A method for preparing a low internal stress, high hardness alloy die

Abstract

A method for preparing a low-stress, high-hardness alloy mold includes the following steps: first, annealing and stress-relief treatment of the mold blank to improve the microstructure and eliminate processing stress; then heating the mold to 1030-1050℃ and holding it at that temperature according to its thickness; then oil quenching to 250-300℃ and immediately transferring it to an isothermal furnace at 250-350℃ for 30-60 minutes; removing it and cooling it to room temperature before deep cryogenic cycling treatment; followed by tempering three times, each time applying a 50Hz, 0.5T pulsed magnetic field for 30 minutes, to obtain a low-stress, high-hardness alloy mold. This application utilizes a three-stage synergistic heat treatment process of "graded quenching—stepped deep cryogenic cycling—pulse magnetic field-assisted tempering," combined with a specific alloy composition design, to produce an alloy mold with a hardness ≥58 HRC, residual stress ≤-80 MPa, residual austenite content ≤5 vol%, and core-to-surface hardness difference ≤2 HRC. The preparation method is controllable and applicable to complex structure molds, representing a significant technological advancement and broad industrial application prospects.

Description

Technical Field

[0001] This invention belongs to the field of alloy mold preparation, specifically relating to a method for preparing a low-internal-stress, high-hardness alloy mold. Background Technology

[0002] Alloy molds, as an indispensable basic process equipment in modern industrial production, are widely used in automobile manufacturing, aerospace, electronics and communications, and precision machinery. With the increasingly stringent requirements of high-end manufacturing for component precision, surface quality, and production efficiency, the working environment of molds is becoming increasingly complex. Molds are required not only to possess extremely high hardness and wear resistance to resist abrasion, but also excellent strength and toughness to withstand high-load impacts and alternating thermal stresses. Therefore, developing alloy mold manufacturing technologies that combine "high hardness" and "low internal stress" has always been a research hotspot and challenge in the field of materials science and engineering.

[0003] Currently, traditional mold manufacturing processes typically employ a "quenching + tempering" heat treatment method. However, existing technologies still have the following drawbacks in practical applications: 1. The contradiction between hardness and toughness is difficult to reconcile: To obtain high hardness, traditional processes often employ rapid and drastic cooling, such as water quenching. While this method can ensure hardness, it generates enormous thermal and structural stresses within the mold, easily leading to mold deformation or even cracking, severely impacting the mold's service life and safety; 2. Incomplete transformation of retained austenite: After conventional heat treatment, mold steel often retains a certain amount of retained austenite. This structure is unstable and gradually transforms during mold storage or use, causing changes in mold dimensions and failing to meet the stringent dimensional accuracy requirements of precision molds.

[0004] Therefore, there is an urgent need for a method for preparing alloy molds that can simultaneously achieve low internal stress, high hardness, and dimensional stability. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing alloy molds with low internal stress and high hardness.

[0006] The present invention adopts the following technical solution: A method for preparing a low-internal-stress, high-hardness alloy mold includes the following steps: Step 1: Heat the mold blank to 850℃ and hold for 2 hours. Then, cool it in the furnace to 600℃ at a cooling rate of 10-20℃ / h, and then air cool it after removing it from the furnace. Step 2: After rough machining, heat the mold to 500-550℃, keep it at that temperature for 4 hours, and then cool it in the furnace. Step 3: Place the mold in a vacuum furnace and heat it to 1030-1050℃. The holding time is calculated based on an effective thickness of 1.5 min / mm. Step 4: After the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. When the surface temperature of the mold drops to 250-300℃, the oil is removed and the mold is immediately placed in an isothermal furnace preheated to 250-350℃ for isothermal treatment for 30-60 minutes. Step 5: After the isothermal treatment is completed, remove the mold from the isothermal furnace and cool it to room temperature; Step 6: After cooling in step 5, the mold is transferred into a cryogenic chamber for cryogenic circulation treatment. Step 7: Heat the cryogenically treated mold to 560°C for the first tempering. During the heat preservation period, apply a pulsed magnetic field with a frequency of 50Hz and a magnetic induction intensity of 0.5T for 30 minutes. Repeat the tempering twice to obtain the low internal stress and high hardness alloy mold.

[0007] Furthermore, in step 6, the specific operations of the cryogenic cycle treatment are as follows: First stage: The cryogenic chamber is cooled to -80℃ at a rate of 2℃ / min and held for 2 hours; Second stage: The cryogenic chamber continues to cool down to -196℃ at a rate of 1℃ / min and is held at that temperature for 12 hours; In the third stage, the temperature is naturally raised to room temperature, then cooled to -80°C and held for 1 hour, before being naturally raised to room temperature again to complete the deep cryogenic cycle treatment of the mold.

[0008] Furthermore, the mold blank comprises the following components by weight percentage: C 0.35-0.50%, Si 0.2-1.0%, Mn 0.2-0.50%, Cr 4.00-5.50%, Mo 1.00-2.00%, V 0.50-1.20%, Ni 0.65-0.85%, W 0.5-1.0%, La 0.02-0.08%, Nb 0.01-0.05%, B 0.001-0.005%, P ≤0.015%, S ≤0.010%, and Fe balance.

[0009] Furthermore, in step 4, the oil-quenched mold is placed in an isothermal furnace preheated to 250-350℃ within ≤30 seconds.

[0010] Furthermore, in step 4, after the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. At this time, the temperature of the quenching oil is pre-controlled at 40-60℃. The mold is completely immersed in the quenching oil, and the temperature is monitored in real time by thermocouples embedded on the surface of the mold. When the surface temperature of the mold drops to 250-300℃, the oil is removed.

[0011] Furthermore, in step 4, the isothermal treatment time is determined according to the effective thickness of the mold: 30 min for ≤50mm, 45 min for 50-150mm, and 60 min for ≥150mm.

[0012] Furthermore, after step 7, the mold is subjected to low-temperature aging treatment. The specific operation of the low-temperature aging treatment is as follows: heat the mold to 150-180℃, keep it at that temperature for 4-8 hours, and then air-cool it to room temperature.

[0013] Furthermore, in step 6, before the cooled mold undergoes deep cryogenic cycling treatment, the mold is first mechanically polished.

[0014] As can be seen from the above description of the present invention, compared with the prior art, the beneficial effects of the present invention are: First, this application, through a three-stage synergistic heat treatment process of "graded quenching—stepped deep cryogenic cycle—pulse magnetic field assisted tempering," combined with a specific alloy composition design, successfully breaks through the technical bottleneck of the mutual constraint between high hardness and low internal stress in traditional mold preparation. The alloy mold prepared by the method of this invention has a hardness ≥58 HRC, residual stress ≤-80MPa, and residual austenite content ≤5 vol%, with a hardness difference between the core and the surface ≤2 HRC. The preparation method is controllable and applicable to complex structure molds, demonstrating significant technological progress and broad industrial application prospects. Secondly, this application specifically defines the preparation method of the alloy mold. By precisely controlling the quenching in stages, the oil quenching at 250-300℃ combined with the isothermal treatment at 250-350℃ effectively avoids the temperature range of the most intense martensitic phase transformation, significantly reducing phase transformation stress and thermal stress, and laying a low-stress foundation for subsequent processing. Furthermore, it defines the combination of pulsed magnetic field and three-stage tempering, using alternating magnetic field to promote uniform dislocation movement and dispersed precipitation of carbides, achieving deep elimination of residual stress while maintaining high hardness. Third, a cryogenic cycle treatment is introduced between the quenching and tempering processes, and the specific method of the cryogenic cycle treatment is further defined. On the one hand, it promotes the full and stable transformation of the retained austenite into martensite, and on the other hand, it avoids thermal shock through step temperature control, further releasing the micro-stress in the cryogenic process. Fourth, this application further defines the raw material composition of the mold blank. By introducing La and B elements in synergy, La purifies grain boundaries and improves carbide morphology, while B segregates at grain boundaries to further enhance hardenability. This synergy allows the mold to obtain a uniform and sufficient martensitic / bainitic multiphase structure even under relatively mild cooling conditions during vacuum oil quenching, fundamentally reducing the tendency for quenching cracks caused by excessively rapid cooling. Simultaneously, the introduction of Nb effectively prevents abnormal grain growth at high temperatures during the vacuum heating and holding stage, resulting in ultra-fine grains that significantly improve hardenability. This process enhances the mold's toughness and provides a large number of martensitic nucleation sites for subsequent cryogenic cycling, resulting in a more uniform and thorough cryogenic transformation and significantly improving the mold's dimensional stability. In addition, the introduction of Mo in combination with W allows for the synergistic precipitation of a large number of dispersed and fine special carbides (such as Mo2C and WC) during the tempering stage, generating an extremely strong "secondary hardening" peak. Furthermore, the magnetic field can further accelerate this precipitation process, ensuring that both the mold surface and core reach the specified hardness and greatly improving the mold's high-temperature tempering stability, thus completely eliminating temper brittleness. Detailed Implementation

[0015] The present invention will be further described below through specific embodiments.

[0016] A method for preparing a low-internal-stress, high-hardness alloy mold includes the following steps: Step 1: Heat the mold blank to 850℃ and hold for 2 hours. Then, cool it in the furnace to 600℃ at a cooling rate of 10-20℃ / h, and then air cool it after removing it from the furnace. Step 2: After rough machining, heat the mold to 500-550℃, keep it at that temperature for 4 hours, and then cool it in the furnace. Step 3: Place the mold in a vacuum furnace and heat it to 1030-1050℃. The holding time is calculated based on an effective thickness of 1.5 min / mm. Step 4: After the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. When the surface temperature of the mold drops to 250-300℃, the oil is removed and the mold is immediately placed in an isothermal furnace preheated to 250-350℃ within ≤30 seconds for isothermal treatment for 30-60 minutes. Step 5: After the isothermal treatment is completed, remove the mold from the isothermal furnace and cool it to room temperature; Step 6: After cooling in step 5, the mold is first mechanically polished, then transferred to a cryogenic chamber for cryogenic circulation treatment. The specific operation is as follows: First stage: The cryogenic chamber is cooled to -80℃ at a rate of 2℃ / min and held for 2 hours; Second stage: The cryogenic chamber continues to cool down to -196℃ at a rate of 1℃ / min and is held at that temperature for 12 hours; The third stage involves naturally heating up to room temperature, then cooling down to -80℃, holding at that temperature for 1 hour, and then naturally heating up to room temperature to complete the deep cryogenic cycle treatment of the mold. Step 7: Heat the cryogenically treated mold to 560°C for the first tempering. During the heat preservation period, apply a pulsed magnetic field with a frequency of 50Hz and a magnetic induction intensity of 0.5T for 30 minutes. Repeat the tempering twice to obtain the low internal stress and high hardness alloy mold.

[0017] The mold blank comprises the following components by weight percentage: C 0.35-0.50%, Si 0.2-1.0%, Mn 0.2-0.50%, Cr 4.00-5.50%, Mo 1.00-2.00%, V 0.50-1.20%, Ni 0.65-0.85%, W 0.5-1.0%, La 0.02-0.08%, Nb 0.01-0.05%, B 0.001-0.005%, P ≤0.015%, S ≤0.010%, and Fe balance.

[0018] In step 4, after the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. At this time, the temperature of the quenching oil is pre-controlled at 40-60℃. The mold is completely immersed in the quenching oil, and the temperature is monitored in real time by thermocouples embedded on the surface of the mold. The oil is removed when the surface temperature of the mold drops to 250-300℃. The isothermal treatment time is determined according to the effective thickness of the mold: ≤50mm for 30min, 50-150mm for 45min, and ≥150mm for 60min.

[0019] Following step 7, the mold undergoes a low-temperature aging treatment. This treatment further stabilizes the dimensions, eliminates residual stress, and improves fatigue resistance. The specific steps for the low-temperature aging treatment are as follows: heat the mold to 150-180℃, hold for 4-8 hours, and then air-cool to room temperature. Example

[0020] A method for preparing a low-internal-stress, high-hardness alloy mold includes the following steps: Step 1: Heat the mold blank to 850℃ and hold for 2 hours. Then, cool it in the furnace at a rate of 10℃ / h to 600℃ and air cool it after removing it from the furnace. Step 2: After rough machining and shaping, heat the mold to 500℃, keep it at that temperature for 4 hours, and then cool it in the furnace. Step 3: Place the mold in a vacuum furnace and evacuate it to a vacuum level of ≤1*10. -1 Pa, heated to 1030℃ at a rate of 8℃ / min, with holding time calculated based on an effective thickness of 1.5min / mm, and maintaining a vacuum degree ≤5*10 during the holding period. -2pa, wherein the effective thickness of the mold is ≥150mm; Step 4: After the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. When the surface temperature of the mold drops to 250℃, the oil is removed and the mold is immediately placed in an isothermal furnace preheated to 250℃ within ≤30 seconds for isothermal treatment for 60 minutes. Step 5: After the isothermal treatment is completed, remove the mold from the isothermal furnace and cool it to room temperature; Step 6: After cooling in step 5, the mold is first mechanically polished, then transferred to a cryogenic chamber for cryogenic circulation treatment. The specific operation is as follows: First stage: The cryogenic chamber is cooled to -80℃ at a rate of 2℃ / min and held for 2 hours; Second stage: The cryogenic chamber continues to cool down to -196℃ at a rate of 1℃ / min and is held at that temperature for 12 hours; The third stage involves naturally heating up to room temperature, then cooling down to -80℃, holding at that temperature for 1 hour, and then naturally heating up to room temperature to complete the deep cryogenic cycle treatment of the mold. Step 7: Heat the cryogenically treated mold to 560°C for the first tempering. During the heat preservation period, apply a pulsed magnetic field with a frequency of 50Hz and a magnetic induction intensity of 0.5T for 30 minutes. Repeat the tempering twice, and then perform low-temperature aging treatment to obtain the low internal stress and high hardness alloy mold.

[0021] The mold blank comprises the following components by weight percentage: C 0.35%, Si 1.0%, Mn 0.2%, Cr 4.00%, Mo 2.00%, V 0.50%, Ni 0.85%, W 0.5%, La 0.02%, Nb 0.05%, B 0.001%, P 0.014%, S 0.008%, and Fe balance.

[0022] In step 4, after the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. At this time, the temperature of the quenching oil is pre-controlled at 40°C. The mold is completely immersed in the quenching oil, and the temperature is monitored in real time by the thermocouple embedded on the surface of the mold. The oil is removed when the surface temperature of the mold drops to 250°C.

[0023] In step 7, the low-temperature aging treatment is performed as follows: heat the mold to 150°C, keep it at that temperature for 8 hours, and then air-cool it to room temperature. Example

[0024] A method for preparing a low-internal-stress, high-hardness alloy mold includes the following steps: Step 1: Heat the mold blank to 850℃ and hold for 2 hours. Then, cool it in the furnace at a rate of 20℃ / h to 600℃ and air cool it after removing it from the furnace. Step 2: After rough machining, heat the mold to 550℃, hold for 4 hours, and then cool it in the furnace. Step 3: Place the mold in a vacuum furnace and evacuate it to a vacuum level of ≤1*10. -1 Pa, heated to 1050℃ at a rate of 8℃ / min, with holding time calculated based on an effective thickness of 1.5min / mm, and maintaining a vacuum degree ≤5*10 during the holding period. -2 pa, wherein the effective thickness of the mold is ≤50mm; Step 4: After the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. When the surface temperature of the mold drops to 300℃, the oil is removed and the mold is immediately placed in an isothermal furnace preheated to 350℃ within ≤30 seconds for isothermal treatment for 30 minutes. Step 5: After the isothermal treatment is completed, remove the mold from the isothermal furnace and cool it to room temperature; Step 6: After cooling in step 5, the mold is first mechanically polished, then transferred to a cryogenic chamber for cryogenic circulation treatment. The specific operation is as follows: First stage: The cryogenic chamber is cooled to -80℃ at a rate of 2℃ / min and held for 2 hours; Second stage: The cryogenic chamber continues to cool down to -196℃ at a rate of 1℃ / min and is held at that temperature for 12 hours; The third stage involves naturally heating up to room temperature, then cooling down to -80℃, holding at that temperature for 1 hour, and then naturally heating up to room temperature to complete the deep cryogenic cycle treatment of the mold. Step 7: Heat the cryogenically treated mold to 560°C for the first tempering. During the heat preservation period, apply a pulsed magnetic field with a frequency of 50Hz and a magnetic induction intensity of 0.5T for 30 minutes. Repeat the tempering twice, and then perform low-temperature aging treatment to obtain the low internal stress and high hardness alloy mold.

[0025] The mold blank comprises the following components by weight percentage: C 0.50%, Si 0.2%, Mn 0.50%, Cr 5.50%, Mo 1.00%, V 1.20%, Ni 0.65%, W 1.0%, La 0.08%, Nb 0.01%, B 0.005%, P 0.012%, S 0.010%, and Fe balance.

[0026] In step 4, after the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. At this time, the temperature of the quenching oil is pre-controlled at 60°C. The mold is completely immersed in the quenching oil, and the temperature is monitored in real time by the thermocouple embedded on the surface of the mold. The oil is removed when the surface temperature of the mold drops to 300°C.

[0027] In step 7, the low-temperature aging treatment is performed as follows: heat the mold to 180°C, keep it at that temperature for 4 hours, and then air-cool it to room temperature. Example

[0028] A method for preparing a low-internal-stress, high-hardness alloy mold includes the following steps: Step 1: Heat the mold blank to 850℃ and hold for 2 hours. Then, cool it in the furnace at a rate of 15℃ / h to 600℃ and air cool it after removing it from the furnace. Step 2: After rough machining, heat the mold to 520℃, hold for 4 hours, and then cool it in the furnace. Step 3: Place the mold in a vacuum furnace and evacuate it to a vacuum level of ≤1*10. -1 Pa, heated to 1040℃ at a rate of 8℃ / min, with holding time calculated based on an effective thickness of 1.5min / mm, and maintaining a vacuum degree ≤5*10 during the holding period. -2 pa; where the effective thickness of the mold is 80mm; Step 4: After the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. When the surface temperature of the mold drops to 280℃, the oil is removed and the mold is immediately placed in an isothermal furnace preheated to 300℃ within ≤30 seconds for isothermal treatment for 45 minutes. Step 5: After the isothermal treatment is completed, remove the mold from the isothermal furnace and cool it to room temperature; Step 6: After cooling in step 5, the mold is first mechanically polished, then transferred to a cryogenic chamber for cryogenic circulation treatment. The specific operation is as follows: First stage: The cryogenic chamber is cooled to -80℃ at a rate of 2℃ / min and held for 2 hours; Second stage: The cryogenic chamber continues to cool down to -196℃ at a rate of 1℃ / min and is held at that temperature for 12 hours; The third stage involves naturally heating up to room temperature, then cooling down to -80℃, holding at that temperature for 1 hour, and then naturally heating up to room temperature to complete the deep cryogenic cycle treatment of the mold. Step 7: Heat the cryogenically treated mold to 560°C for the first tempering. During the heat preservation period, apply a pulsed magnetic field with a frequency of 50Hz and a magnetic induction intensity of 0.5T for 30 minutes. Repeat the tempering twice, and then perform low-temperature aging treatment to obtain the low internal stress and high hardness alloy mold.

[0029] The mold blank comprises the following components by weight percentage: C 0.42%, Si 0.60%, Mn 0.35%, Cr 4.80%, Mo 1.50%, V 0.85%, Ni 0.75%, W 0.75%, La 0.05%, Nb 0.03%, B 0.003%, P 0.010%, S 0.008%, and Fe balance.

[0030] In step 4, after the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. At this time, the temperature of the quenching oil is pre-controlled at 50°C. The mold is completely immersed in the quenching oil, and the temperature is monitored in real time by the thermocouple embedded on the surface of the mold. The oil is removed when the surface temperature of the mold drops to 280°C.

[0031] In step 7, the low-temperature aging treatment is performed as follows: heat the mold to 160°C, keep it at that temperature for 6 hours, and then air-cool it to room temperature.

[0032] Comparative Example 1 The mold blank is the same as in Example 3, the main difference being that the preparation method follows the traditional process: Step 1: Heat the mold blank to 850℃ and hold for 2 hours. Then, cool it in the furnace at a rate of 15℃ / h to 600℃ and air cool it after removing it from the furnace. Step 2: After rough machining, heat the mold to 520℃, hold for 4 hours, and then cool it in the furnace. Step 3: Place the mold in a vacuum furnace and evacuate it to a vacuum level of ≤1*10. -1 Pa, heated to 1040℃ at a rate of 8℃ / min, with holding time calculated based on an effective thickness of 1.5min / mm, and maintaining a vacuum degree ≤5*10 during the holding period. -2 pa, where the effective thickness of the mold is 80mm; Step 4: After the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. The temperature of the quenching oil is pre-controlled at 50℃. The mold is completely immersed in the quenching oil. The temperature is monitored in real time by the thermocouple embedded on the surface of the mold. The oil is removed when the surface temperature of the mold drops to room temperature. Step 5: Place the mold after oiling in still air and allow it to return to room temperature naturally; Step 6: After cooling in Step 5, the mold is first mechanically polished, then heated to 560℃ for the first tempering, and held at that temperature for 2 hours; repeat the above tempering operation twice. Step 7: Place the mold after three tempering treatments in still air and allow it to cool naturally to room temperature to obtain the alloy mold.

[0033] Comparative Example 2 Its preparation method and mold blank composition are basically the same as those in Example 3. The main difference is that its preparation method does not include the deep cryogenic cycle treatment in step 6.

[0034] Comparative Example 3 Its preparation method and mold blank composition are basically the same as those in Example 3. The main difference is that in step 7, no pulsed magnetic field is applied during tempering.

[0035] Comparative Example 4 The preparation method is basically the same as that in Example 3. The main difference is that the mold blank does not contain the four elements La, Nb, B and W, and the composition is C 1.0%, Cr 12.0%, Mo 0.5% and V 0.2%, while the rest is the same as the mold blank composition in Example 3.

[0036] The alloy molds prepared in Examples 1-3 and Comparative Examples 1-4 were tested, and the specific results are shown in Table 1. Hardness testing: A Rockwell hardness tester (HRC) was used, with 5 points taken from both the mold surface and core, and the average value was recorded. Residual stress testing: X-ray diffraction (XRD) was used to measure the residual stress value on the mold surface. Residual austenite content testing: Referring to the Chinese standard YB / T5338-2019 "Quantitative Determination of Austenite in Steel by X-ray Diffraction," X-ray diffraction was used. The volume fraction of residual austenite was calculated by comparing the integrated intensity of the diffraction peaks on specific crystal planes of martensite (body-centered cubic, BCC) and austenite (face-centered cubic, FCC) due to their different crystal structures. Grain size determination: The austenite grain size was determined using the intercept method according to ASTM E112 standard. Dimensional stability testing: The mold was heated to 200℃ and held for 24 hours, and the dimensional change rate before and after heat treatment was measured.

[0037] Table 1 Test Data Table

[0038] As shown in Table 1, the alloy mold prepared in this application has a hardness ≥58 HRC, residual stress ≤-80 MPa, and residual austenite content ≤5 vol%. The difference in hardness between the core and the surface is ≤2 HRC. Furthermore, a comparison between Example 3 and Comparative Examples 1-4 shows that there is a synergistic enhancement effect between the graded quenching, deep cryogenic cycle treatment, pulsed magnetic field assisted tempering, and the composition of the specific mold blank in this application.

[0039] Based on Example 3, a supplementary comparative experiment was set up. The specific results are shown in Table 2, where the martensite conversion rate = 1 - the volume fraction of residual austenite (vol%).

[0040]

[0041] As shown in Table 2, Example 1, which adds La, B, and Nb simultaneously, exhibits the best performance. Omitting any one of these elements leads to a decrease in hardness, an increase in residual stress, or grain coarsening. In particular, the synergistic addition of La and B, under the same oil quenching cooling conditions, increases the content of residual austenite, i.e., decreases the martensite conversion rate, demonstrating the synergistic effect of "La+B" in improving hardenability and obtaining a more complete martensitic structure.

[0042] In summary, this application, through a three-stage synergistic heat treatment process of "graded quenching—stepped deep cryogenic cycle—pulse magnetic field assisted tempering," combined with a specific alloy composition design, successfully breaks through the technical bottleneck of the mutual constraint between high hardness and low internal stress in traditional mold preparation. The alloy mold prepared by the method of this invention has a hardness ≥58 HRC, residual stress ≤-80MPa, and residual austenite content ≤5 vol%, with a hardness difference between the core and the surface ≤2 HRC. The preparation method is controllable and applicable to complex structure molds, demonstrating significant technological progress and broad industrial application prospects.

[0043] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the scope of the present invention and the contents of the specification should still fall within the scope of the present invention.

Claims

1. A method for preparing a low-internal-stress, high-hardness alloy mold, characterized in that: Includes the following steps: Step 1: Heat the mold blank to 850℃ and hold for 2 hours. Then, cool it in the furnace to 600℃ at a cooling rate of 10-20℃ / h, and then air cool it after removing it from the furnace. Step 2: After rough machining, heat the mold to 500-550℃, keep it at that temperature for 4 hours, and then cool it in the furnace. Step 3: Place the mold in a vacuum furnace and heat it to 1030-1050℃. The holding time is calculated based on an effective thickness of 1.5 min / mm. Step 4: After the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. When the surface temperature of the mold drops to 250-300℃, the oil is removed and the mold is immediately placed in an isothermal furnace preheated to 250-350℃ for isothermal treatment for 30-60 minutes. Step 5: After the isothermal treatment is completed, remove the mold from the isothermal furnace and cool it to room temperature; Step 6: After cooling in step 5, the mold is transferred into a cryogenic chamber for cryogenic circulation treatment. Step 7: Heat the cryogenically treated mold to 560°C for the first tempering. During the heat preservation period, apply a pulsed magnetic field with a frequency of 50Hz and a magnetic induction intensity of 0.5T for 30 minutes. Repeat the tempering twice to obtain the low internal stress and high hardness alloy mold.

2. The method for preparing a low-internal-stress, high-hardness alloy mold according to claim 1, characterized in that: In step 6, the cryogenic cycle process is performed as follows: First stage: The cryogenic chamber is cooled to -80℃ at a rate of 2℃ / min and held for 2 hours; Second stage: The cryogenic chamber continues to cool down to -196℃ at a rate of 1℃ / min and is held at that temperature for 12 hours; In the third stage, the temperature is naturally raised to room temperature, then cooled to -80°C and held for 1 hour, before being naturally raised to room temperature again to complete the deep cryogenic cycle treatment of the mold.

3. The method for preparing a low-internal-stress, high-hardness alloy mold according to claim 1, characterized in that: The mold blank comprises the following components by weight percentage: C 0.35-0.50%, Si 0.2-1.0%, Mn 0.2-0.50%, Cr 4.00-5.50%, Mo 1.00-2.00%, V 0.50-1.20%, Ni 0.65-0.85%, W 0.5-1.0%, La 0.02-0.08%, Nb 0.01-0.05%, B 0.001-0.005%, P ≤0.015%, S ≤0.010%, and Fe balance.

4. The method for preparing a low-internal-stress, high-hardness alloy mold according to claim 1, characterized in that: In step 4, the oil-quenched mold is placed into an isothermal furnace preheated to 250-350℃ within ≤30 seconds.

5. The method for preparing a low-internal-stress, high-hardness alloy mold according to claim 1, characterized in that: In step 4, after the heat preservation is completed, quenching oil is introduced into the vacuum furnace for oil quenching. At this time, the temperature of the quenching oil is pre-controlled at 40-60℃. The mold is completely immersed in the quenching oil, and the temperature is monitored in real time by the thermocouple embedded on the surface of the mold. When the surface temperature of the mold drops to 250-300℃, the oil is removed.

6. The method for preparing a low-internal-stress, high-hardness alloy mold according to claim 1, characterized in that: In step 4, the isothermal treatment time is determined according to the effective thickness of the mold: 30 min for ≤50mm, 45 min for 50-150mm, and 60 min for ≥150mm.

7. The method for preparing a low-internal-stress, high-hardness alloy mold according to claim 1, characterized in that: After step 7, the mold is subjected to low-temperature aging treatment. The specific operation of the low-temperature aging treatment is as follows: heat the mold to 150-180℃, keep it at that temperature for 4-8 hours, and then air-cool it to room temperature.

8. The method for preparing a low-internal-stress, high-hardness alloy mold according to claim 1, characterized in that: In step 6, before the cooled mold undergoes deep cryogenic cycling, it is first mechanically polished.

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