Stellite 21 Cobalt Bar
Product Introduction
Stellite 21 (Alloy 21, UNS R30021, AMS 5385, ISO 5832-3) is a cobalt-chromium-molybdenum alloy with an extremely low carbon content (≤0.25%), making it the toughest grade in the Stellite family.
To briefly compare Stellite 6 and 21: Stellite 6 has a carbon content of around 1%; while it is very hard, it is also brittle and prone to cracking under impact or thermal cycling. Alloy 21, with a carbon content of only 0.25%, sacrifices a small amount of wear resistance to achieve toughness and thermal shock resistance far superior to that of Stellite 6. Stellite 21 is the industry-standard choice for nuclear power valve sealing surfaces, which must withstand frequent thermal cycling and the mechanical shocks of opening and closing; it is also the material selected for artificial joints, which must endure decades of repeated friction within the human body. The fact that a single material can be used in both nuclear reactors and the human body demonstrates that its reliability and biocompatibility have withstood the most rigorous validation.
Lork Group offers the following material selection recommendations: choose Alloy 21 for operating conditions involving impact, thermal cycling, or thermal fatigue; choose Stellite 6 for applications involving pure abrasive wear and no impact where extreme hardness is required.
Machining and Welding
Stellite 21 is slightly less difficult to machine than Stellite 6, though it remains a challenging material. Just keep the following points in mind.
1. Heat Treatment-Solution treatment + aging; properties are adjustable.
It offers flexibility in heat treatment. After solution treatment, the material is relatively soft (HRC 28–35), making it suitable for machining. Aging increases the hardness to HRC 35–42 and boosts strength, though toughness decreases slightly. The choice depends on the operating conditions: prioritize less aging for impact resistance, or more aging for wear resistance.
2. Machining-Easy to machine in the solution-treated state; hard on cutting tools in the aged state.
3. Forging-Forgeable, but with a narrow temperature window.
4. Welding/Hardfacing-Preheating is critical.
When welding Alloy 21, preheating to at least 200°C is essential; welding without preheating makes the material prone to cracking. After welding, the material must be cooled slowly rather than left to cool naturally. This requirement is stricter than that for Stellite 6 because the molybdenum content in Stellite 21 makes it more sensitive to welding heat input.
For more information, please contact:
Email: susan@lorkgroup.com
WhatsApp: +86 199 3707 5488
Chemical Composition
(UNS R30021 / Stellite 21 Typical Values)
| element | Content Range |
| Ni | 2.0-4.0% |
| Mo | 4.5-6.0% |
| Cr | 26-30% |
| Co | 60-65% |
| Fe | ≤3.0% |
| C | 0.20-0.30% |
| Mn | ≤1.0% |
Mechanical Properties
(Typical values )
| Material | Annealed state |
| Tensile Strength | 700-900 MPa |
| Yield Strength (0.2%) | 400-550 MPa |
| Elongation | 8-15% |
| Material | Age-hardened state |
| Tensile Strength | 900-1100 MPa |
| Yield Strength (0.2%) | 550-650 MPa |
| Elongation | 8-12% |
High-temperature strength retention: Stellite 21 maintains excellent high-temperature strength at temperatures up to 816°C. Its cobalt matrix is resistant to phase transformation at high temperatures and exhibits good oxidation resistance.
Typical Applications
Stellite 21's typical applications span a range of high-end sectors:
- Nuclear Power: Valve seats and plugs for main steam isolation valves and pressurizer safety valves-the preferred choice for environments involving thermal shock and radiation.
- Aerospace: Turbine engine seal rings, blade tips, and guide vanes-wear-resistant components for high-temperature service (up to 850°C).
- Medical Devices: Wear-resistant contact surfaces for artificial hip and knee joints, and dental restorations-biocompatibility validated by decades of clinical use.
- Petrochemicals: High-pressure pump plungers, sleeves, valve internals, and agitator bushings-meeting dual requirements for corrosion and wear resistance.
- Hot-Work Tooling: Wear-resistant inserts for die-casting and glass molds-a "lifesaver" in environments prone to thermal fatigue.
Technical Consultation Before Quotation
We have compiled a summary of key technical parameters regarding Stellite 21-points frequently discussed during past inquiries and procurement orders to facilitate rapid quotation.
Dimensions: Diameter (typically Φ10–200 mm), length
Form: Forged rod (dense microstructure, superior properties) vs. cast rod (flexible shape)
Condition: Solution-treated (soft; customer performs subsequent aging) vs. aged (hardened finished product)
Surface finish: Black (as-forged/cast), turned, or ground
Certification requirements: AMS 5385 certification required? Intended for nuclear or medical applications?
Guidance on condition selection:
For machining complex shapes → Solution-treated (soft and easy to machine; age after machining)
For direct use as finished parts → Aged (hard; already possesses design-specified properties)
For customers without heat treatment capabilities, → Must purchase aged finished products
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