Method for prolonging fatigue life of WC-Co hard alloy tail clamping die

Abstract

The invention discloses a method for prolonging the fatigue life of a WC-Co hard alloy tail clamping die, which comprises the following steps: firstly, increasing the radius of a fillet at the variable cross section of the tail clamping die so as to reduce stress concentration, then adopting a hard alloy material in which coarse grain and fine grain WC are matched, and adding rare earth elements so as to improve the toughness of a binding phase, so that the fatigue life of the tail clamping die is prolonged. And finally, shot blasting or high-frequency induction heat treatment is conducted on the mold, and through collaborative optimization of the structure, materials and surface treatment, the fatigue life of the tail clamping mold can be systematically prolonged.

Description

Technical Field

[0001] This invention relates to the field of mold technology, specifically to a method for improving the fatigue life of WC-Co cemented carbide tail molds. Background Technology

[0002] WC-Co cemented carbide is widely used in cold extrusion and cold heading molds due to its high hardness, high wear resistance, and good compressive strength. However, in actual service, especially under alternating load conditions such as tail dies, its fatigue performance is particularly problematic. This is mainly due to the following reasons: First, traditional molds often use small fillet radii at variable cross-sections, leading to significant stress concentration in these areas, which easily becomes the initiation source of fatigue cracks. Second, commonly used cemented carbide materials often employ WC grains of a single size, making it difficult to simultaneously achieve high strength and fatigue toughness. While fine grains offer high strength, densification is difficult, while coarse grains easily become crack propagation paths. Third, the binder phase Co is prone to embrittlement under cyclic loading, accelerating crack initiation and propagation. In addition, residual tensile stress often exists on the surface of molds after conventional machining, further promoting fatigue failure. Therefore, existing technologies lack a comprehensive method that systematically addresses structural design, material microstructure control, and surface stress state improvement to significantly enhance the fatigue life of WC-Co cemented carbide tail dies. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention relates to a method for improving the fatigue life of WC-Co cemented carbide tail dies. This method is simple, reliable, and effectively solves the above-mentioned technical problems, making it suitable for widespread use. To achieve the above objectives, this invention employs the following technical solution: A method for improving the fatigue life of WC-Co cemented carbide tail dies includes the following steps: S1, Structural design: The fillet radius at the variable cross-section of the tail mold is designed to be 0.5 mm to 1 mm; S2, Mixing: WC powder and Co powder are provided, wherein the WC powder comprises coarse-grained WC powder with an average particle size of 4 to 8 μm and fine-grained WC powder with an average particle size of 0.5 to 2 μm, and the content of the WC powder is 85 wt% to 92 wt% and the content of the Co powder is 8 wt% to 15 wt% based on the total mass of the mixture, and rare earth element powder is added in a content of 0.1 wt% to 0.5 wt%, wherein the rare earth element powder comprises at least one of La, Ce or Y; S3, forming and sintering: the mixture obtained in step S2 is pressed into a green blank in the shape of a clamping mold, and then the green blank is sintered in a vacuum environment to obtain a clamping mold substrate. S4, Surface strengthening: The tail clamping mold substrate obtained in step S3 is subjected to surface strengthening treatment, namely shot peening or high-frequency induction heat treatment, to form a residual compressive stress of -200 to -600 MPa on the surface of the tail clamping mold substrate. Based on the above scheme and as a preferred embodiment of the above scheme: in step S3, the sintering is completed by holding at 1350°C to 1450°C for 0.5h to 2h.

[0004] Based on the above scheme and as a preferred embodiment of the above scheme: In step S4, when performing shot peening, shot with a diameter of 0.1 mm to 0.5 mm is sprayed at a speed of 40 m / s to 80 m / s.

[0005] Based on the above scheme and as a preferred embodiment of the above scheme: the shot material used in the shot peening treatment is cast steel or ceramic.

[0006] Based on the above scheme and as a preferred option of the above scheme: In step S4, when performing high-frequency induction heat treatment, the surface is heated to 800°C to 1100°C and held for 2s to 30s before water cooling.

[0007] The outstanding and beneficial technical effects of this invention compared to the prior art are: by synergistically optimizing the structural design, material system and surface strengthening process of the tail mold, this invention achieves a significant improvement in fatigue life.

[0008] Specifically, by increasing the radius of the variable cross-section fillet to 0.5 to 1 mm, stress concentration is effectively reduced; a bimodal microstructure of coarse and fine WC composite is adopted, and trace amounts of rare earth elements are added to improve the strength, toughness, and binder phase toughness of the material while maintaining high hardness; further, combined with shot peening or high-frequency induction heat treatment processes, a residual compressive stress layer of -200 to -600 MPa is formed on the mold surface, which effectively inhibits the initiation and propagation of fatigue cracks. The above measures work together systematically from three levels: structure, organization, and stress state, which increases the fatigue life of the tail mold by 50% to 150%, and has outstanding industrial application value. Attached Figure Description

[0009] Figure 1 This is a diagram showing the fillet radius at the variable cross-section of the tail mold in Embodiment 1 of the present invention.

[0010] Figure 2 This is the principal stress cloud diagram of the tail mold in Embodiment 1 of the present invention.

[0011] Figure 3 This is a metallographic image of the tail mold in Embodiment 1 of the present invention.

[0012] Figure 4This is a diagram showing the fillet radius at the variable cross-section of the tail mold in Embodiment 2 of the present invention.

[0013] Figure 5 This is the principal stress cloud diagram of the tail mold in Embodiment 2 of the present invention.

[0014] Figure 6 This is a metallographic image of the tail mold in Embodiment 2 of the present invention. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. However, the specific implementation methods and embodiments described below are for illustrative purposes only and are not intended to limit the present invention.

[0016] In the description of this invention, it should be understood that the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the appendix. Figure 1 The directions or positional relationships shown are for the purpose of describing the present invention only, and are not intended to indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the present invention.

[0017] In the description of this application, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated.

[0018] To address the technical problem, this invention presents a method for improving the fatigue life of WC-Co cemented carbide tail dies, comprising the following steps: S1, Structural Design: The fillet radius at the variable cross-section of the tail mold is designed to be 0.5 mm to 1 mm. According to the theory of elasticity, significant stress concentration will occur at the abrupt structural changes (such as small fillets). Increasing the fillet radius can smooth the transition of cross-section changes. The purpose is to directly reduce the stress concentration coefficient of the mold at the key part of the variable cross-section, and suppress the initiation of fatigue cracks from the design source.

[0019] S2, Mixing: WC powder and Co powder are provided. The WC powder comprises coarse-grained WC powder with an average particle size of 4 to 8 μm and fine-grained WC powder with an average particle size of 0.5 to 2 μm. The content of WC powder is 85 wt% to 92 wt% and the content of Co powder is 8 wt% to 15 wt% based on the total mass of the mixture. Fine-grained WC provides high hardness and strength through a fine-grained strengthening mechanism (Hall-Page relationship), while coarse-grained WC can consume fracture energy and improve the fracture toughness of the material through mechanisms such as crack deflection and bridging. This "bimodal structure" achieves an optimized match between strength and toughness.

[0020] Adding rare earth element powder at a content of 0.1 wt% to 0.5 wt%, wherein the rare earth element powder contains at least one of La, Ce or Y, can purify phase boundaries, reduce impurity segregation, and refine cobalt phase grains, thereby improving the strength and toughness of the binder phase itself. This enhances the binder phase's ability to hold WC particles, delays the process of interface debonding or cobalt phase self-cracking under fatigue load, and thus improves the overall fatigue crack initiation resistance of the material.

[0021] S3, Forming and Sintering: The mixture obtained in step S2 is pressed into a green blank in the shape of a clamping die. The green blank is then sintered in a vacuum environment to obtain a clamping die matrix. The vacuum sintering environment can prevent oxidation, which is conducive to better wetting and wrapping of WC particles by the Co liquid phase, promoting material diffusion and eliminating pores, thereby obtaining a high-density alloy blank.

[0022] The sintering process was designed to be completed at 1350℃ to 1450℃ for 0.5h to 2h. Within this temperature range, the Co binder phase fully melts to form a liquid phase, effectively wetting the WC particles. This liquid-phase sintering mechanism promotes material migration and pore filling, achieving densification. The holding time (0.5-2h) is crucial for process control. Too short a time results in insufficient densification and diffusion, while too long a time leads to excessive dissolution of fine-grained WC, which transforms into coarse-grained WC through the Ostwald ripening mechanism, disrupting the designed bimodal structure. This optimized parameter balances densification and grain size control, optimizing the intrinsic mechanical properties of the material.

[0023] S4, Surface strengthening: The tail mold substrate obtained in step S3 is subjected to surface strengthening treatment, which is shot peening or high-frequency induction heat treatment, so that a residual compressive stress of -200 to -600 MPa is formed on the surface of the tail mold substrate. A high-amplitude residual compressive stress layer is actively introduced into the working surface of the mold to effectively resist the propagation of fatigue cracks by external mechanical means.

[0024] Preferably, in step S4, when performing shot peening, shot with a diameter of 0.1 mm to 0.5 mm is used at a velocity of 40 m / s to 80 m / s. The essence of shot peening in introducing residual compressive stress is through plastic deformation. The smaller shot diameter (0.1-0.5 mm) ensures the concentration of impact energy and controllable surface depth of influence, while the higher peening velocity (40-80 m / s) gives the shot sufficient kinetic energy to overcome the material's yield strength, causing plastic deformation of the surface. The matching of velocity and diameter ensures that a suitable impact energy is obtained per unit area per unit time, which is sufficient to induce plastic deformation and generate compressive stress, while avoiding damage to surface integrity caused by excessive impact.

[0025] Preferably, the shot used in the shot peening process is made of cast steel or ceramic. Cast steel shot has high hardness and good toughness, effectively transferring kinetic energy and resisting deformation and breakage. Ceramic shot (such as zirconium oxide) has even higher hardness, wear resistance, and chemical stability, exhibiting almost no wear or metal contamination during impact, making it particularly suitable for precision molds requiring high surface purity. Both types of shot provide stable and repeatable elastoplastic impact, ensuring a consistent and high-quality residual compressive stress layer.

[0026] Preferably, in step S4, during high-frequency induction heat treatment, the surface is heated to 800°C to 1100°C and held for 2 to 30 seconds before water cooling. High-frequency induction heating has a skin effect, which can selectively heat the surface layer to the temperature at which the Co phase undergoes austenitization transformation (800-1100°C) in a very short time (2-30 seconds), while the core remains at a low temperature. The subsequent immediate water cooling (quenching) rapidly transforms the high-temperature austenitized Co phase into martensite. The martensitic transformation is accompanied by a volume expansion of about 1-3%, but this expansion is strongly constrained by the core material, which is still in an elastic state due to the slower cooling rate, thus generating a large residual compressive stress in the surface layer. The shorter holding time avoids excessive heat conduction to the core and the growth of WC grains, ensuring the enhanced surface layer positioning and the stability of the matrix properties.

[0027] To illustrate the technical solution and effects of the present invention, the following embodiments are provided for detailed explanation. The relevant test methods in the embodiments are as follows: a residual stress analyzer equipped with a Co target was used to perform residual stress testing on the sample with the (101) crystal plane of WC under the conditions of 35kV voltage and 30mA current; the metallographic analysis was performed in accordance with the GB / T 3488.3-2021 standard.

[0028] Example 1 like Figure 1-3As shown, this embodiment demonstrates the specific application of the method of the present invention combined with shot peening surface strengthening process. First, the tail die is structurally optimized, with the fillet radius at its variable cross-section set to 0.66 mm. Finite element stress analysis results show that after this optimization, the maximum principal stress at this location significantly decreases from 520 MPa in the comparative example to 350 MPa, effectively alleviating stress concentration. Second, in the material preparation stage, a vacuum sintering process is used, holding at 1400 degrees Celsius for 1.5 hours to obtain a cemented carbide with a bimodal microstructure. In this microstructure, the average grain size of coarse-grained WC is approximately 6.5 μm, and the average grain size of fine-grained WC is approximately 1.6 μm. The material composition is 91% WC, 9% Co, and 0.2% Ce is specifically added to improve the toughness of the cobalt binder phase. Subsequently, the formed tail die is shot peened using 0.3 mm diameter cast steel shot at a blasting speed of 60 m / s to uniformly impact its working surface. Post-treatment testing revealed a residual compressive stress of -305 MPa on the mold surface. Fatigue life testing showed that the tail clamp mold achieved approximately 700,000 cycles, representing a 75% improvement in fatigue life compared to conventional processes without this invention.

[0029] Example 2 like Figure 4-6 As shown, this embodiment provides another implementation combining the method of the present invention with a high-frequency induction heat treatment surface strengthening process. In terms of structural design, the radius of the corner radius of the variable cross-section of the tail mold is further increased to 0.8 mm; finite element analysis confirms that the maximum principal stress at this location decreases to 280 MPa, showing a better stress homogenization effect. Regarding materials, a finer bimodal hard alloy structure is obtained through a vacuum sintering process at 1380 degrees Celsius for 1 hour, with an average grain size of approximately 5.5 micrometers for coarse-grained WC and approximately 1.2 micrometers for fine-grained WC. The alloy composition includes 88% WC, 12% Co, and 0.3% La to enhance material toughness. Surface strengthening employs high-frequency induction heat treatment, rapidly heating the working surface of the mold to 950 degrees Celsius, holding for 5 seconds, and then immediately water-quenching. This process successfully introduces a residual compressive stress as high as -530 MPa on the mold surface. Fatigue tests have verified that the service life of this tail clamping die has been extended to approximately 900,000 cycles, representing a significant increase of 125% in fatigue life compared to the comparative example.

[0030] The comparison between the above embodiments and the reference cases fully demonstrates that the present invention, through the synergistic implementation of three core technical means—structural fillet optimization, material bimodal microstructure and rare earth toughening design, and surface compressive stress introduction—can systematically and significantly improve the fatigue resistance of WC-Co cemented carbide tail molds, resulting in a 50%-150% significant increase in fatigue life, effectively overcoming the long-standing problem of early fatigue failure of molds in this field.

[0031] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made by those skilled in the art based on the structure, shape, and principle of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for improving the fatigue life of a WC-Co cemented carbide tail die, characterized in that: Includes the following steps, S1, Structural design: The fillet radius at the variable cross-section of the tail mold is designed to be 0.5 mm to 1 mm; S2, Mixing: Providing WC powder and Co powder, wherein the WC powder comprises coarse-grained WC powder with an average particle size of 4 to 8 μm and fine-grained WC powder with an average particle size of 0.5 to 2 μm, wherein the content of the WC powder is 85 wt% to 92 wt% and the content of the Co powder is 8 wt% to 15 wt% based on the total mass after mixing, and adding rare earth element powder in a content of 0.1 wt% to 0.5 wt%, wherein the rare earth element powder comprises at least one of La, Ce or Y; S3, forming and sintering: the mixture obtained in step S2 is pressed into a green blank in the shape of a clamping mold, and then the green blank is sintered in a vacuum environment to obtain a clamping mold substrate. S4, Surface strengthening: The tail mold substrate obtained in step S3 is subjected to surface strengthening treatment, which is shot peening or high-frequency induction heat treatment, so that the surface of the tail mold substrate forms a residual compressive stress of -200 to -600 MPa.

2. The method for improving the fatigue life of a WC-Co cemented carbide tail die according to claim 1, characterized in that: In step S3, the sintering is completed by holding at 1350°C to 1450°C for 0.5 h to 2 h.

3. The method for improving the fatigue life of a WC-Co cemented carbide tail die according to claim 1, characterized in that: In step S4, when shot peening is performed, shot with a diameter of 0.1 mm to 0.5 mm is sprayed at a speed of 40 m / s to 80 m / s.

4. The method for improving the fatigue life of a WC-Co cemented carbide tail die according to claim 3, characterized in that: The shot peening process uses cast steel or ceramic shot.

5. The method for improving the fatigue life of a WC-Co cemented carbide tail die according to claim 1, characterized in that: In step S4, when performing high-frequency induction heat treatment, the surface is heated to 800°C to 1100°C and held for 2 to 30 seconds before being water-cooled.

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