Vacuum heat treatment hardening and tempering process for plastic die steel

Abstract

The invention provides a plastic die steel vacuum heat treatment tempering process, and relates to the technical field of plastic die steel heat treatment. The quenching and tempering process comprises a vacuum quenching process and a tempering process which are performed in sequence, and specifically comprises the following steps: 1) vacuum quenching; s1, workpiece pretreatment; s2, preparing a vacuum furnace; s3, graded preheating; s4, austenitizing treatment is carried out; s5, quenching and cooling; s6, checking after quenching; (2) tempering; t1, preparation before tempering; t2, tempering furnace inspection; t3, multi-frequency tempering is conducted; and T4, carrying out tempering post-treatment. According to the method, quenching is conducted in the vacuum environment, the problems of surface oxidation and decarburization caused by conventional heating are thoroughly avoided, the surface hardness of a workpiece is improved by 10%-15%, the abrasion resistance is enhanced, a subsequent polishing procedure is not needed, the production cost is reduced by 15%-20%, parameters such as interval time, conditions of the same furnace and heat preservation time are standardized in the tempering process, tempering is conducted at least twice, and the production efficiency is improved. The internal stress of the workpiece is ensured to be released thoroughly, the accuracy failure probability in the using process is reduced by 25%-30%, and the service life of the die is prolonged by 30%-40%.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for plastic mold steel, specifically a vacuum heat treatment tempering process for plastic mold steel. Background Technology

[0002] Plastic molds must withstand certain temperatures, pressures, and friction during the molding process. The mechanical properties (such as hardness, toughness, and wear resistance) of the core component, the plastic mold steel, directly determine the mold's service life and molding accuracy. Currently, the heat treatment of plastic mold steel mostly adopts conventional heating, quenching, and tempering processes, but these have the following drawbacks: Conventional heating environments can easily lead to oxidation and decarburization of the mold steel surface, reducing surface hardness and wear resistance, requiring additional subsequent grinding processes and increasing production costs; The preheating stage did not accurately grade according to the effective thickness of the workpiece, but only used a single preheating temperature or simple grading, resulting in uneven temperature inside and outside the workpiece. This can easily generate internal stress during the subsequent austenitization process, and lead to deformation and cracking risks after cooling. The control of austenitizing time and heating rate lacks specificity. Using the same parameters for workpieces with different effective thicknesses can easily lead to problems such as insufficient microstructure transformation (insufficient hardness) or excessive transformation (decreased toughness). The nitrogen pressure selection during the quenching and cooling stage has a low matching degree with the effective thickness of the workpiece, making it impossible to balance hardness and deformation control. This is especially true for complex molds, which are prone to local hardness differences. Problems such as excessive dwell time during the tempering stage (leading to fluctuations in workpiece performance), mismatch in parameters of workpieces processed in the same furnace (excessive thickness difference, insufficient spacing), and insufficient tempering times and holding time can result in incomplete stress release of the mold steel, making it prone to cracking or precision failure during use.

[0003] To address the aforementioned issues, vacuum heat treatment technology has been gradually applied to the processing of plastic mold steel. However, existing vacuum heat treatment processes still lack precise parameter design tailored to the characteristics of plastic mold steel, making it difficult to achieve synergistic optimization of mechanical properties and dimensional accuracy.

[0004] Therefore, there is an urgent need for a vacuum heat treatment and tempering process for plastic mold steel with precise parameters and standardized procedures to solve the problems mentioned in the background. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention aims to overcome the performance fluctuations and deformations caused by inaccurate preheating, improper austenitizing parameters, mismatched quenching pressure, and insufficient tempering specifications in existing vacuum heat treatment processes for plastic mold steel. It provides a vacuum heat treatment tempering process for plastic mold steel to improve its hardness, toughness, and dimensional stability, thereby extending the service life of the mold.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A vacuum heat treatment and tempering process for plastic mold steel includes a vacuum quenching process and a tempering process performed sequentially, with the specific steps as follows: 1) Vacuum quenching S1. Workpiece Pretreatment Inspect the surface of the plastic mold steel workpiece for cracks and impurities, measure the effective thickness δ of the workpiece and group it according to the δ value; S2. Vacuum Furnace Preparation Turn on the power to the vacuum quenching furnace, put the grouped workpieces into the furnace, ensuring that adjacent workpieces do not overlap and have gaps, take a picture to record the initial state of the workpieces, close the furnace door, and start the vacuum pump to evacuate the furnace until the vacuum degree inside the furnace is ≤5Pa. S3. Staged preheating Preheating is carried out in 2 to 3 stages according to the effective thickness δ of the workpiece, and the heating rate of the preheating section is ≤200℃ / hour. The preheating parameters for the two stages are: the first stage preheating temperature is 560~650℃, and the second stage preheating temperature is 820~850℃. The parameters for the third-stage preheating are: adding a third-stage preheating on the basis of the second-stage preheating, with a temperature of 920~990℃; The preheating time should meet the following requirements: the first-stage preheating time should be 1.6 to 2.0 times the subsequent austenitizing time, and the second and third-stage preheating times should both be 1.2 to 1.5 times the austenitizing time. S4. Austenitizing treatment After preheating, heat to the corresponding austenitizing temperature of the plastic mold steel at a heating rate of 20~40℃ / min, and control the holding time (i.e., austenitizing time) according to the following standards: When δ≤60mm, austenitizing time = 40+δ / 2 (unit: min); When 61mm < δ ≤ 150mm, austenitization time = (20~40) + δ / 2 (unit: min). When 151mm < δ ≤ 200mm, the austenitizing time = (0.6~0.8) × δ (unit: min); When δ is greater than or equal to 200 mm, the austenitizing time = (0.62~0.78) × δ (unit: min); S5. Quenching and Cooling After the austenitizing heat treatment is completed, nitrogen is introduced into the furnace for pre-cooling. After pre-cooling to 800~850℃, the nitrogen pressure is adjusted for rapid cooling. After rapid cooling is completed, the workpiece temperature is measured. When the effective thickness δ < 60 mm, the quenching pressure is 4.0 bar; When the effective thickness is 60mm≤δ≤100mm, the quenching pressure is 4.5~5.0bar; When the effective thickness is 100mm < δ ≤ 200mm, the quenching pressure is 5~5.5 bar; When the effective thickness δ > 200 mm, the quenching pressure > 5.5 bar; If the workpiece is a large or complex mold, use graded quenching, control the surface thermocouple temperature to 430℃, and continue cooling after isothermal treatment for 15 minutes.

[0007] S6. Inspection after quenching When the workpiece temperature drops to 50~60℃, stop cooling and release the gas into the furnace to normal pressure. Open the furnace door and take out the workpiece. Perform visual inspection, Rockwell hardness test and deformation measurement on the workpiece (using a micrometer or coordinate measuring machine). If the deformation exceeds the design allowable value or cracks, contact the customer immediately to confirm the subsequent process and fill in the "Quenching Abnormality and Analysis Record". 2) Tempering T1. Preparations before tempering The workpieces are classified and packed into baskets according to their hardness and deformation after quenching. During packing, it is ensured that the workpieces do not collide. After packing, each workpiece is inspected for cracks on its surface. If cracks are found, the workpiece is rejected. T2. Tempering furnace inspection Before starting the tempering furnace, check the accuracy of the thermocouples inside the furnace, the status of the instrument display, the sealing performance of the furnace cover, and the operation of the lifting mechanism. It can only be used after confirming that everything is normal. T3. High-frequency tempering First tempering: The workpieces after being placed in the basket are sent into the tempering furnace. The tempering temperature is determined by referring to the "Steel Tempering Temperature Selection Table" according to the plastic mold steel type. The holding time is greater than or equal to 2 hours. After the holding time is completed, the workpieces are taken out of the furnace and air-cooled to 300~350℃ first, and then air-cooled to room temperature. Second tempering: The workpiece after room temperature is sent back into the tempering furnace, the tempering temperature is the same as the first tempering temperature, and the holding time is greater than or equal to 2 hours; after the holding time is completed, it is cooled to room temperature in the same way as the first tempering; after cooling, the hardness and deformation of the workpiece are tested. Third / Fourth tempering: If the hardness of the workpiece does not meet the design requirements after the second tempering, a third tempering is performed. The tempering temperature is reduced by 10-20℃ compared to the second tempering temperature, and the holding time is greater than or equal to 2 hours. After the holding time is completed, the workpiece is removed from the furnace, first furnace-cooled to 400℃, then air-cooled to room temperature, and finally air-cooled to room temperature. If the hardness still does not meet the standard after the third tempering, the third tempering parameters are repeated for a fourth tempering. T4. Post-tempering treatment After tempering, the workpiece is inspected for surface quality, hardness, and deformation. If the surface decarburized layer thickness is greater than 0.1mm or the hardness deviation is greater than HRC2, the customer is contacted in time to confirm the subsequent process and the "Tempering Abnormality Record" is filled out. The workpiece that passes the inspection is cleaned and rust-proofed. And it meets the following process constraints: a. The time interval between the workpiece being taken out of the furnace and entering the tempering furnace after quenching is ≤60 minutes; b. The workpiece must be cooled to room temperature after each tempering before the next tempering can be carried out; c. Before each tempering, each workpiece must be inspected for cracks. If cracks are found, the workpiece must be discarded immediately and the results recorded. d. For workpieces tempered in the same furnace, the effective thickness difference should be ≤50mm; if the thickness difference is >50mm, and temperature uniformity cannot be guaranteed even by squeezing them together or wrapping them with stainless steel paper, they should not be processed in the same furnace. e. A gap should be left between workpieces tempered in the same furnace. The distance between workpieces placed in parallel should be greater than or equal to 10 mm. Only line contact is allowed between workpieces, and the included angle at the line contact point should be greater than or equal to 15 degrees (to avoid uneven local temperature). F. Tempering furnaces should preferably use atmosphere-protected tempering furnaces (such as nitrogen protection) to avoid oxidation of the workpiece surface.

[0008] Furthermore, the pressure parameter in step S5, "adjusting nitrogen pressure for rapid cooling," is controlled according to the following standards: When δ < 60 mm, the nitrogen pressure is 4.0 bar. When 60mm≤δ≤100mm, the nitrogen pressure is 4.5~5.0 bar; When 100mm < δ ≤ 200mm, the nitrogen pressure is 5~5.5 bar; When δ > 200 mm, the nitrogen pressure > 5.5 bar.

[0009] Furthermore, in step S3, the "selection of the number of stages for graded preheating" satisfies the following: when δ≤100mm, 2-stage preheating is used; when δ>100mm, 3-stage preheating is used.

[0010] Furthermore, the "austenitic heating rate" in step S4 is refined according to the following criteria: When δ≤60mm, the heating rate is 30~40℃ / min; When 61mm < δ ≤ 150mm, the heating rate is 25~35℃ / min; When δ>150mm, the heating rate is 20~30℃ / min.

[0011] Furthermore, in step T2, the "tempering furnace" is an atmosphere-protected tempering furnace. During the tempering process, nitrogen is introduced into the furnace as a protective atmosphere. The purity of the nitrogen in the furnace is greater than or equal to 99.99%, and the oxygen content in the furnace is ≤50ppm.

[0012] Furthermore, the "holding time" in step T3 is further limited: when the effective thickness of the workpiece δ > 100 mm, the holding time for the first and second tempering is greater than or equal to 2.5 hours; the holding time for the third and fourth tempering is greater than or equal to 2.5 hours.

[0013] Furthermore, in step T3, the furnace cooling speed of the "third tempering" is controlled to be ≤100℃ / hour; the wind speed of the "air cooling" in step T3 is controlled to be 2~3m / s, and the temperature drop rate of the workpiece surface during the air cooling process is ≤5℃ / min.

[0014] Furthermore, the "quenching and cooling" step S5 also includes: if the workpiece is a complex plastic mold (defined as: the workpiece structure has 3 or more irregular corners, or the thickness ratio of the largest cross section to the smallest cross section is >3:1), then before adjusting the nitrogen pressure for rapid cooling, the surface temperature of the workpiece is controlled to drop to 430°C (detected by the thermocouple in the furnace), and isothermaled at this temperature for 15 minutes. After isothermaling, rapid nitrogen cooling is then performed.

[0015] Furthermore, the chemical composition of the plastic mold steel includes the following percentage content: C 0.24~0.32%, Si 0.20~0.40%, Mn 0.40~0.60%, Cr 12.80~13.20%, Ni 0.40~0.60%, Mo 0.30~0.50%, V 0.30~0.50%, with the balance being iron.

[0016] Preferably, the chemical composition of the plastic mold steel includes the following percentage content: C 0.28%, Si 0.30%, Mn 0.50%, Cr 13.00%, Ni 0.50%, Mo 0.40%, V 0.40%, with the balance being iron.

[0017] This invention provides a vacuum heat treatment and tempering process for plastic mold steel. It has the following beneficial effects: 1. This invention provides a vacuum heat treatment tempering process for plastic mold steel. This process uses a vacuum environment for quenching, which completely avoids the surface oxidation and decarburization problems caused by conventional heating. The surface hardness of the workpiece is increased by 10%~15%, the wear resistance is enhanced, no subsequent grinding process is required, and the production cost is reduced by 15%~20%.

[0018] 2. This invention provides a vacuum heat treatment tempering process for plastic mold steel. During the preheating stage, the workpiece is precisely graded into 2-3 levels according to its effective thickness. Combined with a limited heating rate, the temperature difference between the inside and outside of the workpiece is controlled within 5°C, significantly reducing internal stress. After quenching, the workpiece deformation is reduced by 20%-30%, and the cracking rate is reduced from 8%-10% in existing processes to below 1%. The austenitizing time and heating rate are designed according to the effective thickness to ensure that the microstructure transformation of workpieces of different thicknesses is sufficient and uniform. The hardness fluctuation range of the workpiece is controlled within HRC2, and the mechanical property stability is significantly improved.

[0019] 3. This invention provides a vacuum heat treatment tempering process for plastic mold steel. The nitrogen pressure during the quenching and cooling stage is precisely matched with the effective thickness of the workpiece. Combined with the graded quenching design for complex molds, it takes into account both hardness and deformation control. The pass rate of complex plastic molds is increased from 75%~80% of the existing process to more than 95%. The tempering process standardizes parameters such as interval time, furnace conditions, and holding time, and at least two tempering processes are performed to ensure that the internal stress of the workpiece is completely released. The probability of precision failure during use is reduced by 25%~30%, and the service life of the mold is extended by 30%~40%. Attached Figure Description

[0020] Figure 1 This is a flowchart of the vacuum quenching process for heat-treated workpieces according to the present invention; Figure 2 This is a flowchart of the heat treatment process for tempering workpieces according to the present invention. Detailed Implementation

[0021] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, the accompanying drawings show only the parts relevant to the present invention, and not all of the structures.

[0022] In the description of this invention, unless otherwise explicitly specified and limited, the terms "connected," "linked," and "fixed" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0023] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0024] In the description of this embodiment, the terms "upper," "lower," "right," etc., refer to the orientation or positional relationship shown in the accompanying drawings. They are used only for ease of description and simplification of operation, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention. In addition, the terms "first" and "second" are used only for distinction in description and have no special meaning.

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0026] like Figure 1-2 As shown in the figure, a specific embodiment of the present invention provides a vacuum heat treatment and tempering process for plastic mold steel, including a vacuum quenching process and a tempering process performed sequentially, and the specific steps are as follows: 1) Vacuum quenching S1. Workpiece Pretreatment Inspect the surface of the plastic mold steel workpiece for cracks and impurities, measure the effective thickness δ of the workpiece and group it according to the δ value; S2. Vacuum Furnace Preparation Turn on the power to the vacuum quenching furnace, put the grouped workpieces into the furnace, ensuring that adjacent workpieces do not overlap and have gaps, take a picture to record the initial state of the workpieces, close the furnace door, and start the vacuum pump to evacuate the furnace until the vacuum degree inside the furnace is ≤5Pa. S3. Staged preheating Preheating is carried out in 2 to 3 stages according to the effective thickness δ of the workpiece, and the heating rate of the preheating section is ≤200℃ / hour. The preheating parameters for the two stages are: the first stage preheating temperature is 560~650℃, and the second stage preheating temperature is 820~850℃. The parameters for the third-stage preheating are: adding a third-stage preheating on the basis of the second-stage preheating, with a temperature of 920~990℃; The preheating time should meet the following requirements: the first-stage preheating time should be 1.6 to 2.0 times the subsequent austenitizing time, and the second and third-stage preheating times should both be 1.2 to 1.5 times the austenitizing time. S4. Austenitizing treatment After preheating, heat to the corresponding austenitizing temperature of the plastic mold steel at a heating rate of 20~40℃ / min, and control the holding time (i.e., austenitizing time) according to the following standards: When δ≤60mm, austenitizing time = 40+δ / 2 (unit: min); When 61mm < δ ≤ 150mm, austenitization time = (20~40) + δ / 2 (unit: min). When 151mm < δ ≤ 200mm, the austenitizing time = (0.6~0.8) × δ (unit: min); When δ is greater than or equal to 200 mm, the austenitizing time = (0.62~0.78) × δ (unit: min); S5. Quenching and Cooling After the austenitizing heat treatment is completed, nitrogen is introduced into the furnace for pre-cooling. After pre-cooling to 800~850℃, the nitrogen pressure is adjusted for rapid cooling. After rapid cooling is completed, the workpiece temperature is measured. When the effective thickness δ < 60 mm, the quenching pressure is 4.0 bar; When the effective thickness is 60mm≤δ≤100mm, the quenching pressure is 4.5~5.0bar; When the effective thickness is 100mm < δ ≤ 200mm, the quenching pressure is 5~5.5 bar; When the effective thickness δ > 200 mm, the quenching pressure > 5.5 bar; If the workpiece is a large or complex mold, use graded quenching, control the surface thermocouple temperature to 430℃, and continue cooling after isothermal treatment for 15 minutes.

[0027] S6. Inspection after quenching When the workpiece temperature drops to 50~60℃, stop cooling and release the gas into the furnace to normal pressure. Open the furnace door and take out the workpiece. Perform visual inspection, Rockwell hardness test and deformation measurement on the workpiece (using a micrometer or coordinate measuring machine). If the deformation exceeds the design allowable value or cracks, contact the customer immediately to confirm the subsequent process and fill in the "Quenching Abnormality and Analysis Record". 2) Tempering T1. Preparations before tempering The workpieces are classified and packed into baskets according to their hardness and deformation after quenching. During packing, it is ensured that the workpieces do not collide. After packing, each workpiece is inspected for cracks on its surface. If cracks are found, the workpiece is rejected. T2. Tempering furnace inspection Before starting the tempering furnace, check the accuracy of the thermocouples inside the furnace, the status of the instrument display, the sealing performance of the furnace cover, and the operation of the lifting mechanism. It can only be used after confirming that everything is normal. T3. High-frequency tempering First tempering: The workpieces after being placed in the basket are sent into the tempering furnace. The tempering temperature is determined by referring to the "Steel Tempering Temperature Selection Table" according to the plastic mold steel type. The holding time is greater than or equal to 2 hours. After the holding time is completed, the workpieces are taken out of the furnace and air-cooled to 300~350℃ first, and then air-cooled to room temperature. Second tempering: The workpiece after room temperature is sent back into the tempering furnace, the tempering temperature is the same as the first tempering temperature, and the holding time is greater than or equal to 2 hours; after the holding time is completed, it is cooled to room temperature in the same way as the first tempering; after cooling, the hardness and deformation of the workpiece are tested. Third / Fourth tempering: If the hardness of the workpiece does not meet the design requirements after the second tempering, a third tempering is performed. The tempering temperature is reduced by 10-20℃ compared to the second tempering temperature, and the holding time is greater than or equal to 2 hours. After the holding time is completed, the workpiece is removed from the furnace, first furnace-cooled to 400℃, then air-cooled to room temperature, and finally air-cooled to room temperature. If the hardness still does not meet the standard after the third tempering, the third tempering parameters are repeated for a fourth tempering. T4. Post-tempering treatment After tempering, the workpiece is inspected for surface quality, hardness, and deformation. If the surface decarburized layer thickness is greater than 0.1mm or the hardness deviation is greater than HRC2, the customer is contacted in time to confirm the subsequent process and the "Tempering Abnormality Record" is filled out. The workpiece that passes the inspection is cleaned and rust-proofed. And it meets the following process constraints: a. The time interval between the workpiece being taken out of the furnace and entering the tempering furnace after quenching is ≤60 minutes; b. The workpiece must be cooled to room temperature after each tempering before the next tempering can be carried out; c. Before each tempering, each workpiece must be inspected for cracks. If cracks are found, the workpiece must be discarded immediately and the results recorded. d. For workpieces tempered in the same furnace, the effective thickness difference should be ≤50mm; if the thickness difference is >50mm, and temperature uniformity cannot be guaranteed even by squeezing them together or wrapping them with stainless steel paper, they should not be processed in the same furnace. e. A gap should be left between workpieces tempered in the same furnace. The distance between workpieces placed in parallel should be greater than or equal to 10 mm. Only line contact is allowed between workpieces, and the included angle at the line contact point should be greater than or equal to 15 degrees (to avoid uneven local temperature). F. Tempering furnaces should preferably use atmosphere-protected tempering furnaces (such as nitrogen protection) to avoid oxidation of the workpiece surface.

[0028] The chemical composition of plastic mold steel includes the following percentage content: C 0.24~0.32%, Si 0.20~0.40%, Mn 0.40~0.60%, Cr 12.80~13.20%, Ni 0.40~0.60%, Mo 0.30~0.50%, V 0.30~0.50%, with the balance being iron.

[0029] Preferably, the chemical composition of the plastic mold steel includes the following percentage content: C 0.28%, Si 0.30%, Mn 0.50%, Cr 13.00%, Ni 0.50%, Mo 0.40%, V 0.40%, with the balance being iron.

[0030] Example 1: P20 steel plastic mold workpiece with δ1=50mm 1) Vacuum quenching S1: Check the workpiece for any impurities or cracks, and measure δ1 = 50mm; S2: Take a picture of the workpiece after it enters the furnace, and then draw a vacuum to 3Pa after closing the furnace door; S3: Two-stage preheating is adopted. The first stage preheating temperature is 600℃, and the austenitizing time is 40 + 50 / 2 = 65 min. Therefore, the first stage preheating time is 65 × 1.8 = 117 min. The second stage preheating temperature is 830℃, and the preheating time is 65 × 1.3 = 84.5 min. The heating rate of the preheating section is 180℃ / hour. S4: Heat to 880℃ (austenitizing temperature of P20 steel) at 30℃ / min, and hold for 65min; S5: After pre-cooling with nitrogen, adjust the nitrogen pressure to 4.0 bar for rapid cooling to 55°C; S6: After exiting the oven, the appearance is free of deformation and cracks, the hardness is HRC32~33, and the deformation is 0.02mm, which meets the requirements.

[0031] 2) Tempering T1: Packed into baskets according to hardness HRC32~33; T2: The tempering furnace was inspected and found to be normal. The tempering temperature was 600℃ and the holding time was 2 hours. After being taken out of the furnace, the furnace was air-cooled to 320℃ and then air-cooled to room temperature. T3: The second tempering temperature is 600℃, and the holding time is 2 hours. The cooling method is the same as T2. After cooling, the hardness is tested to be HRC31~32, and the deformation is 0.01mm. T4: No third tempering required; T5: Passes final inspection and undergoes cleaning and rust prevention treatment.

[0032] Example 2: P20 steel plastic mold workpiece with δ2=80mm 1) Vacuum quenching S1: δ2 = 80mm, appearance is normal; S2: Evacuate the furnace to 4Pa; S3: Two-stage preheating is adopted. The first stage is 620℃, and the austenitizing time is 30 + 80 / 2 = 70 min. The first stage preheating time is 70 × 1.7 = 119 min. The second stage is 840℃, and the preheating time is 70 × 1.4 = 98 min. The heating rate is 190℃ / hour. S4: Heat to 890℃ at 35℃ / min, and hold for 70min; S5: Nitrogen pressure 4.8 bar rapid cooling, cooling to 58℃; S6: After firing, the hardness is HRC33~34 and the deformation is 0.03mm, which is acceptable.

[0033] 2) Tempering T1~T5: Same as Example 1, after two temperings, the hardness is HRC32~33, the deformation is 0.02mm, which is qualified.

[0034] Example 3: P20 steel plastic mold workpiece with δ3=180mm 1) Vacuum quenching S1: δ3=180mm, appearance normal; S2: Evacuate the furnace to 3Pa; S3: Three-stage preheating is adopted. The first stage is 580℃, with an austenitizing time of 0.7×180=126min and a first-stage preheating time of 126×1.9=239.4min; the second stage is 830℃, with a preheating time of 126×1.3=163.8min; the third stage is 950℃, with a preheating time of 163.8min; the heating rate is 200℃ / hour. S4: Heat to 900℃ at 25℃ / min, hold for 126min; S5: Nitrogen pressure 5.2 bar for rapid cooling. Due to the complexity of the workpiece, after isothermal cooling at 430℃ for 15 minutes, continue cooling to 60℃. S6: After firing, the hardness is HRC34~35 and the deformation is 0.04mm, which is acceptable.

[0035] 2) Tempering T1~T3: After two tempering processes, the hardness is HRC33~34 and the deformation is 0.03mm, which is acceptable. No third tempering is required.

[0036] Example 4: P20 steel plastic mold workpiece with δ4=220mm 1) Vacuum quenching S1: δ4=220mm, appearance is normal; S2: Evacuate the furnace to 4Pa; S3: 3-stage preheating; Stage 1: 650℃, austenitizing time = 0.7 × 220 = 154 min, first-stage preheating time = 154 × 2.0 = 308 min; Stage 2: 850℃, preheating time = 154 × 1.5 = 231 min; Stage 3: 980℃, preheating time = 231 min; heating rate: 180℃ / hour. S4: Heat to 910℃ at 20℃ / min, hold for 154min; S5: Rapid cooling with nitrogen pressure of 5.8 bar, isothermal at 430℃ for 15 minutes, then cooling to 58℃; S6: After firing, the hardness is HRC35~36 and the deformation is 0.05mm, which is acceptable.

[0037] 2) Tempering T1~T4: After two tempering processes, the hardness is HRC34~35. Because the customer requires a hardness of HRC33~34, a third tempering process is performed (temperature 590℃, holding for 2 hours). After cooling, the hardness is HRC33~34, and the deformation is 0.04mm, which is qualified.

[0038] In the above embodiments, after all workpieces are processed by this process, the surface is free from oxidation and decarburization, the hardness fluctuation is ≤HRC2, and the deformation is within the design allowable range, which meets the requirements for use of plastic molds.

[0039] The following points should be noted in this article: 1. The accompanying drawings of the embodiments disclosed herein only relate to the structures involved in the embodiments disclosed herein; other structures can be referred to in a general design.

[0040] 2. Where there is no conflict, the embodiments of this disclosure and the features in the embodiments can be combined with each other to obtain new embodiments.

[0041] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations 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. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A vacuum heat treatment and tempering process for plastic mold steel, characterized in that, The process includes sequential vacuum quenching and tempering, with the following specific steps: 1) Vacuum quenching S1. Workpiece Pretreatment Inspect the surface of the plastic mold steel workpiece for cracks and impurities, measure the effective thickness δ of the workpiece and group it according to the δ value; S2. Vacuum Furnace Preparation Turn on the power to the vacuum quenching furnace, put the grouped workpieces into the furnace, ensuring that adjacent workpieces do not overlap and have gaps, take a picture to record the initial state of the workpieces, close the furnace door, and start the vacuum pump to evacuate the furnace until the vacuum degree inside the furnace is ≤5Pa. S3. Staged preheating Preheating is carried out in 2 to 3 stages according to the effective thickness δ of the workpiece, and the heating rate of the preheating section is ≤200℃ / hour. The preheating parameters for the two stages are: the first stage preheating temperature is 560~650℃, and the second stage preheating temperature is 820~850℃. The parameters for the third-stage preheating are: adding a third-stage preheating on the basis of the second-stage preheating, with a temperature of 920~990℃; The preheating time should meet the following requirements: the first-stage preheating time should be 1.6 to 2.0 times the subsequent austenitizing time, and the second and third-stage preheating times should both be 1.2 to 1.5 times the austenitizing time. S4. Austenitizing treatment After preheating, heat to the corresponding austenitizing temperature of the plastic mold steel at a heating rate of 20~40℃ / min, and control the holding time according to the following standards: When δ≤60mm, austenitizing time = 40+δ / 2 (unit: min); When 61mm < δ ≤ 150mm, austenitization time = (20~40) + δ / 2 (unit: min). When 151mm < δ ≤ 200mm, the austenitizing time = (0.6~0.8) × δ (unit: min); When δ is greater than or equal to 200 mm, the austenitizing time = (0.62~0.78) × δ (unit: min); S5. Quenching and Cooling After the austenitizing heat treatment is completed, nitrogen is introduced into the furnace for pre-cooling. After pre-cooling to 800~850℃, the nitrogen pressure is adjusted for rapid cooling. After rapid cooling is completed, the workpiece temperature is measured. S6. Inspection after quenching When the workpiece temperature drops to 50~60℃, stop cooling and release the gas into the furnace to normal pressure. Open the furnace door and take out the workpiece. Perform visual inspection, Rockwell hardness test and deformation measurement on the workpiece (using a micrometer or coordinate measuring machine). If the deformation exceeds the design allowable value or cracks, contact the customer immediately to confirm the subsequent process and fill in the "Quenching Abnormality and Analysis Record". 2) Tempering T1. Preparations before tempering The workpieces are classified and packed into baskets according to their hardness and deformation after quenching, ensuring that there is no collision between the workpieces during packing. After packing, each workpiece is inspected for cracks on its surface. If cracks are found, the workpiece is rejected. T2. Tempering furnace inspection Before starting the tempering furnace, check the accuracy of the thermocouples inside the furnace, the status of the instrument display, the sealing performance of the furnace cover, and the operation of the lifting mechanism. It can only be used after confirming that everything is normal. T3. High-frequency tempering First tempering: The workpieces after being placed in the basket are sent into the tempering furnace. The tempering temperature is determined by referring to the "Steel Tempering Temperature Selection Table" according to the plastic mold steel type. The holding time is greater than or equal to 2 hours. After the holding time is completed, the workpieces are taken out of the furnace and air-cooled to 300~350℃ first, and then air-cooled to room temperature. Second tempering: The workpiece after room temperature is sent back into the tempering furnace, the tempering temperature is the same as the first tempering temperature, and the holding time is greater than or equal to 2 hours; after the holding time is completed, it is cooled to room temperature in the same way as the first tempering; after cooling, the hardness and deformation of the workpiece are tested. Third / Fourth tempering: If the hardness of the workpiece does not meet the design requirements after the second tempering, a third tempering is performed. The tempering temperature is reduced by 10-20℃ compared to the second tempering temperature, and the holding time is greater than or equal to 2 hours. After the holding time is completed, the workpiece is removed from the furnace, first furnace-cooled to 400℃, then air-cooled to room temperature, and finally air-cooled to room temperature. If the hardness still does not meet the standard after the third tempering, the third tempering parameters are repeated for a fourth tempering. T4. Post-tempering treatment After tempering, the workpiece is inspected for surface quality, hardness, and deformation. If the surface decarburized layer thickness is greater than 0.1mm or the hardness deviation is greater than HRC2, the customer is contacted in time to confirm the subsequent process and the "Tempering Abnormality Record" is filled out. The workpiece that passes the inspection is cleaned and rust-proofed.

2. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, The pressure parameter in step S5, "adjusting nitrogen pressure for rapid cooling," is controlled according to the following standards: When δ < 60 mm, the nitrogen pressure is 4.0 bar. When 60mm≤δ≤100mm, the nitrogen pressure is 4.5~5.0 bar; When 100mm < δ ≤ 200mm, the nitrogen pressure is 5~5.5 bar; When δ > 200 mm, the nitrogen pressure > 5.5 bar.

3. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, In step S3, the "selection of the number of stages for graded preheating" satisfies the following: when δ≤100mm, 2-stage preheating is used; when δ>100mm, 3-stage preheating is used.

4. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, The "austenitic heating rate" in step S4 is refined according to the following criteria: When δ≤60mm, the heating rate is 30~40℃ / min; When 61mm < δ ≤ 150mm, the heating rate is 25~35℃ / min; When δ>150mm, the heating rate is 20~30℃ / min.

5. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, In step T2, the "tempering furnace" is an atmosphere-protected tempering furnace. During the tempering process, nitrogen is introduced into the furnace as a protective atmosphere. The purity of the nitrogen in the furnace is greater than or equal to 99.99%, and the oxygen content in the furnace is ≤50ppm.

6. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, The "holding time" in step T3 is further limited as follows: when the effective thickness of the workpiece δ > 100 mm, the holding time for the first and second tempering is greater than or equal to 2.5 hours; the holding time for the third and fourth tempering is greater than or equal to 2.5 hours.

7. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, In step T3, the furnace cooling speed during the third tempering is controlled to be ≤100℃ / hour; the air speed during the air cooling in step T3 is controlled to be 2~3m / s, and the temperature drop rate of the workpiece surface during the air cooling process is ≤5℃ / min.

8. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, The "quenching and cooling" step S5 also includes: if the workpiece is a complex plastic mold, before adjusting the nitrogen pressure for rapid cooling, the surface temperature of the workpiece is first controlled to drop to 430°C, and isothermal at this temperature for 15 minutes. After isothermal isothermation, nitrogen rapid cooling is then performed.

9. The vacuum heat treatment and tempering process for plastic mold steel according to claim 1, characterized in that, The chemical composition of the plastic mold steel includes the following percentage content: C 0.24~0.32%, Si 0.20~0.40%, Mn 0.40~0.60%, Cr 12.80~13.20%, Ni 0.40~0.60%, Mo 0.30~0.50%, V 0.30~0.50%, with the balance being iron.

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