The morphology and coarsening kinetics of spheroidized Fe-C binary alloys

The tempering and spheroidization of martensitic Fe-C binary alloys ranging in carbon content from 0.2% to 1.4% in 0.2% increments have been investigated. The tempering was conducted at 690 C for up to 3 weeks. Quantitative metallography techniques were used to determine average carbide diameter and ferrite grain aspect ratio. Recrystallization was not found to occur during tempering, and the ferrite grain aspect ratio was shown to be dependent on the prior austenite grain size, carbon content, and temper time. Carbides were found to reside almost exclusively in the grain boundaries and the carbide coarsening kinetics were governed by a power law relationship, d = kt{sup n}, where n is determined by the diffusion mechanism. The coarsening kinetic time constant, n, fell between 0.2 and 0.23 for the entire range of Fe-C alloys studied, indicating that the coarsening process during tempering is controlled by carbon diffusion in grain boundaries and dislocation.