Stellite Alloy is a very popular material for hard valve trims that require resistance to sliding wear, corrosion and/or cavitation. It has even been successfully used in some corrosive applications. However, in some areas of boiler feedwater applications, it seems that stellite alloy 6 should perform well and encounter problems. Here are some theories and facts about the problem:
1. Stellite alloy 6 is available in cast, forged and welded covers. The foundry material is named UNS R30006; the forged material is named UNS R30016, but is commonly referred to as stellite alloy 6B; the generic AWS name for case hardened materials is CoCr-A (specific name is ECoCr-A for SMAW electrodes) And ERCoCr-A for bare electrodes.
2. Stellite alloy 6 is a cobalt-chromium-tungsten alloy with a carbon content of approximately 1%. The material consists of a soft cobalt-chromium-tungsten solid solution matrix surrounded by a small amount of hard brittle chromium carbide.
3. Although the carbide phase provides high hardness (about 40 HRC), studies have shown that the cobalt-chromium-tungsten matrix is the cause of the excellent wear resistance and cavitation resistance of the alloy. When high stresses, such as in the case of wear or cavitation, stellite alloy 6 undergoes a phase change (ie, a change in crystal structure). Compared to materials that do not have this “shock-absorbing” property, this phase change absorbs some of the energy that can cause damage, actually reducing the overall damage.
4. Like most corrosion-resistant alloys containing chromium as an alloying element, Stellite Alloy 6 achieves corrosion resistance by forming a stable chromium oxide passivation layer. The passive layer protects the underlying material from reacting with the environment. Certain chemicals can weaken the passive layer and reduce its ability to protect the material from corrosion.