Cobalt-based Alloy Classification and Grades
Cobalt-based alloy is a hard alloy that can withstand various types of wear, corrosion and high temperature oxidation. It is commonly referred to as cobalt-chromium-tungsten (molybdenum) alloy or Stellite alloy (Stellite alloy was invented by American Elwood Hayness in 1907).
Cobalt-based alloys are based on cobalt as the main component, containing a considerable amount of nickel, chromium, tungsten and a small amount of alloying elements such as molybdenum, niobium, tantalum, titanium, lanthanum, and occasionally iron.
According to the different components in the alloy, they can be made into welding wire, powder used for hard surface welding, thermal spraying, spray welding and other processes, and can also be made into castings and forgings and powder metallurgy parts.
Classified by use, cobalt-based alloys can be divided into cobalt-based wear-resistant alloys, cobalt-based high-temperature alloys, and cobalt-based wear-resistant and aqueous solution corrosion alloys.
Under normal operating conditions, in fact, they are both wear-resistant, high-temperature resistant or wear-resistant and corrosion-resistant. Some operating conditions may also require high-temperature, wear-resistant and corrosion-resistant at the same time. Under circumstances, the more it can reflect the advantages of cobalt-based alloys.
In my country, the main research on cobalt-based high-temperature alloys is relatively in-depth and thorough (the domestic typical research and promotion units include the Central Iron and Steel Research Institute).
The typical grades of cobalt-based high temperature alloys are: Hayness188, Haynes25(L-605), Alloy S-816, UMCo-50, MP-159, FSX-414, X-40, Stellite6B, etc.
Chinese grades are: GH5188(GH188 ), GH159, GH605, K640, DZ40M and so on. Different from other superalloys, cobalt-based superalloys are not strengthened by an ordered precipitation phase firmly bonded to the matrix, but are composed of an austenite fcc matrix that has been solid solution strengthened and a small amount of carbides distributed in the matrix.
Casting cobalt-based superalloys relies heavily on carbide strengthening. Pure cobalt crystals have a hexagonal close packed (hcp) crystal structure below 417°C and transform to fcc at higher temperatures. In order to avoid this transformation during use of cobalt-based superalloys, practically all cobalt-based alloys are alloyed with nickel in order to stabilize the structure from room temperature to melting point temperature.
Cobalt-based alloys have a flat fracture stress-temperature relationship, but they show better thermal corrosion resistance than other high temperatures above 1000°C. This may be due to the higher chromium content of the alloy, which is the most important of this type of alloy.
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