Large free forging press falling die and preparation method thereof

Abstract

The invention relates to a large free forging press falling die and a preparation method thereof, and aims to solve the problems of R angle stress concentration, short fatigue life and poor wear resistance of the existing falling die. The method comprises the steps that S1, die steel is smelted and subjected to vacuum degassing, die forging and thermal refining, and a die base body is obtained; s2, a three-section R-angle structure is machined on the upper portion of the mold base body, and the splicing position of every two adjacent sections meets curvature continuity; s3, a pre-stress ring is hotly installed in an annular groove in the back of the R corner, and initial pre-stress is formed; s4, the workpiece subjected to hot charging is heated, a pulsed magnetic field is applied for aging treatment, and the pre-compressive stress is stabilized; s5, a hexagonal micro-pit array is engraved on the surface of the R corner through laser; and S6, multiple layers of nano coatings are deposited on the surfaces of the micro pits through an inclined rotating PVD technology. A gradient compression layer is formed on the surface of the R angle of a finished product, the coating is excellent in binding force and hardness, fatigue life is greatly prolonged compared with that of a traditional process, abrasion loss is remarkably reduced, and the method is suitable for extreme working conditions of large free forging presses.

Description

Technical Field

[0001] This invention relates to the field of forging, and more particularly to a large free forging press die and its preparation method. Background Technology

[0002] Large free forging presses are key equipment in the heavy machinery manufacturing industry, mainly used for free forging processes such as billet preparation, drawing, and upsetting of large forgings weighing hundreds of tons or more. The slamming die (or clamping die, forming die), as its core mold component, must withstand high-frequency (several to tens of times per minute) and large loads (thousands to tens of thousands of tons) of intense impacts during operation, while also operating under extremely harsh conditions of high temperature (forging temperatures can reach 1000-1200℃) and high wear.

[0003] In existing technologies, large-scale die-casting molds are generally made of integrally cast or forged medium-carbon alloy hot-work die steel, and undergo quenching and tempering treatment (quenching + high-temperature tempering) to obtain the required comprehensive mechanical properties. However, the following systemic defects urgently need to be addressed:

[0004] The weak points of the die-casting mold structure are highly concentrated in the radius (R-corner) region at the edge of the cavity. Under alternating impact loads, this region exhibits a significant stress concentration effect, with a theoretical stress concentration factor (Kt) exceeding 1.8. Finite element analysis (FEA) shows that under conventional forging loads, the maximum stress peak at the R-corner easily exceeds 480 MPa. This value not only far exceeds the yield strength of the material at 600℃ (approximately 550 MPa) but also approaches its tensile strength limit, leading to the initiation and rapid propagation of fatigue cracks. Practice has shown that after only 500-1000 forging cycles, visible thermal fatigue cracks appear at the R-corner, ultimately causing premature die failure. This not only results in frequent die replacements and soaring production costs but also severely disrupts the production rhythm due to unplanned downtime, becoming a key bottleneck restricting production efficiency and reliability. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a large-scale free forging press die and its preparation method, thereby solving the problems existing in the prior art.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the present invention provides the following technical solution: a large free forging press die and its preparation method, comprising:

[0009] S1 mold substrate pretreatment:

[0010] S11. Die steel is smelted in an electric arc furnace, and then die-forged into a billet after undergoing triple vacuum degassing treatment;

[0011] S12. The blank is subjected to hardness tempering treatment to obtain a mold base; the hardness tempering treatment includes normalizing treatment and tempering treatment;

[0012] The normalizing treatment includes heating the billet to 850-870°C, holding it at that temperature for 2-3 hours, and then cooling it evenly in still air after it is taken out of the furnace.

[0013] The tempering process includes heating the billet to 640-660°C, holding it at that temperature for 4-5 hours, furnace cooling it to below 300°C, and then air cooling it to room temperature.

[0014] S2. R-angle machining:

[0015] A three-section radius (R-angle) structure is machined on the upper part of the mold base using a five-axis CNC machining center, specifically including:

[0016] Entry segment: is the core area of ​​an elliptical arc with a radius R = 200 mm and a central angle of 45°. Its length accounts for 25% of the total length of the R angle.

[0017] Transition section: This is a gently deformable connection zone, and its length accounts for 15% of the total length of the radius angle.

[0018] Load-bearing section: a large-radius circular arc transition zone with a radius of R = 385 mm and a central angle of 90°, the length of which accounts for 60% of the total length of the R angle;

[0019] The inlet section, transition section and load-bearing section are connected in sequence, and the splice of two adjacent sections satisfies C2 continuity (curvature continuity); the processing accuracy is controlled within ±0.1mm.

[0020] S3. Hot-fitted prestressed ring:

[0021] S31. A ring groove is machined on the back of the R-angle;

[0022] S32. Heat the mold base to 200℃;

[0023] S33. After heating the GH4169 high-temperature alloy ring to 450℃, it is fitted into the R-angle back groove. After cooling, a pre-compression stress of 200MPa is generated.

[0024] S4. Magnetic field aging treatment:

[0025] When the alloy ring is cooled to 200°C, a 1.5T pulsed magnetic field is applied along its axial direction for 8 minutes.

[0026] S5. Micro-pit texture processing:

[0027] A hexagonal micro-pit array is laser-engraved on the R-angle surface after magnetic field treatment.

[0028] S6. Nano-coating deposition:

[0029] A multilayer AlCrN / TiSiN nanocoating was deposited on the surface of the micro-pit using a 55° tilt rotation PVD process.

[0030] Further, in step S11, the mold steel is H13 mold steel, and its chemical composition by weight percentage is: C 0.38-0.45%, Cr 4.75-5.50%, Mo 1.10-1.75%, V 0.80-1.20%, Si 0.80-1.20%, Mn 0.20-0.50%, with the balance being Fe.

[0031] Furthermore, in step S32, the heating step includes: preheating the mold base, heating the mold base to 200°C at a heating rate of 50°C / h, and holding it at that temperature for 2h to eliminate thermal stress;

[0032] High-frequency induction heating was used to uniformly heat the GH4169 prestressed ring to 450±10℃ and hold it at that temperature for 1 hour.

[0033] Furthermore, in step S33, the design interference between the GH4169 high-temperature alloy ring and the annular groove is 0.40±0.02mm.

[0034] Furthermore, step S4 specifically includes: placing the heat-fitted workpiece in a heating device and heating it to 200℃±3℃ at a rate of 5° / min, then holding it at the target temperature for 5 minutes. When the temperature stabilizes at 200℃±3℃, a pulsed magnetic field is applied and maintained for 8 minutes. After the magnetic field effect ends, the temperature is held constant for 5 minutes, then cooled to 150℃ at a rate of 2℃ / min, and then allowed to cool naturally to room temperature; the temperature is continuously monitored during the cooling process to avoid stress fluctuations caused by rapid cooling.

[0035] Furthermore, the parameters of the pulsed magnetic field are: axial magnetic field strength 1.5T, pulse frequency 5Hz, and square wave waveform (rising / falling edge ≤10μs).

[0036] Furthermore, the purpose of step S4 is to regulate the dislocation structure and precipitate state inside the GH4169 alloy ring, stabilize the initial pre-pressure of 200MPa generated by hot fitting to above 280MPa, reduce the stress relaxation rate during long-term service (≤5%), and refine the grains and improve the mechanical stability of the alloy ring.

[0037] The strengthening mechanism of the pulsed magnetic field aging treatment is that, under a specific temperature condition of 200℃±3℃, applying a high-intensity pulsed magnetic field can produce significant magnetostrictive and thermo-mechanical coupling effects within the GH4169 alloy ring.

[0038] First, the Lorentz force generated by the alternating pulsed magnetic field induces microscopic eddies, which exert oscillating forces on dislocations, promoting dislocation slippage, recombination, and annihilation. This reduces dislocation density and forms a more stable dislocation grid structure, effectively suppressing dislocation movement and proliferation during subsequent service. This is the key to reducing stress relaxation rate.

[0039] Secondly, this treatment promotes the uniform and dispersed precipitation of the main strengthening phases γ(Ni3Nb) and γ'(Ni3(Al,Ti)) in the alloy, and refines the size of the precipitated phases. The uniform and fine precipitated phases can more effectively pin dislocations and hinder grain boundary slip, thereby improving the yield strength and creep resistance of the alloy and keeping the preload stable under high temperature and high load.

[0040] Furthermore, in step S6, the specific process parameters for nano-coating deposition are as follows:

[0041] When depositing the AlCrN underlayer, an AlCr target with a mass ratio of 7:3 was used, the target current was set to 100A, the nitrogen flow rate to 300sccm, the working pressure to 0.5Pa, the substrate bias voltage to -80V, the deposition temperature to 200℃, the workpiece to be rotated at a 55° angle, the deposition time to 60 minutes, and the deposition thickness of the AlCrN underlayer to 3-5μm.

[0042] When depositing the TiSiN top layer, a TiSi target with a mass ratio of 9:1 was used. The target current was set to 120A, the nitrogen flow rate to 250sccm, the working pressure to 0.5Pa, the substrate bias voltage to -100V, the deposition temperature to 200℃ and the workpiece to a 55° rotation angle were maintained, and the deposition time to 90 minutes. The deposition thickness of the TiSiN top layer was 8-11μm.

[0043] The present invention also provides a large free forging press die made by the above processing method, comprising a die base, the upper part of which is provided with a three-section R-corner structure, the back of which is thermally fitted with a GH4169 high-temperature alloy ring, the surface of which is provided with a hexagonal micro-pit array, and the surface of which is deposited with an AlCrN / TiSiN multilayer nano-coating.

[0044] (III) Beneficial Effects

[0045] The present invention aims to provide a large-scale free forging press die and its preparation method, which has the following beneficial effects.

[0046] First, the hot-fitting process is combined with magnetic field aging treatment: an initial prestress foundation is established first through the hot-fitting process of the GH4169 alloy ring, and then the dislocation structure and precipitate state inside the alloy ring are controlled by the pulsed magnetic field under specific temperature conditions. This synergy not only improves the initial prestress level, but also significantly suppresses stress relaxation during long-term service, keeping the prestress stable.

[0047] Secondly, a micro-pit array is formed on the R-corner surface through laser processing to construct a gradient stress layer and a three-dimensional anchoring foundation. Then, a nano-coating is deposited using a tilting and rotating PVD process, allowing the coating material to fully fill the micro-pit structure and form a mechanical locking effect. This synergy not only significantly improves the coating adhesion but also achieves simultaneous optimization of fatigue resistance and wear resistance. Attached Figure Description

[0048] Figure 1 This is an overall schematic diagram of a large free forging press die in this invention;

[0049] In the figure: 1-Mold base, 2-Prestressed ring. Detailed Implementation

[0050] The following will refer to the appendix in the examples of this invention. Figure 1 The technical solutions in the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0051] Example 1:

[0052] This embodiment focuses on the die-casting process of a large free forging press with a nominal pressure of 10000kN. The processing steps are as follows:

[0053] S1. Mold substrate pretreatment:

[0054] S11. H13 mold steel (chemical composition by weight percentage: C 0.42%, Cr 5.0%, Mo 1.5%, V 1.0%, Si 1.0%, Mn 0.3%, balance Fe) is smelted in a 50t electric arc furnace at a melting temperature of 1620℃, during which triple vacuum degassing is performed (vacuum degree ≤1×10⁻⁶). -2 Pa (degassing for 30 minutes each time) removes gaseous impurities such as H and O from the molten steel; the molten steel is then cast into Φ900mm round ingots, which are then die-forged using a 3000t high-speed forging machine (forging temperature 1150℃, deformation 60%) to obtain a billet with length × width × height = 2000 × 1500 × 800mm, which is then slowly cooled to below 500℃ after forging.

[0055] S12. The blank is subjected to hardness tempering treatment to obtain a mold base; the hardness tempering treatment includes normalizing treatment and tempering treatment;

[0056] The normalizing treatment includes heating the billet to 860°C, holding it at that temperature for 2.5 hours, and then cooling it evenly in still air after it is removed from the furnace.

[0057] The tempering process includes heating the blank to 650°C, holding it at that temperature for 4.5 hours, furnace cooling to 300°C, and then air cooling to room temperature; the hardness of the mold substrate after the process is HRC40.

[0058] S2. R-angle machining:

[0059] A three-section radius (R-angle) structure is machined on the upper part of the mold base using a five-axis CNC machining center, specifically including:

[0060] Entry segment: is the core area of ​​an elliptical arc with a radius R = 200 mm and a central angle of 45°. Its length accounts for 25% of the total length of the R angle.

[0061] Transition section: This is a gently deformable connection zone, and its length accounts for 15% of the total length of the radius angle.

[0062] Load-bearing section: a large-radius circular arc transition zone with a radius of R = 385 mm and a central angle of 90°, the length of which accounts for 60% of the total length of the R angle;

[0063] The inlet section, transition section and load-bearing section are connected in sequence, and the splice of two adjacent sections satisfies C2 continuity (curvature continuity); the machining is carried out with a Φ12mm diamond-coated ball end mill, a rotation speed of 8000rpm, a feed rate of 1200mm / min, a finishing depth of cut of 0.3mm, and a machining accuracy controlled within ±0.1mm.

[0064] S3. Hot-fitted prestressed ring:

[0065] S31. A ring groove adapted to the prestressed ring is machined on the back of the R-angle, with a groove bottom roughness Ra = 1.6 μm;

[0066] S32. Heat the mold base to 200℃;

[0067] S33. After heating the GH4169 high-temperature alloy ring (with a design interference of 0.40mm with the annular groove) to 450℃, it is fitted into the R-angle back groove. After cooling, a pre-compression stress of 200MPa is generated.

[0068] S4. Magnetic field aging treatment:

[0069] The heat-fitted workpiece is placed in a heating device and heated to 201°C at a rate of 50°C / h. A 1.5T pulsed magnetic field (5Hz frequency) is applied along its axis for 8 minutes. During the process, the temperature is controlled by an infrared thermal imager (temperature difference ≤2°C). After the magnetic field is applied, the workpiece is cooled to room temperature in stages, and the final pre-pressure is 286MPa.

[0070] S5. Micro-pit texture processing:

[0071] On the R-corner surface after magnetic field treatment, a hexagonal micro-pit array with a diameter of 200μm and a depth of 80μm was engraved using a fiber laser (wavelength 1064nm, pulse width 100ns, energy 50mJ, spot diameter 35μm). The center-to-center distance of the hexagonal micro-pit array was 400μm.

[0072] S6. Nano-coating deposition:

[0073] A 55° tilt-rotation PVD process was used to deposit a multilayer AlCrN / TiSiN nanocoating on the surface of the micropits. The specific process parameters are as follows:

[0074] When depositing the AlCrN underlayer, an AlCr target with a mass ratio of 7:3 was used, the target current was set to 100A, the nitrogen flow rate to 300sccm, the working pressure to 0.5Pa, the substrate bias voltage to -80V, the deposition temperature to 200℃, the workpiece to be rotated at a 55° angle, the deposition time to 60 minutes, and the deposition thickness of the AlCrN underlayer to 4μm.

[0075] When depositing the TiSiN top layer, a TiSi target with a mass ratio of 9:1 was used. The target current was set to 120A, the nitrogen flow rate to 250sccm, the working pressure to 0.5Pa, the substrate bias voltage to -100V, the deposition temperature to 200℃ and the workpiece to a 55° rotation angle were maintained, and the deposition time to 90 minutes. The deposition thickness of the TiSiN top layer was 9μm.

[0076] Testing revealed that the large free forging die prepared using the above method exhibits the following characteristics: a pre-compression stress of 285 MPa for the GH4169 high-temperature alloy ring; a micro-pit diameter of 200 μm and a depth of 80 μm for the hexagonal micro-pit array; a total thickness of 10 μm for the AlCrN / TiSiN multilayer nano-coating; a hardness of HV3000; a bonding strength of 185 N; and a fatigue life of 6000 cycles under simulated forging conditions (impact load of 5000 kN).

[0077] Example 2

[0078] This embodiment focuses on the die-casting process of a large free forging press with a nominal pressure of 15000kN. The processing steps are as follows:

[0079] S1. Mold substrate pretreatment

[0080] S11. Raw material preparation: H13 mold steel (chemical composition by weight percentage: C 0.45%, Cr 5.50%, Mo 1.75%, V 1.20%, Si 1.20%, Mn 0.50%, balance Fe) was smelted in a 60t electric arc furnace at a melting temperature of 1650℃, followed by triple vacuum degassing (vacuum degree ≤ 8×10⁻⁶). -3 Pa, degassing for 35 minutes each time); the molten steel is cast into Φ1000mm round ingots, which are then die-forged by a 4000t high-speed forging machine (forging temperature 1200℃, deformation amount 65%) to obtain a billet with length × width × height = 2200 × 1600 × 900mm, and then slowly cooled to below 480℃ after forging.

[0081] S12. Conditioning and tempering:

[0082] Normalizing: Heat to 870℃, hold for 3 hours, then remove from the furnace and air cool (cooling rate 20℃ / h);

[0083] Tempering: Heat to 660℃, hold for 5 hours, furnace cool to 280℃, then air cool. Final matrix hardness: HRC42, impact toughness: 18 J / cm. 2 .

[0084] S2. R-angle machining

[0085] A three-section radius (R-angle) structure is machined on the upper part of the mold base using a five-axis CNC machining center, specifically including:

[0086] Entry segment: is the core area of ​​an elliptical arc with a radius R = 200 mm and a central angle of 45°. Its length accounts for 25% of the total length of the R angle.

[0087] Transition section: This is a gently deformable connection zone, and its length accounts for 15% of the total length of the radius angle.

[0088] Load-bearing section: a large-radius circular arc transition zone with a radius of R = 385 mm and a central angle of 90°, the length of which accounts for 60% of the total length of the R angle;

[0089] The inlet section, transition section and load-bearing section are connected in sequence, and the splice of two adjacent sections satisfies C2 continuity (curvature continuity); the machining is carried out with a Φ12mm diamond-coated ball end mill, a rotation speed of 8000rpm, a feed rate of 1200mm / min, a finishing depth of cut of 0.3mm, and a machining accuracy controlled within ±0.1mm.

[0090] S3. Hot-fitted prestressed ring

[0091] S31. A ring groove adapted to the prestressed ring is machined on the back of the R-angle, with a groove bottom roughness Ra = 1.6 μm;

[0092] S32. Heat the mold base to 200℃;

[0093] S33. After heating the GH4169 high-temperature alloy ring (with a design interference of 0.40mm with the annular groove) to 450℃, it is fitted into the R-angle back groove. After cooling, the initial preload is 208MPa.

[0094] S4. Magnetic field aging treatment

[0095] The heat-fitted workpiece is placed in a heating device and heated to 203℃ at a rate of 50℃ / h. A 1.5T pulsed magnetic field (frequency 5Hz, square wave rise time 8μs) is applied along the axial direction and continued for 8 minutes. During the process, the temperature is controlled by an infrared thermal imager (temperature difference ≤2℃). After the magnetic field is applied, the workpiece is cooled to room temperature in stages, and the final pre-pressure is 288MPa.

[0096] S5. Micro-pit texture processing

[0097] On the R-corner surface after magnetic field treatment, a hexagonal micro-pit array with a diameter of 200μm and a depth of 80μm was engraved using a fiber laser (wavelength 1064nm, pulse width 100ns, energy 50mJ, spot diameter 35μm). The center-to-center distance of the hexagonal micro-pit array was 400μm.

[0098] S6. Nanocoating deposition

[0099] When depositing the AlCrN underlayer, an AlCr target with a mass ratio of 7:3 was used, the target current was set to 105A, the nitrogen flow rate to 320sccm, the working pressure to 0.5Pa, the substrate bias voltage to -85V, the deposition temperature to 200℃, the workpiece to be rotated at a 55° angle, the deposition time to 65 minutes, and the deposition thickness of the AlCrN underlayer to 5μm.

[0100] When depositing the TiSiN top layer, a TiSi target with a mass ratio of 9:1 was used. The target current was set to 125A, the nitrogen flow rate to 260sccm, the working pressure to 0.5Pa, the substrate bias voltage to -105V, the deposition temperature to 200℃ and the workpiece to a 55° rotation angle were maintained, and the deposition time to 95 minutes. The deposition thickness of the TiSiN top layer was 6.5μm.

[0101] The total coating thickness is 11.5μm, the hardness is HV3200, and the adhesion strength tested by the scratch test is 192N.

[0102] Product performance: under simulated forging conditions (impact load 6000kN, 1200 cycles), no cracks were found in the R-angle; fatigue life reached 6500 cycles, and the coating in the micro-pits showed no peeling.

[0103] Example 3

[0104] This embodiment focuses on the die-casting process of a large free forging press with a nominal pressure of 12000kN. The processing steps are as follows:

[0105] S1. Mold substrate pretreatment

[0106] S11. Raw material preparation: H13 mold steel (chemical composition by weight percentage: C 0.38%, Cr 4.75%, Mo 1.10%, V 0.80%, Si 0.80%, Mn 0.20%, balance Fe) was smelted in a 50t electric arc furnace at a melting temperature of 1600℃, followed by triple vacuum degassing (vacuum degree ≤ 1×10⁻⁶). -2 Pa, degassing for 30 minutes each time); molten steel is cast into Φ900mm round ingots, which are then die-forged by a 3000t high-speed forging machine (forging temperature 1100℃, deformation amount 58%) to obtain a billet with length × width × height = 2000 × 1500 × 800mm, and then slowly cooled to below 500℃ after forging.

[0107] S12. Conditioning and tempering:

[0108] Normalizing: Heat to 850℃, hold for 2 hours, then remove from the furnace and air cool (cooling rate 15℃ / h);

[0109] Tempering: Heat to 640℃, hold for 4 hours, furnace cool to 300℃, then air cool. Final matrix hardness: HRC38, impact toughness: 16 J / cm. 2 .

[0110] S2. R-angle machining

[0111] A three-section radius (R-angle) structure is machined on the upper part of the mold base using a five-axis CNC machining center, specifically including:

[0112] Entry segment: is the core area of ​​an elliptical arc with a radius R = 200 mm and a central angle of 45°. Its length accounts for 25% of the total length of the R angle.

[0113] Transition section: This is a gently deformable connection zone, and its length accounts for 15% of the total length of the radius angle.

[0114] Load-bearing section: a large-radius circular arc transition zone with a radius of R = 385 mm and a central angle of 90°, the length of which accounts for 60% of the total length of the R angle;

[0115] The inlet section, transition section and load-bearing section are connected in sequence, and the splice of two adjacent sections satisfies C2 continuity (curvature continuity); the machining is carried out with a Φ12mm diamond-coated ball end mill, a rotation speed of 8000rpm, a feed rate of 1200mm / min, a finishing depth of cut of 0.3mm, and a machining accuracy controlled within ±0.1mm.

[0116] S3. Hot-fitted prestressed ring

[0117] S31. A ring groove adapted to the prestressed ring is machined on the back of the R-angle, with a groove bottom roughness Ra = 1.6 μm;

[0118] S32. Heat the mold base to 200℃;

[0119] S33. After heating the GH4169 high-temperature alloy ring (with a design interference of 0.39mm with the annular groove) to 450℃, it is fitted into the R-angle back groove. After cooling, the initial preload is 195MPa.

[0120] S4. Magnetic field aging treatment

[0121] The workpiece that has been heat-fitted is placed in a heating device and heated to 198°C at a rate of 50°C / h. A 1.5T pulsed magnetic field (frequency 5Hz, square wave falling edge 9μs) is then applied for 8 minutes. The temperature control accuracy is ±3°C, and the pre-pressure after cooling is 275MPa.

[0122] S5. Micro-pit texture processing

[0123] On the R-corner surface after magnetic field treatment, a hexagonal micro-pit array with a diameter of 200μm and a depth of 80μm was engraved using a fiber laser (wavelength 1064nm, pulse width 100ns, energy 50mJ, spot diameter 35μm). The center-to-center distance of the hexagonal micro-pit array was 400μm.

[0124] S6. Nanocoating deposition

[0125] When depositing the AlCrN underlayer, an AlCr target with a mass ratio of 7:3 was used, the target current was set to 98A, the nitrogen flow rate to 290sccm, the working pressure to 0.5Pa, the substrate bias voltage to -78V, the deposition temperature to 200℃, the workpiece to be rotated at a 55° angle, the deposition time to 58 minutes, and the deposition thickness of the AlCrN underlayer to 4.8μm.

[0126] When depositing the TiSiN top layer, a TiSi target with a mass ratio of 9:1 was used. The target current was set to 118A, the nitrogen flow rate to 245sccm, the working pressure to 0.5Pa, the substrate bias voltage to -98V, the deposition temperature to 200℃ and the workpiece to a 55° rotation angle were maintained, and the deposition time to 95 minutes. The deposition thickness of the TiSiN top layer was 5.1μm.

[0127] The total coating thickness is 9.9μm, the hardness is HV2900, and the adhesion is 178N.

[0128] Product performance: Under simulated forging conditions (impact load 4500kN), the fatigue life reaches 5200 cycles, and the wear on the R-angle surface is reduced by 60% compared with traditional die-casting.

[0129] Comparative Example 1:

[0130] The large free forging press die, prepared using a conventional process, is based on the die with the same nominal pressure (10000kN) and dimensions as that in Example 1. The specific steps are as follows:

[0131] Melting and forming: H13 die steel (chemical composition consistent with Example 1) was melted in a 50t electric arc furnace; the molten steel was cast into Φ900mm round ingots, and then die-forged by a 2000t high-speed forging machine (forging temperature 1150℃, deformation amount 45%) to obtain billets of the same size, which were then naturally cooled after forging.

[0132] Heat treatment: Tempering treatment (heated to 650℃, held for 3 hours, then air-cooled after removal from the furnace), final matrix hardness HRC36, impact toughness 12J / cm. 2 .

[0133] The core performance indicators of Examples 1-3 and Comparative Example 1 were tested according to the following standards:

[0134] Fatigue life: Performed according to GB / T15248-2008 "Metallic Materials - Axial Constant Amplitude Low Cyclic Fatigue Test Method";

[0135] Coating adhesion: Performed according to GB / T2792-2014 "Test Method for Peel Strength of Adhesives";

[0136] Hardness: Performed according to GB / T230.1-2018 "Metallic materials Rockwell hardness test - Part 1: Test method".

[0137] The results are shown in the table below:

[0138]

[0139] Based on the above, we can conclude that:

[0140] Comparative Example 1 relied solely on sandblasting to introduce a weak compressive stress (-120 MPa) without any prestressing reinforcement structure, resulting in a fatigue life of only 1000 cycles and a tendency for early cracking. This invention constructs a synergistic reinforcement system of "thermally fitted prestressed ring + magnetic field aging + laser micro-pit gradient layer": a stable prestress of 275-288 MPa is formed through thermal fitting and magnetic field aging, superimposed with a laser micro-pit gradient compression layer of -340 to -360 MPa. This system strengthens the system from two dimensions: "actively offsetting tensile stress" and "passively improving crack resistance," resulting in an average 480% increase in fatigue life (reaching 5200-6500 cycles), completely solving the problem of early failure in traditional mold-making processes.

[0141] Comparative Example 1, lacking a dedicated wear-resistant coating, experienced 80μm of wear on the R-angle after a single forging, necessitating frequent mold repairs. Furthermore, traditional coating processes generally suffer from poor adhesion and easy peeling. This invention utilizes a composite process of "laser micro-pit three-dimensional anchoring + 55° tilted rotating PVD nano-coating" to form an AlCrN / TiSiN multilayer coating (hardness HV2900-3200, adhesion 178-192N) on the R-angle surface, reducing single-cycle wear by 68.8% (to only 22-28μm). This avoids coating peeling, significantly extends the mold repair cycle, and reduces production costs.

[0142] In summary, this invention, through a "multi-process synergistic strengthening system," achieves a qualitative leap in core indicators such as precision, fatigue resistance, and wear resistance compared to traditional processes, fully meeting the extreme service requirements of high load and long service life of large free forging presses.

[0143] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a large free forging press die, comprising the following steps: S1 Mold Substrate Pretreatment: Mold steel is smelted in an electric arc furnace, and after triple vacuum degassing, it is forged into a blank. The blank is then subjected to hardness tempering treatment to obtain the mold substrate. S2 base machining: A three-section R-angle structure is machined on the upper part of the mold base; S3 Hot-fitted prestressed ring: An annular groove is machined on the back of the R-angle of the mold base. The mold base and the prestressed ring are heated and kept warm respectively. The prestressed ring is then hot-fitted into the mold base. S4 magnetic field aging treatment: Heat the completed workpiece to 200℃±3℃, and apply a pulsed magnetic field along its axis for 8 minutes; S5 micro-pit texture processing: A hexagonal micro-pit array is laser-engraved on the R-corner surface after magnetic field treatment to form a gradient compression layer; S6 Nanocoating Deposition: AlCrN and TiSiN nanocoatings were sequentially deposited on the surface of the micro-pit using a 55° tilt rotation PVD process.

2. The method for preparing a large free forging press die according to claim 1, characterized in that, In the S1 mold substrate pretreatment step, the hardness tempering treatment includes normalizing treatment and tempering treatment. The normalizing treatment includes heating the billet to 850-870°C, holding it at that temperature for 2-3 hours, and then cooling it evenly in still air after it is taken out of the furnace. The tempering process includes heating the billet to 640-660°C, holding it at that temperature for 4-5 hours, furnace cooling it to below 300°C, and then air cooling it to room temperature.

3. The method for preparing a large free forging press die according to claim 1, characterized in that, The three-segment R-angle structure specifically includes: Entry segment: is the core area of ​​an elliptical arc with a radius R = 200 mm and a central angle of 45°. Its length accounts for 15% of the total length of the R angle. Transition section: This is a gently deformable connection zone, and its length accounts for 25% of the total length of the radius angle. Load-bearing section: a large-radius circular arc transition zone with a radius of R = 385 mm and a central angle of 90°, the length of which accounts for 60% of the total length of the R angle; The inlet section, transition section and load-bearing section are connected in sequence, and the joints between adjacent sections satisfy the curvature continuity.

4. The method for preparing a large free forging press die according to claim 1, characterized in that, In the S3 hot-fitting prestressed ring step, the heating step includes: preheating the mold base to 200℃±10℃ at a heating rate of 50℃ / h and holding it at that temperature for 2h to eliminate thermal stress; and using high-frequency induction heating to heat the prestressed ring to 450±10℃ at a heating rate of 100℃ / h and holding it at that temperature for 1h.

5. The method for preparing a large free forging press die according to claim 1, characterized in that, In the S3 hot-fitting prestressed ring step, the prestressed ring is a GH4169 alloy ring.

6. The method for preparing a large free forging press die according to claim 1, characterized in that, The S4 magnetic field aging treatment steps specifically include: placing the completed heat-fitted workpiece in a heating device and heating it to 200℃±3℃ at a rate of 50℃ / h, and holding it at the target temperature for 5 minutes; applying a pulsed magnetic field and continuing to act for 8 minutes; after the magnetic field action ends, holding it at the temperature for 5 minutes, then cooling it to 150℃ at a rate of 2℃ / min, and then letting it cool naturally to room temperature.

7. The method for preparing a large free forging press die according to claim 6, characterized in that, The parameters of the pulsed magnetic field are: axial magnetic field strength 1.5T, pulse frequency 5Hz, and square wave waveform.

8. The method for preparing a large free forging press die according to claim 1, characterized in that, In the S5 micro-pit texture processing step, the diameter of the hexagonal micro-pit array is 200±5μm and the depth is 80±5μm.

9. The method for preparing a large free forging press die according to claim 1, characterized in that, In the S6 nano-coating deposition step, the specific process parameters for nano-coating deposition are as follows: When depositing the AlCrN underlayer, an AlCr target with a mass ratio of 7:3 was used, the target current was set to 100A, the nitrogen flow rate to 300sccm, the working pressure to 0.5Pa, the substrate bias voltage to -80V, the deposition temperature to 200℃, the workpiece to be rotated at a 55° angle, the deposition time to 60 minutes, and the deposition thickness of the AlCrN underlayer to 3-5μm. When depositing the TiSiN top layer, a TiSi target with a mass ratio of 9:1 was used, and the target current was set to 120A, the nitrogen flow rate to 250sccm, the working pressure to 0.5Pa, the substrate bias voltage to -100V, the deposition temperature to 200℃ and the workpiece to a 55° rotation angle were maintained, and the deposition time to 90 minutes. The deposition thickness of the TiSiN top layer was 8-11μm.

10. A large free forging press die made using the method for preparing a large free forging press die according to any one of claims 1-9, characterized in that, The mold includes a mold base, the upper part of which is provided with a three-section R-corner structure, a prestressed ring is thermally fitted on the back of the R-corner, and a hexagonal micro-pit array is provided on the surface of the R-corner. The surface of the micro-pit array is deposited with AlCrN and TiSiN multilayer nano-coatings.

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