Stellite 6
Introduction to Stellite 6
Stellite 6 is the most widely used cobalt-chromium alloy in the Stellite family due to its balanced combination of wear resistance, corrosion resistance, and moderate hardness. It performs excellently in metal-to-metal contact, low thermal conductivity environments, and corrosive fluids. Known for its galling resistance and stability at high temperatures, Stellite 6 is particularly well-suited for valve components, pump parts, and bearings that must endure continuous friction and corrosive attack.
This alloy is ideal for CNC-machined parts requiring long service life and dimensional precision in mechanical assemblies exposed to sliding wear and chemical exposure. Its adaptability, machinability (relative to harder Stellite grades), and performance consistency have made it a preferred material in aerospace, nuclear, petrochemical, and marine industries.
Chemical, Physical, and Mechanical Properties of Stellite 6
Stellite 6 (UNS R30006 / AMS 5387 / ISO 5832-4) is a cobalt-based alloy typically produced via casting, welding overlays, or powder metallurgy before CNC finishing.
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Cobalt (Co) | Balance (≥55.0) | Matrix phase providing hot hardness and corrosion resistance |
Chromium (Cr) | 27.0–32.0 | Oxidation resistance and passivation in corrosive environments |
Tungsten (W) | 4.5–6.5 | Carbide reinforcement for wear resistance |
Carbon (C) | 1.0–1.4 | Moderate carbide content for anti-galling behavior |
Nickel (Ni) | ≤3.0 | Increases toughness and ductility |
Iron (Fe) | ≤3.0 | Residual alloying element |
Silicon (Si) | ≤1.2 | Enhances fluidity and surface finish during casting |
Manganese (Mn) | ≤1.0 | Improves hot workability |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.65 g/cm³ | ASTM B311 |
Melting Range | 1240–1345°C | ASTM E1268 |
Thermal Conductivity | 12.5 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 0.96 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 12.7 µm/m·°C (20–400°C) | ASTM E228 |
Specific Heat Capacity | 415 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 210 GPa at 20°C | ASTM E111 |
Mechanical Properties (As-Cast or HIP + Heat Treated)
Property | Value (Typical) | Test Standard |
|---|---|---|
Hardness | 38–44 HRC (as-cast) / up to 46 HRC (HIP treated) | ASTM E18 |
Tensile Strength | 800–1000 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 500–600 MPa | ASTM E8/E8M |
Elongation | 3–5% | ASTM E8/E8M |
Wear Resistance Index | >2× 316 stainless steel | ASTM G65 |
Key Characteristics of Stellite 6
Outstanding Galling and Sliding Wear Resistance: Ideal for surfaces that experience continuous friction, even without lubrication.
Excellent Corrosion Resistance: Suitable for service in acidic, saline, and oxidizing media, including seawater and process chemicals.
Thermal Stability: Performs reliably at temperatures up to 800°C, retaining hardness and dimensional tolerance.
Machinability Advantage: Easier to CNC machine than harder grades like Stellite 3 or Stellite 12, while still offering excellent wear performance.
CNC Machining Challenges and Solutions for Stellite 6
Machining Challenges
Abrasive Carbide Phases
Tungsten carbides can dull tools quickly, especially under high-speed machining or improper tool coatings.
Built-Up Edge Formation
Flooding or material adhesion can occur without proper coolant and edge geometry, impacting tolerance and finish.
Moderate Work Hardening
Repeated tool passes may increase surface hardness, making finishing passes more difficult if not properly sequenced.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | Carbide (K30–K40), PVD-coated; CBN for finishing | Withstands abrasive carbides while providing edge retention |
Coating | AlTiN or TiAlCrN (3–5 µm) | Reduces thermal wear and friction |
Geometry | Neutral rake (0° to +5°), honed edge radius 0.02–0.05 mm | Minimizes cutting forces and tool chipping |
Cutting Parameters (ISO 3685)
Operation | Speed (m/min) | Feed (mm/rev) | DOC (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 12–18 | 0.20–0.30 | 2.0–3.0 | 100–120 |
Finishing | 22–30 | 0.05–0.10 | 0.5–1.0 | 120–150 |
Surface Treatment for Machined Stellite 6 Parts
Hot Isostatic Pressing (HIP)
HIP at 1150°C and 150 MPa eliminates porosity and enhances microstructural homogeneity, improving both fatigue and wear performance.
Heat Treatment
Heat Treatment can be used to relieve stress and fine-tune the distribution of carbides, ensuring long-term stability.
Superalloy Welding
Superalloy Welding using TIG or PTA overlays ensures wear zones remain robust after final CNC machining.
Thermal Barrier Coating (TBC)
TBC Coating increases performance in parts exposed to steam or gas erosion at elevated temperatures.
Electrical Discharge Machining (EDM)
EDM enables precise detail and tight tolerances in hardened parts, with Ra <0.6 µm.
Deep Hole Drilling
Deep Hole Drilling ensures accurate bore profiles in valve seats, nozzles, and wear-critical sleeves.
Material Testing and Analysis
Material Testing includes ASTM G65 wear tests, microstructure analysis, and hardness verification (ASTM E18).
Industry Applications of Stellite 6 Components
Valve Seats, Discs, and Guides
Reliable sealing and anti-galling performance in steam, chemical, and marine service valves.
Pump Bushings and Impellers
Resists cavitation and particulate erosion in slurry and process fluid environments.
Nuclear Power Systems
Control rod wear plates and valve internals operating under radiation and high pressure.
Marine and Oilfield Equipment
Shaft sleeves, gate valve seats, and wear-resistant trim exposed to saltwater and drilling fluids.
FAQs
What are the optimal CNC cutting tools and speeds for machining Stellite 6?
How does Stellite 6 compare to Stellite 3 and Stellite 12 in wear performance?
Can Stellite 6 components be used in seawater or corrosive chemical environments?
Is HIP recommended for cast Stellite 6 parts used in thermal cycling applications?
What inspection and QA documentation is provided for CNC-machined Stellite 6 components?
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