Development and Characterisation of Nanobainite

Abstract

The purpose of the present study is to develop and characterise the novel nanostructured bainite in recently proposed low manganese high chromium silicon steel. An attempt is made to test thoroughly the capability of mentioned steel to transform to nanobainite. Moreover, the effect of heat treatment conditions on the resulting microstructure and hardness are investigated. Scanning electron microscopy (SEM) results indicate the existence of fine sheaves of bainite, which are not resolved. Blocky austenite along with thin film austenite is present in the microstructure. The volume fraction of austenite calculated using x-ray diffraction (XRD) is found to reduce with decreasing transformation temperature. It is mainly associated with the reduced nucleation rate of subunits due to sluggish diffusion of carbon atoms at low temperatures. It is also shown that longer holding time resulted in increasing amount of austenite. The possible explanation is amplification of transformation to bainite for extended holding time due to more diffusion of carbon atoms. It is found that soaking time of 6hrs at 300°C is sufficient to achieve sufficient fraction of nanobainite. At 200°C temperature holding time of 24hrs is not enough to obtain considerable transformation. In addition, the prevalence of martensite becomes more at lower temperatures. Best microstructural characteristics and hardness are achieved at 250°C temperature. A linear relationship between amount of nanobainite and hardness is observed. It is suggested that overall transformation kinetics is excellent at higher temperatures while at low temperatures (e.g. 200°C) transformation rate is poor.