Multi-stage heat treatment process method for refining grains and application of multi-stage heat treatment process method

Abstract

The invention discloses a heat treatment production method for refining grains of a 42CrMo steel motor suspender. 42CrMo steel comprises the following components in percentage by weight: 0.38%-0.45% of C, 0.17%-0.37% of Si, 0.50%-0.80% of Mn, 0.90%-1.20% of Cr, 0.15%-0.25% of Mo, less than or equal to 0.035% of P, less than or equal to 0.035% of S, less than or equal to 0.30% of Cu, less than or equal to 0.30% of Ni and the balance of Fe. According to the process, a fine lamellar pearlitic structure is obtained through normalizing (step heating to 880 + / -10 DEG C, heat preservation and slow cooling), martensite grains are refined and retained austenite is reduced through two times of quenching (840-850 + / -10 DEG C brine rapid cooling and graded oil cooling), and finally grain ultra-fining (smaller than 10 microns) and structure homogenization are achieved through 620 + / -10 DEG C high-temperature tempering. The tensile strength of the final material is larger than or equal to 1000 MPa, the impact energy is larger than or equal to 80 J, the yield strength is larger than or equal to 900 MPa, and the mechanical property of the motor suspender under the extreme working condition is remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat treatment of metal materials, and specifically relates to a systematic heat treatment process for 42CrMo steel motor booms. Through a multi-stage heat treatment process, the grains of 42CrMo steel are refined, the structural uniformity is optimized, and the strength-toughness matching, fatigue resistance, and high-temperature stability are improved. The process is suitable for high-load dynamic load-bearing components in fields such as wind power, ship propulsion, and heavy industry. Background Art

[0002] The motor boom assembly, consisting of a motor boom and rubber nodes, is a key load-bearing component in the vehicle's traction system, responsible for transmitting dynamic force between the bogie frame and the traction power system. Its operating environment is characterized by high loads, strong vibrations, and long service cycles. It must withstand both static loads (such as the motor's own weight) and dynamic loads (such as vibration and impact), placing stringent demands on the material's strength-toughness matching and fatigue resistance. Current materials exhibit significant deficiencies in low-temperature impact toughness, which can easily lead to the risk of brittle fracture under sudden loads, becoming a key technical bottleneck restricting reliability.

[0003] 42CrMo steel is widely used in the manufacture of motor booms due to its high hardenability and the strength advantages of medium-carbon alloy steel. However, the traditional quenching and tempering process (quenching + high-temperature tempering) has significant limitations: improper control of tempering parameters can easily lead to an imbalance between strength and toughness. Although low-temperature tempering can maintain high strength, it causes a sharp drop in toughness; the residual stress and structural defects (coarse martensite, undissolved carbides) caused by uneven quenching and cooling significantly reduce fatigue resistance; insufficient structural stability after high-temperature tempering leads to strength attenuation under working conditions above 150°C. More seriously, microscopic defects (grain coarsening, inclusion segregation) can induce fatigue crack initiation, exacerbating the service risk under dynamic loads. These defects jointly restrict the service life of the material under extreme working conditions, and it is urgent to achieve performance breakthroughs through process optimization.

[0004] This research focuses on controlling quenching and tempering process parameters (temperature gradient and cooling rate), combining austempering and thermomechanical heat treatment techniques to refine grains, eliminate microdefects, and optimize structural uniformity. By improving strength-toughness matching and high-temperature stability, this research effectively addresses the brittle fracture and strength degradation issues associated with traditional processes, providing process support for extending the service life of motor booms under alternating loads. Summary of the Invention

[0005] The present invention aims to provide a 42CrMo steel comprising, by weight percentage, 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: ≤0.035%, S: ≤0.035%, Cu: ≤0.30%, Ni: ≤0.30%, and the balance being Fe;

[0006] Preferably, the 27SiMn steel composition is: 0.24% to 0.32% C, 1.10% to 1.40% Si, 1.10% to 1.40% Mn, 0.030% to 0.035% P, 0.035% S, 0.03% Cu, 0.03% Cr, 0.03% Ni, and the balance is Fe;

[0007] The present invention provides a heat treatment production method for 42CrMo steel after forging, the heat treatment method comprising the following steps:

[0008] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet to form a coarse Widmanstätten structure;

[0009] (2) Normalizing: Stepwise heating to 550±10℃ and holding for 1.5-2 hours, then heating to 880±10℃ and holding for 3-3.5 hours; slowly cooling (≤55℃ / h) to 300℃ and recharging the furnace to form a balanced structure mainly composed of fine lamellar pearlite + a small amount of ferrite, with carbides evenly dispersed, providing a uniform matrix for subsequent quenching.

[0010] (3) First quenching: Raise the temperature to 550±10℃, hold for 1.5-2 hours, then raise the temperature to 840±10℃, austenitizing temperature, hold for 1.5-2 hours, use brine rapid cooling + oil cooling, and then re-load the furnace at 300℃; low-temperature austenitization at 840℃ to inhibit grain growth, use a graded cooling strategy (first brine rapid cooling to avoid pearlite transformation, then oil cooling to reduce thermal stress), and return to the furnace at 200℃ for slow cooling to reduce quenching internal stress and avoid cracking. Finally, fine lath martensite + a small amount of retained austenite is obtained.

[0011] (4) Second quenching: Heat to 550±10℃, keep warm for 1.5-2 hours, heat to austenitizing temperature 850±10℃ and keep warm for 1.5-2 hours, cool in brine at a rate of ≥200℃ / s, and cool in oil to 100℃; the retained austenite is further reduced and the martensite grains are further refined.

[0012] (5) Tempering: Directly heat to 620±10℃ and hold for 3-4 hours, then cool with water or oil. Tempering at 620℃ decomposes martensite into tempered bainite, improving impact toughness. Water / oil cooling avoids slow cooling in the brittle range of 400-550℃, eliminates the risk of impurity elements (P, S) segregating at grain boundaries, and suppresses temper brittleness.

[0013] Preferably, after quenching, the forging blank is oil-cooled to 600-650° C. at a cooling rate of 10-15° C. / s, then water-cooled to 350-400° C., and then oil-cooled to room temperature.

[0014] Preferably, the water cooling quenching method is brine rapid cooling + oil cooling graded cooling,

[0015] Preferably, the water cooling rate is controlled to be 40-50°C / s.

[0016] Preferably, the cooling rate of the brine in step (4) is ≥200°C / s.

[0017] Preferably, the forging blank is kept at 610-630° C. for 3-4 hours and then cooled to room temperature.

[0018] The beneficial effects of the present invention are:

[0019] 1. Double quenching strengthening: Through two austenitizing temperature adjustments (840℃→850℃), the hardenability is improved, the dissolution and redistribution of carbides are optimized, and the grain size is ultra-fine.

[0020] 2. Graded cooling strategy: rapid cooling with brine inhibits pearlite transformation, while oil cooling reduces thermal stress and reduces retained austenite;

[0021] 3. High temperature tempering control: 620℃ tempering combined with rapid cooling to avoid temper brittleness and balance strength and toughness.

[0022] 4. Avoid the risk of deformation and cracking caused by direct high-temperature heating by increasing the temperature in steps (preheating to 550°C and then increasing to 840°C-880°C). BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the metallographic structure diagram after normalizing of the present invention;

[0024] Figure 2 This is the metallographic structure diagram after the first quenching of the present invention;

[0025] Figure 3 This is the metallographic structure diagram after the second quenching of the present invention;

[0026] Figure 4 This is the metallographic structure diagram after high temperature tempering of the present invention;

[0027] Figure 5This is a curve diagram of the yield strength and tensile strength of 42CrMo steel according to the present invention as a function of tempering temperature;

[0028] Figure 6 This is a curve of the elongation and cross-sectional shrinkage of 42CrMo steel according to the present invention as a function of temperature.

[0029] Figure 7 This is a curve of the impact energy of 42CrMo steel according to the present invention as a function of the impact test temperature. DETAILED DESCRIPTION

[0030] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0031] The components of the 42CrMo steel of the present invention are C, Si, Mn, P, S, Cu, Cr, Ni and Fe, as shown in Table 1 by weight percentage:

[0032]

[0033] Example 1

[0034] The 42CrMo steel is directly heat treated after forging. The heat treatment method includes the following steps:

[0035] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet;

[0036] (2) Normalizing: Stepwise heating to 550℃ and holding for 2 hours, then to 880℃ and holding for 3 hours; slowly cool (≤55℃ / h) to 300℃ and reheat the furnace. This forms a balanced structure consisting mainly of fine lamellar pearlite and a small amount of ferrite, providing a uniform matrix for subsequent quenching.

[0037] (3) First quenching: heat to 550℃, keep warm for 1.5 hours, heat to austenitizing temperature of 840℃ and keep warm for 2 hours, use brine rapid cooling + oil cooling to cool to 200℃ and re-load the furnace;

[0038] (4) Second quenching: heat to 550℃, keep warm for 1.5 hours, heat to austenitizing temperature 850℃ and keep warm for 2 hours, cool in brine at a rate of ≥200℃ / s, and cool in oil to 100℃;

[0039] (5) Tempering: Directly heat to 620℃ and keep warm for 4 hours, then cool in oil.

[0040] According to the method of directly heat treating 42CrMo steel after forging in the above embodiment, a 100 mm thick steel material was subjected to tensile test and impact test to test the mechanical properties.

[0041] Comparative Example 1

[0042] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet;

[0043] (2) Normalizing: Raise the temperature to 880℃ and keep it for 3-3.5 hours, then air cool.

[0044] (3) Heating to the austenitizing temperature of 850°C and holding for 2 hours, followed by oil quenching;

[0045] (4) Tempering: directly heat to 600℃ and keep warm for 3 hours, then air cool.

[0046] According to the method of directly heat treating 42CrMo steel after forging in the above comparative example, a 100 mm thick steel material was subjected to tensile test and impact test to conduct mechanical property test.

[0047] Example 2

[0048] The 42CrMo steel is directly heat treated after forging. The heat treatment method includes the following steps:

[0049] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet;

[0050] (2) Normalizing: Stepwise heating to 550℃ and holding for 2 hours, then to 890℃ and holding for 3 hours; slowly cool (≤55℃ / h) to 300℃ and reheat the furnace. This forms a balanced structure consisting mainly of fine lamellar pearlite and a small amount of ferrite, providing a uniform matrix for subsequent quenching.

[0051] (3) First quenching: heat to 560℃, keep warm for 1.5 hours, heat to austenitizing temperature of 840℃ and keep warm for 2 hours, use brine rapid cooling + oil cooling to cool to 200℃ and re-load the furnace;

[0052] (4) Second quenching: heat to 560℃, keep warm for 1.5 hours, heat to austenitizing temperature 8500℃ and keep warm for 2 hours, cool in brine at a rate of ≥200℃ / s, and cool in oil to 100℃;

[0053] (5) Tempering: directly heat to 630℃ and keep warm for 3 hours, then oil cool

[0054] According to the method of directly heat treating 42CrMo steel after forging in the above embodiment, a 100 mm thick steel material was subjected to tensile test and impact test to test the mechanical properties.

[0055] Comparative Example 2

[0056] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet;

[0057] (2) Normalizing: Raise the temperature to 880℃ and keep it for 3 hours, then air cool.

[0058] (3) Heating to the austenitizing temperature of 850°C and holding for 2 hours, followed by oil quenching;

[0059] (4) Tempering: directly heat to 630℃ and keep warm for 3 hours, then air cool.

[0060] According to the method of directly heat treating 42CrMo steel after forging in the above comparative example, a 100 mm thick steel material was subjected to tensile test and impact test to conduct mechanical property test.

[0061] Example 3

[0062] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet;

[0063] (2) Normalizing: Stepwise heating to 550℃ and holding for 2 hours, then to 880℃ and holding for 3 hours; slowly cool (≤55℃ / h) to 300℃ and reheat the furnace. This forms a balanced structure consisting mainly of fine lamellar pearlite and a small amount of ferrite, providing a uniform matrix for subsequent quenching.

[0064] (3) First quenching: heat to 550℃, keep warm for 2 hours, heat to austenitizing temperature 840℃ and keep warm for 2 hours, use brine rapid cooling + oil cooling to cool to 200℃ and re-load the furnace;

[0065] (4) Second quenching: heat to 550℃, keep warm for 2 hours, heat to austenitizing temperature 850℃ and keep warm for 2 hours, cool in brine at a rate of ≥200℃ / s, and cool in oil to 100℃;

[0066] (5) Tempering: Directly heat to 620℃ and keep warm for 4 hours, then cool in oil.

[0067] According to the method of directly heat treating 42CrMo steel after forging in the above embodiment, a 100 mm thick steel material was subjected to tensile test and impact test to test the mechanical properties.

[0068] Comparative Example 3

[0069] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet;

[0070] (2) Normalizing: Raise the temperature to 880℃ and keep it for 3.5 hours, then air cool.

[0071] (3) Heating to the austenitizing temperature of 850°C and holding for 2 hours, followed by oil quenching;

[0072] (4) Tempering: directly heat to 620℃ and keep warm for 4 hours, then air cool.

[0073] According to the method of directly heat treating 42CrMo steel after forging in the above comparative example, a 100 mm thick steel material was subjected to tensile test and impact test to conduct mechanical property test.

[0074] Example 4

[0075] The 42CrMo steel is directly heat treated after forging. The heat treatment method includes the following steps:

[0076] (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet;

[0077] (2) Normalizing: Stepwise heating to 560℃ and holding for 2 hours, then to 890℃ and holding for 3 hours; then slowly cooling (≤55℃ / h) to 300℃ and recharging the furnace. This forms a balanced structure consisting mainly of fine lamellar pearlite and a small amount of ferrite, providing a uniform matrix for subsequent quenching.

[0078] (3) First quenching: heat to 560℃, keep warm for 1.5 hours, heat to austenitizing temperature of 840℃ and keep warm for 2 hours, use brine rapid cooling + oil cooling to cool to 200℃ and re-load the furnace;

[0079] (4) Second quenching: heat to 560℃, keep warm for 1.5 hours, heat to austenitizing temperature 850℃ and keep warm for 2 hours, cool in brine at a rate of ≥200℃ / s, and cool in oil to 100℃;

[0080] (5) Tempering: Directly heat to 620℃ and keep warm for 4 hours, then cool in oil.

[0081] According to the method of directly heat treating 42CrMo steel after forging in the above embodiment, a 100 mm thick steel material was subjected to tensile test and impact test to test the mechanical properties.

[0082] Table 2 Mechanical properties of the embodiments of the present invention and comparative examples

[0083]

[0084]

[0085] The present invention utilizes a low-temperature austenitization at 840°C to inhibit grain growth, and a graded cooling strategy of oil quenching, salt water cooling and oil cooling. First, salt water is used for rapid cooling to avoid pearlite transformation, and then oil cooling is used to reduce thermal stress. The microstructure changes from the quenched state to the tempered state, namely, cryptocrystalline martensite transforms to tempered bainite (ferrite matrix + dispersed carbides), thereby reducing residual stress by more than 90%, and finally obtaining 42CrMo steel with excellent comprehensive mechanical properties.

[0086] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A 42CrMo steel, characterized in that: Contains by weight: 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: ≤0.035%, S: ≤0.035%, Cu: ≤0.30%, Ni: ≤0.30%, and the balance is Fe.

2. The 42CrMo steel composition according to claim 1 is: 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, 0.03% S, 0.03% Cu, 0.1% Cr, 0.03% Ni, and the balance is Fe.

3. The 42CrMo steel according to claim 2 comprises 0.4% C, 0.25% Si, 0.65% Mn, 1% Cr, 0.2% Mo, 0.035% P, 0.035% S, 0.03% Cu, 0.03% Cr, 0.03% Ni, and the balance is Fe.

4. The heat treatment production method for 42CrMo steel after forging according to any one of claims 1 to 3, characterized in that: The heat treatment method comprises the following steps: (1) Forging billet: According to the composition of 42CrMo steel, it is smelted and cast into an ingot, and the ingot is forged into a forging billet; (2) Normalizing: Raise the temperature step by step to 550±10℃ and keep it warm, then raise it to 880±10℃ and keep it warm for 3-3.5 hours, then cool it slowly. (3) First quenching: heat to 550±10℃, keep warm for 1.5-2 hours, heat to austenitizing temperature 840±10℃, keep warm for 1.5-2 hours, cool to 200℃ and reheat; (4) Second quenching: heat to 550±10℃, keep warm for 1.5-2 hours, heat to austenitizing temperature 850±10℃, keep warm for 1.5-2 hours, and then cool; (5) Tempering: Air cool the forging billet to room temperature and then heat it to 620±10℃ and keep it at this temperature for 3-4 hours, then cool it down.

5. The heat treatment production method for 42CrMo steel after forging according to claim 4, characterized in that: The slow cooling rate in step (2) is less than 55°C / h and the furnace is reloaded after slow cooling to 300°C.

6. The heat treatment production method for 42CrMo steel after forging according to claim 5, characterized in that: A balanced structure is formed, which is mainly composed of fine lamellar pearlite and a small amount of ferrite.

7. The heat treatment production method for 42CrMo steel after forging according to claim 4, characterized in that: Step (3) uses brine rapid cooling + oil cooling graded cooling.

8. The heat treatment production method for 42CrMo steel after forging according to claim 4, characterized in that: In step (4), brine rapid cooling + oil cooling graded cooling is adopted, and the brine cooling rate is ≥200°C / s.

9. A heat treatment production method for 42CrMo steel after forging according to claims 7-8, characterized in that: It can reduce quenching internal stress and avoid cracking.

10. The heat treatment production method for 42CrMo steel after forging according to claim 4, characterized in that: Step (5) directly heat the forging to 620±10°C, keep it at that temperature for 3-4 hours, and then cool it.

11. The method according to claim 10, characterized in that: Grain size ≥ grade 9, retained austenite content <2%.

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