Effect of prior austenite grain size on the tensile properties and fracture toughness of nano-structured bainite

Abstract

Prior austenite grain size (PAGS) can significantly impact the bainitic lath thickness formed during austempering of high carbon steels. This study examines the effect of two different PAGS on the microstructure development and in turn the tensile properties and fracture toughness of nano-structured bainitic steels. Two PAGS have been obtained by austenitizing at 900 °C and 1000 °C respectively but the bainite fraction has been maintained to be around the same range by austempering at 250 °C. Less amount of blocky austenite is formed when the PAGS is larger. It is also observed that with an increase in the PAGS, the thickness of bainitic lath and retained austenite (RA) decreases and the tensile strength, tensile ductility and the plane strain fracture toughness is enhanced. The fraction of RA transformed to martensite is less for larger PAGS during a tensile test while showing greater ductility owing to the greater stability of finer RA films. • Higher austenitization temperature results in higher prior austenite grain size (PAGS). • Higher PAGS leads to finer, morphology of bainite and more amount of film type austenite. • Refinement of bainitic lath thickness leads to enhancement of strength, ductility and plane strain fracture toughness. • Bainitic ferrite block size reduces with an increase in PAGS. • Fractography features correspond to bainitic ferrite block size.