High-cobalt stainless powder metallurgy high-speed steel and preparation method and application thereof

Abstract

The invention discloses high-cobalt stainless powder metallurgy high-speed steel and a preparation method and application thereof.The high-cobalt stainless powder metallurgy high-speed steel comprises, by mass, 15%-25% of Cr, 1.4%-1.6% of C, 5%-10% of Co, 0.05%-0.10% of B, 0.1%-0.3% of La and the balance Fe and inevitable impurities, the B and the La are preferably added in the form of lanthanum hexaboride (LaB), and the high-cobalt stainless powder metallurgy high-speed steel is prepared through ball milling, cold pressing, vacuum sintering and multi-section heat treatment processes. The alloy has the advantages of compact structure, fine carbide, pure grain boundary, high hardness of HRC 67-69, salt mist corrosion resistance of more than or equal to 290 hours and excellent wear resistance, and is suitable for manufacturing high-end cutting tools in complex corrosive environments, such as military daggers, dental drill bits, medical cutting instruments, stirring and crushing knives and the like.

Description

Technical Field

[0001] This invention relates to the field of metal materials technology, and in particular to a high-cobalt stainless powder metallurgy high-speed steel, its preparation method, and its application. Background Technology

[0002] With the development of modern manufacturing and equipment industries, the demand for cutting tools in military equipment, medical devices, and high-end industrial manufacturing continues to grow. High-performance cutting tools such as military daggers and dental drills need to operate stably for extended periods in complex service environments, including high humidity, saline solutions, and acid / alkali conditions. This requires not only excellent strength and wear resistance but also long-term corrosion resistance. However, while existing traditional stainless steels (such as AISI 420 and 440C) possess some corrosion resistance, their hardness and toughness are insufficient. Conventional high-speed steels (such as M2 and W18Cr4V), although possessing high hardness, have severely limited corrosion resistance due to their low chromium content, making them unsuitable for the long-term service requirements under these special conditions.

[0003] Currently available stainless steel and high-speed steel materials struggle to achieve a balance between hardness, wear resistance, and corrosion resistance. This is particularly problematic in applications such as medical knives and military daggers, which are exposed to saline solution, sweat, or humid environments for extended periods. Issues such as passivation film failure, blade cracking, and surface pitting frequently arise, leading to frequent maintenance and poor service reliability. Furthermore, while some high-end imported powder metallurgy high-speed steel materials possess excellent properties, their complex composition, high process dependence, and high manufacturing costs make large-scale domestic substitution difficult.

[0004] In recent years, powder metallurgy technology has provided an important avenue for the development of high-performance cutting tool materials. Through powder refinement and dense sintering, a material structure with uniform microstructure and dispersed carbide distribution can be obtained, thereby improving hardness and wear resistance, and to some extent optimizing corrosion resistance. Existing technology discloses a stainless powder metallurgy high-speed steel for wet rotary die-cutting rollers, which improves material hardness and wear resistance by adding high levels of alloying elements such as Cr, Mo, Nb, and VC, supplemented by rare earth treatment. However, this approach relies heavily on complex multi-component strengthening systems and is primarily geared towards contact wear scenarios in die-cutting equipment, still exhibiting limitations in terms of actual corrosion resistance, material system controllability, and adaptability to high-end cutting tool requirements.

[0005] Therefore, there is an urgent need to develop a stainless powder metallurgy high-speed steel material with a simplified alloy composition, uniform microstructure, high hardness, and excellent corrosion resistance, suitable for cutting tools such as military daggers and dental drills that require extremely high corrosion resistance and strength to meet the service requirements of high-performance cutting tools. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the first objective of this invention is to provide a high-cobalt stainless powder metallurgy high-speed steel, which aims to solve the problems of performance degradation and insufficient service life of existing cutting tool materials under high corrosion and high load service environments. The high-cobalt stainless powder metallurgy high-speed steel alloy provided by this invention has a simplified composition, uniform microstructure, high hardness, and excellent corrosion resistance.

[0007] The second objective of this invention is to provide a method for preparing high-cobalt stainless powder metallurgy high-speed steel. The preparation method of this invention is simple and controllable, and suitable for industrial production.

[0008] The third objective of this invention is to provide an application of high-cobalt stainless powder metallurgy high-speed steel.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] This invention discloses a high-cobalt stainless powder metallurgy high-speed steel, which has the following composition by mass percentage: Cr: 15%–25%, C: 1.4%–1.6%, Co: 5%–10%, B: 0.05%–0.10%, La: 0.1%–0.3%, with the balance being Fe and unavoidable impurities.

[0011] The high-cobalt stainless steel powder metallurgy high-speed steel provided by this invention is based on Fe-Cr-C, with the addition of 5-10% high-content Co element. The Co element is dissolved in the Fe matrix, playing a role in solid solution strengthening, improving temperature resistance, and enhancing the high-temperature hardness and red hardness of the matrix. At the same time, it increases the solid solubility of Cr element in the Fe matrix, significantly improving the ability of stainless steel to form a passivation film during service, that is, further improving the corrosion resistance of stainless steel. In addition, by adding B and La elements, on the one hand, it promotes sintering densification, enhances grain boundary bonding, and increases hardness; on the other hand, it promotes grain refinement and purifies grain boundaries. Due to the reduction of impurities at the grain boundaries, the precipitation nucleation sites of Cr after the high Co increases the solid solubility of Cr are greatly reduced, reducing the precipitation of Cr during tempering, thereby inhibiting the precipitation of Cr near the grain boundaries in the form of carbides during the tempering process, resulting in the Cr grain boundary "depletion" phenomenon, and further maintaining its high corrosion resistance.

[0012] In a preferred embodiment, the high-cobalt stainless powder metallurgy high-speed steel has the following composition by mass percentage: Cr: 18%–22%; C: 1.45%–1.55%; Co: 6%–9%; B: 0.05%–0.10%; La: 0.10%–0.25%, with the balance being Fe and unavoidable impurities.

[0013] In a preferred embodiment, in the high-cobalt stainless powder metallurgy high-speed steel, B and La elements are introduced through LaB6 powder, which can further improve the thermal stability and service consistency of the material.

[0014] The preferred embodiment features a high-cobalt stainless powder metallurgy high-speed steel with a refined microstructure of carbide grain size ≤5μm, a hardness of HRC 67~69, and a salt spray corrosion resistance life of over 290 hours. It is suitable for high-end cutting tool applications in highly corrosive environments.

[0015] This invention discloses a method for preparing high-cobalt stainless powder metallurgy high-speed steel. The method involves taking raw materials according to a design ratio, mixing them to obtain a mixed powder, pressing the mixed powder into a compact, sintering the compact to obtain a sintered body, and heat-treating the sintered body to obtain high-cobalt stainless powder metallurgy high-speed steel.

[0016] The heat treatment process is as follows: first, preheating; then, heating to 1000~1050℃ and holding for 30-90 minutes for pre-solidification; then heating to 1150℃~1180℃ for high-temperature solution treatment and holding for 30-60 minutes; cooling; then holding at -80℃~-100℃ for 1-2 hours for cryogenic treatment; finally, tempering treatment is performed 2-3 times, with the temperature of any tempering treatment being 200℃~250℃ and the time of any tempering treatment being 60-90 minutes.

[0017] The preparation method provided by this invention, during the heat treatment process, involves a specific segmented heating heat treatment process. First, Cr is pre-dissolved at 1000-1050℃, where it has the highest solubility in austenite. Then, the furnace temperature is raised to 1150-1180℃ for high-temperature solution treatment and holding. Combined with the synergistic effect of high Co content, this segmented solution treatment of Cr pre-dissolution + high-temperature quenching significantly improves the solubility of Cr in the Fe matrix. During low-temperature tempering, a higher Cr content (greater than 13%) is retained in the martensitic matrix, thus significantly improving the passivation film formation ability of stainless steel during service, further enhancing its corrosion resistance. Finally, during low-temperature tempering, the high Co content stabilizes the tempered martensitic structure and promotes the precipitation and strengthening of nano-scale carbides, further improving the hardness and wear resistance of the stainless steel.

[0018] In a preferred embodiment, LaB6 powder is used as a raw material for elements B and La, and the amount of LaB6 powder added is 0.15% to 0.4% of the mass of the mixed powder.

[0019] In a preferred embodiment, the raw materials for other elements are selected from elemental powders or pre-alloyed metal powders. In this invention, "other elements" refers to elements in the raw materials other than B and La.

[0020] In the preferred embodiment, the particle size D50 of each raw material is 2~10μm to obtain finer sintered crystals.

[0021] In a preferred embodiment, the mixing method is wet ball milling, which is carried out under a protective atmosphere for 24–36 hours, with a ball-to-material ratio of 5–6:1. Wet ball milling ensures uniform element mixing and a reasonable particle size distribution.

[0022] In a preferred embodiment, the pressing and molding method is cold isostatic pressing (CIP) or compression molding.

[0023] In a preferred embodiment, the pressing pressure is 150–200 MPa. By controlling the pressing process within this range, a high-density green body can be obtained.

[0024] In a preferred embodiment, the sintering is carried out under vacuum or a protective atmosphere at a temperature of 1180–1220°C for 2–3 hours, with a heating rate of 5–10°C / min. Through this sintering process, during the heating process, when the temperature reaches 800–1000°C, the oxygen in the powder undergoes a reduction reaction with the carbon in the raw materials, causing the oxygen to be removed in the form of CO or CO2. Finally, the temperature is maintained at 1180–1220°C to obtain a sintered body with a dense structure and low oxygen content.

[0025] In a preferred embodiment, the preheating process involves first holding the temperature at 600°C for 30-60 minutes, and then raising the temperature to 850°C and holding it for 30-60 minutes.

[0026] In a preferred embodiment, the cooling method is oil cooling or air cooling.

[0027] The present invention also provides an application of high-cobalt stainless powder metallurgy high-speed steel, wherein the high-cobalt stainless powder metallurgy high-speed steel is used to prepare cutting tools, wherein the cutting tools are selected from at least one of military daggers, dental drills, medical cutting instruments, and mixing and crushing knives.

[0028] Principles and advantages

[0029] The high-cobalt stainless steel powder metallurgy high-speed steel provided by this invention is based on Fe-Cr-C, with the addition of 5-10% high-content Co element. The Co element is dissolved in the Fe matrix, playing a role in solid solution strengthening, improving temperature resistance, and enhancing the high-temperature hardness and red hardness of the matrix. At the same time, it increases the solid solubility of Cr element in the Fe matrix, significantly improving the ability of stainless steel to form a passivation film during service, that is, further improving the corrosion resistance of stainless steel. In addition, by adding B and La elements, on the one hand, it promotes sintering densification, enhances grain boundary bonding, and increases hardness; on the other hand, it promotes grain refinement and purifies grain boundaries. Due to the reduction of impurities at the grain boundaries, the precipitation nucleation sites of Cr after the high Co increases the solid solubility of Cr are greatly reduced, reducing the precipitation of Cr during tempering, thereby inhibiting the precipitation of Cr near the grain boundaries in the form of carbides during the tempering process, resulting in the Cr grain boundary "depletion" phenomenon, and further maintaining its high corrosion resistance.

[0030] The preparation method provided by this invention, during the heat treatment process, involves a specific segmented heating heat treatment process. First, Cr is pre-dissolved at 1000-1050℃, where it has the highest solubility in austenite. Then, the furnace temperature is raised to 1150-1180℃ for high-temperature solution treatment and holding. Combined with the synergistic effect of high Co content, this segmented solution treatment of Cr pre-dissolution + high-temperature quenching significantly improves the solubility of Cr in the Fe matrix. During low-temperature tempering, a higher Cr content (greater than 13%) is retained in the martensitic matrix, thus significantly improving the passivation film formation ability of stainless steel during service, further enhancing its corrosion resistance. Finally, during low-temperature tempering, the high Co content stabilizes the tempered martensitic structure and promotes the precipitation and strengthening of nano-scale carbides, further improving the hardness and wear resistance of the stainless steel.

[0031] Through the synergistic control of the above-mentioned preferred components and process routes, the material prepared by the present invention can achieve: Rockwell hardness HRC 67~69; salt spray corrosion resistance time >290 hours (5 wt.% NaCl, ASTM B117); carbide particle size ≤5μm, uniformly distributed, dense structure, and no obvious inclusions.

[0032] This invention overcomes the bottleneck problem of unifying high performance and actual manufacturability in existing technologies, under the premise of simple composition system, precise microstructure control and high process window tolerance. It is particularly suitable for the manufacturing needs of cutting tools in long-term service and high corrosion load environments in the military and medical fields. Detailed Implementation

[0033] The component compositions of Examples 1-3 and Comparative Examples 1-3 are shown in Table 1:

[0034]

[0035] The preparation processes for Examples 1-3 and Comparative Examples 1-4 are as follows:

[0036] 1. Raw material preparation (taking the ingredients of Example 2 as an example)

[0037] Weigh out Cr powder, Fe powder, Co powder, graphite powder and LaB6 powder, and mix them in the following proportions: Cr 20%, C 1.5%, Co 8% and LaB6 0.28% (after conversion, B is 0.08% and La is 0.20%). The average particle size of the powder is 2-10 μm and the purity is ≥99.5%.

[0038] 2. Mixing and ball milling

[0039] The mixed powder was added to a ball mill jar with ethanol as the medium and wet ball milled for 30 hours under an argon atmosphere with a ball-to-powder ratio of 5:1. After ball milling, the powder was vacuum dried for 24 hours.

[0040] 3. Compression molding

[0041] The mixed powder was cold isostatically pressed at 180 MPa to obtain a dense compact with a diameter of 20 mm and a height of 10 mm.

[0042] 4. Sintering

[0043] The pressed billet was held at 1220℃ for 2.5 hours in a vacuum sintering furnace, with a vacuum level better than 10. -2 Pa, then slowly cooled to room temperature.

[0044] 5. Heat treatment

[0045] After preheating the sample (30 minutes each at 600℃ and 850℃), it was pre-solidified at 1050℃ for 30 minutes and then at 1160℃ for 45 minutes for high-temperature solution treatment, followed by oil cooling; then it was cryogenically treated at -100℃ for 2 hours, and finally tempered three times at 240℃, each time for 70 minutes, and air-cooled.

[0046] Comparative Example 4

[0047] The other conditions are the same as in Example 3, except that no pre-solution treatment is performed.

[0048] Performance testing

[0049] The wear performance was tested under ASTM B117 salt spray corrosion environment in 5 wt.% NaCl, and the wear performance was tested under 80 N dry friction load. The test results are shown in Table 2.

[0050]

[0051] Examples 1-3 all exhibit superior overall performance compared to the comparative examples and commercially available materials, with Example 2 achieving the best balance between hardness, corrosion resistance, and wear resistance. The introduction of high cobalt (8%) significantly improves red hardness and high-temperature corrosion resistance, while the addition of LaB6 significantly refines carbides, purifies grain boundaries, and stabilizes the passivation film through the synergistic effect of B and La. Compared to imported M390, the material of this invention shows approximately 20% improvement in corrosion resistance and a 15%-20% reduction in manufacturing costs, making it more suitable for localized mass production and widespread adoption.

[0052] In summary, the stainless powder metallurgy high-speed steel for high corrosion-resistant cutting tools provided by this invention achieves optimization of the material's microstructure and significant improvement of its macroscopic properties by introducing high cobalt (5%~10%) strengthening elements into the Fe-Cr-C base system and supplementing it with rare earth lanthanum (La) and boron (B) synergistic microalloying provided by LaB6 powder.

[0053] The experimental results show that:

[0054] After optimized heat treatment, the hardness of the material of this invention can be stabilized between HRC 67 and 69, exhibiting good edge retention and resistance to deformation.

[0055] In the salt spray corrosion test, the sample's corrosion resistance time reached 280-340 hours, which is far superior to traditional stainless steel (440C) and high-speed steel (M2), and also superior to common imported powder steel M390;

[0056] The wear volume is controlled within 5.3–6.1 mm. 3 Within this range, it exhibits excellent wear resistance;

[0057] Microscopic observation shows that the carbide particles are small (≤5μm), uniformly distributed, with pure grain boundaries, dense structure, and no obvious corrosion spots or pores on the surface.

[0058] Compared with existing multi-element reinforced powder metallurgy stainless steels, this invention exhibits significant technical advantages in terms of simplified composition design, more controllable microstructure, superior performance balance, and wider environmental adaptability. It is particularly suitable for high-end cutting tools such as military daggers, dental drills, and surgical blades, which require extremely high strength, corrosion resistance, and stability, and possesses excellent engineering practicality and prospects for industrialization.

Claims

1. A high-cobalt stainless powder metallurgy high-speed steel, characterized in that: The composition by mass percentage is as follows: Cr: 15%–25%, C: 1.4%–1.6%, Co: 5%–10%, B: 0.05%–0.10%, La: 0.1%–0.3%, with the balance being Fe and unavoidable impurities.

2. The high-cobalt stainless powder metallurgy high-speed steel according to claim 1, characterized in that: In the high-cobalt stainless powder metallurgy high-speed steel, B and La elements are introduced through LaB6 powder.

3. The high-cobalt stainless powder metallurgy high-speed steel according to claim 1, characterized in that: The high-cobalt stainless powder metallurgy high-speed steel has a refined microstructure with carbide particle size ≤5μm, a hardness of HRC 67~69, and a salt spray corrosion resistance life of more than 290 hours.

4. The method for preparing high-cobalt stainless powder metallurgy high-speed steel according to any one of claims 1-3, characterized in that: According to the design ratio, each raw material is prepared and mixed to obtain a mixed powder. The mixed powder is pressed into a compact to obtain a compact. The compact is sintered to obtain a sintered body. The sintered body is heat-treated to obtain high cobalt stainless powder metallurgy high-speed steel. The heat treatment process is as follows: first, preheating; then, heating to 1000~1050℃ and holding for 30-90 minutes for pre-solidification; then heating to 1150℃~1180℃ for high-temperature solution treatment and holding for 30-60 minutes; cooling; then holding at -80℃~-100℃ for 1-2 hours for cryogenic treatment; finally, tempering treatment is performed 2-3 times, with the temperature of any tempering treatment being 200℃~250℃ and the time of any tempering treatment being 60-90 minutes.

5. The method for preparing high-cobalt stainless powder metallurgy high-speed steel according to claim 4, characterized in that: LaB6 powder was used as a raw material for elements B and La, and the amount of LaB6 powder added was 0.15% to 0.4% of the mass of the mixed powder. The raw materials for other elements are selected from elemental powders or pre-alloyed powders; The particle size D50 of each raw material is 2~10μm.

6. The method for preparing high-cobalt stainless powder metallurgy high-speed steel according to claim 4, characterized in that: The mixing method is wet ball milling, which is carried out under a protective atmosphere for 24 to 36 hours, with a ball-to-material ratio of 5 to 6:

1.

7. The method for preparing high-cobalt stainless powder metallurgy high-speed steel according to claim 4, characterized in that: The pressing and molding method is cold isostatic pressing or compression molding; The pressure for compression molding is 150–200 MPa.

8. The method for preparing high-cobalt stainless powder metallurgy high-speed steel according to claim 4, characterized in that: The sintering is carried out under vacuum or a protective atmosphere, at a temperature of 1180~1220℃, for a time of 2~3h, and at a heating rate of 5~10℃ / min.

9. The method for preparing high-cobalt stainless powder metallurgy high-speed steel according to claim 4, characterized in that: The preheating process is as follows: first, maintain the temperature at 600℃ for 30-60 minutes, then raise the temperature to 850℃ and maintain the temperature for 30-60 minutes. The cooling method is either oil cooling or air cooling.

10. The application of the high-cobalt stainless powder metallurgy high-speed steel according to any one of claims 1-3, characterized in that: The high-cobalt stainless powder metallurgy high-speed steel is used to prepare cutting tools, which are selected from at least one of military daggers, dental drills, medical cutting instruments, and mixing and crushing knives.

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