Hammer forging die heat treatment method

Abstract

The invention relates to the technical field of metal die heat treatment, and provides a hammer forging die heat treatment method which comprises the following steps that after a blank is forged, spheroidizing annealing pretreatment is conducted, the toughness of a die is improved, and then rough machining is conducted; the mold is placed in a quenching furnace, nitrogen is introduced, and segmented heating quenching is carried out; after the mold is quenched and discharged, tempering is conducted twice, and finish machining is conducted through a numerical control grinding machine; and carrying out gas tufftriding treatment to further improve the wear resistance of the surface of the mold, and finally grinding and polishing. The risks of quenching deformation and cracking are reduced through spheroidizing annealing pretreatment, austenite is homogenized through segmented heating quenching, the thickness of a permeation layer is increased by introducing carbon disulfide during tempering, the surface abrasion resistance is improved through tufftriding treatment, the four improvement points are combined, the impact resistance of the mold is remarkably improved, energy consumption is reduced, and the service life of the mold is prolonged. And an economic and efficient industrial solution is provided.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metal molds, and more particularly to a heat treatment method for hammer forging molds. Background Technology

[0002] Die forging is a forging method in which a heated metal billet is placed into a forging die cavity fixed on a forging machine. The metal is then plastically deformed within the die cavity until it is filled, resulting in a forging of the desired shape and size. Die forging offers several advantages: high productivity; high accuracy in shape and size of forgings with low surface roughness; the ability to forge complex shapes; the ability to achieve ideal metal flow lines, thus improving part lifespan; and simple operation with low skill requirements and low labor intensity. It is suitable for batch and mass production of small and medium-sized forgings. Therefore, die forging is widely used in automobile manufacturing, tractor manufacturing, aircraft manufacturing, defense industry, and power industry.

[0003] Hammer forging dies must withstand high temperatures, high pressures, high impact loads, complex multi-directional stresses, friction, and wear during service. They often fail due to quenching deformation, surface fatigue cracking, and dovetail cracking. Therefore, hammer forging dies must not only possess high strength and fatigue toughness, but also good wear resistance and impact resistance.

[0004] The quality of forging dies directly affects the quality of the finished products, and the die's lifespan determines the production cost and economic benefits of the parts. To ensure the quality and service life of forging dies, proper heat treatment is essential. Current conventional heat treatments for forging dies mainly include annealing, normalizing, quenching, and tempering. Currently, rapidly developing optimization methods include pretreatment and surface nitriding, but both methods have limitations. Annealing pretreatment can improve the die's toughness and optimize its microstructure, but the die surface is prone to oxidation, has low hardness and poor wear resistance, and its impact on the die's impact resistance is limited. Surface nitriding forms a nitrided layer on the die surface, which has high hardness and wear resistance, and minimal workpiece deformation, but its overall strength is low. Small and medium-sized forging dies are often made of 5CrNiMo material, which has high toughness but poor hardenability and thermal fatigue resistance. During use, forging dies experience large deformation and high wear rates. In complex working environments, they are prone to fatigue, wear, fracture, and deformation, seriously affecting the quality and service life of castings, leading to decreased production efficiency and reduced production benefits. Therefore, a new method is urgently needed to address the existing shortcomings. Based on this, a heat treatment method for hammer forging dies is proposed. Summary of the Invention

[0005] The present invention aims to provide a heat treatment method for hammer forging dies to solve the problems of large deformation, high wear rate, and easy occurrence of fatigue, wear, fracture and deformation of hammer forging dies during use.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A heat treatment method for hammer forging dies includes the following steps:

[0008] S1. Spheroidizing annealing pretreatment: After forging, the billet is placed in a furnace at ≤200℃ and heated to 630-670℃, held for 1 hour, then heated to 780-820℃ and held for 2.5 hours. After furnace cooling to 530-570℃, it is taken out of the furnace and air cooled for rough machining.

[0009] S2. Segmented heating and quenching: The pre-treated mold is placed in the furnace at room temperature. Nitrogen gas is introduced into the furnace. The mold is heated to 550-590℃ and held for 3 hours. Then it is heated to 840-860℃ and held for 5 hours. After pre-cooling to 760-800℃, it is oil quenched and cooled to 180-220℃ on the surface before draining the oil.

[0010] S3. Tempering treatment: After quenching and oil removal, immediately place the furnace at ≤200℃ for two tempering treatments.

[0011] The first tempering is done in an air atmosphere, which includes heating the forging die to 300-340℃ and holding it for 4 hours, then heating it to 500-540℃ and holding it for 5 hours, and then air cooling it to 200-240℃.

[0012] The second tempering is carried out in a carbon disulfide atmosphere. The forging die after the first tempering is reheated to 300-340℃ and held for 2 hours, then heated to 440-500℃, and carbon disulfide gas is introduced and held for 6 hours, and then air-cooled to room temperature.

[0013] S4. Soft nitriding treatment: Heat the hammer forging die to 480-520℃, remove the air, add ammonia and methanol, heat to 550-590℃ and hold for 4 hours. After heating, put the die into quenching oil to cool to room temperature, and finally grind and polish the die cavity.

[0014] Furthermore, in step S1, the heating rate of the annealing pretreatment is 60-80℃ / h, and the cooling rate of the furnace cooling is 30-50℃ / h.

[0015] Furthermore, in step S1, a margin of 0.3-0.5mm is reserved on one side of the rough-machined cavity.

[0016] Furthermore, in step S2, the heating rate is 80-100℃ / h, nitrogen gas with a purity of at least 99.99% is introduced into the furnace, and the cooling rate is 40-50℃ / s.

[0017] Furthermore, in step S3, the carbon disulfide gas introduction rate is 0.2 L / h, and the tempering heating rate is 60-80 °C / h.

[0018] Furthermore, in step S4, the heating rate is 40-50℃ / h, the air removal rate is 500L / h, the exhaust time is 3h, the ammonia addition rate is 900L / h, the methanol dropping rate is 120 drops / min, and the cooling rate is 50-60℃ / s.

[0019] Furthermore, in step S4, the surface roughness Ra of the grinding mold is ≤0.4µm.

[0020] The principles and beneficial effects of this technical solution include at least the following:

[0021] 1. Spheroidizing annealing pretreatment ensures uniform temperature inside and outside the mold steel, reduces thermal stress, and avoids deformation and cracking. Adding spheroidizing annealing heat treatment before quenching effectively eliminates forging stress, reduces hardness, improves toughness, improves machinability, and transforms the workpiece structure into spheroidized pearlite, preparing the structure for quenching and reducing the risk of rapid cooling cracking of the workpiece.

[0022] 2. Nitrogen gas is introduced into the furnace during quenching to increase heat transfer and prevent chromium evaporation; it also avoids surface oxidation and decarburization. Segmented heating ensures uniform heat distribution in the mold, reducing thermal stress and promoting homogenization of austenite composition.

[0023] 3. Multiple tempering can further eliminate quenching stress, reduce the tendency of mold deformation and cracking, and fully transform the quenched structure into tempered sorbite with excellent comprehensive mechanical properties. In addition, the subsequent carbon disulfide treatment results in a porous sulfide layer on the surface of the workpiece, which effectively improves the surface activity of the mold alloy and is conducive to the co-diffusion process. At the same time, the formed sulfide layer can reduce the reaction temperature and reduce the difficulty of nitrogen and carbon elements infiltration. The thickness of the infiltrated layer is increased by more than 6.8μm, and the bonding force is stronger and it is not easy to fall off.

[0024] 4. Soft nitriding further improves the wear resistance, fatigue resistance, anti-galling and corrosion resistance of the mold surface, while maintaining sufficient toughness in the mold core. Attached Figure Description

[0025] Figure 1 A process flow diagram for manufacturing a hammer forging die;

[0026] Figure 2 This is a flowchart of the annealing process for a heat treatment method of hammer forging dies;

[0027] Figure 3 This is a flow chart of the quenching process for a heat treatment method of hammer forging dies;

[0028] Figure 4 A flow chart of the tempering process for a heat treatment method of hammer forging dies;

[0029] Figure 5 This is a flow chart of a soft nitriding process for heat treatment of hammer forging dies. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments and accompanying drawings. Here, the illustrative embodiments and descriptions of this invention are used to explain the invention, but are not intended to limit the invention.

[0031] It should also be noted that, in order to avoid obscuring the invention with unnecessary details, only the processing steps closely related to the solution according to the invention are shown in the accompanying drawings, while other details that are not closely related to the invention are omitted.

[0032] It should be emphasized that the term "including / comprises" as used herein refers to the presence of a feature, element, or step, but does not exclude the presence or addition of one or more other features, elements, or steps.

[0033] Example 1

[0034] refer to Figures 1 to 5 A heat treatment method for a hammer forging die includes the following steps:

[0035] 1. Spheroidizing annealing process: After forging, the billet is placed in a box annealing furnace at ≤200℃ and heated to 650℃ at a rate of 80℃ / h. It is preheated for 1 hour, then heated to 800℃ and held for 2.5 hours. Then it is furnace cooled to 550℃ at a rate of 40℃ / h and then air cooled. Finally, it is rough machined using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity.

[0036] 2. Segmented heating and quenching: The pre-treated mold is placed in a bogie-type rapid quenching furnace at room temperature, and nitrogen gas with a purity of 99.99% is introduced into the furnace. It is heated to 560℃ at a rate of 100℃ / h and held for 3 hours, then heated to 860℃ and held for 5 hours to fully austenitize the workpiece. After pre-cooling to 780℃, it is immediately placed in quenching oil for oil quenching, and cooled to 200℃ at a cooling rate of 40℃ / s until the surface temperature is reduced. The oil is then drained.

[0037] 3. Tempering treatment: After quenching and oil removal, the hammer forging die is immediately placed in a vacuum tempering furnace at ≤200℃ and heated to 300℃ at a rate of 80℃ / h and held for 4h. Then it is heated to 510℃ and held for 5h. After air cooling to 220℃, the first tempering is performed. Then it is heated to 300℃ and held for 2h. Then it is heated to 480℃ and carbon disulfide gas is introduced at 450℃ and held for 6h. The carbon disulfide gas introduction rate is 0.2L / h. After air cooling to room temperature, the second tempering is performed.

[0038] 4. Soft nitriding treatment: The hammer forging die is heated to 500℃ at a rate of 50℃ / h, and the exhaust is carried out at a rate of 500L / h for 3h. The soft nitriding media are ammonia and methanol, with the ammonia addition rate at 900L / h and the methanol dripping rate at 120 drops / min. The die is heated to 570℃ and held for 4h. The die is then placed in machine oil and cooled to room temperature at a rate of 50℃ / s. The die cavity is then ground and polished. The final surface roughness of the die is Ra≤0.4µm.

[0039] Comparative Example 1

[0040] 1. Rough machining: Rough machining is performed using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity;

[0041] 2. Segmented heating and quenching: The pre-treated mold is placed in a trolley-type rapid quenching furnace at room temperature. Nitrogen gas with a purity of 99.99% is introduced into the furnace. The mold is heated to 560℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 860℃ and held for 5 hours to fully austenitize the workpiece. After pre-cooling to 780℃, the mold is immediately placed in quenching oil and cooled to 200℃ at a cooling rate of 40℃ / s. The mold is then drained from the oil.

[0042] 3. Tempering treatment: After quenching and oil removal, the forging die is immediately placed in a vacuum tempering furnace at ≤ 200℃ and heated to 300℃ at a rate of 80℃ / h and held for 4h. Then it is heated to 510℃ and held for 5h. After air cooling to 220℃, the first tempering is performed. Then it is heated to 300℃ and held for 2h. Then it is heated to 480℃ and carbon disulfide gas is introduced at 450℃ and held for 6h. The carbon disulfide gas introduction rate is 0.2L / h. After air cooling to room temperature, the second tempering is performed.

[0043] 4. Soft nitriding treatment: The hammer forging die is heated to 500℃ at a rate of 50℃ / h, and the exhaust is carried out at a rate of 500L / h for 3h. The soft nitriding media are ammonia and methanol, with the ammonia addition rate at 900L / h and the methanol dripping rate at 120 drops / min. The die is heated to 570℃ and held for 4h. The die is then placed in machine oil and cooled to room temperature at a rate of 50℃ / s. The die cavity is then ground and polished. The final surface roughness of the die is Ra≤0.4µm.

[0044] Referring to Table 1, the forging die in Comparative Example 1 has lower toughness and higher hardness than the forging die in Example 1, and is at risk of rapid cooling cracking. This indicates that the spheroidizing annealing pretreatment proposed in this invention makes the temperature inside and outside the die steel uniform, which can effectively eliminate forging stress and improve the impact resistance of the die.

[0045] Table 1 Comparison of mold parameters between Example 1 and Comparative Example 1

[0046]

[0047] Example 2

[0048] refer to Figures 1 to 5 A heat treatment method for a hammer forging die includes the following steps:

[0049] 1. Spheroidizing annealing process: After forging, the billet is placed in a box annealing furnace at ≤200℃ and heated to 670℃ at a rate of 80℃ / h. It is preheated for 1 hour, then heated to 820℃ and held for 2.5 hours. After that, it is furnace cooled to 570℃ at a rate of 40℃ / h and then air cooled. Finally, it is rough machined using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity.

[0050] 2. Segmented heating and quenching: The pre-treated mold is placed in a bogie-type rapid quenching furnace at room temperature, and nitrogen gas with a purity of 99.99% is introduced into the furnace. It is heated to 560℃ at a rate of 100℃ / h and held for 3 hours, then heated to 860℃ and held for 5 hours to fully austenitize the workpiece. After pre-cooling to 780℃, it is immediately placed in quenching oil and cooled to 200℃ at a cooling rate of 40℃ / s until the surface temperature is reduced. The oil is then drained.

[0051] 3. Tempering treatment: After quenching and oil removal, the hammer forging die is immediately placed in a vacuum tempering furnace at ≤200℃ and heated to 300℃ at a rate of 80℃ / h and held for 4h. Then it is heated to 510℃ and held for 5h. After air cooling to 220℃, the first tempering is performed. Then it is heated to 300℃ and held for 2h. Then it is heated to 480℃ and carbon disulfide gas is introduced at 450℃ and held for 6h. The carbon disulfide gas introduction rate is 0.2L / h. After air cooling to room temperature, the second tempering is performed.

[0052] 4. Soft nitriding treatment: The hammer forging die is heated to 500℃ at a rate of 50℃ / h, and the exhaust is carried out at a rate of 500L / h for 3h. The soft nitriding media are ammonia and methanol, with the ammonia addition rate at 900L / h and the methanol dripping rate at 120 drops / min. The die is heated to 570℃ and held for 4h. The die is then placed in machine oil and cooled to room temperature at a rate of 50℃ / s. The die cavity is then ground and polished. The final surface roughness of the die is Ra≤0.4µm.

[0053] Comparative Example 2

[0054] 1. Spheroidizing Annealing Process: After forging, the billet is placed in a box-type annealing furnace at ≤ 200℃ and heated to 670℃ at a rate of 80℃ / h. It is preheated for 1 hour, then heated to 820℃ and held for 2.5 hours. After that, it is furnace cooled to 570℃ at a rate of 40℃ / h and then air-cooled. Finally, it is rough-machined using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity.

[0055] 2. Heating and quenching: The pre-treated mold is placed in a trolley-type rapid quenching furnace at room temperature. Nitrogen gas with a purity of 99.99% is introduced into the furnace and heated to 560℃ at a rate of 100℃ / h and held for 3 hours. Then, it is pre-cooled to 380℃ and immediately placed in quenching oil. It is cooled to 200℃ on the surface at a cooling rate of 40℃ / s and then drained.

[0056] 3. Tempering treatment: After quenching and oil removal, the hammer forging die is immediately placed in a vacuum tempering furnace at ≤200℃ and heated to 300℃ at a rate of 80℃ / h and held for 4h. Then it is heated to 510℃ and held for 5h. After air cooling to 220℃, the first tempering is performed. Then it is heated to 300℃ and held for 2h. Then it is heated to 480℃ and carbon disulfide gas is introduced at 450℃ and held for 6h. The carbon disulfide gas introduction rate is 0.2L / h. After air cooling to room temperature, the second tempering is performed.

[0057] 4. Soft nitriding treatment: The hammer forging die is heated to 500℃ at a rate of 50℃ / h, and the exhaust is carried out at a rate of 500L / h for 3h. The soft nitriding media are ammonia and methanol, with the ammonia addition rate at 900L / h and the methanol dripping rate at 120 drops / min. The die is heated to 570℃ and held for 4h. The die is then placed in machine oil and cooled to room temperature at a rate of 50℃ / s. The die cavity is then ground and polished. The final surface roughness of the die is Ra≤0.4µm.

[0058] Referring to Table 2, the forging die in Comparative Example 2 has lower hardness and poorer die uniformity compared to the forging die in Example 2. The segmented heating and quenching proposed in this invention makes the heat distribution of the die uniform to reduce thermal stress and promote the homogenization of austenite composition. Subsequent tempering heating transforms austenite into martensite with higher hardness, thereby giving the die uniform mechanical properties.

[0059] Table 2 Comparison of mold parameters between Example 2 and Comparative Example 2

[0060]

[0061] Example 3

[0062] refer to Figures 1 to 5 A method for manufacturing and heat treating a hammer forging die, comprising the following steps:

[0063] 1. Spheroidizing annealing process: After forging, the billet is placed in a box annealing furnace at ≤200℃ and heated to 650℃ at a rate of 80℃ / h. It is preheated for 1 hour, then heated to 800℃ and held for 2.5 hours. Then it is furnace cooled to 550℃ at a rate of 40℃ / h and then air cooled. Finally, it is rough machined using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity.

[0064] 2. Segmented heating and quenching: The pre-treated mold is placed in a trolley-type rapid quenching furnace at room temperature. Nitrogen gas with a purity of 99.99% is introduced into the furnace. The mold is heated to 560℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 860℃ and held for 5 hours to fully austenitize the workpiece. After pre-cooling to 780℃, the mold is immediately placed in quenching oil and cooled to 200℃ at a cooling rate of 40℃ / s. The mold is then drained from the oil.

[0065] 3. Tempering treatment: After quenching and oil removal, the hammer forging die is immediately placed in a vacuum tempering furnace at ≤200℃ and heated to 300℃ at a rate of 80℃ / h and held for 4h. Then it is heated to 510℃ and held for 5h. After air cooling to 220℃, the first tempering is performed. Then it is heated to 300℃ and held for 2h. Then it is heated to 480℃ and carbon disulfide gas is introduced at 450℃ and held for 6h. The carbon disulfide gas introduction rate is 0.2L / h. After air cooling to room temperature, the second tempering is performed.

[0066] 4. Soft nitriding treatment: The hammer forging die is heated to 520℃ at a rate of 50℃ / h, and the exhaust is carried out at a rate of 500L / h for 3h. The soft nitriding media are ammonia and methanol, with the ammonia addition rate being 900L / h and the methanol dripping rate being 120 drops / min. The die is heated to 590℃ and held for 4h. The die is then placed in machine oil and cooled to room temperature at a rate of 50℃ / s. The die cavity is then ground and polished. The final surface roughness of the die is Ra≤0.4µm.

[0067] Comparative Example 3

[0068] 1. Spheroidizing annealing process: After forging, the billet is placed in a box annealing furnace at ≤200℃ and heated to 650℃ at a rate of 80℃ / h. It is preheated for 1 hour, then heated to 800℃ and held for 2.5 hours. Then it is furnace cooled to 550℃ at a rate of 40℃ / h and then air cooled. Finally, it is rough machined using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity.

[0069] 2. Segmented heating and quenching: The pre-treated mold is placed in a trolley-type rapid quenching furnace at room temperature. Nitrogen gas with a purity of 99.99% is introduced into the furnace. The mold is heated to 560℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 860℃ and held for 5 hours to fully austenitize the workpiece. After pre-cooling to 780℃, the mold is immediately placed in quenching oil and cooled to 200℃ at a cooling rate of 40℃ / s. The mold is then drained from the oil.

[0070] 3. Tempering treatment: After quenching and oil removal, the hammer forging die is immediately placed in a vacuum tempering furnace at ≤200℃ and heated to 300℃ at a rate of 80℃ / h and held for 4 hours. Then it is heated to 510℃ and held for 5 hours. After air cooling to 220℃, it is tempered for the first time. Then it is heated to 300℃ and held for 2 hours. Then it is heated to 480℃ and held for 6 hours. After air cooling to room temperature, it is tempered for the second time.

[0071] 4. Soft nitriding treatment: The hammer forging die is heated to 520℃ at a rate of 50℃ / h, and the exhaust is carried out at a rate of 500L / h for 3h. The soft nitriding media are ammonia and methanol, with the ammonia addition rate being 900L / h and the methanol dripping rate being 120 drops / min. The die is heated to 590℃ and held for 4h. The die is then placed in machine oil and cooled to room temperature at a rate of 50℃ / s. The die cavity is then ground and polished. The final surface roughness of the die is Ra≤0.4µm.

[0072] Referring to Table 3, compared with the hammer forging die in Example 3, the forging die in Example 3 has a lower diffusion layer thickness, poorer surface hardness and wear resistance. The carbon disulfide gas introduced during tempering proposed in this invention effectively improves the surface activity of the die alloy, which is conducive to the co-diffusion process. At the same time, the formed sulfide layer can reduce the reaction temperature and reduce the difficulty of nitrogen and carbon diffusion. The diffusion layer thickness is increased by more than 6.8µm, and the bonding force is stronger and it is not easy to fall off.

[0073] Table 3 Comparison of mold parameters between Example 3 and Comparative Example 3

[0074]

[0075] Example 4

[0076] refer to Figures 1 to 5 A heat treatment method for a hammer forging die includes the following steps:

[0077] 1. Spheroidizing annealing process: After forging, the billet is placed in a box annealing furnace at ≤200℃ and heated to 650℃ at a rate of 80℃ / h. It is preheated for 1 hour, then heated to 800℃ and held for 2.5 hours. Then it is furnace cooled to 550℃ at a rate of 40℃ / h and then air cooled. Finally, it is rough machined using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity.

[0078] 2. Segmented heating and quenching: The pre-treated mold is placed in a trolley-type rapid quenching furnace at room temperature. Nitrogen gas with a purity of 99.99% is introduced into the furnace and heated to 590℃ at a rate of 100℃ / h. The temperature is held for 3 hours, and then heated to 850℃ and held for 5 hours to fully austenitize the workpiece. After pre-cooling to 780℃, it is immediately placed in quenching oil and cooled to 200℃ at a cooling rate of 40℃ / s until the surface temperature is reduced to 200℃. The oil is then drained.

[0079] 3. Tempering treatment: After quenching and oil removal, the hammer forging die is immediately placed in a vacuum tempering furnace at ≤200℃ and heated to 300℃ at a rate of 80℃ / h and held for 4h. Then it is heated to 510℃ and held for 5h. After air cooling to 220℃, the first tempering is performed. Then it is heated to 300℃ and held for 2h. Then it is heated to 480℃ and carbon disulfide gas is introduced at 450℃ and held for 6h. The carbon disulfide gas introduction rate is 0.2L / h. After air cooling to room temperature, the second tempering is performed.

[0080] 4. Soft nitriding treatment: The hammer forging die is heated to 500℃ at a rate of 50℃ / h, and the exhaust is carried out at a rate of 500L / h for 3h. The soft nitriding media are ammonia and methanol, with the ammonia addition rate at 900L / h and the methanol dripping rate at 120 drops / min. The die is heated to 570℃ and held for 4h. The die is then placed in machine oil and cooled to room temperature at a rate of 50℃ / s. The die cavity is then ground and polished. The final surface roughness of the die is Ra≤0.4µm.

[0081] Comparative Example 4

[0082] refer to Figures 1 to 5 A heat treatment method for a hammer forging die includes the following steps:

[0083] 1. Spheroidizing annealing process: After forging, the billet is placed in a box annealing furnace at ≤200℃ and heated to 650℃ at a rate of 80℃ / h. It is preheated for 1 hour, then heated to 800℃ and held for 2.5 hours. Then it is furnace cooled to 550℃ at a rate of 40℃ / h and then air cooled. Finally, it is rough machined using a CNC milling machine, leaving a 0.4mm allowance on each side of the cavity.

[0084] 2. Segmented heating and quenching: The pre-treated mold is placed in a trolley-type rapid quenching furnace at room temperature. Nitrogen gas with a purity of 99.99% is introduced into the furnace. The mold is heated to 590℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 850℃ and held for 5 hours to fully austenitize the workpiece. After pre-cooling to 780℃, the mold is immediately placed in quenching oil and cooled to 200℃ at a cooling rate of 40℃ / s. The mold is then drained from the oil.

[0085] 3. Tempering treatment: After quenching and oil removal, the hammer forging die is immediately placed in a vacuum tempering furnace at ≤200℃ and heated to 300℃ at a rate of 80℃ / h, held for 4h, then heated to 510℃ and held for 5h, air-cooled to 220℃ for the first tempering, then heated to 300℃ and held for 2h, then heated to 480℃, and carbon disulfide gas is introduced at 450℃ and held for 6h at a rate of 0.2L / h, air-cooled to room temperature for the second tempering; then the die cavity is ground and polished, and the final surface roughness Ra of the die is ≤0.4µm.

[0086] Referring to Table 4, the forging die in Comparative Example 4 has lower surface hardness and poorer wear resistance compared to the forging die in Example 4. The soft nitriding treatment after tempering proposed in this invention further improves the wear resistance, fatigue resistance, anti-galling, and corrosion resistance of the die surface, while maintaining sufficient toughness in the die core. After long-term use, the die surface remains smooth and glossy, and the product quality is not adversely affected.

[0087] Table 4 Comparison of mold parameters between Example 4 and Comparative Example 4

[0088]

[0089] In summary, the heat treatment method for hammer forging dies of this invention involves preheating the die followed by high-temperature quenching, which improves the alloy content; multiple tempering processes effectively eliminate residual stress caused by quenching, increasing the die hardness; and soft nitriding further enhances the die surface's wear resistance, fatigue resistance, anti-galling properties, and corrosion resistance. The process is simple, yields a high product qualification rate, and is low in cost. Furthermore, it offers high resistance to deformation, toughness, wear resistance, corrosion resistance, and fatigue resistance, resulting in a long service life, making it an economical and efficient industrial solution.

[0090] It should be clarified that the present invention is not limited to the specific configurations and processes described above and shown in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present invention is not limited to the specific steps described and shown. Those skilled in the art can make various changes, modifications, and additions, or change the order of steps, after understanding the spirit of the present invention.

[0091] In this invention, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with or in place of features of other embodiments.

[0092] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and variations of the embodiments of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A heat treatment method for a hammer forging die, characterized in that, Includes the following steps: S1. Spheroidizing annealing pretreatment: After forging, the billet is placed in a furnace at ≤200℃ and heated to 630-670℃, held for 1 hour, then heated to 780-820℃ and held for 2.5 hours. After furnace cooling to 530-570℃, it is taken out of the furnace and air cooled for rough machining. S2. Segmented heating and quenching: Place the pre-treated mold into the furnace at room temperature, introduce nitrogen into the furnace, heat to 550-590℃, hold for 3 hours, then heat to 840-860℃ and hold for 5 hours. After pre-cooling to 760-800℃, oil quench and cool to 180-220℃ on the surface before draining the oil. S3. Tempering treatment: After quenching and oil removal, immediately place the furnace at ≤200℃ for two tempering treatments. The first tempering is done in an air atmosphere, which includes heating the forging die to 300-340℃ and holding it for 4 hours, then heating it to 500-540℃ and holding it for 5 hours, and then air cooling it to 200-240℃. The second tempering is carried out in a carbon disulfide atmosphere. The forging die after the first tempering is reheated to 300-340℃ and held for 2 hours, then heated to 440-500℃, and carbon disulfide gas is introduced and held for 6 hours, and then air-cooled to room temperature. S4. Soft nitriding treatment: Heat the hammer forging die to 480-520℃, remove the air, add ammonia and methanol, heat to 550-590℃ and hold for 4 hours. After heating, put the die into quenching oil to cool to room temperature, and finally grind and polish the die cavity.

2. The heat treatment method for a hammer forging die according to claim 1, characterized in that, In step S1, the heating rate of the annealing pretreatment is 60-80℃ / h, and the cooling rate of the furnace cooling is 30-50℃ / h.

3. The heat treatment method for a hammer forging die according to claim 1, characterized in that, In step S1, a margin of 0.3-0.5mm is reserved on one side of the rough-machined cavity.

4. The heat treatment method for a hammer forging die according to claim 1, characterized in that, In step S2, the heating rate is 80-100℃ / h, nitrogen gas with a purity of at least 99.99% is introduced into the furnace, and the cooling rate is 40-50℃ / s.

5. The heat treatment method for a hammer forging die according to claim 1, characterized in that, In step S3, the carbon disulfide gas introduction rate is 0.2 L / h, and the tempering heating rate is 60-80 °C / h.

6. The heat treatment method for a hammer forging die according to claim 1, characterized in that, In step S4, the heating rate is 40-50℃ / h, the air removal rate is 500L / h, the exhaust time is 3h, the ammonia addition rate is 900L / h, the methanol dropping rate is 120 drops / min, and the cooling rate is 50-60℃ / s.

7. The heat treatment method for a hammer forging die according to claim 1, characterized in that, In step S4, the surface roughness Ra of the grinding mold is ≤0.4µm.

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