Effect of Carbon on Nano-Crystallization in Steel during Mechanical Milling Treatment

Abstract

Mechanical milling (MM) treatment of metallic powder is suitable for fabricating nano-crystallized materials, because milling action by steel balls enables to give a huge amount of strain with multi-directional plastic deformation to powder particles. In this study, effect of carbon on the grain refining behavior during MM treatment was investigated in high purity iron material and Fe-C materials. The powder used in this study is electrolytic pure (9 ppmC) iron and cementite (6.2 mass%C) powder particles. The powders are mixed to set the chemical composition to be Fe-(0-2)mass%C. The mixed powder is subjected to MM treatment for various times (3.6-360 ks) and then microstructure was examined by means of X-ray diffractometry, TEM observation. With MM treatment, cementite decomposes into ferrite matrix and ferritic single structure is obtained after 360 ks MM treatment. On the other hand, microstructure of ferrite develops from dislocation cells structure to fine-grained structure through dynamic continuous recrystallization (DCR). The grain size is reduced gradually with MM treatment. However the grain size after reaching steady state is different between high purity iron and Fe-C materials. The grain size after 360 ks MM treatment decreases with increasing carbon content, and nano-crystallized structure with about 15 nm grain size was obtained in the Fe-0.8 mass%C. This indicates that carbon addition enhances grain refining and is necessary for nano-crystallization by severe plastic deformation. Considering the interaction between carbon atoms and dislocations, carbon addition would assist the increment of stored dislocations which contributes to DCR. This results in the effectiveness for the formation of nano-crystallized structure in carbon added iron.