2019 Volume 91 Issue 6 Pages 343-348
ADI has outstanding balance between strength and elongation because the retained austenite in ADI enhances ductility by strain induced transformation. Retained austenite is formed by the austempering process consisting of heat treatment in the austenite temperature range and subsequent heat treatment in the bainitic transformation temperature range. It has been reported that heat treatment from (α ＋ γ) range is effective for forming ferrite, bainitic ferrite, and retained austenite rather than heat treatment from the austenite single phase range. However, not many studies have investigated the effects of prior structure on subsequent heat treatment from (α ＋ γ) range. Thus this study focused on fine pearlite as the prior structure for heat treatment from (α ＋ γ) range. The effects of fine pearlite on the formation of fine ferrite, bainitic ferrite, and retained austenite and its mechanical properties were investigated. Fine structure was obtained from the prior structure of fine pearlite and subsequent heat treatment from (α ＋ γ) range. EBSD analysis showed that the fine structure consists of fine and equiaxed ferrite grains. Regarding mechanical properties, 0.2% proof stress and tensile strength improved from 440 to 470MPa and from 740 to 770MPa. However, the total elongation was the same. Although the matrix structures were largely different between coarse and fine pearlite as the prior structure, the difference in the mechanical properties was small. EBSD analysis showed that the strain induced transformation of the retained austenite occurred near spheroidal graphite. This means that spheroidal graphite is the origin of stress concentration. Strain induced transformation near the spheroidal graphite was predominant and did not work effectively to enhance work hardening. Therefore, although two matrix structures of ferrite, bainitic ferrite and retained austenite were largely different, their mechanical properties were almost the same.
