Effect of Prior Austenite Grain Size on the Morphology of Nano-Bainitic Steels
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NANOSTRUCTURED bainitic steels consist of slender plates of ferrite of thickness less than 100 nm[1–5] with carbon-enriched retained austenite. Extremely high hardness has been reported in these steels since the numerous ferrite austenite interfaces block dislocation motion effectively thus reducing the mean free path for dislocation glide.[1,6] This unique microstructure is achieved due to growth by displacive transformation during prolonged isothermal treatments at temperatures as low as 125 to 300 C.[6–9] In addition to the transformation temperature, prior austenite grain size (AGS) can potentially impact the mechanical properties of nano-bainite (NB) by impacting the kinetics of transformation and hence the resulting microstructure. There have been a limited number of investigations on leveraging the prior AGS to control the kinetics and morphology of NB steels. Matsuzaki et al.[10] studied the effect of AGS on the overall kinetics of bainite transformation in a low as well as a high carbon bainitic steel. It was concluded that refinement of prior AGS leads to an acceleration of the rate of transformation when the overall reaction is limited by a slow growth
KRITIKA SINGH, AVANISH KUMAR, and APARNA SINGH are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai, 400076, India. Contact e-mail: [email protected] Manuscript submitted September 3, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
rate. However, for nucleation-limited kinetics, AGS refinement slows down the transformation to bainite. Hu et al.[11] have shown a mild increase in the transformation kinetics of Fe-0.95C-0.91Si-1.30Mn-2.30Cr-0.99Mo-0.17Ti nano-bainite with an increase in AGS and have attributed it to unhindered growth of the sheaves in a larger grained steel. On the contrary, Jiang et al.[12] have shown that a decrease in prior AGS from 53 lm to 3 lm led to acceleration of bainitic transformation significantly. However, when the prior austenite had a grain size of 3 lm, more than 50 pct of the bainite was not nano-structured and was surrounded with chunky blocks of austenite resulting in poor impact toughness. In another study by Yuan et al.,[13] it was reported that the decrease in prior AGS increases incubation time for bainite nucleation which was confirmed by the shifting of nose of the C-curve towards the right. The limited number of studies done to elucidate the effect of prior AGS on bainite transformation clearly show that the prior AGS not only impacts the kinetics of bainite formation but can also affect the morphology of the bainitic sheaves formed and hence the resulting mechanical behavior. However, in most of the previous studies, the isothermal treatment for NB nucleation and growth has not been done for sufficient time to completely transform austenite to the fraction of NB determined by the T’0 line.[14] Hence, the dependence of final bainitic lath morphology on AGS remains unknown. That is a significant impediment to designing structural nano-bainitic ste
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