Carbon Migration Behavior during Rolling Contact in Tempered Martensite and Retained Austenite of Carburized SAE4320 Steel

The changes in the state of carbon in tempered martensite and retained austenite in carburized SAE4320 steel under the rolling contact fatigue (RCF) were investigated using atom probe tomography (APT). In the tempered martensite, the carbons in solid solution and in carbon cluster were readily transferred to the preexisting metastable (epsilon) carbide due to rolling contact, resulting in a localized change from tempered martensite to ferrite accompanied by the growth of carbides. This supports the recently proposed dislocation assisted carbon migration theory. On the other hand, retained austenite with uniformly distributed enriched solute carbon was partially transformed into the very fine deformation-induced martensite due to rolling contact. Furthermore, carbon seemed to be partitioned into retained austenite from the deformation-induced martensite during further rolling contact cycles. This is a new insight into the characteristics of deformation-induced martensite and retained austenite generated by rolling contact. The present study provides a plausible explanation to the phenomenon that the deformation-induced martensitic transformation improves the RCF life.