Retained austenite in silicon-containing bainitic spring steels for trucks

Abstract

Bainitic steels are the type of steels which are widely in use, especially in automotive industries, for its good mechanical behavior, i.e., with coexistence of strength, ductility and fatigue resistance. A better understanding on the mechanism and transformation kinetics of bainite are necessary in order to improve the performance of the bainitic steel further. This research mainly studies the kinetics of bainitic transformation during isothermal treatments. The chemical composition, in particular silicon (Si), affects the transformation kinetics of bainite, and therefore the fractions of retained austenite and bainitic ferrite. The current research focuses on the bainite transformation kinetics during isothermal heat treatment. Steel specimens are austempered at 250°C, 300°C and 350°C and held for 30, 60 and120 minutes, respectively. The steel in the current research has the alloy composition of Fe-0.61C-1.62Si-0.85Mn-0.32Cr (wt.%). Microstructures observed using optical micrographs consist of bainite and retained austenite after the austempering process. Quantitative measurements of the retained austenite (RA) fraction are performed by magnetization technique. The results show that, at austempering temperatures of 250°C and 300°C, the fraction of retained austenite decreases gradually with increasing holding times and increases with increasing austempering temperatures. However, a different affect is observed in the steel austempered at 350°C for 30minutes. The fraction of retained austenite increases from 30 to 60 minutes and subsequently decreases from 60 to 120 minutes. In order to study the effect of retained austenite on hardness resulting in decrease in overall hardness with increasing austempering temperatures and increases with increasing hold duration. The JMAK model has been fitted to the observed fraction of austenite as function of time and temperature, which results in the parameters such as the rate constant and Avrami exponent. The fitted results suggest a one-dimensional grain growth of the bainite in this steel. The predictions from a thermodynamic analysis using the para-equilibrium model and the T0-temperature are compared to the experimentally observed fractions of RA, which results in obtaining higher fractions than obtained experimentally. This thermodynamic analysis predicts an increase in the fraction of remaining austenite with increasing austempering temperature as observed experimentally in most cases. The thermodynamic study on the effect of Si concentration (1.62 wt.%)results shows that there is insufficient Si present in the steel to hinder the formation of cementite.