Super-carburization of low alloy steel and low carbon steel by fluidized-bed furnaces

The present work is concerned with super-carburization treatment on low alloy steel AISI 4135 and plain carbon steel AISI 1020 by fluidized-bed furnaces. Super-carburization is a repetition of carburization and precipitation heat treatment processes to produce a dense spheroidized carbide layer near to the steel surface. Carburization was conducted at 930°C and 800°C in an endothermic atmosphere with a carbon potential higher than 1.3% generated by the direct boosting of propane and air into the bed. The precipitation of carbides in steels was conducted at 600°C in a nitrogen-fluidized bed. The rapid cooling rate associated with the great undercooling of the fluidized bed effectively suppressed the formation of network cementite when carburized steels were cooled from the austenitizing temperature to the precipitation temperature. This phenomenon has made it possible for even a plain carbon steel to be super-carburized without difficulty.