Stellite 31
Introduction to Stellite 31
Stellite 31 is a cobalt-based wear-resistant alloy known for its exceptional performance in high-temperature, high-load, and highly corrosive environments. It features a high carbon and chromium content, with a significant volume of complex carbides, delivering outstanding hardness, thermal stability, and metal-to-metal sliding resistance. Compared to other Stellite alloys, Stellite 31 provides superior abrasive wear resistance and strength retention at elevated temperatures up to 1100°C.
This alloy is typically cast, HIP-processed, or applied as a hard-facing layer and then precision-finished via advanced CNC machining to meet critical tolerances. Stellite 31 is commonly used in aerospace, oil & gas, nuclear, and chemical processing industries for valve trims, seat rings, pump parts, and other high-wear, high-temperature applications.
Chemical, Physical, and Mechanical Properties of Stellite 31
Stellite 31 (UNS R30031 / ASTM F75-based family) is engineered for extreme abrasion and heat resistance, with a high carbide content within a cobalt-chromium solid solution matrix.
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Cobalt (Co) | Balance (≥50.0) | Base matrix for thermal stability and corrosion resistance |
Chromium (Cr) | 25.0–28.0 | Improves oxidation and corrosion resistance |
Carbon (C) | 2.4–3.0 | Promotes high-volume carbide formation for wear resistance |
Tungsten (W) | 5.0–7.0 | Forms W-rich carbides for hardness and hot wear protection |
Nickel (Ni) | ≤3.0 | Enhances ductility and alloy weldability |
Iron (Fe) | ≤3.0 | Residual element |
Silicon (Si) | ≤1.2 | Assists in casting and thermal oxidation resistance |
Manganese (Mn) | ≤1.0 | Improves hot workability and structure control |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.65 g/cm³ | ASTM B311 |
Melting Range | 1320–1400°C | ASTM E1268 |
Thermal Conductivity | 13.5 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 0.94 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.1 µm/m·°C (20–400°C) | ASTM E228 |
Specific Heat Capacity | 420 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 215 GPa at 20°C | ASTM E111 |
Mechanical Properties (As-Cast or HIP-Treated)
Property | Value (Typical) | Test Standard |
|---|---|---|
Hardness | 50–58 HRC | ASTM E18 |
Tensile Strength | 1100–1250 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 600–750 MPa | ASTM E8/E8M |
Elongation | 1.0–2.0% | ASTM E8/E8M |
Wear Resistance | >4× vs. 316 SS (ASTM G65) | ASTM G65 |
Operating Temperature | Up to 1100°C | N/A |
Key Characteristics of Stellite 31
Extreme Abrasion Resistance: High carbide content (~30–35% by volume) delivers exceptional wear resistance under high stress and dry sliding conditions.
High-Temperature Performance: Retains hardness and structural integrity at sustained temperatures up to 1100°C.
Excellent Galling and Friction Resistance: Ideal for metal-to-metal applications with limited lubrication, such as valve seat and stem combinations.
Chemical and Oxidation Resistance: Performs reliably in acids, steam, combustion gases, and high-salinity environments.
CNC Machining Challenges and Solutions for Stellite 31
Machining Challenges
Rapid Tool Wear
The alloy’s complex carbide microstructure causes intense abrasion on cutting tools, especially during continuous engagement operations.
High Cutting Forces
Requires increased power and rigidity from machines due to the alloy’s high tensile strength and limited ductility.
Risk of Surface Cracking
Excessive heat generation and poor thermal conductivity may result in surface microcracks or edge chipping during dry or insufficiently cooled cutting.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | Ultra-fine carbide (K40) or ceramic (SiAlON) for roughing; CBN for finishing | Provides wear resistance under extreme loads |
Coating | AlTiN or TiAlCrN (PVD, 3–5 µm) | Protects against heat and friction |
Geometry | Negative rake, honed edge radius 0.05 mm | Prevents edge wear and tool failure |
Cutting Parameters (ISO 3685 Compliant)
Operation | Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 8–12 | 0.20–0.30 | 1.5–2.0 | 100–120 |
Finishing | 16–20 | 0.05–0.10 | 0.3–0.8 | 120–150 |
Surface Treatment for Machined Stellite 31 Parts
Hot Isostatic Pressing (HIP)
HIP removes internal porosity and enhances fatigue resistance and dimensional consistency in cast or 3D-printed parts.
Heat Treatment
Heat Treatment improves carbide uniformity and microstructural stability, enhancing wear resistance post-machining.
Superalloy Welding
Superalloy Welding using compatible filler maintains wear and oxidation resistance in high-heat joint zones.
Thermal Barrier Coating (TBC)
TBC Coating provides additional heat shielding for parts exposed to combustion gases above 1000°C.
Electrical Discharge Machining (EDM)
EDM enables high-precision detail work in hardened sections without mechanical deformation.
Deep Hole Drilling
Deep Hole Drilling provides tight-tolerance channels and internal passageways in pump components and valves.
Material Testing and Analysis
Material Testing includes hardness profiling, G65 wear testing, metallography, and ultrasonic defect detection.
Industry Applications of Stellite 31 Components
Oil & Gas Equipment
Valve seats, ball valves, and downhole tools are exposed to sand, brine, and pressure cycling.
Aerospace Turbomachinery
Seal rings, wear strips, and guide vanes that require long wear life and oxidation resistance at high altitudes and temperatures.
Power Generation
Boiler components, burner nozzles, and feedwater valves operate in steam and thermal cycling environments.
Metal Forming & Mining
Dies, punches, and liners in metal extrusion and high-impact abrasive systems.
FAQs
What is the best tooling and strategy for CNC machining hardened Stellite 31 components?
How does Stellite 31 compare to Stellite 12 or 6K in terms of wear and thermal resistance?
Can Stellite 31 parts be EDM machined for high-precision applications?
What post-machining heat treatments enhance Stellite 31’s dimensional and surface properties?
What applications require Stellite 31 over traditional stainless or nickel alloys?
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