DIFFUSION OF CARBON DURING THE FORMATION OF LOW-CARBON MARTENSITE

The time required for carbon diffusion from martensite to enrich surrounding austenite from 0.27 to 1.04% C during the formation of low-carbon matensite is 10~(-7)s in order of magnitude as calculated. It does prove that the diffusion of carbon atoms can keep pace with the formation of lath martensite. From thermodynamical calculation, it is reasonable to recognize that the precipitation of carbon from martensite results the enrichment of austenite. TEM observation revealed that the quenched structure in a 0.12C-low Ni-Cr steel mainly contains lath martensite and interlath retained austenite, and also twin martensite. The existence of the latter further confirms the occurrence of carbon diffusion to enrich austenite during the martensite formation, and twin martensite forms at the parent phase where carbon enrichment is not very high. The interface of austenite and martensite is somewhat straight. The typical upper bainite (B_Ⅱ), B_Ⅲ type bainite and carbide-free bainite (B_1) can aU appear in the ame steel and there exists superledges at the interface of austenite and bainitie ferrite which is quite different to that of austenite and martensite. In addition, from the kinetics point of view. the growth rate of low-carbon martensite is 3 to 4 order of magnitude greater than that of upper bainite. It is more likely to conclude that the mechanism of the formation of lath martensite is not identical with that of bainite.