Tempering method for large pre-hardened plastic mold steel
By controlling the low-temperature waiting temperature and the waiting time, the high-temperature tempering time, and the two-stage high-temperature tempering, the problems of uneven hardness and tempering brittleness during the tempering process of large pre-hardened plastic mold steel were solved, achieving both uniform hardness and economy.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- KEJIA (CHANGXING) MOULD BASE MFG CO LTD
- Filing Date
- 2026-03-20
- Publication Date
- 2026-07-07
AI Technical Summary
Large pre-hardened plastic mold steels suffer from uneven surface and core hardness and temper brittleness during tempering, which are difficult to solve effectively with existing technologies and are costly.
By controlling the low-temperature waiting temperature and the waiting time, the high-temperature tempering time, the two-stage high-temperature tempering and cooling after tempering, the temperature and time in each step are strictly controlled to improve the hardness difference between the surface and core of the steel and avoid temper brittleness.
It achieves uniform surface and core hardness of large pre-hardened plastic mold steel, avoids temper brittleness, and does not increase material costs, making it economical and universally applicable.
Smart Images
Figure CN122344653A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of mold steel manufacturing, and more specifically, to a tempering method for large pre-hardened plastic mold steel. Background Technology
[0002] Pre-hardened steels such as P20H and 718H are commonly used in plastic mold manufacturing due to their excellent performance and cost-effectiveness. Because they are pre-hardened by the forging plant, mold manufacturers do not need to perform secondary heat treatment, avoiding problems such as deformation and cracking that can occur during secondary heat treatment. However, steel mills still face the following issues when producing large pre-hardened plastic mold steels: 1. Uneven surface and core hardness: Some steel mills, in order to improve efficiency and save costs when producing large pre-hardened plastic mold steel, use post-forging residual heat quenching followed by tempering. After quenching in this way, there is a large hardness difference between the surface and core of the pre-hardened steel. After tempering at a single temperature, the surface and core hardness are uneven, and the core hardness is lower.
[0003] 2. Temper embrittlement: Pre-hardening generally employs a quenching + tempering process. During quenching, first-type quenching internal stress (thermal stress) and second-type quenching internal stress (structural stress) may be generated. Tempering can eliminate quenching internal stress. However, during tempering, first-type irreversible temper embrittlement and second-type reversible temper embrittlement may occur. According to literature research, temper embrittlement is related to the segregation of carbides and impurity elements such as P, Sn, and Sb at grain boundaries with specific size distributions (see *Journal of Heat Treatment of Metals*, 2018, Vol. 39, No. 5).
[0004] To address the above issues, the existing solutions are as follows: 1. Improve steel quality: Use methods such as electroslag remelting and vacuum smelting to reduce the content of harmful components such as P and As in steel. However, this method will increase the material cost by 20%-30%. Moreover, for mold steel with a thickness greater than 400mm, the method of reducing impurities has limited help in avoiding temper brittleness (see Special Casting and Nonferrous Alloys, 2020, Vol. 40, No. 3).
[0005] 2. Adding alloying elements: Adding elements such as Mo and W to steel can effectively avoid temper brittleness, but at the same time, it will significantly increase the material cost (see Heat Treatment, 2019, Vol. 44, No. 2).
[0006] 3. Rapid cooling after high-temperature tempering: Oil cooling or water cooling can avoid the second type of tempering brittleness. However, for mold steel with a diameter of 400mm or more, rapid cooling of the core area is not possible, and rapid cooling may cause new stress to be generated, causing the steel to crack (see "Heat Treatment of Metal Materials", 2021, Vol. 42, No. 4).
[0007] In summary, existing technologies are significantly insufficient in addressing the issues of uneven surface and core hardness and temper brittleness in large pre-hardened plastic mold steel after tempering, necessitating an economical and effective tempering method. Summary of the Invention
[0008] To address the problems existing in the prior art, the present invention aims to provide a tempering method for large pre-hardened plastic mold steel. This process strictly controls the temperature and time in each step by controlling the low-temperature waiting temperature and waiting time, the high-temperature tempering holding time, the two-stage high-temperature tempering holding, the cooling after tempering, and the temperature control during secondary and multiple tempering processes. This improves the difference in hardness between the surface and core of the steel and avoids tempering brittleness.
[0009] To solve the above problems, the present invention adopts the following technical solution.
[0010] A tempering method for large pre-hardened plastic mold steel includes the following steps: A1: Determine the tempering temperature and total number of tempering cycles s based on the type, thickness H, and hardness requirements of the pre-hardened steel; A2: Low temperature waiting: Determine the heat preservation temperature of the pre-hardened steel based on the current number of tempering cycles i, and after heat preservation at the heat preservation temperature for a certain period of time, raise the temperature to the preset tempering heat preservation temperature at full power. A3: High-temperature tempering and heat preservation: After determining the tempering method and tempering heat preservation time based on the thickness H of the pre-hardened steel and the number of tempering times i, high-temperature tempering is carried out; A4: After a single tempering is completed, the pre-hardened steel is air-cooled to room temperature. When the current tempering number i = the total tempering number s, the tempering ends; otherwise, repeat steps A2-A4 until the tempering ends.
[0011] Furthermore, in step A1, the pre-hardened steel is a pre-hardened plastic mold steel with grades P20, XPM, 1.2714, 718, or 1.2738.
[0012] Furthermore, in step A2, when i=1, the heat preservation temperature is 190-210℃ and the heat preservation time is 3-4 hours; when i>1, the heat preservation temperature is 340-360℃ and the heat preservation time is 3-4 hours.
[0013] Furthermore, in step A3, when the thickness H ≤ 400 mm, a one-stage high-temperature tempering is used; when the thickness H of the pre-hardened steel > 400 mm, a two-stage high-temperature tempering is used.
[0014] Furthermore, the tempering holding time is equal to the product of the tempering holding time coefficient and the thickness H.
[0015] Furthermore, the specific details of the one-stage high-temperature tempering are as follows: When i=1: when the tempering temperature is <575℃, the tempering holding time coefficient is ≤3.25 minutes / mm; preferably 3-3.25 minutes / mm; more preferably 3.25 minutes / mm; When the tempering temperature is ≥575℃, the tempering holding time coefficient is 3.25-4.5 minutes / mm; When i > 1: when the tempering temperature is < 575℃, the tempering holding time coefficient is 1.75-2.25 minutes / mm; when the tempering temperature is ≥ 575℃, the tempering holding time coefficient is ≤ 3.25 minutes / mm; preferably 3 minutes / mm.
[0016] Furthermore, if the tempering time is less than 15 hours, it shall be performed as 15 hours.
[0017] Furthermore, the two-stage high-temperature tempering is specifically described below: When i=1: the first tempering temperature must be less than 575℃, and the tempering holding time coefficient for the first tempering stage is ≤2.75 minutes / mm; preferably 2.5-2.75 minutes / mm; more preferably 2.5 minutes / mm; When the second stage tempering temperature is ≥575℃, the tempering holding time is 8-12 hours; When the second-stage tempering temperature is <575℃, the sum of the tempering holding time coefficient of the first-stage tempering and the tempering holding time coefficient of the second-stage tempering is ≤3.25 minutes / mm; preferably 0.5-0.75 minutes / mm. When i > 1: the tempering holding time coefficient for the first stage of tempering is 1.25 minutes / mm; When the second stage tempering temperature is ≥575℃, the tempering holding time is 8-12 hours; When the second tempering temperature is <575℃, the sum of the tempering holding time coefficient of the first tempering stage and the tempering holding time coefficient of the second tempering stage is ≤2.25 minutes / mm; preferably 0.5-1.0 minutes / mm.
[0018] Furthermore, in step A4, the air cooling is a forced-air air cooling system.
[0019] Furthermore, in step A4, when i=s, the pre-hardened steel is air-cooled to room temperature; when i≠s, the pre-hardened steel is air-cooled to room temperature and then steps A2 to A4 are repeated until i=s.
[0020] Compared with the prior art, the advantages of this invention are: This solution improves the surface and core hardness difference in large pre-hardened plastic mold steel by controlling the low-temperature waiting temperature and holding time, the high-temperature tempering holding time, the two-stage high-temperature tempering holding, the cooling after tempering, and the temperature control during secondary and multiple tempering processes. Simultaneously, it avoids the occurrence of both Type I and Type II tempering brittleness in large plastic mold steel during tempering. This method significantly improves the surface and core hardness difference and avoids tempering brittleness in large plastic mold steel without compromising the steel's inherent quality, offering advantages in terms of economy and versatility. Attached Figure Description
[0021] Figure 1 This is a flowchart of the tempering method in Example 1; Figure 2 This is a process diagram of a single-stage tempering process when i=1 in Example 1; Figure 3 This is a process diagram of a one-stage tempering process when i>1 in Example 1; Figure 4 This is a flowchart of the two-stage tempering process when i=1 in Example 1; Figure 5 This is a diagram of the two-stage tempering process when i>1 in Example 1. Detailed Implementation
[0022] Example 1:
[0023] Tempering process for large pre-hardened plastic mold steel, such as Figure 1 As shown, the specific content is as follows: Step 1: Determine the tempering temperature and tempering cycles (s) based on the type, thickness (H), and hardness requirements of the pre-hardened steel, and in conjunction with the information in Table 1. When using a single-stage tempering process, the tempering temperature should be the temperature of the second stage in Table 1.
[0024] ; Step 2: After quenching, the pre-hardened steel is kept at a low temperature for a certain period of time and then heated to the preset tempering holding temperature at full power. The preset tempering holding temperature is shown in Table 1 (usually not lower than 400℃). When the current tempering number i=1, the low temperature holding temperature is 200℃ and the holding time is 3-4h. When the number of tempering cycles i > 1, the low-temperature holding temperature is 350℃ and the holding time is 3-4 hours.
[0025] Step 3: Determine the tempering process and tempering time based on the thickness H of the steel: (1) When the thickness H≤400mm, a one-stage high-temperature tempering is adopted (tempering holding time = tempering holding time coefficient × thickness H): (A) When i=1, a single-stage tempering method (such as...) Figure 2 Specifically, if the tempering temperature is 450-575℃, the tempering holding time coefficient is 3-3.25 minutes / mm; if the tempering temperature is ≥575℃, the tempering holding time coefficient is 3.25-4.5 minutes / mm.
[0026] In addition, if the tempering and heat preservation time is less than 15 hours, it shall be performed as 15 hours.
[0027] (B) When i>1, a single-stage tempering method (such as...) Figure 3 Specifically, if the tempering temperature is ≥575℃, the tempering holding time coefficient is ≤3.25 minutes / mm; if the tempering temperature is <575℃, the tempering holding time coefficient is 1.75-2.25 minutes / mm.
[0028] In addition, if the tempering and heat preservation time is less than 15 hours, it shall be performed as 15 hours.
[0029] (2) When the thickness H > 400 mm, a two-stage high-temperature tempering process is adopted: (A) When i=1, two-stage high-temperature tempering (such as...) Figure 4 (As shown) Specifically: the first stage tempering temperature is <575℃, and the tempering holding time coefficient for the first stage tempering is 2.5-2.75 minutes / mm; If the second stage tempering temperature is ≥575℃, the tempering holding time is 8-12 hours; If the tempering temperature of the second stage is <575℃, the tempering holding time coefficient of the second stage is 0.5-0.75 minutes / mm.
[0030] (B) When i>1, the two-stage tempering method (such as...) Figure 5 (As shown) Specifically, the tempering holding time coefficient for the first stage of tempering is 1.25 minutes / mm; If the second stage tempering temperature is ≥575℃, the tempering holding time is 8-12 hours; If the second tempering temperature is <575℃, the tempering holding time coefficient is 0.5-1.0 minutes / mm.
[0031] Step 4: After the pre-hardened steel has been kept in the heat for the time specified above, immediately open the furnace door and lift it out of the furnace platform for air cooling; With s=i, air cooling to room temperature completes the tempering process; If s≠i and s>1, after air cooling to room temperature, repeat steps two through four until i=s, at which point tempering is complete.
[0032] Application Example 1: The following pre-hardened die steel was tempered according to the procedure in Example 1: P20 grade pre-hardened die steel with a thickness of 240mm, a width of 1460mm, and a length of 3500mm, and a hardness requirement of 28-34 HRC.
[0033] Tempering operation: After the P20 grade pre-hardened die steel is quenched, when it is put into the furnace for tempering, it is held at 200℃ for 3 hours, and then the furnace temperature is increased at full power to quickly reach the high temperature tempering temperature of 605℃, and held at this temperature for 3.25×240÷60=13 hours. Since it is less than 15 hours, it is executed as 15 hours. After the holding time is completed, it is immediately lifted out of the furnace platform and cooled to room temperature by blowing air.
[0034] After the above tempering process, the surface hardness was measured to be 32 HRC, the core hardness to be 30 HRC, and the V-notch impact energy of the standard sample was 38J, indicating that the hardness was uniform and no first or second type of temper brittleness was produced.
[0035] Application Example 2: The following pre-hardened die steel was tempered according to the procedure in Example 1: P20 grade pre-hardened die steel with a thickness of 780mm, a width of 1240mm, and a length of 1380mm, and a hardness requirement of 28-34 HRC, requiring two-stage tempering.
[0036] Tempering operation: After quenching, hold at 200℃ for 4 hours, then increase the furnace temperature at full power to quickly reach the first stage high-temperature tempering temperature of 540℃, and hold at this temperature for 2.5×780÷60=32.5 hours. Then quickly increase the furnace temperature to 605℃ and hold at this temperature for 12 hours. After the holding period, immediately lift it off the furnace platform and blow it to cool to room temperature.
[0037] After the above tempering process, the surface hardness was measured to be 34 HRC, the core hardness to be 32 HRC, and the V-notch impact energy of the standard sample was 32J, indicating that the hardness was uniform and no first or second type of temper brittleness was produced.
[0038] Application Example 3: The following pre-hardened die steel was subjected to tempering according to the procedure in Example 1: 718 grade pre-hardened die steel with a thickness of 900mm, a width of 1200mm, and a length of 2500mm, with a hardness requirement of 30-36 HRC, requiring two tempering cycles.
[0039] The tempering process involves two stages: After quenching, the furnace is held at 200℃ for 4 hours. Then, the furnace temperature is increased to full power, rapidly reaching the first stage high-temperature tempering temperature of 540℃, and held at this temperature for 2.5 × 900 ÷ 60 = 37.5 hours. Next, the furnace temperature is rapidly increased to 605℃ and held at this temperature for 12 hours. After this holding period, the furnace is immediately removed from the furnace and cooled to room temperature by air. After cooling to room temperature, a second tempering process is performed. First, the furnace is held at 350℃ for 4 hours. Then, the furnace temperature is increased to full power, rapidly reaching the first stage high-temperature tempering temperature of 540℃, and held at this temperature for 1.25 × 900 ÷ 60 = 18.75 hours. Next, the furnace temperature is rapidly increased to 605℃ and held at this temperature for 12 hours. After this holding period, the furnace is immediately removed from the furnace and cooled to room temperature by air.
[0040] After the above tempering process, the surface hardness was measured to be 34 HRC, the core hardness to be 30 HRC, and the V-notch impact energy of the standard sample was 39J, indicating that the hardness was uniform and no first or second type of temper brittleness was produced.
Claims
1. A tempering method for large pre-hardened plastic mold steels, characterized by: Includes the following steps: A1: Determine the tempering temperature and total number of tempering cycles s based on the type, thickness H, and hardness requirements of the pre-hardened steel; A2: Determine the holding temperature of the pre-hardened steel based on the current number of tempering cycles i, and after holding at the holding temperature for a certain period of time, raise the temperature to the preset tempering holding temperature at full power. A3: After determining the tempering method and tempering holding time based on the thickness H of the pre-hardened steel and the current tempering number i, high-temperature tempering is carried out; A4: After a single tempering is completed, the pre-hardened steel is air-cooled to room temperature. When the current tempering number i = the total tempering number s, the tempering ends; otherwise, repeat steps A2-A4 until the tempering ends.
2. The tempering method for large pre-hardened plastic mold steel according to claim 1, characterized in that: In step A1, the pre-hardened steel is a pre-hardened plastic mold steel with grades P20, XPM, 1.2714, 718 or 1.2738.
3. The tempering method for large pre-hardened plastic mold steel according to claim 1, characterized in that: In step A2, when i=1, the heat preservation temperature is 190-210℃ and the heat preservation time is 3-4 hours; when i>1, the heat preservation temperature is 340-360℃ and the heat preservation time is 3-4 hours.
4. The tempering method for large pre-hardened plastic mold steel according to claim 1, characterized in that: In step A3, when the thickness H ≤ 400 mm, a one-stage high-temperature tempering is used; when the thickness H > 400 mm of the pre-hardened steel, a two-stage high-temperature tempering is used.
5. The tempering method for large pre-hardened plastic mold steel according to claim 4, characterized in that: The tempering holding time is equal to the product of the tempering holding time coefficient and the thickness H.
6. The tempering method for large pre-hardened plastic mold steel according to claim 5, characterized in that: The specific details of the one-stage high-temperature tempering are as follows: When i=1: when the tempering temperature is <575℃, the tempering holding time coefficient is ≤3.25 minutes / mm; When the tempering temperature is ≥575℃, the tempering holding time coefficient is 3.25-4.5 minutes / mm; When i > 1: when the tempering temperature is < 575℃, the tempering holding time coefficient is 1.75-2.25 minutes / mm; when the tempering temperature is ≥ 575℃, the tempering holding time coefficient is ≤ 3.25 minutes / mm.
7. The tempering method for large pre-hardened plastic mold steel according to claim 6, characterized in that: If the tempering time is less than 15 hours, it shall be performed as 15 hours.
8. The tempering method for large pre-hardened plastic mold steel according to claim 5, characterized in that: The two-stage high-temperature tempering process is described in detail below: When i=1: the first tempering temperature must be less than 575℃, and the tempering holding time coefficient for the first tempering stage is ≤2.75 minutes / mm; When the second stage tempering temperature is ≥575℃, the tempering holding time is 8-12 hours; When the second tempering temperature is <575℃, the sum of the tempering holding time coefficient of the first tempering stage and the tempering holding time coefficient of the second tempering stage is ≤3.25 minutes / mm; When i > 1: the tempering holding time coefficient for the first stage of tempering is 1.25 minutes / mm; When the second stage tempering temperature is ≥575℃, the tempering holding time is 8-12 hours; When the second tempering temperature is <575℃, the sum of the tempering holding time coefficient of the first tempering stage and the tempering holding time coefficient of the second tempering stage is ≤2.25 minutes / mm.
9. The tempering method for large pre-hardened plastic mold steel according to claim 1, characterized in that: In step A4, the air cooling is a forced-air air cooling system.
10. The tempering method for large pre-hardened plastic mold steel according to claim 1, characterized in that: In step A4, when i=s, the pre-hardened steel is air-cooled to room temperature; when i≠s, the pre-hardened steel is air-cooled to room temperature and then steps A2 to A4 are repeated until i=s.