# Stellite 21 (UNS R30021) Cobalt-Based Alloy
## Introduction to Stellite 21
Stellite 21, also known by its UNS designation R30021, is a cobalt-based alloy renowned for its exceptional wear resistance, corrosion resistance, and high-temperature strength. This alloy belongs to the Stellite family of materials, which have been widely used in demanding industrial applications since their development in the early 20th century.
## Composition and Properties
The chemical composition of Stellite 21 typically includes:
– Cobalt (Co): Balance
– Chromium (Cr): 26-30%
– Molybdenum (Mo): 5-7%
– Nickel (Ni): 2-4%
– Carbon (C): 0.25% max
– Silicon (Si): 1% max
– Iron (Fe): 3% max
– Manganese (Mn): 1% max
This unique combination of elements gives Stellite 21 its remarkable properties:
– Excellent resistance to galling and adhesive wear
– Good corrosion resistance in various environments
– Maintains strength at elevated temperatures
– Good resistance to thermal shock
– Moderate machinability in annealed condition
## Manufacturing and Processing
Stellite 21 can be produced through various manufacturing processes:
### Casting
The alloy is commonly cast using investment casting or sand casting techniques to produce complex shapes with good dimensional accuracy.
### Powder Metallurgy
For certain applications, Stellite 21 can be produced through powder metallurgy routes, offering improved homogeneity and finer microstructures.
### Welding
Stellite 21 is weldable using standard techniques, though preheating and post-weld heat treatment may be required for certain applications to minimize residual stresses.
## Heat Treatment
While Stellite 21 is typically used in the as-cast condition, it can be heat treated to optimize certain properties:
– Solution annealing: 1200-1250°C (2192-2282°F) followed by rapid cooling
– Stress relieving: 650-750°C (1202-1382°F) for 1-2 hours
## Applications of Stellite 21
Stellite 21 finds use in numerous industrial applications where wear resistance and corrosion resistance are critical:
### Valve Components
– Valve seats and stems
– Ball valve components
– Gate valve wedges
### Pump Components
– Shaft sleeves
– Impellers
– Wear rings
### Aerospace Applications
– Bearing surfaces
– Wear pads
– High-temperature fasteners
### Oil and Gas Industry
– Downhole tools
– Wellhead components
– Valve trim
## Comparison with Other Stellite Alloys
While Stellite 21 offers excellent properties, it’s important to understand how it compares to other alloys in the Stellite family:
Alloy | Hardness (HRC) | Wear Resistance | Corrosion Resistance
Stellite 1 | 50-55 | Excellent | Good
Stellite 6 | 40-45 | Very Good | Excellent
Stellite 12 | 45-50 | Excellent | Good
Stellite 21 | 30-35 | Good | Excellent
## Machining Considerations
Machining Stellite 21 requires special considerations due to its work-hardening characteristics:
– Use rigid machine setups to minimize vibration
– Employ sharp, positive rake angle tools
– Maintain adequate coolant flow
– Use moderate cutting speeds and feed rates
– Consider carbide or ceramic cutting tools for best results
## Corrosion Resistance
Stellite 21 exhibits excellent corrosion resistance in various environments:
– Resistant to sulfuric, hydrochloric, and phosphoric acids in moderate concentrations
Keyword: Stellite 21 UNS R30021
– Performs well in seawater and chloride-containing environments
– Maintains resistance to oxidation at elevated temperatures
– Resistant to stress corrosion cracking in many environments
## Future Developments
Research continues to enhance the properties and applications of Stellite 21