Microstructural evolution during tempering of a multiphase steel containing retained austenite

Double cycle heat treatment has been imparted to a low carbon low alloy steel with a view to maximize volume fraction of retained austenite. ε-Carbide has been found to occur in the austenite phase. Its unusual presence has been explained as a result of enrichment by interstitials during isothermal holding. The steel has been subjected to low temperature ageing treatment to assess the stability of retained austenite against thermal severity. Retained austenite phase remained stable up to a temperature of 100°C. Decomposition of the austenitic phase having predominantly chunky morphology commenced at and above 200°C. Tempering at a temperature of 300°C led to the rearrangement of dislocations into parallel arrays. The mismatch between parent austenite and product ferrite phase resulted in the formation of interfacial dislocations. Upon tempering at 400°C, ε-carbide gave way to the formation of ε′(η)-carbide. The observation is extraordinary in view of low carbon content of the steel. Tempering at 500°C eventually led to the formation of stable cementite. Thus a sequence of events leading to the precipitation of cementite (θ) from parent austenite phase can be traced as: γR→α+ε→α+ε′η→α+θ