Mechanism of carburization of high-temperature alloys

An investigation of the mechanism of gaseous carburization in a reducing environment was conducted for selected Fe- and Ni-base alloys. Carburization kinetics were measured as functions of temperature in the range 870-980 °C. Scanning electron microscopy, analytical electron microscopy and X-ray diffractometry were employed for microstructural characterization and microchemical analysis. Changes in mechanical strength produced by carburization were determined from microhardness and tensile property measurements. Kinetic studies indicated that the carburization reaction followed a parabolic rate law. Depending upon the nature of surface scale formed in the presence of a carburizing environment, the rate-determining step of the reaction varied from C diffusion into the alloy in the presence of a carbide scale to that in the presence of an oxide scale. Under reducing carburizing conditions, alloys inherently protected by Cr₂O₃-base scale were found to develop a surface carbide scale which allowed C to penetrate into the alloy with relative ease and, thus, the carburization kinetics was accelerated. In contrast, an alloy capable of forming Al₂O₃ developed and maintained a protective surface oxide scale which acted as an effective barrier to C diffusion into the alloy. Degradation of mechanical strength due to precipitation of carbides in the alloy was correlated with the rate of attack and consequently the nature of the surface scale.