High temperature alloys are divided into two categories, iron-based and nickel-based alloys. Large differences exist between iron- and nickel- based alloys. As iron-based alloys have a higher Curie temperature, they can be used below the Curie temperature. Above the Curie temperature, nickel-based alloys should be used. Iron-based alloys are more economical than nickel- based alloys and have a cost advantage. Depending on the alloys used, their properties can differ considerably.
In GB/T14992-1994, four types of high temperature alloys (Fe-Ni, Fe-Ni-W, Co-Ni, Fe-Ni-Al-Si-Ti) are specified. The Fe-Ni alloys are described as having a low CTE, high temperature strength and corrosion resistance. Fe-Ni-W alloys also have a low CTE, as well as a high melting point, good soldering hot ductility and corrosion resistance. The Co-Ni alloys are described as having high temperature strength, erosion corrosion and oxidation resistance. Finally, the Fe-Ni-Al-Si-Ti alloys are described as having good ductility and weldability, excellent oxidation resistance.
The Fe-Ni alloys feature excellent mechanical property and thermal fatigue resistance at moderately high temperatures. They also possess superior creep resistance, fatigue strength, and resistance to hot cracking, as compared with other alloys. Fe-Ni alloys can be hardenable and have good low temperature toughness. In addition, they have good metal-to-metal wear resistance.
The Fe-Ni-W alloys feature good room temperature strength, high temperature strength and superior thermal fatigue resistance, especially to high frequency stresses. They have superior wear and erosion resistance compared to other alloys, as well as better hot corrosion resistance. Fe-Ni-W alloys possess good formability, weldability and processability.
The Co-Ni alloys possess superior oxidation resistance, corrosion resistance and thermal fatigue resistance at moderately high temperatures. They also have superior wear and erosion aggression compared to other alloys. The Co-Ni alloys possess good temperature strength, high thermal shock resistance, as well as good wear resistance when coupled with appropriate tribological coating.
The Fe-Ni-Al-Si-Ti alloys have superior oxidation resistance, especially at elevated temperatures, as well as good creep resistance and thermal shock resistance. They also have superior weldability and formability, as well as superior corrosion resistance and intermetallic growth, compared to other alloys. Fe-Ni-Al-Si-Ti alloys possess good handling characteristics, so that they can be machined easily and be readily available in various sizes, shapes and forms.
In summary, the GB/T14992-1994 specifies four high temperature alloys, namely Fe-Ni, Fe-Ni-W, Co-Ni and Fe-Ni-Al-Si-Ti. Each of these alloys has its own unique properties and strengths, which should be examined carefully, to ensure the optimum selection for the specific application. Additionally, it is important for the engineer, when selecting these alloys, to be aware of all the parameters associated with their implementation, such as weldability and machining, processability, and corrosion resistance. This will ensure that the best alloy is selected for the desired application.
