A composite heat treatment method for 40CrNiMoA steel rods

Abstract

The present application belongs to the technical field of metal material heat treatment, and discloses a composite heat treatment method of 40CrNiMoA steel rod, which comprises the following steps: S1: sectional step preheating treatment; S2: austenitizing homogenization treatment; S3: graded quenching cooling control; S4: circulating graded tempering treatment; S5: deep cooling treatment and aging strengthening; and S6: surface nanocrystallization grinding treatment. The present application adopts 120 DEG C hot oil graded quenching process, combines with 580 DEG C→520 DEG C→480 DEG C three-stage circulating tempering, finally carries out -80 DEG C deep cooling treatment and mechanical grinding surface nanocrystallization, so that the tensile strength of the 40CrNiMoA steel rod is greater than or equal to 1100 MPa, the yield strength is greater than or equal to 950 MPa, the reduction of area is greater than or equal to 55%, the impact energy is good, the surface hardness reaches HRC 38-42, the core toughness is good, and the present application is especially suitable for manufacturing heavy load gears, crankshafts, connecting rods and other high-end mechanical parts.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and specifically relates to a composite heat treatment method for 40CrNiMoA steel bars. Background Technology

[0002] 40CrNiMoA steel is a typical low-carbon alloy structural steel, belonging to the Cr-Ni-Mo series of quenched and tempered steels, possessing excellent comprehensive mechanical properties. This steel contains multiple alloying elements such as Cr, Ni, and Mo, exhibiting high hardenability, a good balance of strength and toughness, and excellent fatigue resistance and impact resistance. It is widely used in aerospace, automotive, engineering machinery, and petrochemical industries to manufacture key components such as heavy-duty gears, engine crankshafts, connecting rods, high-strength bolts, and drive shafts.

[0003] However, the traditional heat treatment process for 40CrNiMoA steel has the following technical defects: (1) The preheating process is simple and crude: conventional processes usually use a single temperature for preheating, which can easily generate large thermal stress, especially for large diameter steel bars. During the heating process, the temperature difference between the inside and outside is large, which can easily generate internal micro-cracks and affect the fatigue performance of the material.

[0004] (2) Insufficient control of quenching cooling: Traditional processes mostly use water quenching or oil quenching, and the cooling rate is difficult to control precisely. If the water quenching cooling rate is too fast, it is easy to generate quenching cracks and large residual stress; if the ordinary oil quenching cooling rate is insufficient, it is difficult to obtain sufficient martensite structure, which affects the final strength.

[0005] (3) Single tempering process: Conventional process uses single high-temperature tempering, which can obtain a certain toughness, but it is difficult to achieve the best match between strength and toughness, and the residual stress is not completely eliminated, affecting the dimensional stability of the parts.

[0006] (4) Residual austenite problem: 40CrNiMoA steel contains a lot of alloying elements and has a low Ms point. After quenching, there is a lot of residual austenite. During use, the structure will change, resulting in changes in part size and unstable performance.

[0007] (5) Insufficient surface properties: After conventional heat treatment, the surface and core properties of steel are not significantly different. For parts that require wear-resistant surfaces and tough cores, additional surface treatment processes are often required, which increases manufacturing costs.

[0008] In recent years, although some scholars have made some improvements to the heat treatment process of 40CrNiMoA steel, problems such as complex processes, high costs, and limited performance improvement still exist. Therefore, developing a new heat treatment process that can significantly improve the comprehensive mechanical properties of 40CrNiMoA steel, with stable processing and controllable costs, has important engineering application value. Summary of the Invention

[0009] To address the aforementioned technical problems, this invention provides a composite heat treatment method for 40CrNiMoA steel bars. Through a composite process of segmented stepped preheating, graded quenching and cooling, cyclic graded tempering, deep cryogenic treatment, and surface nano-grinding, the strength, toughness, wear resistance, and dimensional stability of 40CrNiMoA steel bars are simultaneously improved.

[0010] The technical solution adopted in this invention is a composite heat treatment method for 40CrNiMoA steel bars, comprising the following steps: S1: Place the 40CrNiMoA steel bar in a heating furnace for segmented step preheating; S2: Heat the preheated steel bar to 860℃±10℃ at a heating rate of 30~40℃ / h, hold for 120~180min, control the furnace atmosphere to be a protective atmosphere, and control the carbon potential at 0.40%~0.45%; S3: Transfer the steel bar treated in step S2 to a hot oil quenching tank at a temperature of 120℃±5℃ within 15s for quenching. The stirring speed of the oil is controlled at 0.8~1.2m / s. After holding at the temperature for 15~25min, take it out and air cool to room temperature. S4: The quenched steel bar is subjected to cyclic stage tempering; S5: The tempered steel bar is placed in a cryogenic device for cryogenic treatment. It is cooled to -80℃±5℃ at a cooling rate of 5~10℃ / min and held for 120~180min. Then it is heated back to room temperature at a heating rate of 3~5℃ / min and aged at 160℃±10℃ for 180~240min. S6: The steel bar is subjected to high-speed grinding, with the grinding line speed controlled at 15-20m / s and the feed rate at 0.02-0.05mm / r, forming a nanocrystalline reinforcement layer with a thickness of 50-100μm on the surface of the steel bar.

[0011] Furthermore, the segmented step preheating is specifically as follows: first, heat to 600℃±10℃ at a heating rate of 80~100℃ / h and hold for 60~90min; then continue heating to 680℃±10℃ at a heating rate of 50~60℃ / h and hold for 90~120min.

[0012] Furthermore, for steel bars with a diameter greater than 80mm, the heat preservation time at each stage is extended by 30 minutes for every 20mm increase in diameter.

[0013] Furthermore, the protective atmosphere in S2 is a nitrogen-based protective atmosphere, wherein the nitrogen content is ≥95%, the propane content is 3% to 5%, and the balance is hydrogen.

[0014] Furthermore, the heat transfer oil in S3 is L-QC320 type heat transfer oil.

[0015] Furthermore, S4 specifically involves: first heating the quenched steel bar to 580℃±10℃, holding it at that temperature for 120~150min, and then oil-cooling it to room temperature; then heating it to 520℃±10℃, holding it at that temperature for 90~120min, and then air-cooling it; finally heating it to 480℃±10℃, holding it at that temperature for 180~240min, and then furnace-cooling it to 200℃ before air-cooling it.

[0016] Furthermore, after the first tempering, the oil cooling temperature drops below 100°C before a second tempering is performed.

[0017] Furthermore, the cryogenic treatment medium is a mixture of liquid nitrogen and alcohol as the cold source, and the hard abrasive is a mixture of boron carbide and alumina with a particle size of W20~W40.

[0018] Furthermore, the steel bar has a tensile strength ≥1100MPa, a yield strength ≥950MPa, a reduction of area ≥55%, and an impact energy of ≥1100MPa. The surface hardness is HRC 38~42, and the thickness of the surface nanocrystalline layer is 50~100μm.

[0019] The beneficial effects of this invention are: (1) This invention uses a composite heat treatment process to significantly improve toughness while maintaining high strength, solving the problem of "strength and toughness contradiction" in traditional processes and achieving simultaneous improvement of strength and toughness.

[0020] (2) The multiple effects of segmented preheating, graded quenching, cyclic tempering and deep cryogenic aging in this invention reduce residual stress by more than 80% and greatly improve the dimensional stability of parts.

[0021] (3) The present invention improves the surface hardness by 15% to 20% and the wear resistance by more than 30% through surface nano-grinding, and significantly improves the fatigue resistance.

[0022] (4) Compared with traditional processes, the method of the present invention reduces energy consumption, requires no additional chemical surface treatment, meets the requirements of green manufacturing, and is more energy-saving and environmentally friendly. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0025] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0026] like Figure 1 As shown, this invention provides a composite heat treatment method for 40CrNiMoA steel bars, the steps of which are as follows: (1) Segmented stepped preheating treatment: After surface cleaning, the 40CrNiMoA steel bar is placed in a bogie-type heating furnace. First, it is slowly heated to 600℃±10℃ at a heating rate of 80~100℃ / h and held for 60~90 minutes to ensure uniform temperature throughout the bar and eliminate cold working stress. Then, it is heated further at a slower heating rate of 50~60℃ / h to 680℃±10℃ and held for 90~120 minutes to complete the preliminary transformation of pearlite to austenite and reduce thermal stress during subsequent high-temperature heating. For large-diameter steel bars greater than 80mm, the holding time at each stage is extended by 30 minutes for every 20mm increase in diameter.

[0027] Technical principle: Segmented stepped preheating is adopted to avoid the huge thermal stress caused by rapid heating. The 600℃ holding is mainly used for stress release and temperature homogenization, while the 680℃ holding completes the microstructure preparation below the Ac1 point, creating uniform initial conditions for subsequent austenitization and effectively preventing the generation of heating cracks.

[0028] (2) Austenitizing and homogenization treatment: After preheating, continue heating at a rate of 30~40℃ / h to 860℃±10℃ and hold for 120~180min. During heating, a nitrogen-based protective atmosphere is introduced, with a nitrogen content ≥95%, a propane content of 3%~5%, and the remainder being hydrogen. The carbon potential in the furnace is controlled at 0.40%~0.45% to prevent decarburization and oxidation of the steel bar surface.

[0029] Technical principle: 860℃ is the optimal austenitizing temperature for 40CrNiMoA steel, ensuring sufficient dissolution of alloy carbides while preventing excessive austenite grain growth. Precise carbon potential control effectively prevents surface decarburization, ensuring consistency in composition and properties between the surface and core of the steel bar.

[0030] (3) Staged quenching and cooling control: After the austenitizing heat treatment is completed, the steel bar is quickly transferred to an L-QC320 type heat transfer oil quenching tank that has been preheated to 120℃±5℃ within 15 seconds. The stirring speed of the oil is controlled at 0.8~1.2m / s. After heat treatment for 15~25 minutes, the bar is taken out and air-cooled to room temperature.

[0031] Technical Principle: The core innovation of this invention lies in the 120℃ hot oil staged quenching. Isothermal holding near the Ms point reduces thermal and structural stress during quenching, significantly lowering the risk of quenching cracks. Furthermore, it stabilizes the austenite to a certain extent, resulting in a more uniform martensitic structure. Precise transfer time control ensures that proeutectoid transformation does not occur before quenching.

[0032] (4) Cyclic staged tempering treatment: The quenched steel bar is first heated to 580℃±10℃, held for 120~150min and then oil-cooled to below 100℃; then heated to 520℃±10℃, held for 90~120min and then air-cooled; finally heated to 480℃±10℃, held for 180~240min and then cooled in the furnace to 200℃ before being removed from the furnace and air-cooled.

[0033] Technical Principle: A three-stage cyclic tempering process of "high temperature - medium temperature - low temperature" is employed. High-temperature tempering at 580℃ rapidly eliminates quenching stress, allowing for initial carbide precipitation; medium-temperature tempering at 520℃ adjusts the morphology and distribution of carbides; and low-temperature tempering at 480℃ stabilizes the microstructure and further eliminates residual stress. This cyclic tempering process results in a more uniform and stable tempered martensite microstructure, achieving the optimal balance between strength and toughness.

[0034] (5) Cryogenic treatment and aging enhancement: After tempering, the steel bars are placed in a cryogenic apparatus and cooled to -80℃±5℃ at a rate of 5~10℃ / min, held at that temperature for 120~180min, and then slowly heated back to room temperature at a rate of 3~5℃ / min. A mixture of liquid nitrogen and alcohol is used as the cryogenic medium. After cryogenic treatment, the bars are aged at 160℃±10℃ for 180~240min.

[0035] Technical principle: -80℃ cryogenic treatment causes the residual austenite remaining after quenching to almost completely transform into martensite, while simultaneously precipitating ultrafine dispersed η-carbides, significantly improving the material's hardness and dimensional stability. Slow heating and cooling avoid thermal shock, and low-temperature aging further stabilizes the microstructure and eliminates cryogenic stress.

[0036] (6) Surface nano-grinding treatment: A mixture of boron carbide and alumina abrasive with a particle size of W20~W40 is used to perform high-speed grinding on the steel rod. The grinding linear speed is controlled at 15~20m / s and the feed rate is 0.02~0.05mm / r, forming a nanocrystalline reinforcement layer with a thickness of 50~100μm on the surface of the steel rod.

[0037] Technical principle: High-speed mechanical grinding produces intense plastic deformation on the material surface, refining the surface grains to the nanoscale, significantly improving surface hardness and wear resistance. At the same time, it forms beneficial residual compressive stress on the surface, greatly improving the fatigue resistance of the parts.

[0038] The 40CrNiMoA steel bar prepared using the process of this invention has the following excellent properties: tensile strength ≥1100MPa, yield strength ≥950MPa, reduction of area ≥55%, U-notch, and impact energy at room temperature. The surface hardness is HRC 38~42, the surface nanocrystalline layer thickness is 50~100μm, and the comprehensive mechanical properties are significantly better than those of traditional processes.

[0039] The 40CrNiMoA steel bars treated using the method of this invention were used to manufacture heavy-duty gears for a heavy-duty vehicle transmission. Bench tests verified that the contact fatigue strength of the gears increased by 40%, the bending fatigue strength increased by 35%, and the service life was extended from 8000 hours to over 15000 hours, demonstrating significant improvement.

[0040] In addition, this technology has been successfully applied to the manufacturing of key components such as engine crankshafts, wind turbine main shafts, and high-strength bolts for oil drilling platforms, all of which have achieved good results.

[0041] The following examples provide further details.

[0042] Example 1: This embodiment involves the heat treatment of Φ50mm 40CrNiMoA steel bars.

[0043] Raw materials: Φ50mm hot-rolled 40CrNiMoA steel bar, with the following chemical composition by mass fraction percentage: C: 0.39%, Si: 0.27%, Mn: 0.72%, Cr: 0.85%, Ni: 1.45%, Mo: 0.25%, P≤0.015%, S≤0.010%, balance Fe.

[0044] Heat treatment process steps: 1. Segmented stepped preheating: After cleaning the surface of the steel bar with alcohol, it is placed in the furnace and heated to 600℃ at 90℃ / h and held for 75min; then heated to 680℃ at 55℃ / h and held for 100min.

[0045] 2. Austenitizing treatment: Heat to 860℃ at 35℃ / h, hold for 150min, purify the furnace with a nitrogen-based protective atmosphere, and control the carbon potential at 0.42%.

[0046] 3. Graded quenching: Within 12 seconds after exiting the furnace, transfer to a 120℃ hot oil bath, stir at a speed of 1.0 m / s, hold for 20 minutes, and then remove and air cool.

[0047] 4. Cyclic stage tempering: Hold at 580℃ for 130 min and oil cool; hold at 520℃ for 100 min and air cool; hold at 480℃ for 210 min, then furnace cool to 200℃ and air cool.

[0048] 5. Cryogenic treatment and aging: Cool down to -80℃ at 8℃ / min, hold for 150min, and then heat up to room temperature at 4℃ / min; age at 160℃ for 210min.

[0049] 6. Surface grinding: W28 mixed abrasive, linear velocity 18m / s, feed rate 0.03mm / r.

[0050] The steel bars that have undergone the above steps were subjected to performance testing, and the test results are shown in Table 1.

[0051] Table 1: Performance Test Results .

[0052] Example 2: This embodiment involves the heat treatment of Φ100mm 40CrNiMoA steel bars.

[0053] Raw materials: Φ100mm hot-rolled 40CrNiMoA steel bar, with the same chemical composition as in Example 1.

[0054] Heat treatment process steps: 1. Segmented stepped preheating: After cleaning the surface of the steel bar with alcohol, it is placed in the furnace and heated to 600℃ at 85℃ / h and held for 135min; then heated to 680℃ at 50℃ / h and held for 180min.

[0055] 2. Austenitizing treatment: Heat to 860℃ at 30℃ / h, hold for 180min, purify the furnace with a nitrogen-based protective atmosphere, and control the carbon potential to 0.43%.

[0056] 3. Graded quenching: Within 10 seconds after exiting the furnace, transfer to a 120℃ hot oil bath, stir at a speed of 1.1m / s, hold for 25 minutes, and then air cool.

[0057] 4. Cyclic stage tempering: Hold at 580℃ for 150 min and oil cool; hold at 520℃ for 120 min and air cool; hold at 480℃ for 240 min and furnace cool to 200℃ before unloading.

[0058] 5. Cryogenic treatment and aging: Cool down to -80℃ at 6℃ / min, hold for 180min, and then restore to room temperature at 3℃ / min; age at 160℃ for 240min.

[0059] 6. Surface grinding: W40 mixed abrasive, linear velocity 16m / s, feed rate 0.04mm / r.

[0060] The steel bars that have undergone the above steps were subjected to performance testing, and the test results are shown in Table 2.

[0061] Table 2: Performance Test Results ; Comparative example: Traditional heat treatment process The conventional quenching and tempering process is adopted: oil quenching at 860℃ + tempering at 600℃, without deep cryogenic treatment and surface nano-processing.

[0062] Performance test results: tensile strength 980MPa, yield strength 830MPa, reduction of area 45%, impact energy 55J, surface hardness HRC32~34.

[0063] As can be seen from the comparison, the process of the present invention significantly improves the comprehensive mechanical properties of 40CrNiMoA steel bars, especially the yield strength is increased by more than 18%, the impact energy is increased by more than 50%, and the surface hardness is increased by more than 20%, which fully meets the requirements of high-end mechanical parts.

[0064] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

[0065] To facilitate understanding by those skilled in the art of the improvements of this invention over the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this invention.

Claims

1. A composite heat treatment method for 40CrNiMoA steel bars, characterized in that, Includes the following steps: S1: Place the 40CrNiMoA steel bar in a heating furnace for segmented step preheating; S2: Heat the preheated steel bar to 860℃±10℃ at a heating rate of 30~40℃ / h, hold for 120~180min, control the furnace atmosphere to be a protective atmosphere, and control the carbon potential at 0.40%~0.45%; S3: Transfer the steel bar treated in step S2 to a hot oil quenching tank at a temperature of 120℃±5℃ within 15s for quenching. The stirring speed of the oil is controlled at 0.8~1.2m / s. After holding at the temperature for 15~25min, take it out and air cool to room temperature. S4: The quenched steel bar is subjected to cyclic stage tempering; S5: The tempered steel bar is placed in a cryogenic device for cryogenic treatment. It is cooled to -80℃±5℃ at a cooling rate of 5~10℃ / min and held for 120~180min. Then it is heated back to room temperature at a heating rate of 3~5℃ / min and aged at 160℃±10℃ for 180~240min. S6: The steel bar is subjected to high-speed grinding, with the grinding line speed controlled at 15-20m / s and the feed rate at 0.02-0.05mm / r, forming a nanocrystalline reinforcement layer with a thickness of 50-100μm on the surface of the steel bar.

2. The composite heat treatment method for a 40CrNiMoA steel bar as described in claim 1, characterized in that, The segmented step preheating process is as follows: first, heat to 600℃±10℃ at a heating rate of 80~100℃ / h and hold for 60~90min; then continue heating to 680℃±10℃ at a heating rate of 50~60℃ / h and hold for 90~120min.

3. The composite heat treatment method for a 40CrNiMoA steel bar as described in claim 2, characterized in that, For steel bars with a diameter greater than 80mm, the heat preservation time at each stage should be extended by 30 minutes for every 20mm increase in diameter.

4. The composite heat treatment method for a 40CrNiMoA steel bar as described in claim 1, characterized in that, The protective atmosphere in S2 is a nitrogen-based protective atmosphere, with a nitrogen content of ≥95%, a propane content of 3% to 5%, and the balance being hydrogen.

5. The composite heat treatment method for a 40CrNiMoA steel bar as described in claim 1, characterized in that, The heat transfer oil in S3 is L-QC320 type.

6. The composite heat treatment method for a 40CrNiMoA steel bar as described in claim 1, characterized in that, S4 specifically involves: first heating the quenched steel bar to 580℃±10℃, holding it at that temperature for 120-150 minutes, and then oil-cooling it to room temperature; then heating it to 520℃±10℃, holding it at that temperature for 90-120 minutes, and then air-cooling it; finally heating it to 480℃±10℃, holding it at that temperature for 180-240 minutes, and then furnace-cooling it to 200℃ before air-cooling it out of the furnace.

7. The composite heat treatment method for a 40CrNiMoA steel bar as described in claim 6, characterized in that, After the first tempering, the oil cooling temperature drops below 100°C before the second tempering is performed.

8. The composite heat treatment method for a 40CrNiMoA steel bar as described in claim 1, characterized in that, The cryogenic treatment medium is a mixture of liquid nitrogen and alcohol as the cold source, and the hard abrasive is a mixture of boron carbide and alumina with a particle size of W20~W40.

9. A composite heat treatment method for a 40CrNiMoA steel bar as described in any one of claims 1-8, characterized in that, The steel bar has a tensile strength ≥1100MPa, a yield strength ≥950MPa, a reduction of area ≥55%, and an impact energy of ≥1100MPa. Surface hardness HRC 38~42, surface nanocrystalline layer thickness 50~100μm.

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