Cobalt Based Super Alloy Investment Casting

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, which transforms 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 show superior thermal corrosion resistance at temperatures above 1000°C than other high temperatures.

Common Grades of Cobalt-based Alloys

The typical grades of common cobalt-based high temperature alloys are: 2.4778 (according to DIN EN 10295), Hayness 188, Haynes 25 (L-605), Alloy S-816, UMCo-50, MP-159, FSX-414, X-40, Stellite 6B, Grade 31, etc. , Chinese brands are: GH5188 (GH188), GH159, GH605, K640, DZ40M and so on.

Applications of Cobalt-based Alloy Castings

Generally, cobalt-based superalloys lack coherent strengthening phases. Although the strength at medium temperature is low (only 50-75% of nickel-based alloys), they have higher strength, good thermal fatigue resistance, abrasion resistance, better weldabilityand thermal corrosion resistance above temperature of 980°C. Therefore, cobalt-based alloy castings are mainly suitable for making guide vanes and nozzle guide vanes for aviation jet engines, industrial gas turbines, naval gas turbines, and diesel engine nozzles, etc.