Cobalt-based Alloy 21
CO 21 is a cobalt-chromium-molybdenum (CoCrMo)-based wear-resistant and corrosion-resistant superalloy, which monopolizes the field of extreme working conditions by virtue of “high wear resistance × full environmental corrosion resistance× high temperature strength and toughness”. The following is a systematic analysis from six dimensions:
I. Standards and brand system
1. Implementing standards
- Aviation/high-end manufacturing: AMS 5898 (aviation material specification, defining chemical/mechanical properties of forgings and bars), SAE J467 (automotive engineering standard, specifying technical requirements for wear-resistant parts);
- General reference: auxiliary reference is ASTM B438 (general specification for cobalt-based alloy bars, testing composition and properties), but the core manufacturing standard is AMS/SAE.
II. Chemical composition (mass fraction%, typical range)
CO 21 is designed by “Mo strengthening corrosion resistance+W/C forming hard phase”, and the synergistic effect of elements is remarkable:
| element | Content range | Core role |
|---|---|---|
| Co | Allowance (~60%) | Stabilize the matrix at high temperature to provide toughness and corrosion resistance. |
| Cr | 12~14 | Cr₂O₃ oxide film is formed, which is resistant to high temperature oxidation and acid/alkali/seawater corrosion; Generate Cr₇C₃ carbide reinforced matrix |
| Mo | 28~32 | Improve the corrosion resistance of reducing medium (such as sulfuric acid and hydrochloric acid) and inhibit pitting/crevice corrosion. |
| W | 3~5 | WC/WC hard phase (hardness HV1500+) is precipitated to enhance wear resistance. |
| C | 0.3~0.5 | Carbide is formed with Cr/W to regulate the balance between hardness and toughness (C↑→ hardness ↑, plasticity ↓) |
| Si | 1.5~3.0 | Deoxidation+optimization of casting fluidity |
| Fe/Ni | ≤2.0 | Impurity elements, exceeding the limit to reduce corrosion resistance (strict control) |
III. Mechanical properties (typical value, cast/heat-treated state)
CO 21 properties crush conventional alloys, focusing on the core advantages of “wear resistance+high temperature+corrosion resistance”;
| Performance index | typical value | Contrast 316L stainless steel. |
|---|---|---|
| density | 8.42 g/cm³ | 15% higher (316l ≈ 7.93g/cm) |
| melting point | 1330℃ | 15% higher (316l ≈ 1370℃? No, the melting point of 316L is 1370-1398℃, where the melting point of Stellite 21 is lower. Correction: the melting point of Stellite 21 is 1330℃, and 316L≈1370℃, so the melting point of Stellite 21 is slightly lower, but the high-temperature strength is better). |
| tensile strength | 1200 MPa | 100% higher （316L≈600 MPa） |
| yield strength | 780 MPa | 150% higher （316L≈310 MPa） |
| Elongation （δ₅） | 8~10% | 75% lower (316l ≈ 40%) |
| hardness | 45~53 HRC | 125% higher （316L≈20 HRC） |
| heat resistance | Long-term tolerance to 900℃ | 125% higher （316L 长期≤400℃） |
IV. Requirements for Heat Treatment and Processing
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Heat treatment (optimizing structure and releasing property):
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Solution+aging:
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Solid solution: heat preservation at 1200~1240℃ and air cooling (dissolving carbide and homogenizing structure);
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Aging: quenching at 700~1150℃+aging (precipitation of fine carbide to improve strength and wear resistance);
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Tempering strengthening: preheating at 870~980℃ → hardening at 1100 ~ 1175℃ → tempering at 650 ~ 750℃ (balance strength and toughness, improve crack resistance).
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Machining characteristics (high hardness challenge):
- Casting: lost wax casting is preferred (precision molding of valve seats, nozzles and other complex parts);
- Machining: cubic boron nitride (CBN)/ diamond tool, low speed+high pressure cooling (to avoid tool wear, the surface roughness should be Ra ≤ 1.6μ m);
- Welding: TIG/MIG welding (ERCoCr welding wire), preheating at 300~400℃ before welding (cobalt-based alloy has poor thermal conductivity to prevent hot cracking), and annealing at 750~850℃ after welding (restoring corrosion resistance).
V. Main application fields (“ultimate solution” for extreme working conditions)
With the advantage of “life× performance” (the cost is 3 times that of stainless steel and the life is 10 times), CO 21 monopolizes high-value scenarios:
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Chemical industry and energy:
- Valves/seals: valve seat of blowout preventer for oil drilling, plunger of chemical pump (resistant to high-pressure mud+hydrochloric acid/sulfuric acid corrosion, with service life exceeding 3 times that of tungsten carbide);
- High-temperature components: geothermal well pipes, gas turbine combustion chamber liners (resistant to 900℃ high temperature+corrosion, replacing nickel-based alloy to reduce cost by 40%).
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Aerospace:
- Engine: turbine blade edge (anti-800℃ gas erosion, wear life is twice that of titanium alloy), landing gear hinge bushing (high impact+wear resistance);
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Medical field:
- Implants: artificial hip joint ball head (CoCrMo has excellent biocompatibility, its service life is over 20 years, and it can reduce the cost by 50% instead of ceramics) and dental implants (body fluid corrosion resistance);
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Moulds and tools:
- Hot working dies: hot forging dies and hot extrusion dies (thermal shock resistance+wear resistance, life 5 times of ultra-high manganese steel);
- Cutting tools: tool tips made of hard-to-machine materials (dry cutting and wear resistance, life is 3 times longer than that of cemented carbide).
Key summary
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Core advantages:
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Breakthrough in corrosion resistance: High Mo content endows reducing medium (sulfuric acid and hydrochloric acid) with corrosion resistance, which exceeds Stellite 6(CoCrW system, more suitable for oxidizing environment);
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Performance balance: hardness 45-53 HRC (wear resistance)+elongation 8-10% (crack resistance), suitable for impact conditions;
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Biocompatibility: CoCrMo system has passed medical certification and can be implanted into human body for a long time.
Limited challenges:
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Cost-sensitive (cobalt price accounts for 70%, and the price fluctuates greatly);
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Complex processing (high hardness requires special technology, and the manufacturing cost increases by 50%).
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CO 21 is the “performance benchmark” of Co-Cr-Mo-based alloys. By replacing W with Mo, the double breakthrough of “reducing medium corrosion resistance and high-temperature wear resistance” has been achieved, and it has become an irreplaceable core material in chemical corrosion, medical implantation and high-temperature impact scenarios.