Stellite 25
Introduction to Stellite 25
Stellite 25, or Alloy L-605 or UNS R30605, is a high-performance cobalt-based superalloy designed for excellent strength, wear resistance, and oxidation stability at elevated temperatures. Unlike other Stellite grades optimized for extreme hardness, Stellite 25 offers a well-rounded mechanical profile—combining moderate hardness with superior creep resistance, fatigue life, and thermal oxidation performance up to 1100°C.
Due to its solid-solution strengthening with chromium, nickel, and tungsten, Stellite 25 is widely used for structural components subjected to thermal fatigue, high-pressure loads, and chemically aggressive conditions. Components are typically forged or cast and then precision-finished through CNC machining, especially for tight-tolerance aerospace, power generation, and chemical processing applications.
Chemical, Physical, and Mechanical Properties of Stellite 25
Stellite 25 (UNS R30605 / AMS 5537 / ASTM F90) is a cobalt-nickel-chromium-tungsten alloy known for excellent high-temperature tensile and fatigue strength, as well as corrosion and oxidation resistance.
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Cobalt (Co) | Balance (≥50.0) | Matrix for high-temperature strength and oxidation resistance |
Chromium (Cr) | 19.0–21.0 | Oxidation resistance via Cr₂O₃ layer formation |
Nickel (Ni) | 9.0–11.0 | Improves ductility, toughness, and corrosion resistance |
Tungsten (W) | 14.0–16.0 | Enhances strength through solid-solution hardening |
Iron (Fe) | ≤3.0 | Residual element |
Carbon (C) | ≤0.10 | Limits carbide formation to retain toughness |
Manganese (Mn) | ≤2.0 | Improves hot workability and microstructure uniformity |
Silicon (Si) | ≤1.0 | Aids oxidation resistance and casting fluidity |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 9.13 g/cm³ | ASTM B311 |
Melting Range | 1330–1410°C | ASTM E1268 |
Thermal Conductivity | 12.6 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 1.00 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.1 µm/m·°C (20–400°C) | ASTM E228 |
Specific Heat Capacity | 400 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 210 GPa at 20°C | ASTM E111 |
Mechanical Properties (Wrought or Heat-Treated Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Hardness | 25–32 HRC (annealed) / 33–40 HRC (aged) | ASTM E18 |
Tensile Strength | 930–1100 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 430–550 MPa | ASTM E8/E8M |
Elongation | 25–35% | ASTM E8/E8M |
Fatigue Strength | ≥275 MPa (rotating beam at 10⁷ cycles) | ASTM E466 |
Creep Rupture Strength | 230 MPa at 870°C (1000h) | ASTM E139 |
Key Characteristics of Stellite 25
Excellent Fatigue and Creep Resistance: Suitable for long-term service at 800–1000°C under cyclic or sustained mechanical loading.
Superior Oxidation and Sulfidation Resistance: Retains oxide scale integrity in aggressive environments such as hot air, steam, or sulfur-rich combustion gases.
Good Ductility and Fabricability: Easily cold- or hot-worked and welded, making it viable for complex structural designs.
Corrosion Resistance in Acidic and Saline Media: Withstands hydrochloric acid, nitric acid, and chloride stress environments, ideal for chemical and marine components.
CNC Machining Challenges and Solutions for Stellite 25
Machining Challenges
High Work Hardening Tendency
Surface hardness increases rapidly due to strain-induced dislocation density during machining, leading to tool deflection and chatter.
Tool Wear from Tough Matrix
The tough, solid-solution strengthened matrix resists shearing, accelerating insert wear and reducing tool life.
Dimensional Instability from Thermal Expansion
Precision machining operations require strict thermal control to maintain tolerances within ±0.01 mm due to high expansion coefficients.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | Fine-grain carbide (K30 grade), ceramic or CBN inserts | High edge retention and wear resistance |
Coating | AlTiN or TiSiN (PVD 3–5 µm) | Reduces cutting heat and thermal shock |
Geometry | Negative rake with 0.05 mm edge hone | Prevents micro-chipping in hardening zones |
Cutting Parameters (ISO 3685 Compliant)
Operation | Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 10–15 | 0.20–0.25 | 1.5–2.5 | 100–120 |
Finishing | 18–25 | 0.05–0.10 | 0.5–1.0 | 120–150 |
Surface Treatment for Machined Stellite 25 Parts
Hot Isostatic Pressing (HIP)
HIP eliminates porosity and enhances cast or additive-processed parts' fatigue strength and creep resistance.
Heat Treatment
Heat Treatment improves grain uniformity, dimensional stability, and stress relief, especially post-machining or forming.
Superalloy Welding
Superalloy Welding using matching filler rods ensures structural integrity for welded joints in high-temperature systems.
Thermal Barrier Coating (TBC)
TBC Coating enables thermal insulation for components operating above 1000°C, such as combustor hardware and liners.
Electrical Discharge Machining (EDM)
EDM achieves sub-10 µm tolerance finishes on hardened surfaces without introducing thermal distortion.
Deep Hole Drilling
Deep Hole Drilling for precision bores in rotating components, coolant passages, and nozzle structures with L/D > 20:1.
Material Testing and Analysis
Material Testing includes tensile, creep, oxidation, microhardness profiling, and ultrasonic flaw detection.
Industry Applications of Stellite 25 Components
Aerospace Combustion and Exhaust
Flame holders, transition ducts, and turbine brackets requiring fatigue resistance and oxidation stability at 1000°C.
Nuclear Reactor Systems
Valve stems, seats, and seals exposed to radiation, pressurized steam, and long-term creep stress.
Chemical Process Equipment
Impellers, reaction vessels, and pump hardware subjected to acid chlorides and oxidizing agents.
Medical and Dental Implants
Biocompatible components such as cardiovascular stents and surgical tools with long wear life and corrosion stability.
FAQs
What are the most effective CNC tool types and parameters for machining Stellite 25?
How does Stellite 25 compare to Stellite 21 in fatigue strength and oxidation resistance?
Can Stellite 25 parts be HIP treated or EDM finished for dimensional precision?
What post-machining heat treatments improve Stellite 25's creep resistance and stability?
What quality tests are recommended for Stellite 25 components used in aerospace or nuclear industries?
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