Effects of ausforming on the microstructure and stability of blocky austenite in nanostructured bainite

Abstract

Nanostructured bainitic steels exhibit high strength and toughness. A potential approach to improving their toughness is enhancing the chemical or mechanical stability of blocky austenite while maintaining the volume fractions of austenite and bainitic ferrite. This study investigates the effects of ausforming at 250 °C with a 20 % strain on the microstructure and stability of blocky austenite and contrasts these effects with non-ausformed bainite. The stability of austenite is assessed by cryogenic treatments. Two types of bainitic ferrite are observed in ausformed bainite. The fine bainitic ferrite forms around austenite twins and has the Kurdjumov-Sachs (K-S) orientation relationship with austenite. In contrast, the coarse bainitic ferrite, which has the Nishiyama-Wassermann (N-W) orientation relationship, creates an interlocking microstructure where blocky austenite is refined and has a high dislocation density. The blocky austenite in the ausformed bainite remains untransformed after the cryogenic treatment, while some blocky austenite in non-ausformed bainite transforms into martensite. These results suggest that two types of bainitic ferrite may form via different mechanisms, and that the interlocking microstructure enhances mechanical stability of blocky austenite by dislocations and block size refinement.