Cobalt-based superalloys play a very significant role in the material industry. These alloys are mostly used in high-temperature and acidic conditions due to their excellent high-temperature strength.
Cobalt-based superalloys are mainly composed of elements such as cobalt, chromium, tungsten, nickel and aluminum.
Cobalt is a very important metal with atomic number 27 in the periodic table. It is heavier than iron, and its melting point is 1493 °C. Due to the higher melting point, this metal can resistance to high temperatures very well than other metals.
Cobalt-based alloys typically have favorable mechanical characteristics.
As a result of their resistance to rust, wear, and heat, they are used for equipment in harsh environments, including hot gas turbines. Additionally, cobalt exhibits strong resistance to sulfides.
Let’s take a closer look at why cobalt-based superalloys are the popular materials.
Composition of Cobalt-based Superalloys
The composition of cobalt-based superalloys is shown in the table:
Elements | Min | Max |
Cobalt | 35 % | 75 % |
Nickel
| 5 % | 25 % |
Iron
| 9 % | 20 % |
Chromium
| 20 % | 20 % |
Tungsten
| 7 % | 15 % |
Carbon | 0.25% | 1 % |
Niobium
| The small amount used for solid solution strengthening
|
|
Aluminum
| The small amount used for solid solution strengthening
|
|
Molybdenum
| The small amount used for solid solution strengthening
|
|
Tantalum
| The small amount used for solid solution strengthening
|
Cobalt
Cobalt is the basic element for cobalt-based superalloys. It provides alloy-positive high-temperature performance, welding performance, and other benefits. Cobalt-based superalloys typically include 35% to 70% cobalt.
Nickel
It is very important to add 5% to 25% nickel in cobalt-based superalloys to stabilize the austenitic structure of the alloy. In addition, the alloy gains some corrosion resistance due to the nickel element.
Iron
Iron can play a role in cobalt-based superalloys in terms of solid solution strengthening. Cobalt undergoes allotrope changes as well. At room temperature, it has a close-packed hexagonal structure; at high temperatures, it has a face-centered cubic structure. Iron’s primary function in cobalt-based alloys is to increase the austenite region.
Chromium
To improve the oxidation resistance of cobalt-based superalloys, it is very important to add about 20% chromium to the composition. Chromium creates a number of carbides depending on the amount of carbon present, which enhances precipitation.
Tungsten and Molybdenum
Both of these elements play a very important role in the composition of cobalt-based nickel alloys. These elements can maintain the solid solution strengthening in the alloy.
Niobium and Tantalum
The amount of Niobium and tantalum is very small in the cobalt-based alloys, but they also have the result of making the antioxidant capacity stronger.
Carbon
Cobalt-based superalloys have more carbon content than other alloys. As a result, these alloys have more carbide precipitation strength than other alloys. Usually, 0.25% to 1% carbon contents are present in cobalt-based superalloys.
Characteristics of Cobalt-based Superalloys
Superalloys must maintain their shape at extreme temperatures near their melting points, so they are designed for high-temperature applications.
Superalloys may keep their high strength, stability, corrosion resistance, and oxidation resistance when alloyed with specific elements at extremely high temperatures.
Cobalt-based alloys have greater melting points than nickel- or iron-based alloys. Superior heat corrosion resistance when compared to alloys based on nickel or iron.
This alloy has better thermal fatigue resistance and weldability than nickel-based alloys.
Excellent corrosion resistance
Cobalt-based superalloys have a greater percentage of chromium concentration than other alloys, making them more resistant to high-temperature corrosion. At high temperatures, cobalt-based superalloys can generate a stable oxide coating. It can keep its material’s strength and stability under the high temperature and corrosive environments. Due to this property, cobalt-based alloys are mostly used in engine blades.
Extreme temperature durability
Nickel-based superalloys have more durability than cobalt-based superalloys under 900°C. When temperatures reach above 900°C, nickel-based superalloys’ durability intensely drops. At this specific point, cobalt-based superalloys have clear advantages in terms of stability. As a result, cobalt-based superalloys are appropriate for high-temperature components.
High melting points
Cobalt-based superalloys have an initial melting temperature of more than 1300 °C. Most nickel-based superalloys have an initial melting temperature of less than 1280°C. This is generally because refractory metal is frequently included in cobalt-based alloys for solid solution strengthening. Because of their high melting point, cobalt-based alloys are commonly used to create aircraft engine guide vanes. It prevents flaws like burning or guide vane distortion at high temperatures.
Excellent welding characteristics
Cobalt-based superalloys also have superior welding performance than nickel-based superalloys. It is appropriate for a variety of welding procedures. Furthermore, it is important to choose a welding technique and parameters carefully for cobalt-based superalloys due to the high melting point.
Applications of Cobalt-based Superalloys
Gas turbine components
Cobalt-based superalloys are mostly used in gas turbine blades and turbine engines because these alloys can maintain their strength, toughness, and corrosion resistance properties at high temperatures.
Aerospace industry
Cobalt-based alloys are mostly used in the aerospace industry. These alloys are used to make the components of jet engines, combustion chambers, and afterburner parts.
Medical sector
Cobalt-based superalloys are also playing a vital role in the medical sector for making medical implants like orthopedic implants and dental prosthetics.
Industrial machinery and equipment
Cobalt-based alloys are used for making industrial equipment which can be utilized in extreme weather environments. Some important components are valves, bearings, pumps, and cutting tools. These alloys are also used in chemical processing equipment like vessels and reactors.
Conclusion
Cobalt is the main ingredient of the cobalt-based superalloys; nickel and chromium are also added with different compositions for the required properties.
Unlike nickel-based superalloys, cobalt-based superalloys use more tungsten for solid solution strengthening and precipitation strengthening.
Cobalt-based superalloys have gained a special place in the industry due to their outstanding properties.
We are dealing in high quality superalloys. If you want to discuss any query according to your project, feel free to discuss it with our experienced team. We will educate you about your alloy type and your application from start to end.
