Preparation method and application of surface-strengthened anti-corrosion steel mold

Abstract

The invention belongs to the field of material processing and manufacturing, and particularly relates to a preparation method and application of a surface-strengthened anti-corrosion steel mold. One purpose of the invention is to provide a material for a surface-strengthened anti-corrosion steel mold. The material comprises the following chemical components in percentage by weight: 75 to 80 percent of Fe, 11 to 13 percent of Cr, 0.5 to 2.5 percent of Ni, 1 to 5 percent of Mo, 2 to 5 percent of Co, 0.3 to 0.6 percent of Cu, 0.1 to 0.2 percent of C, 0 to 2 percent of W, 0.3 to 0.6 percent of Cu, 0.1 to 0.2 percent of C, 0 to 0.3 percent of V, 0 to 0.1 percent of Nb and 0 to 0.005 percent of La. The invention provides the preparation method of the steel mold for compression molding of the PBX raw material, which is simple and convenient to operate and can be industrially applied. Through combination of alloy matrix design, high-temperature and high-pressure nitriding and subzero treatment, collaborative optimization of corrosion resistance and mechanical properties of the steel die is achieved, and it is guaranteed that the surface of the steel die has long service life and high stability in the complex forming environment where the surface makes contact with PBX raw materials.

Description

Technical Field

[0001] This invention belongs to the field of materials processing and manufacturing, and specifically relates to a method for preparing and applying a surface-strengthened anti-corrosion steel mold. Background Technology

[0002] In the field of energetic materials, polymer-bonded explosives (PBX), as a class of high-performance, insensitive energetic materials, are widely used in high-end equipment such as missile warheads, aerospace propellants, and precision explosive devices due to their high energy output, good safety, and excellent chemical stability. The molding quality of PBX directly determines the reliability and performance of the end product, and in its manufacturing process, the mold, as a key tool for imparting the shape and dimensional accuracy of the explosive charge, is particularly crucial. The mold material must maintain structural integrity and dimensional stability under high pressure and corrosive process environments, thus placing dual requirements on the material's mechanical properties and corrosion resistance.

[0003] The raw material molding environment for PBX has significant unique characteristics. During the synthesis and pretreatment of energetic materials, trace amounts of corrosive media such as chloride ions and nitrogen oxides are often introduced or remain. Under high temperature and high pressure molding processes, these components easily undergo electrochemical reactions with the mold steel matrix, inducing pitting corrosion, intergranular corrosion, or crevice corrosion. This type of corrosion not only damages the surface finish of the mold cavity but also leads to dimensional deviations in the molded propellant cartridges and a decline in surface quality, severely affecting product performance and batch consistency.

[0004] Currently, widely used mold materials mainly include traditional mold steels such as Cr12, 45 steel, and 42CrMo. These materials possess high strength, hardness, and wear resistance, enabling them to withstand the high-pressure loads during PBX molding. However, the corrosion resistance of these steels is generally insufficient, making them highly susceptible to corrosion failure in process media containing chloride ions and acidic substances. Corrosion pits and peeling on the mold surface not only reduce mold life but may also introduce safety hazards due to the mixing of detached metal particles with PBX powder, affecting product purity and molding quality.

[0005] To improve corrosion resistance, austenitic stainless steels (such as 304 and 316) are sometimes used. These stainless steels perform well in mild corrosive environments due to their low carbon content (typically ≤0.08%) and the passivation film formed by chromium. However, these stainless steels have significant limitations in mechanical properties: the low carbon design restricts the solid solution strengthening effect, and the austenitic phase cannot be transformed into high-strength martensite during conventional heat treatment, resulting in insufficient overall hardness, compressive strength, and wear resistance. In high-pressure forming processes such as PBX, the molds are prone to plastic deformation or wear, making it difficult to meet the requirements of long-cycle, high-precision forming.

[0006] Therefore, existing mold materials face a contradiction when dealing with PBX molding conditions: "corrosion resistance is not enough strength, and strength is not corrosion resistance." Developing a new type of mold steel that can maintain surface stability in corrosive media while possessing high hardness, high strength, and excellent wear resistance has become a key technological requirement for improving PBX manufacturing levels and ensuring product quality and production safety.

[0007] Furthermore, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the inventors studied a large number of documents and patents when making this invention, but due to space limitations, not all details and contents were listed in detail. However, this does not mean that the present invention does not possess the features of these prior art. On the contrary, the present invention already possesses all the features of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0008] This invention belongs to the field of materials processing and manufacturing, specifically relating to a method for preparing and applying a surface-strengthened anti-corrosion steel mold. The purpose of this invention is to prepare anti-corrosion, high-strength, and wear-resistant steel molds for PBX molding using deep surface strengthening technology, aiming to solve the problem that existing steel molds are easily corroded and have their service life and performance affected when in contact with specific raw materials and in specific working environments during the manufacturing process.

[0009] To address the aforementioned technical problems, this invention provides a simple and industrially applicable method for preparing steel molds for PBX raw material compression molding. By combining alloy matrix design, high-temperature and high-pressure nitriding, and cryogenic treatment, the steel mold achieves synergistic optimization in corrosion resistance and mechanical properties, ensuring its surface maintains long service life and high stability in the complex molding environment where it comes into contact with PBX raw materials.

[0010] One of the objectives of this invention is to provide a surface-strengthened, corrosion-resistant steel mold material, which, by weight percentage, contains Fe 75-80%, Cr 11-13%, Ni 0.5-2.5%, Mo 1-5%, Co 2-5%, Cu 0.3-0.6%, C 0.1-0.2%, W 0-2%, Cu 0.3-0.6%, C 0.1-0.2%, V 0-0.3%, Nb 0-0.1%, and La 0-0.005%.

[0011] According to a preferred embodiment, the surface-strengthened anti-corrosion steel mold material contains, by weight percentage, 79.5% Fe, 12.5% ​​Cr, 2% Ni, 1.5% Mo, 4% Co, 0.5% Cu, and 0.15% C.

[0012] One of the objectives of this invention is to provide a method for preparing a surface-strengthened, corrosion-resistant steel mold material, comprising the following steps: Preparation of S1 base steel; S2 Rough machining; S3 strengthening: Apply pressure in a vacuum chamber, nitriding temperature 1000-1200℃, nitriding time 8-10 h; S4 Cooling: Deep cryogenic treatment for 4-6 hours, then warm up to room temperature; S5 Low-temperature tempering: Heat treatment at 450~500℃.

[0013] According to a preferred embodiment, in S3, the vacuum degree of the vacuum chamber is 0.1-1 Pa.

[0014] According to a preferred embodiment, in S3, the applied pressure is 0.8~1.5 MPa.

[0015] According to a preferred embodiment, in S4, the temperature range of the cryogenic treatment is -200 to -50°C.

[0016] According to a preferred embodiment, in S2, rough machining: the steel mold base material is rough machined according to the mold dimensions, and surface treated to facilitate nitrogen atom adsorption and diffusion. Preferably, the surface treatment is one or more of pickling, sandblasting, or mechanical polishing.

[0017] According to a preferred embodiment, in S1, the base steel is prepared by melting in a vacuum induction furnace to obtain a steel ingot, forging it at 850~1100℃, and cooling it to obtain a steel mold base material.

[0018] Preferably, the step further includes finishing, i.e., removing the brittle surface compound layer and finishing to the required dimensional accuracy of the mold. The finishing method is one or more of mechanical polishing, electrolytic polishing, ultrasonic polishing, and magnetorheological polishing.

[0019] One of the objectives of this invention is to provide a surface-strengthened anti-corrosion steel mold, which is made using the above-described surface-strengthened anti-corrosion steel mold material or the surface-strengthened anti-corrosion steel mold material prepared by the above-described preparation method.

[0020] One of the objectives of this invention is to provide a surface-strengthened anti-corrosion steel mold material prepared by the above-described preparation method, which has an anti-corrosion and wear-resistant layer of not less than 1.8 mm.

[0021] According to a preferred embodiment, the tensile strength of the surface-strengthened anti-corrosion steel mold material is not less than 1550 MPa, and the surface hardness is not less than 830 HV.

[0022] One of the objectives of this invention is to provide the application of the above-mentioned surface-strengthened anti-corrosion steel mold material or the surface-strengthened anti-corrosion steel mold material prepared by the above-mentioned preparation method in the compression molding of PBX energetic materials.

[0023] The beneficial effects of this invention are: 1) The alloy design achieves synergistic optimization of corrosion resistance and mechanical properties, and effectively promotes the deepening of the nitriding layer. This invention controls the chromium (Cr) content at 11-13%, enhancing the corrosion resistance of the matrix while providing a good foundation for nitriding. During high-pressure nitriding, molybdenum (Mo) in the alloy combines with nitrogen atoms to form fine molybdenum nitride particles. Their dispersed precipitation has a dual effect: on the one hand, it consumes local nitrogen atoms to maintain the nitrogen concentration gradient, promoting continuous inward diffusion of nitrogen; on the other hand, it pins grain boundaries to refine the grains, and the increased number of grain boundaries provides a rapid diffusion channel for nitrogen atoms. Furthermore, 0.3-0.6% copper (Cu) induces the formation of a relatively loosely arranged martensitic structure, further increasing the nitrogen diffusion path. Molybdenum and copper work synergistically through different mechanisms to jointly promote the increase in the depth of the nitriding layer.

[0024] 2) A composite process of nitriding and cryogenic treatment was employed to obtain a reinforced layer with superior depth and performance compared to conventional treatments. Following nitriding under specific parameters, a cryogenic treatment at -200 to -50°C was applied. This process not only improved the surface hardness, wear resistance, and dimensional stability of the mold but also promoted further diffusion of nitrogen atoms into the core through micro-stress adjustment. Compared to conventional nitriding processes, this method ultimately forms a microstructure on the steel mold surface dominated by high-nitrogen martensite with a depth exceeding 2 mm, demonstrating a significant advantage in reinforced layer depth.

[0025] 3) The overall performance of the mold effectively meets the stringent requirements of PBX molding. The steel mold obtained after the above treatment has a dense and deep nitrided layer on its surface, which can effectively resist the corrosive erosion of TATB-based PBX raw materials under high temperature and high pressure environments. At the same time, the deep reinforcing layer gives the mold matrix high hardness, high strength, and good deformation resistance, which is sufficient to withstand the mechanical load of PBX raw material compression molding. Therefore, the steel mold of the present invention achieves a good balance between corrosion resistance and mechanical properties, and is more suitable for compression molding of high-performance PBX energetic materials than existing mold steels, which helps to ensure product accuracy and production safety, and extend the service life of the mold. Attached Figure Description

[0026] Figure 1 This is a comparison diagram of the enhancement depth of Embodiment 1 and Comparative Example 1 of the present invention; Figure 2 The graph shows the corrosion resistance test results of high-temperature salt spray tests for Examples 1, 2, and Comparative Example 1 of the present invention. Figure 3 The graph shows the wear resistance test results of Example 1 and Comparative Example 2 of the present invention; Figure 4 This is the microstructure of the anti-corrosion reinforcement layer in Embodiment 1 of the present invention. Detailed Implementation

[0027] In the description of this invention, terminology is used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly defined.

[0028] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods; the materials, reagents or instruments used, unless otherwise specified by the manufacturer, are all commercially available reagents and materials; the conditions not specified in the examples are all carried out according to conventional conditions or conditions recommended by the manufacturer. At the same time, the present invention does not limit the source of the raw materials used. Unless otherwise specified, the raw materials used in the present invention are all commercially available products in this technical field.

[0029] In the stainless steel substrate described in this application, due to uncontrollable factors such as ore raw materials and smelting processes, the following trace impurity elements are often present: W 0–2%, Cu 0.3–0.6%, C 0.1–0.2%, V 0–0.3%, Nb 0–0.1%, and La 0–0.005%. These elements exist in solid solution or fine dispersed phases, with extremely low content and uniform distribution. They do not form continuous brittle phases, nor do they significantly alter the matrix structure, corrosion-resistant film structure, or mechanical properties. Extensive process verification and service data indicate that these impurities have no substantial impact on the corrosion resistance, strength, formability, and subsequent surface treatment effects of stainless steel. Therefore, they can be considered harmless accompanying components and do not require additional removal or composition adjustment.

[0030] Example 1 This embodiment relates to a method for preparing stainless steel. This embodiment also relates to a method for preparing a surface-strengthened, corrosion-resistant steel mold.

[0031] 1) Preparation of base steel: After melting in a vacuum induction furnace, steel ingots are obtained, forged at 1100℃, and cooled to obtain the base material of the steel mold. By weight percentage, the melted material contains Fe 79.5%, Cr 12.5%, Ni 2%, Mo 1.5%, Co 4%, Cu 0.5%, and C 0.15%; 2) Rough machining: The steel mold base material is rough machined according to the mold size and surface treated to facilitate nitrogen atom adsorption and diffusion; 3) Strengthening: Pressure is applied in a vacuum chamber, the nitriding temperature is 1200℃, and the nitriding time is 10 h, so that active nitrogen atoms penetrate into the surface to form a nitride layer; 4) Cooling: Deep cryogenic treatment for 6 hours, then slowly warm to room temperature; 5) Low-temperature tempering: Heat treatment at 500℃ to eliminate residual stress and stabilize dimensions.

[0032] Preferably, the surface treatment in this embodiment is polishing. More preferably, the specific steps of polishing are as follows: placing the metal in an abrasive suspension (e.g., diamond suspension) and using high-frequency oscillation of 20-50 kHz to cause the abrasive to impact the surface for 5-10 minutes.

[0033] Example 2 The process method in this embodiment is the same as that in embodiment 1. The difference is that, by weight percentage, the smelted material contains 82% Fe, 10% Cr, 2% Ni, 1.5% Mo, 4% Co, 0.5% Cu, and 0.15% C.

[0034] Example 3 The process method in this embodiment is the same as that in embodiment 1. The difference is that, by weight percentage, the smelted material contains 77% Fe, 15% Cr, 2% Ni, 1.5% Mo, 4% Co, 0.5% Cu, and 0.15% C.

[0035] Example 4 The process method in this embodiment is the same as that in embodiment 1. The difference is that, by weight percentage, the smelted material contains 77.8% Fe, 12% Cr, 2% Ni, 4% Mo, 4% Co, and 0.2% C.

[0036] Example 5 The process method in this embodiment is the same as that in embodiment 1, except that, by weight percentage, the smelted material contains 80.3% Fe, 13% Cr, 2% Ni, 4% Co, 0.5% Cu, and 0.2% C.

[0037] Example 6 The process method of this embodiment is the same as that of embodiment 1, except that 1) the forging temperature is 850°C in the preparation of the base steel.

[0038] Example 7 The process method in this embodiment is the same as that in embodiment 1, except that in 3) the nitriding temperature is 1000℃.

[0039] Example 8 The process method in this embodiment is the same as that in embodiment 1, except that in 3) the nitriding time is 8 hours.

[0040] Example 8 The process method of this embodiment is the same as that of embodiment 1, except that 4) during cooling, deep cryogenic treatment is performed for 4 hours.

[0041] Example 9 The process method in this embodiment is the same as that in embodiment 1, except that in 5) the heat treatment temperature is 450°C during low-temperature tempering.

[0042] Comparative Example 1 The 42CrMo mold was subjected to conventional nitriding treatment.

[0043] The 42CrMo material composition uses iron as the matrix. The process is as follows: the 42CrMo matrix is ​​rough machined, the surface is polished, and then a pressure of 0.1 MPa is applied in a vacuum chamber. The nitrogen-hydrogen mixed gas is ionized, and the ions bombard the workpiece. The active nitrogen atoms penetrate into the surface to form a nitride layer. The material is then heat-treated and finely machined to the required dimensional accuracy of the mold.

[0044] Comparative Example 2 The mold currently in use is made of 42CrMo. 42CrMo is a commonly used alloy structural steel with iron as its chemical matrix, and it is a commonly used mold material in this field. The surface has not undergone any strengthening treatment.

[0045] Comparative Example 3 The steel mold was prepared according to the preparation method in Example 1. After nitriding strengthening, it was directly and slowly cooled (0.5℃ / min) without cryogenic treatment.

[0046] Comparative Example 4 The steel mold was prepared according to the preparation method of Example 1, and after nitriding strengthening, it was directly and slowly cooled (2℃ / min) without deep cryogenic treatment.

[0047] The components of the examples and comparative examples are shown in Table 1.

[0048] Table 1

[0049] Test case 1) Finishing: Remove the brittle compound layer on the surface (e.g., grinding or polishing) and finish to the dimensional accuracy required for the mold.

[0050] 2) High-temperature salt spray test: The corrosion of the sample is detected by placing the finely processed sample in a salt spray corrosion test chamber at 80°C.

[0051] 3) Tensile strength test: The specimen is stretched at a constant speed until it breaks using a universal testing machine, and the force-displacement data is recorded to calculate the tensile strength.

[0052] 4) Surface hardness: Measured using a microhardness tester.

[0053] 5) Wear resistance: Wear volume loss is determined by ASTM testing, which involves rubbing an abrasive against a metal surface and measuring the wear volume loss to obtain the metal's wear resistance.

[0054] 6) Depth of anti-corrosion reinforcement layer: The depth of the reinforcement layer can be determined by testing the hardness of materials at different depths.

[0055] The hardness of Comparative Example 1 and Example 1 varies with surface distance as follows: Figure 1 As shown.

[0056] The hardness curve of the nitriding layer directly quantifies the distance the strengthening effect extends into the material. Deep nitriding ensures that even after heavy loads, surface wear, or grinding, parts retain a hard support layer, preventing collapse and fatigue spalling, significantly improving service life and reliability. The hardness curve of the nitriding layer directly shows the depth of the nitrided layer in the material.

[0057] Compared to Comparative Example 1, the hardness curve of the infiltration layer in the embodiment of the present invention is generally more to the right (deeper), decreases more gently (better gradient), and is at a higher overall hardness level. This demonstrates that the depth of the infiltration layer in this embodiment is significantly increased, providing stronger durability and heavy-load resistance to the parts composed of the material. The overall improved hardness level ensures extremely high strength and wear resistance from the surface to the core. The extremely gentle hardness gradient indicates a smooth transition in performance, reducing the risk of surface peeling. The results show that by optimizing the matrix formulation and improving the nitriding process, the depth of the anti-corrosion nitriding layer can be significantly increased; simultaneously, the mechanical properties (hardness, wear resistance) and anti-corrosion performance of the mold surface are significantly improved compared to ordinary strengthening processes.

[0058] As shown in Table 2, optimizing the Cr content in the steel mold matrix material can synergistically improve the corrosion resistance and mechanical properties of the steel. Adding molybdenum and copper to the steel mold matrix material enhances the nitriding depth, which is beneficial for increasing the thickness of the anti-corrosion reinforcement layer. A comparison of the product performance of Comparative Example 3 and Example 1 with and without cryogenic treatment shows that the surface hardness, wear resistance, and deformation resistance of the material are significantly improved after cryogenic treatment, and the depth of the anti-corrosion reinforcement layer is increased, indicating that cryogenic treatment can promote deeper diffusion of nitrogen atoms. Compared with the ordinary mold steel and ordinary nitriding process used in the prior art (Comparative Examples 1 and 2), the deep surface strengthening process used in this invention is more suitable for the compression molding of PBX energetic materials.

[0059] like Figure 2 As shown in Table 3, the materials involved in this application (Examples 1-5) exhibited strong corrosion resistance, especially for PBX, in high-temperature corrosion tests and high-temperature salt spray tests on PBX raw material powder. This result indicates that the materials have strong surface chemical inertness, stable microstructure, and well-matched high-temperature mechanical properties, making them more suitable for high-quality, high-safety, and long-cycle stable manufacturing of PBX.

[0060] like Figure 3As shown, the wear resistance of the deep-strengthened sample remained almost unchanged after different corrosion times in PBX, while the wear resistance of the mold steel currently used was significantly lower than that of Example 1, and the wear resistance decreased significantly after corrosion occurred as the contact time with PBX increased.

[0061] like Figure 4 As shown, the anti-corrosion reinforcement layer of Example 1 has a microstructure with finely dispersed nitride grains. This structure provides more pathways for nitrogen atom diffusion, which is beneficial for nitrogen atoms to diffuse into deeper layers.

[0062] Table 2

[0063] Table 3

[0064] It should be noted that the specific embodiments described above are exemplary, and those skilled in the art can devise various solutions inspired by the disclosure of this invention. These solutions all fall within the scope of this invention and its protection. Those skilled in the art should understand that this specification and its accompanying drawings are illustrative and not intended to limit the scope of the claims. The scope of protection of this invention is defined by the claims and their equivalents.

Claims

1. A surface-reinforced, corrosion-resistant steel mold material, characterized in that, By weight percentage, the chemical composition includes Fe 75-80%, Cr 11-13%, Ni 0.5-2.5%, Mo 1-5%, Co 2-5%, Cu 0.3-0.6%, C 0.1-0.2%, W 0-2%, Cu 0.3-0.6%, C 0.1-0.2%, V 0-0.3%, Nb 0-0.1%, and La 0-0.005%.

2. The surface-strengthened anti-corrosion steel mold material according to claim 1, characterized in that, The chemical composition, by weight percentage, includes Fe 79.5%, Cr 12.5%, Ni 2%, Mo 1.5%, Co 4%, Cu 0.5%, and C 0.15%.

3. A method for preparing a surface-strengthened, corrosion-resistant steel mold material, characterized in that, Includes the following steps: Preparation of S1 base steel; S2 Rough machining; S3 strengthening: Apply pressure in a vacuum chamber, nitriding temperature 1000-1200℃, nitriding time 8-10 h; S4 Cooling: Deep cryogenic treatment for 4-6 hours, then warm up to room temperature; S5 Low-temperature tempering: Heat treatment at 450~500℃.

4. The method for preparing deep-nitrided high-strength and high-toughness stainless steel according to claim 4, characterized in that, In S3, the vacuum level of the vacuum chamber is 0.1-1 Pa.

5. The method for preparing deep-nitrided high-strength and high-toughness stainless steel according to claim 4, characterized in that, In S3, the applied pressure is 0.8~1.5 MPa.

6. The method for preparing deep-nitrided high-strength and high-toughness stainless steel according to claim 4, characterized in that, During solution nitriding treatment, the temperature range for cryogenic treatment in S4 is -200 to -50℃.

7. A surface-reinforced corrosion-resistant steel mold, characterized in that, The surface-strengthened anti-corrosion steel mold material is prepared by the material according to any one of claims 1 to 2 or by the preparation method according to any one of claims 3 to 6.

8. A surface-strengthened, corrosion-resistant steel mold material prepared according to the preparation method of any one of claims 3 to 6, characterized in that, The surface-reinforced anti-corrosion steel mold material is provided with an anti-corrosion and wear-resistant layer of not less than 1.8 mm.

9. The surface-strengthened anti-corrosion steel mold material according to claim 8, characterized in that, The surface-strengthened anti-corrosion steel mold material has a tensile strength of not less than 1550 MPa and a surface hardness of not less than 830 HV.

10. The application of the surface-strengthened anti-corrosion steel mold material according to any one of claims 1 to 2 or the surface-strengthened anti-corrosion steel mold material prepared by any one of claims 3 to 6 in the compression molding of PBX energetic materials.

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