# 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 family of Stellite materials, which have been widely used in demanding industrial applications for over a century.
## Chemical Composition
The chemical composition of Stellite 21 plays a crucial role in its performance characteristics:
– Cobalt (Co): Balance
– Chromium (Cr): 26-30%
– Molybdenum (Mo): 5-7%
– Nickel (Ni): 2-3%
– Carbon (C): 0.25% max
– Iron (Fe): 3% max
– Silicon (Si): 1% max
– Manganese (Mn): 1% max
## Key Properties
Stellite 21 exhibits several remarkable properties that make it suitable for challenging applications:
### Wear Resistance
The alloy demonstrates excellent resistance to various forms of wear, including adhesive, abrasive, and erosive wear. This property makes it ideal for components subjected to sliding or rubbing contact.
### Corrosion Resistance
With its high chromium content, Stellite 21 offers good resistance to many corrosive environments, including oxidizing acids and saltwater conditions.
### High-Temperature Performance
The alloy maintains its mechanical properties at elevated temperatures, making it suitable for applications where thermal stability is required.
### Mechanical Properties
Typical mechanical properties of Stellite 21 include:
– Tensile Strength: 90-110 ksi (620-760 MPa)
– Yield Strength: 45-65 ksi (310-450 MPa)
– Elongation: 8-15%
– Hardness: 28-35 HRC
## Manufacturing and Processing
Stellite 21 can be produced and processed using various methods:
### Casting
The alloy is commonly produced through investment casting or sand casting processes to create complex shapes and components.
### Powder Metallurgy
Stellite 21 can also be manufactured using powder metallurgy techniques for applications requiring precise dimensional control.
### Machining
While machinable, Stellite 21 requires special tooling and techniques due to its work-hardening characteristics.
## Applications
Stellite 21 finds applications across various industries:
### Aerospace
– Turbine blades and vanes
– Combustion chamber components
– Exhaust valves
### Industrial
– Pump components for corrosive fluids
– Valve seats and stems
– Bearing surfaces
### Medical
– Surgical instruments
– Dental tools
– Implant components
## Comparison with Other Stellite Alloys
Keyword: Stellite 21 UNS R30021
While Stellite 21 shares many characteristics with other cobalt-based alloys, it offers some distinct advantages:
– Better machinability than Stellite 6
– Higher corrosion resistance than Stellite 1
– Improved weldability compared to some other Stellite grades
## Maintenance and Care
Proper maintenance can extend the service life of Stellite 21 components:
– Regular inspection for wear and corrosion
– Proper lubrication in moving applications
– Avoidance of excessive thermal cycling when possible
Stellite 21 (UNS R30021) continues to be a material of choice for engineers facing demanding wear, corrosion, and high-temperature challenges. Its balanced combination of properties makes it versatile across multiple industries, ensuring its continued relevance in advanced engineering applications.
