A heat treatment process method for reducing the fracture sensitivity of 20Mn2A round link chain

Abstract

This invention discloses a heat treatment process for reducing the fracture sensitivity of 20Mn2A circular link chains, comprising the following steps: (1) Pretreatment: heating to 650±10℃ at a rate of 4.8-5.2℃ / min in a vacuum annealing furnace, holding for 1-3 hours, and then cooling to room temperature in the furnace; (2) Graded quenching; (3) Two-stage tempering: the first stage is low-temperature tempering: holding at 200-210℃ for 2-3 hours, followed by oil cooling; the second stage is medium-temperature tempering: holding at 380-400℃ for 1-2 hours, followed by oil cooling. This invention, through a scientific three-stage treatment, achieves an optimized match between the strength and toughness of 20Mn2A circular link chains, with the specific objectives being: tensile strength ≥1050N / mm², low-temperature impact energy at -40℃ ≥48J, and internal residual stress ≤85MPa, significantly reducing fracture sensitivity and extending service life.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment technology, and particularly relates to a heat treatment process for reducing the susceptibility of 20Mn2A circular chain to breakage. Background Technology

[0002] 20Mn2A steel, as a typical representative of medium-carbon alloy structural steel, possesses excellent strength properties, good weldability and hardenability, making it one of the preferred materials for manufacturing round link chains. Thanks to its reasonable carbon-manganese alloy ratio, it can achieve high tensile strength after heat treatment, meeting the core requirements of round link chains for bearing heavy loads and transmitting power. Therefore, it is widely used in key components such as conveyor traction chains and lifting chains for hoisting equipment.

[0003] However, 20Mn2A circular link chains frequently experience fracture failures during actual service, severely impacting equipment safety and service life. Industry experience has shown that the fracture risks primarily stem from inherent defects in traditional heat treatment processes: Firstly, the circular link chains retain significant internal stress after welding, and traditional processes fail to address this stress during subsequent heat treatment, leading to stress accumulation and potential crack formation at chain bends. Secondly, traditional heat treatment employs a "single-stage quenching + single tempering" method. During quenching, austenite grains tend to grow, forming coarse martensite and reducing material toughness. Furthermore, the tempering temperature, within the brittle range of 450-550℃, easily triggers impurity element segregation at grain boundaries, further exacerbating embrittlement.

[0004] Traditional heat treatment process

[0005] Traditional processes are simple and mainly consist of three steps:

[0006] Pretreatment: The welded circular chain is directly loaded into the heating furnace from room temperature, rapidly heated to 880-900℃, and held for 30-40 seconds to complete the austenitization transformation;

[0007] Single-stage quenching: After austenitization, the circular chain is directly immersed in quenching oil and cooled to room temperature. The cooling rate is controlled at 15-20℃ / second to obtain a martensitic structure.

[0008] Single tempering: Place the quenched circular chain into a tempering furnace, heat it to 450-550℃, hold it for 2-3 hours, and then slowly cool it to room temperature with the furnace to reduce the quenching internal stress.

[0009] Although the process is convenient to operate and has low production costs, it has obvious shortcomings: insufficient grain refinement during quenching results in poor material toughness; the tempering temperature is in the brittle range, which easily leads to second-type temper brittleness; improper cooling rate control and incomplete elimination of internal stress ultimately make the circular chain prone to early fracture under alternating loads, failing to meet the requirements of high-strength working conditions. Summary of the Invention

[0010] To address the shortcomings of traditional processes, the purpose of this invention is to provide a simple, easy-to-operate, and highly adaptable heat treatment process. Through a scientific three-stage treatment, the strength and toughness of 20Mn2A circular link chains are optimized and matched. The specific targets are: tensile strength ≥1050N / mm², low-temperature impact energy at -40℃ ≥48J, and internal residual stress ≤85MPa, which significantly reduces fracture sensitivity and extends service life.

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0012] This invention discloses a heat treatment process for reducing the susceptibility of 20Mn2A circular chain breakage, comprising the following steps:

[0013] (1) Pretreatment: Heat to 650±10℃ in a vacuum annealing furnace at a rate of 4.8-5.2℃ / min, hold for 1-3 hours, and cool to room temperature with the furnace;

[0014] (2) Graded quenching: First stage quenching: first heat to 920-930℃, hold for 15-16 seconds, and oil cool to 350-380℃; Second stage quenching: heat to 870-880℃ again, hold for 10-11 seconds, and oil and water cool to room temperature.

[0015] (3) Two-stage tempering: First stage low temperature tempering: 200-210℃ for 2-3 hours, oil cooling; Second stage medium temperature tempering: 380-400℃ for 1-2 hours, oil cooling, cooling rate ≥10℃ / second, to suppress the accumulation of impurity elements at the grain boundary, and at the same time further release residual internal stress.

[0016] Furthermore, the pretreatment process involves heating in a vacuum annealing furnace at a rate of 5.0°C / min.

[0017] Furthermore, the temperature is raised to 650°C during the pretreatment.

[0018] Furthermore, the second stage of quenching involves first oil cooling for 3-4 seconds to rapidly reduce the surface temperature, then transferring it to a water cooling bath to cool to room temperature. The cooling rate is increased to 30-32℃ / second to ensure core hardenability and ultimately obtain a uniform and fine martensitic structure.

[0019] Furthermore, after the first stage of low-temperature tempering, oil cooling is used to bring the temperature to room temperature at a rate of ≥10℃ / second to prevent embrittlement caused by excessively slow cooling.

[0020] Furthermore, the strength and toughness of the 20Mn2A circular link chain are optimized: tensile strength ≥1050N / mm², low-temperature impact energy ≥48J at -40℃, and internal residual stress ≤85MPa, significantly reducing fracture sensitivity and extending service life.

[0021] Furthermore, specifically:

[0022] Pretreatment: Place the welded circular chain into a vacuum annealing furnace, close the furnace door and start the equipment to heat to 650°C at a rate of 5°C / minute, hold for 2 hours, and then cool to room temperature with the furnace. After taking it out, observe that there is no deformation or oxidation on the surface of the chain.

[0023] Graded quenching: Start the high-frequency induction heating equipment, feed the chain into the heating coil, heat up to 925℃, hold for 15 seconds, then immerse in quenching oil to cool to 360℃; take it out and immediately heat it again to 875℃, hold for 10 seconds, first oil cool for 3 seconds, then transfer to a water cooling tank to cool to room temperature.

[0024] Two-stage tempering: The quenched chain is placed in a tempering furnace, heated to 205℃, held for 2.5 hours, and then oil-cooled to room temperature; then the temperature is raised again to 390℃, held for 1.5 hours, and then oil-cooled to room temperature.

[0025] Furthermore, specifically: the mechanical properties and microstructure of the treated circular ring chain were tested, and the results are as follows:

[0026] Mechanical properties: tensile strength 1065 N / mm², yield strength 855 N / mm², impact energy at -40℃ 49 J, elongation after fracture 13.5%, reduction of area 46%;

[0027] Stress level: Residual internal stress 82MPa, uniformly distributed, with no obvious stress concentration areas;

[0028] Microstructure: The austenite grain size is grade 11, the microstructure is uniform and fine tempered martensite, the cementite particles are dispersed and there is no obvious segregation.

[0029] Fatigue performance: Under alternating loads with a stress ratio R=0.1 and a maximum stress of 800 N / mm², the fatigue life reaches 1.05 × 10⁻⁶. 6 Secondly, the fracture surface exhibits typical ductile fracture characteristics, with dimples evenly distributed.

[0030] Summary of the core parameters of this heat treatment process:

[0031] Process Stage Temperature range Insulation time Cooling method Cooling speed core role Preprocessing 650℃ 2h Cooling with furnace - Eliminate welding internal stress and homogenize the microstructure. First-stage quenching 920-930℃ 15-16s oil cooling 20-22℃ / second Preliminary austenitization, fine grain base Secondary quenching 870-880℃ 10-11s Oil and water cooling 30-32℃ / second Refine grains to ensure hardenability Low temperature tempering 200-210℃ 2.5h oil cooling ≥10℃ / second Eliminate quenching stress and stabilize the microstructure intermediate temperature tempering 380-400℃ 1.5h oil cooling ≥10℃ / second Improve toughness, prevent embrittlement, and relieve secondary stress.

[0032] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0033] Strength performance meets the standards: tensile strength ≥1050N / mm², yield strength ≥850N / mm², meeting the requirements for high strength load-bearing capacity;

[0034] Significantly improved toughness: -40℃ low temperature impact energy ≥48J, more than 50% higher than traditional process, less prone to brittle fracture in low temperature environment;

[0035] Effective control of internal stress: residual internal stress ≤85MPa, which is 45% lower than that of traditional processes, and the risk of crack initiation is greatly reduced;

[0036] Good structural stability: It obtains a uniform and fine tempered martensite structure with a grain grade of 10 or higher, and excellent performance consistency;

[0037] Extended service life: fatigue life under repeated loads ≥ 1 million cycles, which is more than 40% longer than that of traditional processes, and fracture sensitivity is significantly reduced. Detailed Implementation

[0038] Example

[0039] A standard 20Mn2A circular chain was selected, with the following chemical composition by mass fraction: carbon (C) 0.17–0.24%, silicon (Si) 0.17–0.37%, manganese (Mn) 1.40–1.80%, phosphorus (P) ≤0.035%, sulfur (S) ≤0.035%, and the remainder being Fe and impurities. The specific measured composition in this embodiment was: carbon (C) 0.20%, silicon (Si) 0.25%, manganese (Mn) 1.60%, phosphorus (P) 0.025%, sulfur (S) 0.025%, and the remainder being Fe and impurities. The process of this invention was implemented according to the following steps:

[0040] Pretreatment: Place the welded circular chain into a vacuum annealing furnace, close the furnace door and start the equipment to heat to 650°C at a rate of 5°C / minute, hold for 2 hours, and then cool to room temperature with the furnace. After taking it out, observe that there is no deformation or oxidation on the surface of the chain.

[0041] Graded quenching: Start the high-frequency induction heating equipment, feed the chain into the heating coil, heat it to 925℃, hold it for 15 seconds, then immerse it in quenching oil to cool to 360℃; after taking it out, immediately feed it into the heating coil again, heat it to 875℃, hold it for 10 seconds, first oil cool for 3 seconds, then transfer it to the water cooling tank to cool to room temperature.

[0042] Two-stage tempering: The quenched chain is placed in a tempering furnace, heated to 205℃, held for 2.5 hours, and then oil-cooled to room temperature; then the temperature is raised again to 390℃, held for 1.5 hours, and then oil-cooled to room temperature.

[0043] Test results

[0044] The mechanical properties and microstructure of the treated circular ring chain were tested, and the results are as follows:

[0045] Mechanical properties: tensile strength 1065 N / mm², yield strength 855 N / mm², impact energy at -40℃ 49 J, elongation after fracture 13.5%, reduction of area 46%;

[0046] Stress level: Residual internal stress 82MPa, uniformly distributed, with no obvious stress concentration areas;

[0047] Microstructure: The austenite grain size is grade 11, the microstructure is uniform and fine tempered martensite, the cementite particles are dispersed and there is no obvious segregation.

[0048] Fatigue performance: Under alternating loads with a stress ratio R=0.1 and a maximum stress of 800 N / mm², the fatigue life reaches 1.05 × 10⁻⁶. 6 Secondly, the fracture surface exhibits typical ductile fracture characteristics, with dimples evenly distributed.

[0049] Compared with traditional craftsmanship

[0050] Using the traditional "single-stage quenching + single tempering" process to treat circular link chains of the same material and batch, the test results show:

[0051] Tensile strength 960 N / mm², impact energy at -40℃ only 30 J, residual internal stress 155 MPa;

[0052] The austenite grain size is grade 6, the microstructure is coarse and uneven, and there is obvious grain boundary segregation;

[0053] The fatigue life is only 700,000 cycles, and the fracture surface exhibits intergranular brittle fracture characteristics with rapid crack propagation.

[0054] As can be seen from the comparison, the circular link chain treated by the process of the present invention (removing the cryogenic step) is still significantly better than the traditional process in terms of strength, toughness, stress control and service life. The fracture sensitivity is greatly reduced, and the process is simpler and the production cost is lower, which can better meet the use needs under complex working conditions.

[0055] 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 reducing the susceptibility of 20Mn2A circular chain breakage, characterized in that, Includes the following steps: (1) Pretreatment: Heat to 650±10℃ in a vacuum annealing furnace at a rate of 4.8-5.2℃ / min, hold for 1-3 hours, and cool to room temperature with the furnace; (2) Graded quenching: First stage quenching: first heat to 920-930℃, hold for 15-16 seconds, and oil cool to 350-380℃; Second stage quenching: heat to 870-880℃ again, hold for 10-11 seconds, and oil and water cool to room temperature. (3) Two-stage tempering: First stage low temperature tempering: 200-210℃ for 2-3 hours, oil cooling; Second stage medium temperature tempering: 380-400℃ for 1-2 hours, oil cooling, cooling rate ≥10℃ / second, to suppress the accumulation of impurity elements at the grain boundary, and at the same time further release residual internal stress.

2. The heat treatment process for reducing the susceptibility of 20Mn2A circular chain breakage according to claim 1, characterized in that, The pretreatment process involves heating in a vacuum annealing furnace at a rate of 5.0°C / min.

3. The heat treatment process for reducing the susceptibility of 20Mn2A circular chain breakage according to claim 2, characterized in that, The temperature is raised to 650°C during the pretreatment.

4. The heat treatment process for reducing the susceptibility to breakage of 20Mn2A circular chain according to claim 1, characterized in that, The second stage of quenching involves first oil cooling for 3-4 seconds to rapidly reduce the surface temperature, then transferring it to a water cooling bath to cool to room temperature. The cooling rate is increased to 30-32℃ / second to ensure the hardenability of the core, ultimately obtaining a uniform and fine martensitic structure.

5. The heat treatment process for reducing the susceptibility to breakage of 20Mn2A circular chain according to claim 1, characterized in that, After the first stage of low-temperature tempering, oil cooling is used to bring the temperature to room temperature at a rate of ≥10℃ / second to prevent embrittlement caused by slow cooling.

6. The heat treatment process for reducing the susceptibility to breakage of 20Mn2A circular ring chains according to claim 1, characterized in that, Achieve optimized matching of strength and toughness in 20Mn2A circular link chains: tensile strength ≥1050N / mm², low-temperature impact energy ≥48J at -40℃, internal residual stress ≤85MPa, significantly reducing fracture sensitivity and extending service life.

7. The heat treatment process for reducing the susceptibility to breakage of 20Mn2A circular ring chains according to claim 1, characterized in that, Specifically: Pretreatment: Place the welded circular chain into a vacuum annealing furnace, close the furnace door and start the equipment to heat to 650°C at a rate of 5°C / minute, hold for 2 hours, and then cool to room temperature with the furnace. After taking it out, observe that there is no deformation or oxidation on the surface of the chain. Graded quenching: Start the high-frequency induction heating equipment, feed the chain into the heating coil, heat up to 925℃, hold for 15 seconds, then immerse in quenching oil to cool to 360℃; take it out and immediately heat it again to 875℃, hold for 10 seconds, first oil cool for 3 seconds, then transfer to a water cooling tank to cool to room temperature. Two-stage tempering: The quenched chain is placed in a tempering furnace, heated to 205℃, held for 2.5 hours, and then oil-cooled to room temperature; then the temperature is raised again to 390℃, held for 1.5 hours, and then oil-cooled to room temperature.

8. The heat treatment process for reducing the susceptibility to breakage of 20Mn2A circular ring chains according to claim 7, characterized in that, The mechanical properties and microstructure of the treated circular ring chain were tested, and the results are as follows: Mechanical properties: tensile strength 1065 N / mm², yield strength 855 N / mm², impact energy at -40℃ 49 J, elongation after fracture 13.5%, reduction of area 46%; Stress level: Residual internal stress 82MPa, uniformly distributed, with no obvious stress concentration areas; Microstructure: The austenite grain size is grade 11, the microstructure is uniform and fine tempered martensite, the cementite particles are dispersed and there is no obvious segregation. Fatigue performance: Under alternating loads with a stress ratio R=0.1 and a maximum stress of 800 N / mm², the fatigue life reaches 1.05 × 10⁻⁶. 6 Secondly, the fracture surface exhibits typical ductile fracture characteristics, with dimples evenly distributed.