Transformation kinetics and microstructural features in Low-Temperature Bainite after ausforming process

Abstract

Acceleration of the bainitic transformation and modifying the microstructural characteristics are believed to be of major concerns in production of Low-Temperature Bainite. Ausforming prior to isothermal bainitic transformation seems to be an effective procedure in this regard. This article aims to study the influence of different ausforming strains on transformation kinetics and microstructural features of low temperature nano bainite obtained after austempering at 300 °C. Results indicated that bainitic transformation tended to be accelerated by plastic deformation of primary austenite depending on applied strain level. In addition, the volume fraction and morphology of the microstructural constituents have been shown to be affected by ausforming after which higher volume fraction of bainite, finer austenite micro blocks and thinner bainitic ferrites could be achieved. Finally, quantitative analysis demonstrated that ausforming favoured the distribution of bainitic sheaves in limited crystallographic directions whereas they spread randomly in different crystallographic orientations in non-ausformed specimens. This was more evident at higher ausforming strain levels at which bainitic sheaves' orientation distributions were narrower relative to the rolling direction regardless of the isothermal bainitic heat treatment time.