The Stellite alloy has excellent high-temperature and corrosion-resistant properties.

The Stellite alloy has excellent high-temperature and corrosion-resistant properties.

Generally, cobalt-based high temperature alloys lack coherent strengthening phases. Although they have low intermediate temperature strength (only 50-75% of nickel based alloys), they exhibit high strength, good resistance to thermal fatigue, hot corrosion, and wear above 980°C.

Additionally, they have good weldability. They are suitable for making aerospace jet engine components, industrial gas turbine components, such as turbine blades and nozzle guide vanes, as well as diesel engine nozzles. In cobalt based high temperature alloys, the main carbides are MC, M23C6, and M6C.

Among them, M23C6 precipitates at grain boundaries and dendrite interfaces during slow cooling. In some alloys, fine M23C6 forms eutectics with the matrix y phase. MC carbides have larger particle sizes and do not directly affect dislocations significantly, resulting in less noticeable strengthening effects on the alloy. However, fine and dispersed carbides have a significant strengthening effect. Carbides located at grain boundaries (mainly M23C6) can inhibit grain boundary slip, thereby improving creep strength. The microstructure of cobalt based high temperature alloy HA-31 (X40) consists of dispersed strengthening phases of (CoCrW)6C carbides.

In some Stellite alloys, harmful topologically closepacked phases such as sigma phase and Laves phase may appear, leading to embrittlement ofthe alloy.

Stellite alloys are less likely to use intermetallic compounds for strengthening because compounds like Co3(Ti,Al) and Co3Ta are not stable enough at high temperatures. However, in recent years, Stellite alloys strengthened with intermetallic compounds have also been developed. The carbides in Stellite alloys have good thermal stability. As the temperature rises, the aggregation and growth rate of carbides are slower than that of the γ phase in nickel   based alloys. The temperature at which they redissolve into the matrix is also higher (up to 1100°C).

Therefore, the decrease in strength of Stellite alloys as the temperature rises is generally slower. Stellite alloys have excellent resistance to hot corrosion. It is generally believed that Stellite alloys outperform nickel based alloys in this regard because the melting point of cobalt sulfides (e.g., Co-Co4S3 eutectic, 877°C) is higher than that of nickel sulfides (e.g., Ni-Ni3S2 eutectic, 645°C), and sulfur diffusion in cobalt is much lower than in nickel. Moreover, since most Stellite alloys have higher chromium content than nickel based alloys, they can form a protective layer of Cr2O3 against alkali metal sulfate corrosion (e.g., Na2SO4).

However, Stellite alloys generally have much lower oxidation resistance than nickel-based alloys. Early Stellite alloys were produced using non-vacuum refining and casting processes.

Later alloys, such as MarM509, contain more active elements like zirconium and boron and are produced using vacuum refining and casting processes.