Heat treatment process for improving microstructure and mechanical properties of 35CrMnSi steel

Abstract

This invention discloses a heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel, belonging to the field of heat treatment technology for medium-carbon alloy structural steel. The process sequentially includes base material pretreatment, precise austenitizing quenching, gradient segmented tempering, and low-temperature stabilization treatment, through synergistic control of "precise quenching at 920-940℃ + gradient tempering at 520-560℃". After quenching, a uniform lath martensite structure is formed. During the tempering stage, M3C-type carbides are dispersed and precipitated, refining the grains and inhibiting the formation of network cementite. After treatment by this invention, the microstructure consists of fine tempered martensite and dispersed carbides, with a tensile strength ≥1450MPa, yield strength ≥1250MPa, elongation after fracture ≥12%, impact absorption energy at -20℃ ≥55J, hardness HRC 45-48, and fatigue life ≥1.8×10⁷ cycles, representing a comprehensive performance improvement of over 30% compared to traditional processes.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for medium carbon alloy structural steel, and is applicable to 35CrMnSi medium carbon alloy structural steel used in machinery manufacturing, aerospace and other fields. In particular, it relates to a heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel. Background Technology

[0002] 5CrMnSi steel, as a typical medium-carbon alloy structural steel, is widely used in machinery manufacturing and aerospace fields due to its high strength, hardness, and good hardenability. However, traditional heat treatment processes have significant drawbacks: the heating rate during quenching is too fast and the temperature fluctuation is large, resulting in uneven austenitization and the formation of coarse martensite; the tempering parameters lack precise matching, leading to coarsened carbide precipitation (size reaching 3-5μm), uneven distribution, and even the formation of network cementite, resulting in an imbalance between the strength and toughness of the steel (tensile strength ≤1200MPa and impact absorption energy ≤35J in traditional processes); at the same time, large-section components are prone to residual stress due to uneven cooling, resulting in short fatigue life and difficulty in meeting the stringent reliability requirements of high-end equipment for structural components.

[0003] Existing research lacks a systematic control over the "austenitization parameters-quenching cooling-tempering gradient-microstructure evolution-properties" process, and the correlation between carbide refinement and toughness improvement in 35CrMnSi steel is unclear, resulting in poor process stability and large fluctuations in product performance. Therefore, there is an urgent need to develop a precisely controlled composite heat treatment process to achieve simultaneous improvement in microstructure and overall performance. Summary of the Invention

[0004] This invention addresses the core problems of coarse microstructure, coarse carbides, and a mismatch between strength and toughness in 35CrMnSi steel. The aim is to provide a heat treatment process that improves the microstructure and mechanical properties of 35CrMnSi steel, proposing a composite heat treatment process of "base material pretreatment + precise austenitizing quenching + gradient segmented tempering + low-temperature stabilization." By precisely controlling the austenitizing process to refine the initial microstructure, gradient tempering to regulate carbide precipitation behavior, and low-temperature stabilization to release residual stress, the optimal match between microstructure and mechanical properties is ultimately achieved.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention discloses a heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel, comprising:

[0007] (1) Preparation and pretreatment of raw materials:

[0008] Base material pretreatment: The steel billet is forged into the target shape and then subjected to a pre-heat treatment of "880℃×2h air cooling" to eliminate forging stress, refine the initial grains, remove surface oxide scale and oil stains, and ensure that there are no cracks or surface defects.

[0009] (2) Precision austenitizing quenching:

[0010] Heating stage: The pretreated steel billet is placed in a box-type resistance furnace and heated to 650±10℃ at a rate of 8-10℃ / min, and held for 55-65min; then heated to 920-940℃ at a rate of 5-6℃ / min, and held for 40-60min to ensure complete austenitization and uniform and refined grains.

[0011] Cooling stage: After the heat preservation is completed, the steel billet is immediately immersed in rapid quenching oil at 20-30℃ and cooled to below 150℃. Then it is taken out and air-cooled to room temperature to obtain a uniform and fine lath martensite structure, avoiding bainite transformation and coarse martensite.

[0012] (3) Gradient-based piecewise reflash:

[0013] First stage tempering: The quenched steel billet is heated to 400±10℃ at a rate of 6-8℃ / min and held for 85-95min to release the residual stress of quenching and lay the foundation for uniform precipitation of carbides.

[0014] Second stage tempering: Heat to 520-560℃ at a rate of 4-5℃ / min, hold for 120-150min to promote the dispersion and precipitation of M3C type carbides and inhibit carbide coarsening and networking.

[0015] Cooling stage: After tempering, a segmented cooling method of "oil cooling to 200±5℃ + air cooling to room temperature" is adopted. The cooling rate of the oil cooling stage is 10-15℃ / s to avoid uneven secondary microstructure transformation.

[0016] 4. Low-temperature stabilization treatment:

[0017] The tempered steel billet is placed in a low-temperature tempering furnace and heated to 180±5℃ at a rate of 3-5℃ / min. It is held for 55-65 minutes to further release residual stress, stabilize carbide distribution and martensite structure, and ensure stable performance during service.

[0018] Furthermore, the chemical composition of the parent material by mass fraction is as follows: C 0.32-0.38%, Si 1.10-1.40%, Mn 1.00-1.30%, Cr 1.10-1.40%, P≤0.025%, S≤0.025%, with the balance being Fe and impurities.

[0019] Furthermore, the target shape includes a φ30mm round bar and a 10×50mm flat steel bar.

[0020] Furthermore, the pretreated steel billet is heated to 650℃ at a rate of 8-10℃ / min and held at that temperature for 60min.

[0021] Furthermore, the cooling rate in the rapid quenching oil is 25-30℃ / s.

[0022] Furthermore, the first stage of tempering involves heating the quenched steel billet to 400℃ at a rate of 6-8℃ / min and holding it at that temperature for 90min.

[0023] Furthermore, the tempered steel billet is placed in a low-temperature tempering furnace and heated to 180±5℃ at a rate of 3-5℃ / min, and held for 60min.

[0024] Furthermore, it also includes (5) post-processing of finished products:

[0025] The components undergo surface grinding to remove the oxide layer; the microstructure is examined using an optical microscope and a scanning electron microscope; the mechanical properties are tested using a universal testing machine; and non-destructive testing is performed to ensure there are no internal defects before the components are put into use.

[0026] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0027] 1. Significantly optimized microstructure: Grain size is refined to 8-12μm (compared to 20-30μm in traditional processes), carbides are dispersed M3C type with a size ≤1.5μm, no network cementite, uniform and fine martensite laths, and reasonable dislocation density (1.2-1.8×10¹¹cm⁻²).

[0028] 2. Precisely matched strength and toughness: tensile strength ≥1450MPa, yield strength ≥1250MPa, elongation after fracture ≥12%, reduction of area ≥45%, impact absorption energy at -20℃ ≥55J, hardness stable at 45-48HRC, with strength increased by more than 20% and toughness increased by more than 57% compared with traditional processes.

[0029] 3. Excellent fatigue performance: 10 7 Fatigue life ≥ 1.8 × 10⁻⁶ cycles 7 The residual stress is ≤80MPa, which extends the fatigue life by 60% compared with the traditional process and effectively reduces the risk of cracking during service.

[0030] 4. Strong process adaptability: Based on existing heat treatment equipment, no new special equipment is required. The parameters are highly controllable and can be adapted to components of different specifications from φ10-100mm, making it suitable for industrial mass production. Detailed Implementation

[0031] The core of this invention lies in the synergistic mechanism of "precise austenitization + gradient tempering": the first stage of preheating and slow heating avoids internal temperature gradients in the billet; the quenching temperature of 920-940℃ ensures complete austenitization and prevents grain coarsening; rapid oil cooling ensures fine martensite; gradient tempering, through two-stage heating and holding, first releases residual stress and then promotes the dispersed precipitation of carbides, avoiding carbide aggregation caused by one-time high-temperature tempering; low-temperature stabilization treatment further enhances the stability of the microstructure. The synergistic effect of each stage fundamentally solves the microstructure and performance defects of traditional processes for 35CrMnSi steel.

[0032] Example

[0033] Example 1

[0034] Base material specifications and chemical composition by mass fraction: φ50mm 35CrMnSi steel round bar was selected, with the following chemical composition (mass fraction %): C 0.35%, Si 1.25%, Mn 1.15%, Cr 1.25%, P 0.018%, S 0.015%, balance Fe and impurities; Pre-forging heat treatment: 880℃×2h air cooling, initial grain size 22μm.

[0035] Precision austenitizing quenching: Heat to 650℃ at 9℃ / min and hold for 60min, then heat to 930℃ at 5.5℃ / min and hold for 50min; the cooling medium is 25℃ rapid quenching oil, cool to 120℃ and then air cool to room temperature. The microstructure after quenching is uniform lath martensite with a grain size of 15μm.

[0036] Gradient segmented tempering: Heat to 400℃ at 7℃ / min and hold for 90min, then heat to 540℃ at 4.5℃ / min and hold for 130min; oil cool to 200℃ (cooling rate 12℃ / s) and then air cool to room temperature.

[0037] Low-temperature stabilization treatment: Heat to 180℃ at 4℃ / min and hold for 60min, then air cool to room temperature.

[0038] Post-treatment properties and microstructure: Microstructure consists of fine tempered martensite + dispersed M3C type carbides (size 1.0-1.2μm), grain size 10μm; tensile strength 1480MPa, yield strength 1280MPa, elongation after fracture 13.5%, reduction of area 48%; impact energy absorbed at -20℃ 62J, hardness 46.5HRC; residual stress 72MPa; 10 7 Fatigue life per cycle: 1.95 × 10⁻⁶ 7 Second-rate.

[0039] Example 2

[0040] Chemical composition of base material specifications and mass fraction: 15×60mm 35CrMnSi steel flat bar was selected, with chemical composition (mass fraction %): C 0.33%, Si 1.18%, Mn 1.08%, Cr 1.18%, P 0.020%, S 0.016%, balance Fe and impurities; Pre-forging heat treatment: 880℃×2h air cooling, initial grain size 23μm.

[0041] Precision austenitizing quenching: Heat to 650℃ at 8.5℃ / min and hold for 60min, then heat to 920℃ at 5℃ / min and hold for 55min; the cooling medium is 22℃ rapid quenching oil, cool to 130℃ and then air cool to room temperature. The microstructure after quenching is uniform lath martensite with a grain size of 16μm.

[0042] Gradient segmented tempering: Heat to 400℃ at 6.5℃ / min and hold for 90min, then heat to 530℃ at 4℃ / min and hold for 140min; oil cool to 200℃ (cooling rate 11℃ / s) and then air cool to room temperature.

[0043] Low-temperature stabilization treatment: Heat to 180℃ at 3.5℃ / min and hold for 60min, then air cool to room temperature.

[0044] Post-treatment properties and microstructure: Microstructure consists of fine tempered martensite + dispersed M3C type carbides (size 1.1-1.3μm), grain size 11μm; tensile strength 1460MPa, yield strength 1260MPa, elongation after fracture 12.8%, reduction of area 46%; impact absorption energy at -20℃ 58J, hardness 45.8HRC; residual stress 75MPa; 10 7 Fatigue life per cycle: 1.85 × 10⁻⁶ 7 Second-rate.

[0045] Example 3

[0046] Chemical composition of base material specifications and mass fraction: φ80mm 35CrMnSi steel round bar was selected, with chemical composition (mass fraction %): C 0.37%, Si 1.32%, Mn 1.22%, Cr 1.30%, P 0.016%, S 0.014%, balance Fe and impurities; Pre-forging heat treatment: 880℃×2h air cooling, initial grain size 21μm.

[0047] Precision austenitizing quenching: Heat to 650℃ at 9.5℃ / min and hold for 60min, then heat to 940℃ at 6℃ / min and hold for 45min; the cooling medium is 28℃ rapid quenching oil, cool to 110℃ and then air cool to room temperature. The microstructure after quenching is uniform lath martensite with a grain size of 14μm.

[0048] Gradient segmented tempering: Heat to 400℃ at 7.5℃ / min and hold for 90min, then heat to 550℃ at 5℃ / min and hold for 120min; oil cool to 200℃ (cooling rate 13℃ / s) and then air cool to room temperature.

[0049] Low-temperature stabilization treatment: Heat to 180℃ at 4.5℃ / min and hold for 60min, then air cool to room temperature.

[0050] Post-treatment properties and microstructure: Microstructure consists of fine tempered martensite + dispersed M3C type carbides (0.9-1.1 μm in size), with a grain size of 9 μm; tensile strength 1500 MPa, yield strength 1300 MPa, elongation after fracture 14.2%, reduction of area 50%; impact energy absorbed at -20℃ 65 J, hardness 47.2 HRC; residual stress 68 MPa; 10 7 Fatigue life per cycle: 2.05 × 10⁻⁶ 7 Second-rate.

[0051] Comparative Example

[0052] Comparative Example 1 (Traditional single quenching + tempering process)

[0053] The base material was the same as in Example 1, and no preheating treatment was performed after forging.

[0054] Process parameters: directly heat to 900℃ at 15℃ / min and hold for 30min, then air cool to room temperature (insufficient quenching); then heat to 500℃ at 20℃ / min and hold for 60min, then air cool to room temperature.

[0055] Post-treatment properties and microstructure: Microstructure consists of coarse martensite + network cementite (carbide size 3.5-4.5μm), grain size 28μm; tensile strength 1180MPa, yield strength 1050MPa, elongation after fracture 8.5%, reduction of area 32%; impact absorption energy at -20℃ 32J, hardness 43HRC; residual stress 150MPa; 10 7 Fatigue life per cycle: 1.2 × 10⁻⁶ 7 Second-rate.

[0056] Comparative Example 2 (no gradient tempering, only single high-temperature tempering)

[0057] The base material is the same as in Example 1, and the preheat treatment after forging is the same as in Example 1.

[0058] Process parameters: The quenching process is the same as in Example 1; the tempering process is "heating to 540℃ at 10℃ / min and holding for 130min", followed by direct air cooling to room temperature.

[0059] Post-treatment properties and microstructure: Microstructure consists of tempered martensite + aggregated M3C carbides (size 2.0-2.5μm), grain size 16μm; tensile strength 1320MPa, yield strength 1150MPa, elongation after fracture 10.2%, reduction of area 38%; impact energy absorbed at -20℃ 45J, hardness 44.5HRC; residual stress 105MPa; 10 7 Fatigue life per cycle: 1.4 × 10⁻⁶ 7 Second-rate.

[0060] Comparative Example 3 (Insufficient Quenching Cooling Rate)

[0061] The base material is the same as in Example 1, and the preheat treatment after forging is the same as in Example 1.

[0062] Process parameters: The austenitizing and tempering processes are the same as in Example 1; the quenching cooling medium is ordinary machine oil at 35℃, and the cooling rate is 15℃ / s.

[0063] Post-treatment properties and microstructure: Microstructure consists of martensite with a small amount of bainite (carbide size 1.8-2.2 μm), grain size 18 μm; tensile strength 1350 MPa, yield strength 1180 MPa, elongation after fracture 11.0%, reduction of area 40%; impact energy absorbed at -20℃ 48 J, hardness 45 HRC; residual stress 98 MPa; 10 7 Fatigue life per cycle: 1.5 × 10⁻⁶ 7 Second-rate.

[0064] Performance Comparison Table

[0065] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Grain size (μm) 10 11 9 28 16 18 Carbide size (μm) 1.0-1.2 1.1-1.3 0.9-1.1 3.5-4.5 2.0-2.5 1.8-2.2 Tensile strength (MPa) 1480 1460 1500 1180 1320 1350 Yield strength (MPa) 1280 1260 1300 1050 1150 1180 Elongation after fracture (%) 13.5 12.8 14.2 8.5 10.2 11.0 Reduction of area (%) 48 46 50 32 38 40 -20℃ Impact Absorbed Energy (J) 62 58 65 32 45 48 Hardness (HRC) 46.5 45.8 47.2 43 44.5 45 Residual stress (MPa) 72 75 68 150 105 98 <![CDATA[Fatigue life (×10 7 times)]]> 1.95 1.85 2.05 1.2 1.4 1.5

[0066] As can be seen from the examples and comparative examples: (1) The examples effectively eliminated forging stress and refined the initial grains (≤25μm) through air cooling preheating at 880℃ for 2h, laying a uniform microstructure foundation for subsequent heat treatment; while Comparative Example 1 did not undergo preheating, and the final grain size reached 28μm, with carbides coarsening in a network pattern (3.5-4.5μm), resulting in significant deterioration of strength, toughness, and fatigue performance; (2) The examples adopted a segmented heating mode of "650℃ preheating + 920-940℃ holding" combined with rapid oil cooling at 25-30℃ / s to obtain fine lath martensite (grain size 8-11μm); Comparative Example 1 used rapid heating at 15℃ / min and short holding time, resulting in insufficient quenching, leading to a coarse microstructure and a tensile strength of only 1180MPa, which is more than 20% lower than the examples; (3) The examples achieved stress relief at 400℃ + carbide precipitation at 520-560℃. Two-stage tempering resulted in the dispersed distribution of M3C carbides (size ≤ 1.3 μm); Comparative Example 2 used a single high-temperature tempering, resulting in carbides agglomerated to 2.0-2.5 μm, with an impact absorption energy of only 45 J, which is more than 20% lower than the example, and the section reduction rate is less than 40%. (4) The example used 20-30℃ rapid quenching oil (cooling rate 25-30℃ / s) to avoid bainite transformation, resulting in pure martensite structure; Comparative Example 3 used ordinary machine oil (cooling rate 15℃ / s), resulting in a small amount of bainite, a tensile strength reduced to 1350 MPa, and a fatigue life 30% lower than the example; (5) The example's system process of "preparatory heat treatment + precise quenching + gradient tempering + low-temperature stabilization" achieved synergistic optimization of grain refinement, carbide dispersion, and residual stress release, ultimately resulting in a tensile strength ≥ 1450 MPa, an impact absorption energy of -20℃ ≥ 55 J, and a fatigue life ≥ 1.8 × 10 7 This method improves overall performance by more than 30% compared to traditional single-process technology (Comparative Example 1), and is suitable for industrial production of components of different specifications.

[0067] 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 35CrMnSi steel, characterized in that, include: (1) Preparation and pretreatment of raw materials: Base material pretreatment: The steel billet is forged into the target shape, and then subjected to a pre-heat treatment of "880℃×2h air cooling" to eliminate forging stress, refine the initial grains, remove surface oxide scale and oil stains, and ensure that there are no cracks or surface defects. (2) Precision austenitizing quenching: Heating stage: The pretreated steel billet is placed in a box-type resistance furnace and heated to 650±10℃ at a rate of 8-10℃ / min, and held for 55-65min; then heated to 920-940℃ at a rate of 5-6℃ / min, and held for 40-60min to ensure complete austenitization and uniform and refined grains. Cooling stage: After the heat preservation is completed, the steel billet is immediately immersed in rapid quenching oil at 20-30℃ and cooled to below 150℃. Then it is taken out and air-cooled to room temperature to obtain a uniform and fine lath martensite structure, avoiding bainite transformation and coarse martensite. (3) Gradient-based piecewise reflash: First stage tempering: The quenched steel billet is heated to 400±10℃ at a rate of 6-8℃ / min and held for 85-95min to release the residual stress of quenching and lay the foundation for uniform precipitation of carbides. Second stage tempering: Heat to 520-560℃ at a rate of 4-5℃ / min, hold for 120-150min to promote the dispersion and precipitation of M3C type carbides and inhibit carbide coarsening and networking. Cooling stage: After tempering, a segmented cooling method of "oil cooling to 200±5℃ + air cooling to room temperature" is adopted. The cooling rate of the oil cooling stage is 10-15℃ / s to avoid uneven secondary microstructure transformation. (4) Low-temperature stabilization treatment: The tempered steel billet is placed in a low-temperature tempering furnace and heated to 180±5℃ at a rate of 3-5℃ / min. It is held for 55-65 minutes to further release residual stress, stabilize carbide distribution and martensite structure, and ensure stable performance during service.

2. The heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel according to claim 1, characterized in that, The chemical composition of the parent material by mass fraction is: C 0.32-0.38%, Si 1.10-1.40%, Mn 1.00-1.30%, Cr 1.10-1.40%, P≤0.025%, S≤0.025%, with the balance being Fe and impurities.

3. The heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel according to claim 1, characterized in that, The target shape includes φ30mm round bars and 10×50mm flat steel.

4. The heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel according to claim 1, characterized in that, The pretreated steel billet is heated to 650℃ at a rate of 8-10℃ / min and held for 60min.

5. The heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel according to claim 1, characterized in that, The cooling rate in the rapid quenching oil is 25-30℃ / s.

6. The heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel according to claim 1, characterized in that, First stage of tempering: The quenched steel billet is heated to 400℃ at a rate of 6-8℃ / min and held for 90min.

7. The heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel according to claim 1, characterized in that, The tempered steel billet is placed in a low-temperature tempering furnace and heated to 180±5℃ at a rate of 3-5℃ / min, and held for 60min.

8. The heat treatment process for improving the microstructure and mechanical properties of 35CrMnSi steel according to claim 1, characterized in that, It also includes (5) post-processing of finished products: The components undergo surface grinding to remove the oxide layer; the microstructure is examined using an optical microscope and a scanning electron microscope; the mechanical properties are tested using a universal testing machine; and non-destructive testing is performed to ensure there are no internal defects before the components are put into use.