Heat treatment process for improving microstructure and mechanical properties of 42CrMo steel

Abstract

The application discloses a heat treatment process for improving the structure and mechanical properties of 42CrMo steel and belongs to the technical field of heat treatment of alloy structural steel. The application proposes a whole-chain process of "quenching + precise tempering synergistic regulation" aiming at the core pain points of the traditional process, obtains a uniform lath martensite structure through specific temperature full austenitization + water quenching, promotes full decomposition of the martensite through precise tempering, changes the lamellar cementite into granular dispersion precipitation, releases internal stress (KAM value is reduced), improves the proportion of high-angle grain boundaries, and synchronously maintains high strength and excellent toughness. The process is suitable for existing heat treatment equipment, has high standardization degree, and can be directly applied to industrial batch production.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for alloy structural steel, and particularly relates to a heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel. It is applicable to the manufacture of heavy-duty gears in mining machinery, speed reducers, wind turbine main units, and other fields, and can be widely used in the production of large gears, shafts, and other components requiring high strength and toughness. Background Technology

[0002] 42CrMo steel, due to its excellent hardenability and creep resistance, has become a core material for heavy-duty machinery. However, traditional processes have significant drawbacks: improper tempering temperatures lead to uneven, lamellar distribution of cementite, which tears the matrix; there is an imbalance between strength and toughness, either the strength meets the requirements but the impact toughness is insufficient (≤70J), or the strength decreases after the plasticity is improved; it is difficult to meet the service requirements of heavy-duty gears in mining machinery that bear high loads and complex working conditions. Existing technologies lack precise tempering schemes for this steel under heavy-duty conditions, and there is an urgent need to optimize the process to achieve uniform microstructure and synergistic improvement in strength and toughness. Summary of the Invention

[0003] The purpose of this invention is to provide a heat treatment process to improve the microstructure and mechanical properties of 42CrMo steel. Addressing the core pain points of traditional processes, this invention proposes a full-chain process of "quenching + precise tempering synergistic control." Through sufficient austenitization at a specific temperature followed by water quenching, a uniform lath martensite microstructure is obtained. Precise tempering then promotes the full decomposition of martensite, causing cementite to transform from lamellar to granular and dispersed precipitation, releasing internal stress (reducing the KAM value), increasing the proportion of large-angle grain boundaries, and simultaneously maintaining high strength and excellent toughness. This process is compatible with existing heat treatment equipment, has a high degree of standardization, and can be directly applied to industrial mass production.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel, comprising:

[0006] I. Substrate Pretreatment

[0007] (1) Substrate selection

[0008] 42CrMo alloy structural steel is selected, and its chemical composition by mass fraction meets the following requirements: C 0.38%~0.45%, Si 0.17%~0.37%, Mn 0.50%~0.80%, Cr 0.90%~1.20%, Mo 0.15%~0.25%, P≤0.030%, S≤0.030%, balance Fe and unavoidable impurities. The base material is hot-rolled plate or forging.

[0009] (2) Preprocessing operations

[0010] Surface cleaning: Remove oxide scale, oil and impurities from the substrate surface, degrease with 4% nitric acid alcohol solution, and dry at 60℃ for 1 hour;

[0011] Material cutting: Cut the blank according to the size of the part to avoid overheating during cutting and generating secondary stress;

[0012] Preheating treatment: Preheat at 200~250℃ for 30 minutes to reduce quenching stress and deformation risk;

[0013] II. Austenitic quenching

[0014] (1) Equipment selection

[0015] It adopts a box-type resistance furnace, equipped with a water quenching cooling device, and has a temperature control accuracy of ±5℃;

[0016] (2) Quenching process parameters

[0017] Heating: Increase to 850℃±10℃ at a rate of 4-6℃ / min to match the phase transformation characteristics of the steel.

[0018] Heat preservation: Hold for 25-35 minutes to ensure complete austenitization and uniform solid solution of carbon and alloying elements;

[0019] Cooling: After heat preservation, the material is rapidly quenched in water to room temperature at a cooling rate of ≥20℃ / s to obtain lath martensite + a small amount of lower bainite structure.

[0020] III. Precise Tempering

[0021] (1) Tempering process parameters

[0022] Heating: Place the quenched workpiece into a box furnace and heat it to 570℃±10℃ at a rate of 2.5-3.5℃ / min;

[0023] Insulation: Insulate for 2.5-3.5 hours to promote the full decomposition of martensite and the morphological transformation of cementite, and release residual stress;

[0024] Cooling: Air cool to room temperature to avoid rapid cooling that could generate new thermal stress;

[0025] (2) Critical Control

[0026] During the tempering process, the furnace temperature fluctuation should be ≤±5℃ to prevent abnormal agglomeration of cementite or insufficient tempering.

[0027] Furthermore, the substrate reference dimensions are 300mm × 150mm × 12mm.

[0028] Furthermore, the quenching is performed by heating to 850°C at a rate of 5°C / min.

[0029] Furthermore, the quenching process involves holding the temperature for 30 minutes.

[0030] Furthermore, the tempering process involves heating to 570°C at a rate of 3°C / min.

[0031] Furthermore, the tempering process involves holding the temperature for 3 hours.

[0032] Furthermore, post-processing is also included:

[0033] Surface cleaning: Remove oxide scale and deposits from the surface after tempering;

[0034] Finishing: Turning and grinding are performed on gears and shafts to ensure that dimensional tolerances and surface roughness meet design requirements;

[0035] Non-destructive testing: Ultrasonic testing is used to ensure there are no internal cracks or inclusions.

[0036] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0037] Significantly optimized microstructure: a typical tempered sorbite microstructure is formed, the cementite changes from lamellar to granular and uniformly dispersed, without aggregation, the microstructure uniformity is improved by 40%, and the internal strain is effectively released.

[0038] Synergistic effect of strength and toughness: tensile strength of 1104MPa and yield strength of 1009MPa meet the strength requirements of heavy-duty gears; elongation of 14.0% and impact absorption energy of 86J, with a 21% improvement in toughness compared to tempering at 550℃, achieving the optimal balance of strength, plasticity and toughness.

[0039] Strong adaptability to working conditions: It has outstanding impact resistance and fracture resistance, and can withstand the high load and complex stress conditions of mining machinery, extending the service life of components by more than 1.5 times.

[0040] Stable and controllable process: Based on existing box furnaces and water quenching equipment, the temperature control accuracy is ±5℃, and the performance consistency of different batches of products is ≥98%, which is suitable for the mass production of thick plates and complex-shaped parts.

[0041] Simple and efficient operation: No new special equipment is required, process parameters are standardized, production costs are controllable, and production efficiency is increased by 20% compared with traditional processes.

[0042] Tissue testing:

[0043] Observation using optical microscope (OM) and scanning electron microscope (SEM): The tissue is typical tempered sorbite, with cementite distributed in a granular and diffuse manner, and no platy carbide residue.

[0044] Electron backscatter diffraction (EBSD) analysis: No obvious texture orientation, significantly increased proportion of large-angle grain boundaries (15°~45°), decreased KAM value, and sufficient release of internal strain.

[0045] Mechanical property testing:

[0046] Tensile test (GB / T228.1-2021): Tensile strength ≥1100MPa, yield strength ≥1000MPa, elongation ≥14%;

[0047] Impact test (GB / T229-2020): Impact absorbed energy ≥85J;

[0048] Hardness testing (GB / T230.1-2018): Rockwell hardness 32~35HRC;

[0049] Meets the mechanical performance requirements of heavy-duty gears for mining machinery (tensile strength ≥ 930 MPa, yield strength ≥ 785 MPa, elongation ≥ 12%, impact absorption energy ≥ 63 J). Detailed Implementation

[0050] The core of this invention lies in the "synergistic effect of precise tempering at 570℃": at this temperature, it promotes the full decomposition of martensite, enabling cementite to transform from lamellar to granular form and avoiding matrix fragmentation; it also inhibits excessive grain coarsening, enhancing crack propagation resistance through a higher proportion of large-angle grain boundaries, ultimately achieving a synergistic effect of strength and toughness. All parameters of this process are verified based on industrial test data, and the holding time can be adjusted according to the thickness of the component (for every 10mm increase in thickness, the tempering holding time is extended by 30 minutes).

[0051] Example

[0052] Example 1 (Optimal process: tempering at 570℃)

[0053] The chemical composition of the selected 42CrMo alloy structural steel by mass fraction is: C 0.40%, Si 0.27%, Mn 0.65%, Cr 1.05%, Mo 0.20%, P 0.020%, S 0.020%, balance Fe and unavoidable impurities;

[0054] Substrate pretreatment: 42CrMo steel hot-rolled plate, preheated at 220℃ for 30 minutes after surface cleaning;

[0055] Austenitizing quenching: Hold at 850℃ for 30 min, then water quench and cool to room temperature;

[0056] Precision tempering: Hold at 570℃ for 3 hours, then air cool to room temperature;

[0057] Post-processing: Surface cleaning + precision grinding + ultrasonic testing;

[0058] Performance testing:

[0059] Microstructure: Typical tempered sorbite with granular cementite dispersed in a diffuse manner;

[0060] Mechanical properties: tensile strength 1104 MPa, yield strength 1009 MPa, elongation 14.0%;

[0061] Impact performance: Impact energy absorbed: 86J;

[0062] Hardness: 33 HRC;

[0063] Microstructure characteristics: High proportion of large-angle grain boundaries and small internal strain.

[0064] Comparative examples (processes with different tempering temperatures)

[0065] 550 (Traditional) 1138 1047 12.8 71 It is mainly in the form of flakes, with uneven distribution. 34 570 (This invention) 1104 1009 14.0 86 Granular, diffusely distributed 33 590 (Traditional) 1043 945 16.5 82 Granular, with coarsened grains 32

[0066] Performance Comparison Table:

[0067] Tensile strength (MPa) 1104 1138 1043 -2.9% (moderate decline) Yield strength (MPa) 1009 1047 945 -3.6% (moderate decline) Elongation (%) 14.0 12.8 16.5 +9.4% Impact absorbed energy (J) 86 71 82 +21.1% uniformity of carbide distribution Excellent (dispersed particles) Poor (uneven in shape) Medium (grainy + coarse-grained) - Proportion of large-angle grain boundaries (%) higher medium high +15% Service life under heavy load conditions (relative value) 2.5 1.0 1.8 +150%

[0068] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel, characterized in that, include: I. Substrate Pretreatment (1) Substrate selection 42CrMo alloy structural steel is selected, and its chemical composition by mass fraction meets the following requirements: C 0.38%~0.45%, Si 0.17%~0.37%, Mn 0.50%~0.80%, Cr 0.90%~1.20%, Mo 0.15%~0.25%, P≤0.030%, S≤0.030%, balance Fe and unavoidable impurities. The base material is hot-rolled plate or forging. (2) Preprocessing operations Surface cleaning: Remove oxide scale, oil and impurities from the substrate surface, degrease with 4% nitric acid alcohol solution, and dry at 60℃ for 1 hour; Material cutting: Cut the blank according to the size of the part to avoid overheating during cutting and generating secondary stress; Preheating: Preheat at 200~250℃ for 30 minutes to reduce quenching stress and deformation risk; II. Austenitic quenching (1) Equipment selection It adopts a box-type resistance furnace, equipped with a water quenching cooling device, and has a temperature control accuracy of ±5℃; (2) Quenching process parameters Heating: Increase to 850℃±10℃ at a rate of 4-6℃ / min to match the phase transformation characteristics of the steel. Heat preservation: Hold for 25-35 minutes to ensure complete austenitization and uniform solid solution of carbon and alloying elements; Cooling: After heat preservation, the material is rapidly quenched in water to room temperature at a cooling rate of ≥20℃ / s to obtain lath martensite + a small amount of lower bainite structure. III. Precise Tempering (1) Tempering process parameters Heating: Place the quenched workpiece into a box furnace and heat it to 570℃±10℃ at a rate of 2.5-3.5℃ / min; Insulation: Insulate for 2.5-3.5 hours to promote the full decomposition of martensite and the morphological transformation of cementite, and release residual stress; Cooling: Air cool to room temperature to avoid rapid cooling that could generate new thermal stress; (2) Critical Control During the tempering process, the furnace temperature fluctuation should be ≤±5℃ to prevent abnormal agglomeration of cementite or insufficient tempering.

2. The heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel according to claim 1, characterized in that, The base material has a reference size of 300mm × 150mm × 12mm.

3. The heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel according to claim 1, characterized in that, The quenching process involves heating the temperature to 850°C at a rate of 5°C / min.

4. The heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel according to claim 1, characterized in that, The quenching process involves holding the temperature for 30 minutes.

5. The heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel according to claim 1, characterized in that, The tempering process involves heating the temperature to 570°C at a rate of 3°C / min.

6. The heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel according to claim 1, characterized in that, The tempering process involves holding the temperature for 3 hours.

7. The heat treatment process for improving the microstructure and mechanical properties of 42CrMo steel according to claim 1, characterized in that, It also includes post-processing: Surface cleaning: Remove oxide scale and deposits from the surface after tempering; Finishing: Turning and grinding are performed on gears and shafts to ensure that dimensional tolerances and surface roughness meet design requirements; Non-destructive testing: Ultrasonic testing is used to ensure there are no internal cracks or inclusions.