A process for the thermo-mechanical treatment of a CoCrMo alloy

Abstract

This invention discloses a deformation heat treatment process for CoCrMo alloys, belonging to the field of non-ferrous metal processing technology. The process includes heating the CoCrMo alloy billet to 1200-1250℃ and holding it at that temperature; rolling the held CoCrMo alloy in multiple passes, with the initial and final rolling temperatures for each pass being 1150-1200℃; between adjacent rolling passes, the alloy is returned to a holding furnace at the initial rolling temperature for holding; after rolling, the alloy is water-cooled to room temperature; and the rolled CoCrMo alloy is heated to 450-600℃ for tempering, held at that temperature, and then furnace-cooled to room temperature to obtain the finished product. This invention solves the problems of easy cracking, complex processing, and poor finished product performance of CoCrMo alloys by eliminating the σ phase through homogenization treatment, precise temperature-controlled continuous rolling, water cooling to suppress secondary precipitation of the σ phase, and stress relief and microstructure optimization through tempering. The resulting alloy has no σ phase residue, fine grains, and good strength-toughness matching, making it suitable for processing high-performance CoCrMo alloy products in the medical, aerospace, and petrochemical industries.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal processing technology, specifically relating to a deformation heat treatment process for cobalt-chromium-molybdenum (CoCrMo) alloys to improve the overall mechanical properties of the alloys. Background Technology

[0002] CoCrMo alloy is a difficult-to-deform high-temperature alloy with excellent wear resistance, corrosion resistance, high-temperature strength, and good biocompatibility, and is widely used in high-end manufacturing fields such as aerospace, petrochemicals, and medical devices. However, the high Cr and Mo content of this alloy makes it prone to the precipitation of brittle σ phases during hot working, resulting in a narrow plastic processing window, high deformation resistance, and difficulty in conventional cold working. It is also prone to cracking and uneven microstructure during processing. Currently, the forming of CoCrMo alloy still mainly relies on single hot working processes such as forging and hot rolling, which cannot fundamentally solve the technical problems of brittle phase residue, easy cracking during processing, and poor overall performance of finished products. Summary of the Invention

[0003] In view of the defects or deficiencies of the prior art, the present invention provides a deformation heat treatment process for CoCrMo alloy.

[0004] Therefore, the deformation heat treatment process for CoCrMo alloy provided by this invention includes the following steps: (1) Heat the CoCrMo alloy billet to 1200~1250℃ and hold it for 1~3 hours; (2) The CoCrMo alloy treated in step (1) is subjected to multi-pass rolling. The starting rolling temperature of a single pass is 1150~1200℃, the rolling process temperature is 1150~1200℃, and the final rolling temperature is 1150~1200℃, and the final rolling temperature is not higher than the starting rolling temperature. Between adjacent rolling passes, the alloy is returned to the furnace and held in a holding furnace at the starting rolling temperature for 2~5 minutes. After rolling, the alloy is cooled to room temperature by water. (3) The CoCrMo alloy treated in step (2) is heated to 450~600℃ for tempering, and then cooled to room temperature in the furnace after holding at the temperature to obtain the finished product.

[0005] An optional approach is to use a heating rate of 5~20℃ / min in step (1).

[0006] An optional approach is to have a total reduction of 60% to 80% in the multi-pass rolling in step (2).

[0007] An optional approach is to use the following single-pass rolling process parameters: reduction of 10%~20%, and strain rate of 0.01~0.1s. - ¹.

[0008] An alternative is to water-cool the alloy at a cooling rate of ≥50℃ / s after rolling.

[0009] An optional option is that the rolling method in step (2) is synchronous rolling or asynchronous rolling, and the speed ratio of the asynchronous rolling is 1.05~1.25.

[0010] An optional approach is to use a tempering treatment in step (3) with a holding time of 1 to 4 hours and a heating rate of 3 to 15 °C / min.

[0011] An alternative is to cast CoCrMo alloy billets with a diameter of 100~300mm and a length of 300~600mm.

[0012] An optional configuration is that the chemical composition of the CoCrMo alloy is: Cr 25%~30%, Mo 5%~7%, C 0.1%~0.3%, Co 62.2%~69.3%, and impurity content ≤0.5%.

[0013] An alternative is that the CoCrMo alloy obtained after the above process has no σ phase residue, a yield strength of 820-880 MPa, a tensile strength of 1050-1100 MPa, and an elongation after fracture of 22%-24%.

[0014] Compared with the prior art, the present invention has the following beneficial effects: 1. Eliminating the influence of brittle phases at the source: This invention uses a high-temperature homogenization treatment at 1200~1250℃ to completely dissolve the brittle σ phase in the CoCrMo alloy, thereby eliminating the risk of processing cracking and performance degradation caused by residual brittle phases at the source, and providing a reliable microstructure guarantee for the subsequent high-quality forming of the alloy.

[0015] 2. Achieving efficient continuous rolling of alloys: This invention uses a combination strategy of "precise temperature control during initial rolling and final rolling + short-term reheating between passes" to keep the alloy deformation process within the optimal temperature window, effectively reducing the alloy deformation resistance, avoiding the problem of long-term and multiple reheating due to temperature drop in traditional processes, simplifying the process flow, significantly shortening the production cycle, reducing energy consumption, and achieving quasi-continuous and efficient rolling of CoCrMo alloys.

[0016] 3. Synergistic optimization of alloy comprehensive properties: This invention combines rapid water cooling after rolling with subsequent tempering treatment. Rapid water cooling effectively suppresses the secondary precipitation of harmful σ phases and preserves the fine-grained structure of the alloy. Subsequent tempering treatment effectively eliminates the internal stress of the alloy, and by controlling the precipitated phases, a good match between the material's strength, plasticity and toughness is achieved, ultimately obtaining a CoCrMo alloy product with fine grains, uniform structure, and excellent mechanical properties and dimensional stability.

[0017] The deformation heat treatment process for continuous rolling of CoCrMo alloys provided by this invention is simple in process and has a wide parameter control window, making it compatible with conventional industrial heat treatment equipment and rolling mills. This process effectively solves the technical bottlenecks of high processing difficulty and poor finished product performance of CoCrMo alloys by synergistically optimizing homogenization, rolling, and heat treatment steps, achieving efficient and low-cost processing and forming of the alloy. The resulting product has a fine microstructure and excellent properties, fully meeting the stringent requirements of high-performance CoCrMo alloy materials in high-end fields such as medical devices, aerospace, and petrochemicals, and has promising industrial application prospects. Attached Figure Description

[0018] Figure 1 SEM microstructure of CoCrMo alloy billet.

[0019] Figure 2 The image shows the EBSD microstructure of the CoCrMo alloy prepared in Example 1.

[0020] Figure 3 The image shows the EBSD microstructure of the CoCrMo alloy prepared in Example 2.

[0021] Figure 4 The image shows the EBSD microstructure of the CoCrMo alloy prepared in Example 3.

[0022] Figure 5 The surface defect diagram (Om) of the CoCrMo alloy prepared in Comparative Example 1 is shown.

[0023] Figure 6 The image shows the SEM microstructure of the CoCrMo alloy prepared in Comparative Example 2.

[0024] Figure 7 The image shows the EBSD microstructure of the CoCrMo alloy prepared in Comparative Example 3. Detailed Implementation

[0025] Unless otherwise specified, the scientific and technical terms used in this article are intended for understanding by those skilled in the art.

[0026] The heat treatment process of this invention, through the process concept of "high-temperature homogenization dissolution + precise temperature-controlled continuous rolling + rapid cooling to suppress precipitation + tempering to eliminate stress and optimize microstructure," solves the problems of brittle σ-phase residue, easy deformation and cracking, complex process flow, and poor overall performance of finished products in traditional hot working of CoCrMo alloys. It achieves high-efficiency and low-cost processing of the alloy, while obtaining high-performance CoCrMo alloy materials with fine grains, uniform microstructure, and good strength-toughness matching. Specifically: Homogenization treatment fully dissolves the brittle σ phase formed by compositional segregation in cast or forged alloys, resulting in a homogeneous supersaturated solid solution microstructure, which lays a good microstructural foundation for subsequent plastic deformation. The homogenization treatment equipment can be either a box-type resistance furnace or a muffle furnace, adaptable to different industrial production conditions.

[0027] The homogenized CoCrMo alloy billet is immediately subjected to continuous hot rolling, with the initial rolling temperature controlled at 1150~1200℃ to ensure deformation of the alloy in the high-temperature γ-phase region. The final rolling temperature is not higher than the initial rolling temperature and is controlled at 1150~1200℃ to prevent σ-phase precipitation during deformation. Between adjacent rolling passes, the alloy is returned to the set rolling temperature and held for 2~5 minutes. This short-time return-to-heat holding measure effectively compensates for the temperature drop during rolling, keeping the alloy within the ideal deformation temperature range, significantly reducing deformation resistance, and achieving a continuous and stable rolling process. After rolling deformation is completed, the alloy is immediately subjected to rapid water cooling to freeze the fine-grained structure after high-temperature deformation, while effectively suppressing secondary precipitation of the σ-phase during cooling. Preferably, asynchronous rolling is used in this step. When using asynchronous rolling, the speed ratio is controlled at 1.05~1.25, which can introduce additional shear deformation in the deformation zone, further promoting grain refinement and microstructure uniformity.

[0028] After rolling and water cooling, the CoCrMo alloy is placed back into the heat treatment furnace for tempering. This aims to eliminate the internal stress caused by rapid cooling and severe plastic deformation. At the same time, by controlling the tempering temperature and time, the alloy microstructure is fine-tuned to optimize the match between strength and toughness, and finally obtain a CoCrMo alloy product with excellent comprehensive performance.

[0029] The CoCrMo alloy billet described in this invention is a CoCrMo alloy casting or forging. Its initial microstructure generally exhibits severe dendritic segregation, with brittle σ phases and eutectic carbides precipitating at grain boundaries. Macroscopically, it exhibits extremely high resistance to high-temperature deformation and is highly susceptible to crack initiation during conventional hot working. The billets used in the following examples are as-cast alloy billets prepared using vacuum induction melting (VIM). The invention will be further described in detail below with reference to specific embodiments and comparative examples. The scope of protection of this invention is not limited to the following embodiments.

[0030] The heat treatment equipment used in the examples and comparative examples is a box-type resistance furnace, the rolling equipment is a two-roll hot rolling mill, the water cooling rate is controlled by adjusting the water cooling nozzle flow rate, and the asynchronous rolling speed ratio is achieved by adjusting the speed of the upper and lower rolls.

[0031] The specific testing methods for each embodiment and comparative example of this invention are uniformly as follows: surface defects are observed and analyzed using an optical microscope (Om); microstructure is observed using a scanning electron microscope (SEM) to confirm whether there is any brittle σ phase residue; and statistical analysis of grain orientation and average grain size is performed using electron backscatter diffraction (EBSD) technology. The room temperature mechanical property test is strictly performed in accordance with the national standard GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Room temperature test method", and the yield strength, tensile strength, and elongation after fracture of the alloy are measured on an electronic universal testing machine.

[0032] Example 1: The deformation heat treatment process for continuous rolling of CoCrMo alloy in this embodiment includes the following specific steps: (1) Homogenization treatment: The CoCrMo alloy billet (Cr 25%, Mo 5%, C 0.2%, Co 69.3%, the remainder being impurities; the initial size of the billet is φ150mm×300mm) is placed in a box-type resistance furnace and heated to 1200℃ at a heating rate of 5℃ / min. It is then held at 1200℃ for 1h to allow the brittle σ phase in the alloy to dissolve completely. (2) Temperature-controlled continuous rolling: Conventional synchronous rolling method is adopted, with an initial rolling temperature of 1150℃, a rolling process temperature controlled at 1150℃, and a final rolling temperature of 1150℃; the average reduction per pass is 10.9%, the total reduction is 60%, and the strain rate is 0.01s. -1 In this embodiment, the alloy is rolled in 8 passes; between adjacent passes, the alloy is returned to the furnace and held at a temperature of 1150°C for 5 minutes; after rolling, the flow rate of water at 20°C is adjusted by a water-cooling spray device to cool the material at a rate of 50°C / s until room temperature, thereby suppressing the secondary precipitation of the σ phase. (3) Tempering treatment: The water-cooled alloy is placed in a box-type resistance furnace and heated to 450°C at a heating rate of 3°C / min. After holding at 450°C for 4 hours, it is cooled to room temperature with the furnace to obtain the CoCrMo alloy product.

[0033] The results showed that the alloy prepared in this example had no residual σ phase, a grain size of 15~20μm, and a uniform and dense structure. The yield strength at room temperature was 820MPa, the tensile strength was 1050MPa, the elongation after fracture was 22%, the rolling process was stable and defect-free, and the finished product had no problems such as cracking or uneven deformation.

[0034] Example 2: The specific steps of the deformation heat treatment process for the CoCrMo alloy in this embodiment are as follows: (1) Homogenization treatment: The CoCrMo alloy billet (Cr 28%, Mo 6%, C 0.1%, Co 65.6%, the remainder being impurities; billet size φ200mm×400mm) was placed in a box-type resistance furnace and heated to 1225℃ at a heating rate of 12℃ / min. It was then held at 1225℃ for 2 hours to allow the brittle σ phase in the alloy to dissolve completely. (2) Temperature-controlled continuous rolling: Asynchronous rolling is adopted with a speed ratio of 1.15. The initial rolling temperature is 1175℃, the rolling process temperature is controlled at 1160-1176℃, and the final rolling temperature is 1160℃. The average reduction per pass is 15.6%, the total reduction is 70%, and the strain rate is 0.05s. -1 In this embodiment, the alloy is rolled in 7 passes; between adjacent passes, the alloy is returned to the furnace and held at a temperature of 1175°C for 3 minutes; after rolling, the flow rate of 20°C water is adjusted by a water-cooling spray device to cool the material at a rate of 60°C / s until room temperature, thereby suppressing the secondary precipitation of the σ phase. (3) Tempering treatment: The water-cooled alloy is placed in a box-type resistance furnace and heated to 525°C at a heating rate of 9°C / min. After holding at 525°C for 2.5 hours, it is cooled to room temperature with the furnace to obtain the CoCrMo alloy product.

[0035] The microstructure and mechanical properties of the finished product obtained in this embodiment were observed and tested. The results showed that there was no σ phase residue in the alloy microstructure, the grain size was 12~18μm, and the microstructure was uniform and dense. The yield strength at room temperature was 850MPa, the tensile strength was 1080MPa, the elongation after fracture was 24%, and the rolling process was stable and defect-free.

[0036] Example 3: This embodiment provides a specific step-by-step process for achieving deformation heat treatment of CoCrMo alloy: (1) Homogenization treatment: The CoCrMo alloy billet (Cr 30%, Mo 7%, C 0.3%, Co 62.2%, the remainder being impurities; billet size φ280mm×500mm) was placed in a box-type resistance furnace and heated to 1250℃ at a heating rate of 20℃ / min. It was then held at 1250℃ for 3 hours to allow the brittle σ phase in the alloy to dissolve completely. (2) Temperature-controlled continuous rolling: Asynchronous rolling is adopted with a speed ratio of 1.25. The initial rolling temperature is 1200℃, the rolling process temperature is controlled at 1180-1200℃, and the final rolling temperature is 1180℃. The average reduction per pass is 18.2%, the total reduction is 80%, and the strain rate is 0.1s. -1 In this embodiment, the alloy is rolled in 8 passes; between adjacent passes, the alloy is returned to the furnace and held at a temperature of 1200°C for 2 minutes; after rolling, the flow rate of 20°C water is adjusted by a water-cooling spray device to cool the material at a rate of 70°C / s until room temperature, thereby suppressing the secondary precipitation of the σ phase. (3) Tempering treatment: The water-cooled alloy is placed in a box-type resistance furnace and heated to 600°C at a heating rate of 15°C / min. After holding at 600°C for 1 hour, it is cooled to room temperature with the furnace to obtain the CoCrMo alloy product.

[0037] The microstructure and mechanical properties of the finished product obtained in this embodiment were observed and tested. The results showed that there was no σ phase residue in the alloy microstructure, the grain size was 8~12μm, and the microstructure was uniform and dense. The yield strength at room temperature was 880MPa, the tensile strength was 1100MPa, the elongation after fracture was 23%, and the rolling process was stable and defect-free.

[0038] Comparative Example 1: The homogenization process was omitted in this comparative example, but the remaining process steps and parameters were completely consistent with those in Example 1.

[0039] Comparative Example 2: In this comparative example, the reheating process between each pass in step (2) is eliminated, and the remaining process steps and parameters are completely consistent with those in Example 1.

[0040] Comparative Example 3: In this comparative example, step (2) uses air cooling instead of water cooling after rolling. That is, after rolling in step (2), natural air cooling is used for cooling. The remaining process steps and parameters are completely consistent with those in Example 1.

[0041] The microstructure and mechanical properties of the products prepared in the above embodiments and comparative examples were tested and analyzed. Specific results are shown in [the table below]. Figure 1-7 And Table 1.

[0042] Table 1 Comparison of process and product performance between the examples and comparative examples.

[0043] Combining microstructure evolution and macroscopic mechanical properties, the toughening mechanism of the deformation heat treatment process of this invention and comparative analysis are as follows: Figure 1 The results showed that the untreated CoCrMo alloy billet had severe dendritic segregation and a large amount of network brittle σ phase precipitates at the grain boundaries. Corresponding to the data in Table 1, this extremely poor inherent microstructure resulted in the material exhibiting extremely low room temperature yield strength (450 MPa) and extremely poor plasticity (elongation after fracture of only 4%).

[0044] Figures 2 to 4The results show that after the complete process of this invention, the alloy underwent sufficient dynamic recrystallization, and the original coarse dendrites were completely broken down and transformed into fine, uniform equiaxed crystals of 8-20 μm, with no σ phase residue. This reconstruction of the microstructure directly led to a leapfrog qualitative change in mechanical properties: the yield strength doubled to 820-880 MPa, and the elongation after fracture jumped to 22-24%, breaking the problem of the alloy's inability to simultaneously achieve high strength and high plasticity.

[0045] Figure 5 According to the results in Table 1, in Comparative Example 1, due to the cancellation of high-temperature homogenization, the original segregated phase induced severe stress concentration in the early stage of deformation, resulting in macroscopic through cracks on the surface. The material completely lost its processing capability and could not be effectively tested mechanically.

[0046] Figure 6 The results showed that in Comparative Example 2, the elimination of inter-pass heat preservation led to uncontrolled deformation temperature, resulting in a large number of elongated, non-recrystallized deformed grains with extremely high internal residual stress. This structural defect is reflected in Table 1, causing grain coarsening to 22-35 μm, a significant decrease in yield strength to 690 MPa, and a dramatic increase in processing resistance.

[0047] like Figure 7 The results showed that Comparative Example 3, which used air cooling instead of rapid water cooling, resulted in a very slow cooling rate that caused severe secondary precipitation of the σ phase at the grain boundaries of the solid solution elements. Table 1 shows that these brittle precipitates severely disrupted the continuity of the matrix, leading to a precipitous drop in elongation after fracture (down to 12%) and a significant deterioration in the toughness of the material.

[0048] As can be seen from the comparison results of the above embodiments and comparative examples, the high-temperature homogenization treatment, short-time heat preservation between passes, and rapid water cooling after rolling are the core process steps for achieving continuous and stable rolling of CoCrMo alloy and obtaining high-performance finished products. The absence of any step will lead to defects in the rolling process or a significant deterioration in the performance of the finished product.

[0049] This invention, through the synergistic combination of the above processes, fundamentally solves the problems of brittle phase residue, easy cracking, and high deformation resistance in the hot working of CoCrMo alloys, and realizes efficient continuous rolling of the alloy. The resulting finished product has a uniform structure, fine grains, and good strength and toughness matching, which can fully meet the application requirements of high-end manufacturing fields in the medical industry.

Claims

1. A deformation heat treatment process for a CoCrMo alloy, characterized in that, Includes the following steps: (1) Heat the CoCrMo alloy billet to 1200~1250℃ and hold it for 1~3 hours; (2) The CoCrMo alloy treated in step (1) is subjected to multi-pass rolling. The starting rolling temperature of a single pass is 1150~1200℃, the rolling process temperature is 1150~1200℃, the final rolling temperature is 1150~1200℃, and the final rolling temperature is not higher than the starting rolling temperature. Between adjacent rolling passes, the alloy is returned to the furnace and held in a holding furnace at the starting rolling temperature for 2~5 minutes. After rolling, the alloy is cooled to room temperature by water. (3) The CoCrMo alloy treated in step (2) is heated to 450~600℃ for tempering, and then cooled to room temperature in the furnace after holding at the temperature to obtain the finished product.

2. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, The heating rate in step (1) is 5~20℃ / min.

3. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, The total reduction in the multi-pass rolling in step (2) is 60%~80%.

4. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, The single-pass rolling process parameters are: reduction 10%~20%, strain rate 0.01~0.1s. - ¹.

5. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, After rolling, the alloy is water-cooled at a cooling rate of ≥50℃ / s.

6. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, The rolling method in step (2) is synchronous rolling or asynchronous rolling, and the speed ratio of asynchronous rolling is 1.05~1.

25.

7. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, The holding time for tempering in step (3) is 1~4h, and the heating rate is 3~15℃ / min.

8. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, CoCrMo alloy ingots with a diameter of 100~300mm and a length of 300~600mm.

9. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, The chemical composition of the CoCrMo alloy is: Cr 25%~30%, Mo 5%~7%, C 0.1%~0.3%, Co 62.2%~69.3%, and impurity content ≤0.5%.

10. The deformation heat treatment process for the CoCrMo alloy according to claim 1, characterized in that, After the aforementioned process, the CoCrMo alloy has no residual σ phase, a yield strength of 820–880 MPa, a tensile strength of 1050–1100 MPa, and an elongation after fracture of 22%–24%.

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