Microstructure and Mechanical Properties of a Harmonic Structure Designed Fe-0.3 mass%C Steel

The microstructure and mechanical properties of harmonic structure designed Fe-0.3mass% carbon steel was investigated. The compacts of Fe-0.3 mass% carbon steel with conventional Homogeneous structure (Homo), and Harmonic Structure (HS) consisting of fine grains (Shell) and coarse grains (Core) were fabricated by a powder metallurgy method. The mechanical milling (MM) leads to the formation of nano ferrite grains at the deformed surface of MM powder particles. After sintering, the Homo and HS compacts had ferrite (alpha) and perlite (P) phases. The Shell had finer alpha + P phases than Core, and the fraction of the P in the Shell was larger than that in the Core. It was considered that the carbon segregation occurs at the deformed surface of MM powder particles due to nano ferrite formation. As a result, the number of austenite nuclei increases in Shell. Therefore, the HS compact has both the grain size gradient as well as a phase constituent gradient. As-sintered HS indicated superior mechanical properties compared to the Homo counterparts. The mechanical properties were improved by further heat treatments. Those as-sintered and heat-treated HS compacts indicated a large increase of ductility and tensile toughness. Such outstanding and unique mechanical properties of the HS were attributed to the enhancement of the local elongation after necking. These superior mechanical properties are consid- ered to be due to the micro and macro synergy effects.