Stellites are Cobalt-based alloys with exceptional corrosion, abrasion, oxidation, and heat resistance properties. They are primarily used in valve seats and acid-resistant machine parts.
The alloys are usually precision cast, requiring minimal machining. However, machining can be challenging due to the alloy’s high hardness and toughness.
Corrosion Resistance
Corrosion resistance is a critical factor in the success of the delivery of any material and Stellite parts. These cobalt-based alloys contain additions of chromium, iron, and tungsten. They can resist corrosion from chemical liquids and gases, including acidic solutions. They are also resistant to abrasion and impact.
X-ray analysis of the coating shows that the deposited layer consists of a-Co and (Cr, Fe)7C6 phases. These are challenging phases that give the Stellite alloys excellent wear resistance.
Low-carbon alloys have better cavitation and sliding wear resistance. In contrast, the higher-carbon ones are primarily recommended for applications involving severe low-angle erosion or galling with other metals. Adding rhenium improves the alloy’s resistance to pitting corrosion in a 3% NaCl solution. This is attributed to its ability to stabilize the surface and anticipate pit formation. This allows the alloy to maintain a lower corrosion potential than the substrate. This is important in the harsh environment of the engine and machine gun barrels.
Wear Resistance
The cobalt-based Stellite alloys are often used in applications requiring high wear resistance levels. The wear resistance of a Stellite alloy can be improved by varying its constituents. For example, lower carbon Stellite alloys are often recommended for cavitation, sliding wear, and moderate galling. In contrast, higher carbon alloys can be selected for abrasion, severe galling, and low-angle erosion.
It is also possible to mold a specific level of wear resistance into a Stellite alloy by introducing elements such as nickel, carbon, iron, silicon, and sulfur. This allows designers to create a material ideally suited to a particular requirement.
To further enhance the wear resistance of a Stellite part, it can be coated with a protective layer using sintering or hot isostatic pressing. This can be achieved with additive manufacturing technology to produce fully finished wear components. This is particularly useful for applications that require toughness and wear resistance.
High-Temperature Resistance
Cobalt-chromium-based Stellite alloys containing complex carbides manufacture hard-facing and acid-resistant machine parts. They also have a good level of wear resistance at elevated temperatures and are non-magnetic.
The Stellite alloy can withstand high temperatures, making it ideal for steam turbines where components must be protected from corrosion and erosion. The alloys are usually coated onto base materials by welding, laser cladding, or HVOF thermal spray processes.
These coatings can be deposited on the surfaces of existing components to restore their original level of performance. Cobalt-based Stellite can be deposited on nuclear reactor components damaged by casting and manufacturing defects or used as the replacement valve seats in industrial steam turbines. It has even been used as cage material for the first artificial heart valve and as the cutting blade on medical drills and surgical instruments. The alloy is non-reactive to human body fluids and a good choice for teeth and bone implants.
High Strength
In addition to its strength and durability, Stellite is non-magnetic and has an extremely high melting point. These unique characteristics make it suitable for various industrial applications requiring heat resistance, corrosion resistance, and hardness.
Stellite parts are generally fabricated using gas-shielded tungsten (TIG) welding, shielded metal arc (MIG) welding, or oxyacetylene welding under 25 CFH of argon gas flow. It is essential to ensure sufficient ventilation to control dust, smoke, and particulate matter exposure.
Some studies have been performed on fabricating Stellite components with various techniques. However, few papers describe the performance of Stellite parts manufactured through additive manufacturing processes. The HVOF method produces coatings with pores and oxides that compromise the mechanical properties of the Stellite coating. On the other hand, the LENS technique has been shown to create dense and crack-free Stellite coatings with comprehensive mechanical properties.
