Stellite 21
Introduction to Stellite 21
Stellite 21 is a wrought cobalt-based alloy known for its exceptional toughness, corrosion resistance, and high-temperature mechanical stability. Unlike other Stellite grades prioritizing hardness and abrasion resistance, Stellite 21 is engineered for applications requiring wear resistance, superior impact, and thermal shock performance. Its balanced composition of cobalt, chromium, nickel, and molybdenum provides excellent metallurgical stability and resistance to galling, making it suitable for sliding contact and mechanical shock.
Stellite 21 is often produced via forging or casting, then precision-finished using advanced CNC machining techniques. It is commonly used in aerospace, nuclear, petrochemical, and medical industries for components like valve seats, engine sleeves, joint implants, and turbine hardware—particularly where dimensional stability and low-friction metal-to-metal contact are essential.
Chemical, Physical, and Mechanical Properties of Stellite 21
Stellite 21 (UNS R30021 / AMS 5385 / ISO 5832-3) is a corrosion- and wear-resistant cobalt-chromium-molybdenum alloy with enhanced impact strength and thermal fatigue resistance. It is available in wrought, cast, and powder metallurgy forms.
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Cobalt (Co) | Balance (≥60.0) | Base matrix for corrosion, toughness, and thermal strength |
Chromium (Cr) | 26.0–30.0 | Enhances oxidation resistance and forms a passive protective layer |
Molybdenum (Mo) | 5.0–6.0 | Increases pitting and crevice corrosion resistance |
Nickel (Ni) | 2.0–4.0 | Improves toughness and weldability |
Carbon (C) | 0.20–0.30 | Forms carbides for wear resistance while maintaining ductility |
Iron (Fe) | ≤3.0 | Residual element |
Silicon (Si) | ≤1.0 | Improves surface finish and flow during casting |
Manganese (Mn) | ≤1.0 | Refines grain structure and improves hot workability |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.33 g/cm³ | ASTM B311 |
Melting Range | 1385–1435°C | ASTM E1268 |
Thermal Conductivity | 14.0 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 0.98 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.4 µm/m·°C (20–400°C) | ASTM E228 |
Specific Heat Capacity | 430 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 210 GPa at 20°C | ASTM E111 |
Mechanical Properties (Wrought or HIP + Heat Treated)
Property | Value (Typical) | Test Standard |
|---|---|---|
Hardness | 30–35 HRC (annealed) / 35–42 HRC (aged) | ASTM E18 |
Tensile Strength | 900–1100 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 400–600 MPa | ASTM E8/E8M |
Elongation | 8–15% | ASTM E8/E8M |
Impact Toughness | ≥80 J (Charpy V-notch at RT) | ASTM E23 |
Galling Resistance | Excellent | ASTM G98 |
Key Characteristics of Stellite 21
High Toughness and Crack Resistance: Superior to other Stellite grades due to lower carbon content, enabling resistance to thermal fatigue, impact loading, and mechanical shocks.
Corrosion Resistance in Chloride and Acidic Media: Excellent pitting and crevice corrosion resistance due to molybdenum and chromium synergy.
Good Galling Resistance: Performs well in metal-to-metal contact applications under pressure without lubrication.
Dimensional Stability at High Temperature: Operates reliably up to 850°C in cyclic thermal environments, with minimal distortion or degradation.
CNC Machining Challenges and Solutions for Stellite 21
Machining Challenges
Moderate Carbide Content
While less abrasive than Stellite 6 or 12, tool wear remains an issue due to distributed carbides and the alloy’s toughness.
Low Thermal Conductivity
Causes localized heating and risk of work hardening, especially in finishing operations or during long tool engagements.
Springback and Tool Deflection
High modulus and strength can lead to tool chatter and dimensional inaccuracies during roughing and profile cuts.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | PVD-coated carbide (K20–K30) or CBN for finishing | Balances wear resistance and toughness |
Coating | TiSiN or AlCrN (3–5 µm) | Reduces tool temperature and friction |
Geometry | Neutral to slightly negative rake, honed cutting edge (0.03 mm) | Enhances edge integrity and surface finish |
Cutting Parameters (ISO 3685 Compliant)
Operation | Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 12–18 | 0.15–0.25 | 1.5–2.5 | 100–120 |
Finishing | 20–28 | 0.05–0.10 | 0.5–1.0 | 120–150 |
Surface Treatment for Machined Stellite 21 Parts
Hot Isostatic Pressing (HIP)
HIP improves fatigue life and removes internal porosity, especially important for cast or 3D-printed parts under dynamic loading.
Heat Treatment
Heat Treatment stabilizes grain structure and enhances hardness while relieving residual stress from machining.
Superalloy Welding
Superalloy Welding using matching filler (Stellite 21 rod) ensures high joint integrity without compromising corrosion or wear resistance.
Thermal Barrier Coating (TBC)
TBC Coating extends part life in applications exposed to combustion or hot gases above 800°C.
Electrical Discharge Machining (EDM)
EDM achieves complex geometries with Ra <0.5 µm while avoiding mechanical stress during finishing.
Deep Hole Drilling
Deep Hole Drilling ensures accuracy in wear-resistant bores, seal cavities, and lubrication channels.
Material Testing and Analysis
Material Testing includes tensile testing, hardness verification, metallography, and NDT (ultrasonic, dye penetrant, X-ray).
Industry Applications of Stellite 21 Components
Aerospace & Power Generation
Valve guides, bushings, and high-load sliding wear components for turbine engines and hot section assemblies.
Nuclear & Petrochemical
Reactor valve seats, pump shafts, and control hardware resistant to high-pressure corrosion and radiation exposure.
Medical & Orthopedic Devices
Hip and knee joint components due to excellent biocompatibility and mechanical durability under dynamic loading.
General Industry
Tools, wear pads, and bearing sleeves used in reciprocating or oscillating wear conditions under pressure.
FAQs
What machining strategies help minimize tool wear and deformation in Stellite 21 components?
How does Stellite 21 perform in corrosive and high-impact operating environments?
Can Stellite 21 be used in medical applications requiring biocompatibility?
What heat treatment and post-processing steps enhance Stellite 21's performance after CNC machining?
What dimensional tolerances and surface finishes can be achieved with Stellite 21 using EDM?
Explore Related Blogs
