A high-strength cobalt-based alloy and a method for producing the same

Abstract

The application relates to a high-strength cobalt-based alloy and a preparation method thereof, and belongs to the technical field of special alloy material preparation. The chemical composition and mass content of the cobalt-based alloy are as follows: Ni: 25-38%, Cr: 15-25%, Mo: 6-13.5%, Ti: 0.5-1.5%, Al<=0.5%, C<=0.03%, B<=0.08%, and the rest is Co and inevitable impurities. The preparation method comprises homogenization treatment, forging, hot rolling and a quenching and tempering process. The molybdenum and titanium elements are reasonably added, in-situ formation of fine and dispersed Mo-Ti nano precipitated phases is promoted, and the structure form is optimized, so that the tensile strength of the alloy is greater than or equal to 1900 MPa, the elongation is greater than or equal to 12%, and the alloy has both ultrahigh strength and excellent ductility. The technical method is suitable for the preparation and production of high-strength and non-magnetic structure materials which bear high stress, strong corrosion and other severe service environments.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature alloy material preparation technology, specifically relating to a high-strength cobalt-based alloy and its preparation method. Background Technology

[0002] Cobalt-based superalloys are widely used in aerospace, marine engineering and other fields due to their high strength, good corrosion resistance and fatigue resistance.

[0003] Traditional cobalt-based alloys primarily improve their properties through solid solution strengthening and carbide strengthening. However, as strength increases, the alloy's plasticity and toughness decrease significantly, making it prone to fracture under impact loads. Furthermore, traditional processes result in poor microstructure uniformity and localized segregation, which not only affects the stability of the alloy's mechanical properties but also reduces its corrosion resistance and fatigue life. Therefore, developing a preparation method that can synergistically improve the strength and plasticity of cobalt-based alloys while ensuring process stability and ease of industrial production has become an important direction for exploration in the field of cobalt-based alloys.

[0004] Chinese invention patent CN105296809B discloses a method for manufacturing a high-strength precipitation-strengthened cobalt-based single-crystal superalloy. The chemical composition of this cobalt-based alloy is: C 0.05-0.9%, Cr 3.0-9.0%, Al 2.0-8.0%, Ti 1.0-4.0%, Ta 2.0-10.0%, W 10.0-16.0%, Ni 2.0-18.0%, with Co as the balance. This invention significantly improves the tensile properties and creep ductility of the alloy by controlling the content range of elements such as Al, Ti, and Cr, combined with a suitable heat treatment process. However, the single-crystal preparation process and the long-term two-stage aging treatment process employed are, to some extent, detrimental to improving production efficiency and cost control.

[0005] Chinese invention patent application CN105088018A discloses a method for manufacturing a high-strength, oxidation-resistant cobalt-based superalloy. The chemical composition of this cobalt-based alloy is: C 0.02-0.10%, Al 2.5-6%, W 12-18%, Cr 5-10%, Ni 25-45%, Ti 0.5-4%, B 0.001-0.008%, S < 0.03%, P < 0.03%, with the balance being Co. This invention improves the alloy's strength and hot workability by utilizing the γ' phase for strengthening; however, the introduction of large amounts of Al and Ti elements results in poor machinability and plasticity. Summary of the Invention

[0006] The purpose of this invention is to provide a high-strength cobalt-based alloy with good ductility and its preparation method through reasonable composition design and process control.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A high-strength cobalt-based alloy has the following chemical composition and mass percentage: Ni: 25-38%, Cr: 15-25%, Mo: 6-13.5%, Ti: 0.5-1.5%, Al≤0.5%, C≤0.03%, B≤0.08%, Si+Mn≤0.2%, with the remainder being Co and unavoidable impurities.

[0008] An appropriate amount of Si+Mn can effectively remove oxygen from molten steel, reduce oxide inclusions, suppress segregation tendency, and avoid the formation of harmful phases. Therefore, this invention controls Si+Mn ≤ 0.2%.

[0009] The mass percentage content of Ti and Mo, and Ti and Al elements in the cobalt-based alloy satisfies the following relationship: 0.01≤Ti / Mo≤0.1, 1%≤Ti+Al≤2%.

[0010] Excessive Ti / Mo ratio leads to the formation of unstable Co3Ti phases, resulting in reduced strength. Conversely, insufficient Ti / Mo ratio promotes the precipitation of other harmful topologically close-packed phases, increasing the number of crack initiation sites. Therefore, this invention controls the Ti / Mo ratio to be 0.01 ≤ Ti / Mo ≤ 0.1.

[0011] Too low a Ti+Al content will result in insufficient volume fraction of the γ' phase, leading to weak precipitation strengthening effect; too high a Ti+Al content will decrease plasticity and toughness, and the excessive driving force will cause the γ' phase to precipitate rapidly and coarsely during solidification or heat treatment, deteriorating performance. Therefore, this invention controls the content to 1% ≤ Ti+Al ≤ 2%.

[0012] The cobalt-based alloy has a diameter of 6–40 mm; at room temperature, it has a tensile strength ≥1900 MPa, a hardness ≥51 HRC, and an elongation ≥12%.

[0013] The functions of the main components in this invention are as follows: Co: reduces stacking fault energy, inhibits dislocation cross-slip, provides thermodynamic conditions for cold deformation-induced γ-phase martensitic transformation, and achieves phase transformation strengthening; solid solution strengthening, as a solvent atom to accommodate high concentrations of alloying elements (Cr, Mo, W, etc.), increasing lattice distortion energy; and creep suppression, significantly reducing the creep rate by hindering dislocation climb and related recovery processes. In this invention, Co is the balance.

[0014] Ni: A constituent element of the Ni3(Ti,Al) type L12 ordered precipitate phase, which is semi-coherent or coherent with the matrix. The precipitate phase improves the strength of the alloy, balances the brittleness caused by precipitation strengthening, maintains room temperature elongation, improves austenite stability, and slows down the phase transformation rate. In this invention, the Ni content is controlled at 25-38%.

[0015] Cr: In an oxidizing / corrosive environment, it forms a dense Cr2O3 film, which improves the corrosion resistance of the alloy. As a medium-sized atom, it causes lattice distortion, which enhances the strength of the matrix and plays a role in solid solution strengthening. In this invention, the Cr content is controlled at 15-25%.

[0016] Mo forms solid carbides to improve alloy strength, suppress brittle phases, delay the precipitation of σ phase, and promote the precipitation of MoTi nanophase, further improving alloy strength. In this invention, the Mo content is controlled at 6-13.5%.

[0017] Ti is a key component of the γ' strengthening phase. This phase maintains a coherent relationship with the γ matrix and hinders dislocation movement through a dislocation cutting mechanism, significantly improving the high-temperature strength of the alloy. In this invention, the Ti content is controlled at 0.5–1.5%.

[0018] B: By altering the grain boundary state and precipitate behavior, the plasticity of the alloy during hot working is improved. Boron segregates at the grain boundaries, suppressing the tendency for early cracking caused by grain boundary sliding or impurity segregation during hot working, thereby widening the hot working window of the alloy and improving the yield.

[0019] The present invention also provides a method for preparing the above-mentioned high-strength cobalt-based alloy, which includes the following steps: S1, Vacuum induction melting process; S2. Demold and anneal the ingot obtained in S1, polish the surface, and then remelt it by electroslag remelting or vacuum arc remelting. S3. The ingot obtained in S2 is subjected to high-temperature homogenization treatment; S4. Forge the ingot obtained in S3 into a rolled billet; S5. Hot-roll the billet obtained in S4 into a bar. S6. The hot-rolled bar obtained in S5 is subjected to quenching and tempering treatment.

[0020] In step S3, when the furnace temperature is ≥300℃, the ingot is put into the furnace and heated to 750℃ at a rate of 100-150℃ / h and held for 8-12h. Then, the temperature is raised to 1140-1180℃ at a rate of 60-90℃ / h and held for 20-40h.

[0021] In step S4, the initial forging temperature is 1120~1160℃, the final forging temperature is ≥900℃, the deformation per pass is 25~35%, and the cross-sectional size of the forging billet is 90mm×90mm~120mm×120mm.

[0022] In step S5, the initial rolling temperature is ≥1050℃, the final rolling temperature is ≥950℃, the rolling ratio is ≥10, and the bar size is φ6~40mm.

[0023] Step S6 includes cryogenic treatment and aging treatment. The cryogenic treatment temperature is -80 to -120℃ and the holding time is 1 to 4 hours. The aging treatment temperature is 420 to 560℃ and the aging time is 4 to 6 hours.

[0024] The beneficial effects of the above technical solution are as follows: By controlling the types and contents of elements added to the alloy, and rationally adding Mo and Ti elements, the present invention promotes the in-situ formation of fine, dispersed Mo-Ti nano-precipitates, optimizes the microstructure, and improves the alloy strength. By controlling the homogenization and forging processes, a fine and uniform grain structure is obtained, laying the foundation for high strength and high toughness; combined with aging treatment, while refining the grains and improving strength, residual stress is effectively controlled and embrittlement is prevented, ultimately achieving a dual optimization of strength and plasticity.

[0025] This invention achieves synergistic optimization of high strength and excellent ductility through integrated and coordinated control of component design and preparation process. The cobalt-based alloy exhibits a room temperature tensile strength ≥1900MPa, a hardness ≥51HRC, and an elongation ≥12%. It is suitable for the preparation and production of high-strength, non-magnetic structural materials that can withstand harsh service environments such as high stress and strong corrosion. Attached Figure Description

[0026] Figure 1 This is a metallographic diagram of the cobalt-based alloy obtained in Example 1 of the present invention; Figure 2 This is a SEM image of the cobalt-based alloy obtained in Example 1 of the present invention. Detailed Implementation

[0027] The chemical composition and mass percentage of the high-strength cobalt-based alloy of this invention are as follows: Ni: 25-38%, Cr: 15-25%, Mo: 6-13.5%, Ti: 0.5-1.5%, Al≤0.5%, C≤0.03%, B≤0.08%, Si+Mn≤0.2%, with the remainder being Co and unavoidable impurities; wherein the mass percentage of Ti and Mo, and Ti and Al elements satisfy the following relationships: 0.01≤Ti / Mo≤0.1, 1%≤Ti+Al≤2%.

[0028] The preparation method and process steps of this cobalt-based alloy are as follows: S1. Vacuum induction melting: Select cobalt, chromium, nickel and other metal raw materials with a purity of ≥99.95%, weigh them accurately according to the composition in Table 1, and cast them into vacuum ingots after vacuum induction melting.

[0029] S2. Demold and anneal the ingot obtained in S1, polish the surface, and obtain electroslag ingots or consumable ingots through electroslag remelting or vacuum consumable melting.

[0030] S3. The ingot obtained in S2 is subjected to high-temperature homogenization treatment. When the furnace temperature is ≥300℃, the ingot is put into the furnace and heated to 750℃ at a rate of 100~150℃ / h and held for 8~12h. Then, the temperature is raised to 1140~1180℃ at a rate of 60~90℃ / h and held for 20~40h.

[0031] S4. Forge the ingot obtained in S3 into a billet and roll it out. The initial forging temperature is 1120-1160℃, the final forging temperature is ≥900℃, the deformation per pass is 25-35%, and the cross-sectional size of the forging billet is 90mm×90mm-120mm×120mm.

[0032] S5. The billet obtained in S4 is hot rolled into a bar with an initial rolling temperature ≥1050℃, a final rolling temperature ≥950℃, a rolling ratio ≥10, and a bar size of φ6~40mm.

[0033] S6. The hot-rolled bars obtained in S5 are subjected to quenching and tempering treatment. This includes cryogenic treatment and aging treatment. The cryogenic treatment temperature is -80 to -120℃, and the holding time is 1 to 4 hours. The aging treatment temperature is 420 to 560℃, and the aging time is 4 to 6 hours.

[0034] Comparative example: The conventional preparation method was used, namely vacuum smelting, electroslag remelting, homogenization heat treatment, forging, and hot rolling, followed by aging and tempering.

[0035] The chemical composition and mass content of the cobalt-based alloys in Examples 1-6 and the comparative examples are shown in Table 1; the parameter control of each production process is shown in Tables 2 and 3; and the mechanical properties characterization of the obtained cobalt-based alloys are shown in Table 4.

[0036] Table 1. Chemical composition (%) of cobalt-based alloys in each embodiment and comparative example

[0037] Table 2 Homogenization process parameters for each embodiment and comparative example

[0038] Table 3. Parameters of forging, hot rolling, and aging and tempering processes in each embodiment and comparative example.

[0039] Table 4. Performance of cobalt-based alloys in various embodiments and comparative examples

[0040] Depend on Figure 1 It can be seen that the cobalt-based alloy matrix of the present invention exhibits a uniform and fine equiaxed crystal structure with clear and complete grain boundaries and no obvious structural defects. This provides a reliable microstructural basis for achieving a high strength and high toughness match in the cobalt-based alloy. Figure 2 It is known that the nanophase in the matrix is ​​the Mo- and Ti-rich precipitates caused by the composition design of this invention, which have a significant precipitation strengthening effect on the alloy.

Claims

1. A high-strength cobalt-based alloy, characterized in that, The chemical composition and mass percentage of the cobalt-based alloy are as follows: Ni: 25-38%, Cr: 15-25%, Mo: 6-13.5%, Ti: 0.5-1.5%, Al≤0.5%, C≤0.03%, B≤0.08%, Si+Mn≤0.2%, with the remainder being Co and unavoidable impurities.

2. The ultra-high strength cobalt-based alloy according to claim 1, characterized in that, The mass percentage content of Ti and Mo, and Ti and Al elements in the cobalt-based alloy satisfies the following relationship: 0.01≤Ti / Mo≤0.1, 1%≤Ti+Al≤2%.

3. The high-strength cobalt-based alloy according to claim 1, characterized in that, The cobalt-based alloy has a diameter of 6–40 mm; at room temperature, it has a tensile strength ≥1900 MPa, a hardness ≥51 HRC, and an elongation ≥12%.

4. A method for preparing a high-strength cobalt-based alloy according to any one of claims 1-3, characterized in that, Includes the following steps: S1, Vacuum induction melting process; S2. Demold and anneal the ingot obtained in S1, polish the surface, and then remelt it by electroslag remelting or vacuum arc remelting. S3. The ingot obtained in S2 is subjected to high-temperature homogenization treatment; S4. Forge the ingot obtained in S3 into a rolled billet; S5. Hot-roll the billet obtained in S4 into a bar. S6. The hot-rolled bar obtained in S5 is subjected to quenching and tempering treatment.

5. The method for preparing the high-strength cobalt-based alloy according to claim 4, characterized in that, In step S3, when the furnace temperature is ≥300℃, the ingot is put into the furnace and heated to 750℃ at a rate of 100-150℃ / h and held for 8-12h. Then, the temperature is raised to 1140-1180℃ at a rate of 60-90℃ / h and held for 20-40h.

6. The method for preparing the high-strength cobalt-based alloy according to claim 4, characterized in that, In step S4, the initial forging temperature is 1120~1160℃, the final forging temperature is ≥900℃, the deformation per pass is 25~35%, and the cross-sectional size of the forging billet is 90mm×90mm~120mm×120mm.

7. The method for preparing the high-strength cobalt-based alloy according to claim 4, characterized in that, In step S5, the initial rolling temperature is ≥1050℃, the final rolling temperature is ≥950℃, the rolling ratio is ≥10, and the bar size is φ6~40mm.

8. The method for preparing the high-strength cobalt-based alloy according to claim 4, characterized in that, Step S6 includes cryogenic treatment and aging treatment. The cryogenic treatment temperature is -80 to -120℃ and the holding time is 1 to 4 hours. The aging treatment temperature is 420 to 560℃ and the aging time is 4 to 6 hours.

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