A variable cooling rate vacuum air quenching process suitable for FS450 die casting mould

Abstract

The present application belongs to the technical field of heat treatment process of die casting mold, and particularly relates to a variable cooling rate vacuum air quenching heat treatment method suitable for FS450 die casting mold. The steps are as follows: the mold is put into a vacuum high-pressure air quenching furnace, and is heated to austenitizing temperature in sections and is kept warm; nitrogen is filled for precooling treatment, and then nitrogen is filled to reach 14000 mbar of furnace pressure, and the fan speed is set to 1600 r / min to start cooling; when the sample temperature cools to 540-570 DEG C, the furnace pressure is reduced to 5000-7000 mbar, the fan speed is set to 1000-1200 r / min, and the cooling continues until the predetermined discharge temperature; after the mold is discharged and transferred to a tempering furnace for three-stage tempering treatment, the mold is discharged and cooled. The method effectively avoids excessive quenching stress caused by the violent martensite phase transition under the traditional full-speed cooling mode, significantly reduces the cracking risk and deformation trend of the mold, thereby prolonging the service life of the mold.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for die casting molds, and specifically relates to a variable cooling rate vacuum quenching heat treatment method suitable for FS450 die casting molds. Background Technology

[0002] FS450 is currently the most widely used die-casting mold steel both domestically and internationally. It is an improved version of H13 steel, possessing excellent hardenability, hot strength, thermal fatigue resistance, and wear resistance, and is widely used in the manufacture of die-casting molds. During use, die-casting molds must withstand significant mechanical impacts and the impact and compressive stresses of high-temperature solid and liquid metals, as well as the tensile stress generated by the tightness of the die-cast metal during demolding. Therefore, die-casting molds require high high-temperature strength, thermal fatigue resistance, and a good balance of strength and toughness.

[0003] Currently, vacuum high-pressure gas quenching is the mainstream heat treatment process for die-casting mold steels such as FS450. This process effectively avoids oxidation and decarburization of the mold during heat treatment and achieves a smooth surface finish. However, to ensure sufficient hardenability of the mold, traditional processes generally employ a cooling strategy of high pressure and high fan speed throughout the entire process. While this intensive cooling method can ensure that the core of large-section molds obtains a martensitic structure, it maintains an extremely high cooling rate in the martensitic transformation zone (near the Ms point), resulting in an overly intense martensitic transformation process and concentrated phase transformation expansion stress. Although hot-working molds prepared in this way have good hardness uniformity, their impact toughness is significantly reduced, leading to an imbalance between strength and toughness, making the molds prone to early brittle fracture during service. At the same time, the intense phase transformation stress also increases the mold's deformation tendency and cracking risk, especially for large-section, complex-structured die-casting molds, where this problem is more prominent. To avoid quenching stress and deformation, some processes also use a lower cooling rate throughout the entire process. However, while FS450 steel has excellent hardenability, for thicker die-casting molds (such as 200mm cross-sections), slow cooling throughout the process often results in insufficient cooling rate in the core to completely avoid the pearlite and bainite transformation zones. This leads to the presence of non-martensitic transformation products or coarse martensite laths in the core structure, while carbides precipitate and aggregate along grain boundaries. The resulting hot-working mold has acceptable surface hardness, but significantly lower core hardness, poor microstructure uniformity, and insufficient overall hardenability. This severely weakens the mold's high-temperature strength and thermal fatigue resistance, failing to meet the requirement of consistent performance across the entire cross-section of the die-casting mold.

[0004] In summary, there is an inherent contradiction between "ensuring sufficient hardenability and uniform hardness" and "controlling quenching stress to avoid cracking and deformation": strong cooling throughout the process can achieve high hardness uniformity, but sacrifices toughness and dimensional stability; slow cooling throughout the process or relying on medium switching, while aiming to control stress, makes it difficult to guarantee the hardenability and microstructure uniformity of the core of large-section molds. To solve these problems, the industry has adopted methods such as staged quenching or isothermal quenching. For example, Chinese patent CN101818234A discloses a quenching process for H13 steel used in die-casting molds. Its core technology is a dual-liquid quenching method of "gas quenching + oil quenching," that is, first air-cooling to 590~620℃ in a vacuum furnace, and then transferring to a quenching oil furnace for rapid cooling, aiming to increase the cooling rate of the core to avoid the bainite transformation zone. However, its operation is complex and it is prone to structural differences and stress concentration due to uneven temperature field; at the same time, the oil quenching medium generates oil fumes at high temperature, which poses environmental pollution and fire safety hazards; more importantly, this type of solution aims to accelerate cooling in the low temperature zone, but instead aggravates the martensitic phase transformation stress and fails to fundamentally solve the problem of phase transformation cracking.

[0005] In addition, there are reports of using isothermal quenching in salt baths, but salt bath media also have problems such as high energy consumption, difficulty in waste salt treatment, and complex equipment maintenance, making it difficult to promote industrialization.

[0006] In summary, how to effectively control quenching stress and deformation while ensuring high strength and good toughness of the mold without introducing additional media such as oil cooling or salt bath, and how to adapt it to the material properties of FS450 steel and typical large-section mold specifications, is a technical problem that has not yet been solved in the existing technology. Summary of the Invention

[0007] In order to solve the problems in the related technology, the purpose of this invention is to provide a variable cooling rate vacuum quenching treatment method for FS450 die casting molds, which can effectively solve the problem of premature failure of hot work molds.

[0008] Technical solution The first aspect of this invention proposes a variable-rate vacuum quenching treatment method for FS450 die-casting molds, comprising the following steps: The mold is placed in a vacuum high-pressure gas quenching furnace, heated in sections to the austenitizing temperature and held at that temperature. Nitrogen gas was introduced for pre-cooling, and then nitrogen gas was introduced until the furnace pressure reached 13000~15000 mbar and the fan speed was set to 1500~1700 r / min to begin cooling. When the sample temperature cooled to 540~570℃, the furnace pressure was reduced to 5000~7000 mbar and the fan speed was set to 1000~1200 r / min to continue cooling to the predetermined furnace exit temperature. After the mold is removed from the furnace, it is transferred to a tempering furnace for a three-stage tempering process before being removed from the furnace and cooled.

[0009] Furthermore, the components of the FS450, by mass fraction, are C 0.38%, Si 0.2%, Mn 0.5%, Cr 5.05%, V 0.56%, Mo 1.82%, P ≤ 0.015%, S ≤ 0.003%, with the balance being Fe.

[0010] Furthermore, the segmented heating involves raising the temperature to 600~700℃ and holding it for 230~280 minutes, then heating it to 800~900℃ and holding it for 230~280 minutes, and then raising the temperature to 1010℃ and holding it for 250~350 minutes.

[0011] Furthermore, the vacuum degree in the vacuum high-pressure gas quenching furnace is 1.0~1.1 mbar.

[0012] Furthermore, the three-stage tempering process involves first heating the mold to 540~560℃ and holding it for 700~900 minutes before air cooling, then heating it to 550~570℃ and holding it for 600~650 minutes before air cooling, and finally heating it to 520~540℃ and holding it for 600~650 minutes before air cooling.

[0013] The second aspect of this invention provides an FS450 die-casting mold, which is prepared using the variable cooling rate vacuum quenching treatment method suitable for FS450 die-casting molds.

[0014] Beneficial effects This invention achieves a segmented cooling strategy by significantly reducing furnace pressure and fan speed during the gas quenching process, particularly when the mold temperature drops above the martensitic transformation initiation point (Ms point). This strategy enables rapid cooling in the high-temperature zone to ensure hardenability, while allowing for gentle and slow cooling in the low-temperature zone to control phase transformation stress. This strategy effectively avoids the excessive quenching stress caused by the intense martensitic phase transformation in traditional high-speed cooling methods throughout the entire process, significantly reducing the risk of mold cracking and deformation, thereby extending the mold's service life.

[0015] This invention effectively ensures that the FS450 mold material achieves sufficient and uniform hardenability, thus guaranteeing its excellent mechanical properties and reducing heat treatment deformation. It enables the formation of fine tempered martensite structures from the mold's edges to its core, resulting in good structural uniformity. This ensures the mold as a whole possesses excellent high-temperature strength, toughness, and resistance to thermal fatigue, meeting the performance requirements of die-casting molds under complex service conditions.

[0016] This invention achieves segmented cooling simply by adjusting the furnace pressure and fan speed during the gas quenching process, without the need for additional media such as oil cooling or salt baths. The process is simple to operate and can be directly implemented on existing vacuum high-pressure gas quenching equipment, exhibiting good process compatibility and adaptability for widespread application.

[0017] This invention targets FS450, a specific die-casting mold steel, and its typical specifications. The proposed segmented cooling process parameters have been systematically verified and have good process stability and reproducibility. They can provide a reliable technical solution for the heat treatment of FS450 die-casting molds and effectively guide actual production. Attached Figure Description

[0018] Figure 1 The images are scanning electron microscope (SEM) images of FS450 hot work dies after vacuum quenching and tempering using the process parameters selected in this invention, wherein (a) is the edge structure of Example 1, (b) is the core structure of Example 1, (c) is the edge structure of Example 2, (d) is the core structure of Example 2, (e) is the edge structure of Example 3, and (f) is the core structure of Example 3.

[0019] Figure 2 Electron backscatter diffraction (EBSD) patterns of FS450 hot work dies after vacuum quenching and tempering using the process parameters selected in this invention, where (a) is the microstructure of Example 1, (b) is the microstructure of Example 2, and (c) is the microstructure of Example 3. Detailed Implementation

[0020] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0021] The experimental samples in this invention are FS450 die-casting molds: the chemical composition is shown in Table 1, and the sample size is 400mm×400mm×400mm.

[0022] Table 1. Chemical composition of the experimental steel, wt%

[0023] The experimental steel was prepared by electric arc furnace smelting + ladle refining + vacuum degassing + protective atmosphere electroslag remelting process to effectively reduce gas content and inclusion levels, improve steel purity and as-cast microstructure uniformity, and obtain molds through multi-directional forging and spheroidizing treatment processes.

[0024] This invention uses high-purity inert gas nitrogen (N2) for high-pressure convection quenching and cooling, and then undergoes different temperatures and different tempering cycles to obtain a high-performance mold with a hardness of 50HRC, which is required for high wear resistance applications.

[0025] The specific process is as follows: The mold was placed in a vacuum high-pressure gas quenching furnace, and the vacuum was evacuated to a degree of 1.05 mbar. The temperature was raised to 650℃ and held for 260 minutes. The temperature was then raised to 850℃ and held for 260 minutes. The temperature was then raised to 1010℃ and held for 300 minutes. After the holding period, a certain amount of N2 was introduced for 2 minutes of pre-cooling. Then, nitrogen was introduced and the furnace pressure reached 14000 mbar. The fan speed was set to 1600 r / min to begin cooling. When the sample temperature cooled to 540~570℃, the furnace pressure was reduced to 5000~7000 mbar and the fan speed was set to 1000~1200 r / min. The sample was removed from the furnace when the temperature dropped to 50℃.

[0026] After the mold is taken out of the furnace, it is immediately transferred to a tempering furnace for tempering treatment. The high hardness (50HRC) mold is first heated to 550℃ and held for 800 minutes before being taken out of the furnace and air-cooled. Then it is heated to 560℃ and held for 620 minutes before being taken out of the furnace and air-cooled. Finally, it is heated to 530℃ and held for 620 minutes before being taken out of the furnace and air-cooled.

[0027] Example 1 (Low Speed) (1) Place the mold sample in a vacuum high-pressure gas quenching furnace, evacuate to a vacuum degree of 1.05 mbar, heat to 650℃ and hold for 260 min, then heat to 850℃ and hold for 260 min, continue to heat to 1010℃ and hold for 300 min, after the holding is completed, fill with nitrogen, the furnace pressure reaches 14000 mbar and the fan speed is set to 1600 r / min, when the sample temperature cools to 550℃, reduce the furnace pressure to 4000 mbar and the fan speed is set to 1000 r / min, and the temperature drops to 50℃ before taking it out of the furnace.

[0028] (2) After the mold is taken out of the furnace, it is immediately transferred to the tempering furnace for tempering treatment and tempered to 50HRC. First, it is heated to 550℃ and held for 800 minutes before being taken out of the furnace and air-cooled. Then it is heated to 560℃ and held for 620 minutes before being taken out of the furnace and air-cooled. Then it is heated to 530℃ and held for 620 minutes before being taken out of the furnace and air-cooled.

[0029] Example 2 (Medium Speed) (1) Place the mold sample in a vacuum high-pressure gas quenching furnace, evacuate to a vacuum degree of 1.05 mbar, heat to 600~700℃ (preferably 650℃) and hold for 230~280min (preferably 260min), then heat to 800~900℃ (preferably 850℃) and hold for 230~280min (preferably 260min), then continue to heat to 1010℃ and hold for 250~350min (preferably 300min). After the holding period, purge with nitrogen gas and bring the furnace pressure to 1010℃. The pressure is set to 13000~15000 mbar, preferably 14000 mbar in this embodiment; the fan speed is set to 1500~1700 r / min, preferably 1600 r / min in this embodiment; when the sample temperature is cooled to 540~570℃, preferably 550℃ in this embodiment, the furnace pressure is reduced to 5000~7000 mbar, preferably 7000 mbar in this embodiment; the fan speed is set to 1000~1200 r / min, preferably 1200 r / min in this embodiment; and the temperature is reduced to 50℃ before the sample is removed from the furnace.

[0030] (2) After the mold is taken out of the furnace, it is immediately transferred to the tempering furnace for tempering treatment and tempered to 50HRC. First, heat it to 540~560℃ (preferably 550℃) and hold it for 700~900min (preferably 800min). Then, remove it from the furnace and air cool it. Then, heat it to 550~570℃ (preferably 560℃) and hold it for 600~650min (preferably 620min). Then, remove it from the furnace and air cool it. Then, heat it to 520~540℃ (preferably 530℃) and hold it for 600~650min (preferably 620min). Then, remove it from the furnace and air cool it.

[0031] Example 3 (High Speed) (1) Place the mold sample in a vacuum high-pressure gas quenching furnace, evacuate to a vacuum degree of 1.05 mbar, heat to 650℃ and hold for 260 min, then heat to 850℃ and hold for 260 min, continue to heat to 1010℃ and hold for 300 min, after the holding is completed, fill with nitrogen, the furnace pressure reaches 14000 mbar, the fan speed is set to 1600 r / min, and the temperature drops to 50℃ before taking it out of the furnace.

[0032] (2) After the mold is taken out of the furnace, it is immediately transferred to the tempering furnace for tempering treatment and tempered to 50HRC. First, it is heated to 550℃ and held for 800 minutes before being taken out of the furnace and air-cooled. Then it is heated to 560℃ and held for 620 minutes before being taken out of the furnace and air-cooled. Then it is heated to 530℃ and held for 620 minutes before being taken out of the furnace and air-cooled.

[0033] The molds after heat treatment in Examples 1-3 were sampled by wire cutting, ground, and polished. The samples, measuring 15mm × 15mm × 10mm, were etched with a 4% nitric acid-alcohol solution. The tissue was observed under a scanning electron microscope (1000x magnification). The results are shown below. Figure 1 .

[0034] pass Figure 1 Figures (a), (b), (c), (d), (e), and (f) of the heat-treated samples show that the microstructure after quenching and tempering at 50 HRC at the three cooling rates is mainly tempered martensite. In Example 1, the microstructure after low-rate quenching and tempering shows coarse tempered martensite laths and a large number of large carbides in the core, which is detrimental to toughness. In Examples 2 and 3, the microstructure after medium- and high-rate quenching and tempering shows fine tempered martensite laths with less difference between the edge and core microstructures. High-speed cooling during quenching has the effect of refining the lath martensite microstructure.

[0035] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that the sample temperature was cooled to 530°C instead of 550°C in step 1.

[0036] Comparative Example 2 The difference between Comparative Example 1 and Example 2 is that the sample temperature was cooled to 580°C instead of 550°C in step 1.

[0037] Wire cutting samples were taken from the heat-treated molds of Examples 1-3 and Comparative Examples 1-2. Rockwell hardness was measured at 10 mm intervals along the depth direction from the surface. Five points were measured at each position and the average value was taken to obtain the hardness distribution data from the surface to the core, as shown in Table 2.

[0038] Table 2 Hardness distribution data

[0039] Table 2 shows that the range of hardness in Examples 3, 1, and 2 is less than 1 HRC, indicating good uniformity. The range of hardness in Example 1 is 2.1 HRC, and in Example 2 it is 2.0 HRC, showing lower uniformity than in Examples 3, 1, and 2. However, the hardness values ​​at all measuring points remained stable within the range of 49.1–52.0 HRC, falling within the normal hardness fluctuation range for FS450 mold steel tempered to 50 HRC. No sudden drops in localized hardness or "soft spots" caused by non-martensitic structures were observed, indicating sufficient hardenability across the entire mold cross-section under these process conditions, and no excessive hardness drop due to insufficient cooling rate.

[0040] Three room temperature tensile test specimens were prepared from the molds of Examples 1-3 and Comparative Examples 1-2 after heat treatment. Each room temperature tensile test specimen was a standard round bar tensile test specimen with a gauge length of 30 mm, an original gauge diameter of 5 mm, and a total length of 80 mm. Three room temperature impact test specimens were also prepared, each being a V-notch standard room temperature impact test specimen with a diameter of 10 mm × 10 mm × 55 mm. The mechanical property data of the specimens after vacuum quenching and tempering are shown in Table 3.

[0041] Table 3 Mechanical property data after vacuum quenching and tempering

[0042] As shown in Table 3, compared with Example 1, which had a slow cooling rate in the low-temperature section, Example 2 had a yield strength 46 MPa higher, a tensile strength 81 MPa higher, and an impact energy 8 J higher. This indicates that an excessively low cooling rate in the low-temperature section leads to insufficient driving force for martensitic transformation, resulting in coarsening of the microstructure, carbide aggregation, and simultaneous deterioration of strength and toughness. Compared with Example 3, which had high-speed cooling throughout, Example 2 had a tensile strength 34 MPa higher and an impact energy only 2 J lower. However, Example 2 reduced energy consumption and quenching stress risk by actively reducing the cooling rate in the low-temperature section, resulting in better process economy and safety. Compared with Comparative Example 1, Example 2 had a tensile strength only 98 MPa lower (approximately 5%), but an impact energy 2 J higher (an increase of over 15%). This indicates that reducing the cooling rate slightly above the Ms point in advance can effectively control the martensitic transformation. The bulk phase transformation stress and protective toughness are achieved by maintaining a sufficient cooling rate in the high-temperature section to ensure that the strength is not significantly lost. Compared with Comparative Example 2, the tensile strength of Example 2 is 27 MPa higher, the yield strength is 28 MPa higher, and the impact energy is only 3 J lower. This shows that if the cooling rate is reduced too early, hardenability will be impaired and the strength and hardness will be insufficient. Example 2 precisely controls the switching time above the Ms point, taking into account both hardenability and toughness. In summary, Example 2 achieves a synergistic match between high strength and good toughness by maintaining sufficient cooling intensity in the high-temperature section to ensure hardenability and actively switching the cooling intensity to a moderate level above the Ms point (540~570℃) to control phase transformation stress. It finds the optimal balance between strength and toughness and fundamentally solves the technical bottleneck of the difficulty in balancing hardenability and stress control in traditional processes.

[0043] The molds from Examples 1-3, after heat treatment, were sampled by wire cutting, ground, and polished. The samples were 5mm × 5mm × 2mm in size. After vibration polishing, the tissue was observed under an electron microscope (500x magnification). The results are shown below. Figure 2 .

[0044] from Figure 2It can be seen that at a cooling rate of 2.37℃ / min, there are large grains, coarse martensite laths, wide lath bundles, relatively low grain boundary density, and small local orientation differences. When the cooling rate is increased to 3.57℃ / min, grain coarsening is suppressed, martensite laths are significantly refined, lath bundle size decreases, grain boundary density increases, and the microstructure becomes more compact. When the cooling rate is further increased to 4.17℃ / min, the grains are further refined, the grain distribution is more uniform and fine, and the number of grain boundaries per unit area is the highest. Figure 2 It can also be concluded that the microstructure refinement effect is significant when the cooling rate is increased from 2.37℃ / min to 3.57℃ / min, while the benefit is limited when the rate is further increased to 4.17℃ / min. Although Example 3 has the finest microstructure, its strength is actually lower than that of Example 2 (Table 3), meaning that Example 2 represents the optimal balance between microstructure refinement and stress control. This scheme significantly refines the martensitic laths and significantly improves the microstructure density while avoiding excessive phase transformation stress caused by high-speed cooling, thus achieving the best match between microstructure and macroscopic mechanical properties. This verifies the scientific validity and rationality of the variable cooling rate technology scheme of this application at the microstructure level.

[0045] In summary, under austenitizing conditions at 1010℃, this application employs a segmented variable cooling rate strategy to proactively reduce the cooling intensity before the martensitic transformation occurs, allowing the martensitic transformation to be fully completed under mild conditions. This effectively solves the technical problem of excessive martensitic transformation stress leading to decreased toughness and increased cracking risk without sacrificing hardenability and strength. Therefore, this application utilizes variable cooling rate vacuum quenching technology to achieve rapid cooling in the high-temperature zone to ensure hardenability and gentle, slow cooling in the low-temperature zone to control phase transformation stress, without introducing additional media such as oil cooling or salt baths. Ultimately, this achieves a synergistic improvement in both high strength and good toughness, meeting the comprehensive service requirements of die-casting molds for high strength and toughness.

[0046] The above-described embodiments are merely illustrative of several implementation methods of the present invention, facilitating a detailed and specific understanding of the technical solutions of the present invention. However, they should not be construed as limiting the scope of protection of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. It should be understood that technical solutions obtained by those skilled in the art based on the technical solutions provided by the present invention through logical analysis, reasoning, or limited experimentation are all within the scope of protection of the appended claims. Therefore, the scope of protection of this invention patent should be determined by the content of the appended claims, and the specification can be used to interpret the content of the claims.

Claims

1. A variable-rate vacuum air quenching treatment method suitable for FS450 die-casting molds, characterized in that, Includes the following steps: The mold is placed in a vacuum high-pressure gas quenching furnace, heated in sections to the austenitizing temperature and held at that temperature. Nitrogen gas was introduced for pre-cooling, and then nitrogen gas was introduced until the furnace pressure reached 13000~15000 mbar and the fan speed was set to 1500~1700 r / min to begin cooling. When the sample temperature cooled to 540~570℃, the furnace pressure was reduced to 5000~7000 mbar and the fan speed was set to 1000~1200 r / min to continue cooling to the predetermined furnace exit temperature. After the mold is removed from the furnace, it is transferred to a tempering furnace for a three-stage tempering process before being removed from the furnace and cooled.

2. The variable-rate vacuum quenching heat treatment method for FS450 die-casting molds according to claim 1, characterized in that, The components of the FS450, by mass fraction, are C 0.38%, Si 0.2%, Mn 0.5%, Cr 5.05%, V 0.56%, Mo 1.82%, P ≤ 0.015%, S ≤ 0.003%, with the balance being Fe.

3. The variable-rate vacuum quenching heat treatment method for FS450 die-casting molds according to claim 1, characterized in that, The segmented heating process involves raising the temperature to 600-700℃ and holding it for 230-280 minutes, then heating it to 800-900℃ and holding it for 230-280 minutes, and finally raising the temperature to 1010℃ and holding it for 250-350 minutes.

4. The variable cooling rate vacuum gas quenching heat treatment method for FS450 die-casting molds according to claim 1, wherein the vacuum degree in the vacuum high-pressure gas quenching furnace is 1.0~1.1mbar.

5. The variable-speed vacuum air quenching treatment method for FS450 die-casting molds according to claim 1, wherein the three-stage tempering involves first heating the mold to 540~560℃ and holding it for 700~900 min, then removing it from the furnace and air-cooling it; then heating it to 550~570℃ and holding it for 600~650 min, then removing it from the furnace and air-cooling it; and finally heating it to 520~540℃ and holding it for 600~650 min, then removing it from the furnace and air-cooling it.

6. An FS450 die-casting mold, characterized in that, It is prepared by the variable cooling rate vacuum quenching treatment method for FS450 die casting mold as described in any one of claims 1-5.

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