at a temperature within the lower bainite transformation zone after quenching and heating, allowing the austenite to undergo an isothermal transformation and resulting in a lower bainite structure. Isothermal quenching is essentially a further development of staged quenching, the difference being that it yields a lower bainite structure. Lower bainite exhibits high strength, hardness, and good toughness; therefore, isothermal quenching can significantly improve the overall mechanical properties of steel. The heating temperature for isothermal quenching is typically higher than that for ordinary quenching to enhance the stability of austenite and increase its cooling rate, preventing pearlite transformation during isothermal cooling.

[0020] Tempering is a technique in steelmaking that involves immersing hardened or normalized steel in a temperature below its critical range for a period of time, followed by rapid cooling to increase the material's toughness. According to metallurgical principles, immersing hardened and normalized steel in a medium-temperature environment (aging) for a period promotes the precipitation of some carbides and eliminates some residual stress caused by rapid cooling, thus improving the material's toughness and flexibility.

[0021] In a preferred embodiment of the present invention, the temperature of the secondary isotherm in S3 is 320-340°C, and the time of the secondary isotherm in S3 is 120-200 min. When the temperature of the secondary isotherm is lower than 320-340°C and the time is longer than 120-200 min, the comprehensive mechanical properties of 40CrNiMoA steel cannot be effectively improved.

[0022] In a preferred embodiment of the present invention, the heating temperature in S1 is 800-900°C.

[0023] In a preferred embodiment of the present invention, the heating time in S1 is 45-75 min.

[0024] In a preferred embodiment of the present invention, the tempering temperature in step S4 is 170-190°C, and the tempering time in step S4 is 100-140 minutes. When the tempering step is omitted, the overall mechanical properties of 40CrNiMoA steel cannot be effectively improved.

[0025] In a preferred embodiment of the present invention, the 40CrNiMoA steel composite microstructure comprises the following chemical composition by mass fraction:

[0026]

[0027] The present invention also discloses a 40CrNiMoA steel composite microstructure obtained by the heat treatment method described above.

[0028] The present invention also discloses the application of the 40CrNiMoA steel composite structure in the preparation of flexible wheels.

[0029] Compared to traditional bainitic processes, this invention produces a composite microstructure with higher matrix hardness and strength. Before the research, the microstructure of the raw materials was analyzed, and the TTT curve was simulated using software. The theoretical Ms point was measured, and experiments were conducted using reasonable process parameters. Finally, through this process method and its influence on mechanical properties, a martensite composite microstructure with high strength and good impact toughness was obtained. Because the composite microstructure combines the advantages of each material, the martensite composite microstructure involves a brief stay in the martensitic region after quenching, forming some lath martensite, followed by the transformation of the high-carbon portion of the austenite into bainite. In engineering, there is also a corresponding process scheme for bainite-martensite composite microstructures. Both belong to composite microstructures, but in bainite-martensite composite microstructures, the martensite is mostly large, high-carbon twinned substructure martensite. Furthermore, the large, high-carbon martensite is prone to internal stress and microcracks during martensite lamination, resulting in greater brittleness. This invention designs the above experimental method based on the engineering characteristics and application conditions of 40CrNiMoA steel used in the flexible wheel of harmonic reducers.

[0030] Compared with existing technologies, the beneficial effects of this invention are as follows: After undergoing this heat treatment process, the material possesses high hardness, good toughness, and wear resistance, as well as a high elastic modulus, meeting the requirements for use in harmonic reducer flexures. 40CrNiMoA steel, a commonly used mechanical structural steel, exhibits a Mapey composite structure with excellent strength and toughness through optimized heat treatment parameters in this process. The material's properties and microstructure were analyzed using an electronic universal testing machine and metallographic microscopy. The results show that this method can effectively improve the comprehensive performance of 40CrNiMoA steel used in harmonic reducer flexures, providing effective engineering support for performance enhancement in the robot harmonic reducer industry. Attached Figure Description

[0031] Figure 1 The microstructure of 40CrNiMoA steel obtained in Examples 1-2 and Comparative Examples 1-2 of this invention is shown. Detailed Implementation

[0032] The sample used in this experiment was 40CrNiMoA steel, initially in a normalized state, and its chemical composition is shown in Table 1:

[0033] Table 1. Chemical composition (mass fraction, %) of 40CrNiMoA steel used in the experiment.

[0034]

[0035] Example 1

[0036] A heat treatment method for a 40CrNiMoA steel composite microstructure includes the following steps:

[0037] S1. Heat treatment is performed on 40CrNiMoA steel; the heating temperature is 850±10℃ and the heating time is 60min.

[0038] S2. Perform a first isothermal test on the 40CrNiMoA steel after heat treatment in S1; the temperature of the first isothermal test is 180±10℃, and the time of the first isothermal test is 30sec.

[0039] S3. Perform a second isothermal treatment on the 40CrNiMoA steel after the first isothermal treatment in S2; the temperature of the second isothermal treatment is 330±10℃, and the time of the second isothermal treatment is 180min.

[0040] S4. The 40CrNiMoA steel after the secondary isothermal treatment in S3 is subjected to tempering treatment; the tempering temperature is 180±10℃, and the tempering is repeated twice for 120 minutes to obtain 1. # 40CrNiMoA steel sample.

[0041] Example 2

[0042] A heat treatment method for a 40CrNiMoA steel composite microstructure includes the following steps:

[0043] S1. Heat treatment is performed on 40CrNiMoA steel; the heating temperature is 850±10℃ and the heating time is 60min.

[0044] S2. Perform a first isothermal test on the 40CrNiMoA steel after heat treatment in S1; the temperature of the first isothermal test is 280±10℃, and the time of the first isothermal test is 30sec.

[0045] S3. Perform a second isothermal treatment on the 40CrNiMoA steel after the first isothermal treatment in S2; the temperature of the second isothermal treatment is 330±10℃, and the time of the second isothermal treatment is 180min.

[0046] S4. The 40CrNiMoA steel after the secondary isothermal treatment in S3 is subjected to tempering treatment; the tempering temperature is 180±10℃, and the tempering is repeated twice for 120 minutes to obtain 2. # 40CrNiMoA steel sample.

[0047] Comparative Example 1

[0048] A heat treatment method for a 40CrNiMoA steel composite microstructure includes the following steps:

[0049] S1. Heat treatment is performed on 40CrNiMoA steel; the heating temperature is 850±10℃ and the heating time is 60min.

[0050] S2. Perform a first isothermal test on the 40CrNiMoA steel after heat treatment in S1; the temperature of the first isothermal test is 330±10℃, and the time of the first isothermal test is 10min.

[0051] S3. Perform a second isothermal treatment on the 40CrNiMoA steel after the first isothermal treatment in S2; the temperature of the second isothermal treatment is 180±10℃, and the time of the second isothermal treatment is 240min.

[0052] S4. The 40CrNiMoA steel after the secondary isothermal treatment in S3 is subjected to tempering treatment; the tempering temperature is 180±10℃, and the tempering is repeated twice for 120 minutes to obtain 3. # 40CrNiMoA steel sample.

[0053] Comparative Example 2

[0054] A heat treatment method for a 40CrNiMoA steel composite microstructure includes the following steps:

[0055] S1. Heat treatment is performed on 40CrNiMoA steel; the heating temperature is 850±10℃ and the heating time is 60min.

[0056] S2. The 40CrNiMoA steel after heat treatment in S1 is subjected to a first isothermal treatment; the temperature of the first isothermal treatment is 330±10℃, and the time of the first isothermal treatment is 240min, thus obtaining 2. # 40CrNiMoA steel sample.

[0057] The heat treatment processes used for the 40CrNiMoA steel samples obtained in Examples 1-2 and Comparative Examples 1-2 are summarized in Table 2:

[0058] Table 2 Heat treatment process parameters of 40CrNiMoA steel used in the experiment

[0059]

[0060] 1. Performance Testing

[0061] The 40CrNiMoA steel samples prepared in Examples 1-2 and Comparative Examples 1-2 were subjected to hardness tests, tensile tests, impact tests, and microstructure observations, respectively.

[0062] 1.1 Hardness Test

[0063] Hardness is not a single physical quantity as commonly understood; it is related to a material's strength and plasticity, and its mechanical properties largely depend on its hardness. Hardness is a material's ability to resist plastic deformation and failure under external forces, and it is one of the important indicators for evaluating the performance of metallic materials. The hardness of this invention conforms to GB / T...

[0064] The test was conducted according to Method 230.1-2009, using a digital Rockwell hardness tester (200HRS-150). To ensure the accuracy of the test, the sample was first sanded to make the surface smooth. Two samples were used in each group, and each sample was tested four times. The average value of the eight test points was taken as the hardness value of the sample in the corresponding process.

[0065] 1.2 Tensile Test

[0066] Standard specimens were selected for tensile testing, and the test method was performed according to GB / T 228.1-2010. An electronic universal testing machine (TH-8110S) with a rated load of 100KN was used. The original diameter of the circular specimen was 5mm and the original gauge length was 35mm. Four specimens were selected for tensile testing in each process group, and the average value of the four results was taken.

[0067] 1.3 Impact Test

[0068] Impact test specimens were tested according to GB / T 229—2007 "Metallic Materials - Charpy Pendulum Impact Test Method" using a Charpy impact testing machine (JB-300) at room temperature. The model used was 10mm×10mm×55mm. The impact specimen notch was U-shaped with a notch size of 2mm. To meet the surface roughness requirements, the specimen surface was ground on a surface grinder to achieve a final surface finish of Ra≤0.6μm. Four specimens were selected for each process, and the impact toughness value was taken as the average of the four results.

[0069] 1.4 Microscopic Tissue Observation

[0070] Metallographic samples were ground and polished with sandpaper (grit from 180 to 2000) after heat treatment. To distinguish between bainite and martensite and estimate their corresponding volume fractions, a 10 vol.% sodium metabisulfite aqueous solution was used during etching. The microstructure was then observed using an optical microscope (LEICA DM18) at four fields of view (100X, 200X, 500X, and 1000X) for each process. The metallographic images used in this invention are at 1000X. The volume fraction of bainite was then analyzed using software and calculated based on the average value.

[0071] 2 Results and Discussion

[0072] 2.1 Hardness Results and Discussion

[0073] The heat-treated samples were subjected to hardness tests, and the test results are shown in Table 3.

[0074] Table 3 Hardness results of 40CrNiMoA steel after different processing methods

[0075]

[0076] As shown in Table 3: Process 4 # The hardness of the sample with conventional bainitic structure was 45.7 HRC, while the hardness of the sample with Marbe microstructure was 1. # and process 2 # The hardness increased by 1.4 and 0.8 respectively. This is because after austenitization, the microstructure preferentially generates a small amount of lath martensite through quenching at different temperatures within a short period of time, which improves the hardness.

[0077] 2.2 Mechanical property results and analysis

[0078] The heat-treated samples were subjected to performance tests, and the test results are shown in Table 4.

[0079] Table 4. Mechanical properties of 40CrNiMoA steel after different processing methods

[0080]

[0081] Theoretically, the smaller the yield strength ratio of a material, the greater its elongation after fracture, the greater its reduction of area, and the better its plasticity. Experimental data shows that: Process 1 # Process 2 # and process 3 # After treatment with different heat treatment processes, the yield strength ratio did not change significantly, with process 1 showing the best results. # and process 2 # Elongation and reduction of area have been improved, Process 1 #The elongation was 14%, and the reduction of area was 61.9%; Process 2 # The elongation is 14.5% and the reduction of area is 62%, therefore process 1 # and process 2 # The plasticity of both processes was improved. Process 1... # The yield strength ratio was relatively low at 0.84, while the yield strength ratios of the other experimental groups were improved. (Process 2) # Process 3 # and process 4 # The yield strength ratios of the materials are 0.849, 0.853, and 0.905, respectively. Process 2 # Process 3 # The tensile strength of the materials is 1504 MPa, and the yield strengths are 1284 MPa and 1272 MPa, respectively. The impact toughness also shows significant differences, at 72.4 J and 59.9 J, respectively. Under similar strength conditions, process 3... # The worst impact toughness performance; Process 1 # Process 2 # The impact toughness was 73.6% and 72.4% respectively, with improved plasticity. Process 1# had a tensile strength of 1546 MPa and a yield strength of 1299 MPa, higher than the other groups. Therefore, Process 1#... # It exhibits good impact toughness at higher strengths.

[0082] 2.3 Effects of different heat treatment processes on microstructure

[0083] Microstructure composition has an important influence on the mechanical properties of metallic materials. By strictly controlling the phase and microstructure composition, Dimitry V Bubnoff et al. [1] established a martensite formation kinetic model using the end-quenching method. In order to accurately grasp the amount of martensite and bainite transformation after complete austenitization and control the relationship between cooling rate and time, the influence of different heat treatment processes on microstructure was analyzed by controlling the proportion of martensite and bainite phases.

[0084] Figure 1 # 2 # 3 # 4 # The corresponding test samples were from four different processes. The microstructure of 40CrNiMoA steel under different processes was observed by metallographic microscope. The overall microstructure was uniform and fine. Han Bo et al. [2] used binary analysis to calculate the bainite phase content in the study of color metallography and used the image processing system of metallographic microscope to calculate the volume fraction of bainite. In this experiment, 10 fields of view were selected for each process. The volume fraction of bainite was calculated for each field of view, so as to obtain the average value of each group of samples under the corresponding field of view and calculate the volume fraction of bainite. Figure 1The image shown is a standard metallographic image. The brown areas represent martensite, the blue areas represent bainite, and the white areas represent martensite and retained austenite. After different processing steps, the results for Process 1 are calculated. # The lower bainite content is 36.69%, process 2 # The lower bainite content is 40.51%, process 3 # The lower bainite content is 26.57%, process 4 # The lower bainite content is 71.03%. Figure 1 It can be seen that: Process 1 # and process 2 # The matrix structure mainly consists of lath-like and a small amount of acicular martensite, with the remainder being lower bainite and a small amount of retained austenite. Based on the isothermal transformation TTT curve analysis, the sample first forms lath-like martensite, and then, through isothermal transformation, the high-carbon portion of the austenite transforms into bainite. Process 1 # and process 2 # The martensite laths are fine, the carbides are evenly distributed, and no non-metallic inclusions, banded segregation, or other structural defects were found. After tempering, the retained austenite showed no significant change, and the lath-like morphology of the tempered martensite was well preserved. It is generally believed that this retained austenite exists as a nano-scale thin film between the lath martensite. Therefore, this martensite composite structure has good strength and impact toughness, which can greatly improve the mechanical properties of the steel. Process 3 # The bainitic region is isothermal, while the subsequent martensite formation is mostly large, plate-like, high-carbon twinned substructure martensite. This structure has relatively concentrated stress and is prone to martensite lattice distortion. If the phase transformation stress is not released in time, microcracks are easily generated, resulting in extreme brittleness and reduced impact toughness of the steel. Process 4... # The bainite phase content in this sample was much higher than in other samples, and the impact toughness results showed that process 4... # The impact toughness is lower than that of process 1. # and process 2 # It is also considered that the formation of a large amount of bainite first, followed by the formation of a certain amount of high-carbon twinned martensite, reduces impact toughness. Process 4... # Its impact toughness is superior to that of process 3. # This indicates that bainite is formed simultaneously, and the proportion of the bainite phase has a certain influence on impact toughness.

[0085] Compared with the traditional bainitic process, the Marbe microstructure (Marbe microstructure process 1) # and process 2 # Alternatively, the Marpe-Mabite microstructure can exhibit better hardness and strength. Compared to traditional bainitic microstructures, the Marpe-Mabite composite microstructure offers improved overall mechanical properties, which is of significant engineering importance for the use of flexible gears in harmonic reducers.

Claims

1. A composite microstructure of 40CrNiMoA steel, characterized in that... It includes martensite, lower bainite, and austenite, wherein the volume fraction of lower bainite is 30%-50%, and the martensite includes 80%-95% lath martensite and 5-20% acicular martensite. The heat treatment method for the 40CrNiMoA steel composite microstructure includes the following steps: S1. Heat treatment is performed on 40CrNiMoA steel; S2. Perform an isothermal treatment on the 40CrNiMoA steel after heat treatment in S1. S3. Perform a second isothermal treatment on the 40CrNiMoA steel after the first isothermal treatment in S2. S4. Tempering treatment is performed on the 40CrNiMoA steel after the secondary isothermal treatment in S3. The temperature of the first isotherm is 150-300℃, and the time of the first isotherm is 15-30 seconds; The temperature of the secondary isotherm described in S3 is 320-340℃; The secondary isothermal time mentioned in S3 is 120-200 min.

2. The 40CrNiMoA steel composite microstructure according to claim 1, characterized in that, The heating temperature described in S1 is 800-900℃.

3. The 40CrNiMoA steel composite microstructure according to claim 1, characterized in that, The heating time described in S1 is 45-75 minutes.

4. The 40CrNiMoA steel composite microstructure according to claim 1, characterized in that, The tempering temperature in S4 is 170-190℃, and the tempering time is 100-140 min.

5. The 40CrNiMoA steel composite microstructure according to claim 1, characterized in that, The 40CrNiMoA steel composite microstructure comprises the following chemical composition by mass fraction: 。 6. The application of the 40CrNiMoA steel composite structure as described in claim 1 in the preparation of flexible wheels.

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