Stellite 6K
Introduction to Stellite 6K
Stellite 6K is a wrought cobalt-chromium alloy specifically engineered for enhanced toughness, galling resistance, and wear performance in metal-to-metal contact applications. It shares a similar chemical composition with Stellite 6B but features higher ductility and improved machinability due to its fine-grained wrought microstructure. The "K" variant is optimized for sliding wear under pressure, impact, and thermal stress applications.
Stellite 6K is ideal for CNC-machined components where both dimensional accuracy and resistance to seizing, scuffing, and erosion are critical. Common applications include bushings, bearings, pump sleeves, and valve stems used in aerospace, marine, energy, and oil & gas industries.
Chemical, Physical, and Mechanical Properties of Stellite 6K
Stellite 6K (UNS R30016 / ASTM F90, AMS 5894 derivative) is a solid-solution strengthened cobalt alloy manufactured by hot working, forging, or rolling to achieve a homogeneous, dense structure with superior mechanical integrity.
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Cobalt (Co) | Balance (≥58.0) | Base matrix for corrosion and high-temperature strength |
Chromium (Cr) | 28.0–32.0 | Provides oxidation and corrosion resistance |
Tungsten (W) | 4.5–6.5 | Forms carbides to improve wear resistance |
Carbon (C) | 0.9–1.4 | Controls carbide formation to resist galling |
Nickel (Ni) | ≤3.0 | Enhances toughness and ductility |
Iron (Fe) | ≤3.0 | Residual element |
Manganese (Mn) | ≤1.0 | Supports hot workability |
Silicon (Si) | ≤1.2 | Improves casting fluidity and surface finish |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.42 g/cm³ | ASTM B311 |
Melting Range | 1320–1395°C | ASTM E1268 |
Thermal Conductivity | 13.0 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 0.96 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.2 µm/m·°C (20–400°C) | ASTM E228 |
Specific Heat Capacity | 420 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 205 GPa at 20°C | ASTM E111 |
Mechanical Properties (Wrought Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Hardness | 33–43 HRC (annealed) / up to 45 HRC (aged) | ASTM E18 |
Tensile Strength | 1050–1200 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 550–700 MPa | ASTM E8/E8M |
Elongation | 10–20% | ASTM E8/E8M |
Galling Resistance | Excellent, even against itself | ASTM G98 |
Wear Resistance Index | >2.5× 316 stainless steel | ASTM G65 |
Key Characteristics of Stellite 6K
Superior Galling Resistance: Outstanding performance under metal-to-metal contact, even in dry sliding conditions.
High Toughness and Ductility: More forgiving in impact-prone environments than cast Stellite grades, reducing the risk of cracking or edge chipping.
Thermal and Corrosive Stability: Withstands service temperatures up to 850°C and resists pitting and oxidation in chlorides and acids.
Improved Machinability: Compared to cast Stellite 6, the wrought 6K form provides better dimensional control during CNC machining.
CNC Machining Challenges and Solutions for Stellite 6K
Machining Challenges
Abrasive Carbide Phases
The tungsten-rich carbides increase tool wear, particularly during prolonged roughing passes.
Heat Buildup at the Tool-Work Interface
Low thermal conductivity traps heat in the cutting zone, degrading tool coatings and accuracy.
Work Hardening
Repeated tool passes can raise surface hardness, requiring careful pass planning and tool monitoring.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | PVD-coated carbide (K30–K40); CBN for tight-finish tolerances | Handles abrasive wear while maintaining edge strength |
Coating | AlTiN or TiAlCrN (3–5 µm) | Lowers cutting zone temperature and prolongs tool life |
Geometry | Neutral rake (0°), honed cutting edge 0.03 mm | Enhances cutting edge durability and prevents chip adhesion |
Cutting Parameters (ISO 3685)
Operation | Speed (m/min) | Feed (mm/rev) | DOC (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 10–18 | 0.20–0.30 | 2.0–3.0 | 100–120 |
Finishing | 20–28 | 0.05–0.10 | 0.5–1.0 | 120–150 |
Surface Treatment for Machined Stellite 6K Parts
Hot Isostatic Pressing (HIP)
HIP is generally unnecessary for wrought Stellite 6K but may be used on powder metallurgy or additively manufactured forms to enhance fatigue life.
Heat Treatment
Heat Treatment post-machining can optimize carbide distribution and reduce residual stress from forming or CNC cutting.
Superalloy Welding
Superalloy Welding using low-dilution PTA or TIG welding techniques with matching filler helps retain wear and galling resistance across joints.
Thermal Barrier Coating (TBC)
TBC Coating protects against hot gas erosion and enhances high-temperature durability in rotating and reciprocating parts.
Electrical Discharge Machining (EDM)
EDM ensures precise finishing of tight-tolerance parts with Ra <0.5 µm in hardened or complex geometries.
Deep Hole Drilling
Deep Hole Drilling is suitable for creating wear-resistant bushings and flow-control components with aspect ratios >20:1.
Material Testing and Analysis
Material Testing includes tensile strength, microhardness profiling, metallographic examination, and ASTM G98 galling evaluation.
Industry Applications of Stellite 6K Components
Industrial Valve Systems
Valve stems, guides, and plug components that must resist galling under pressure and temperature extremes.
Aerospace Bearings and Linkages
Precision bushings and control system components operating in dry or lubricated sliding contact.
Oil & Gas Equipment
Drill head stabilizers, pump sleeves, and rotating seals subjected to slurry abrasion and corrosive fluids.
Power Generation and Turbomachinery
Hot section wear pads and sealing surfaces exposed to high thermal and cyclic loading.
FAQs
How does Stellite 6K differ from Stellite 6B regarding machinability and toughness?
What tooling and parameters are best for CNC machining Stellite 6K?
Is Stellite 6K suitable for dry-contact aerospace and industrial wear surfaces?
Can precision features like deep bores and grooves be EDM-machined in Stellite 6K?
What mechanical and wear test certifications are available for machined Stellite 6K parts?
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