Enhancing prior austenite grain boundary coherency by introducing partial bainite in martensitic steel

A method to enhance coherency of prior austenite grain boundary (PAGB) was suggested by exploring the relationship between prior austenite misorientation (θγ) and martensite misorientation (θα′) across PAGB. It was found that a double KS OR (where the variant holds near Kurdjumov-Sachs orientation relationship not only with austenite into which it grows but also with opposite austenite sharing the grain boundary) is a necessary condition for improving PAGB coherency. To achieve a double KS OR, the PAGB should be either an L-PAGB (θγ < 10o) or an H-PAGB (θγ > 45o). L-PAGB always holds double KS OR, whereas H-PAGB requires an initial nucleation event to achieve a double KS OR. The reason why initially nucleated H-PAGB is more likely to hold a double KS OR is related to variant selection to minimize interfacial energy. To make full use of variant selection to enhance PAGB coherency, partial bainite was introduced before martensitic transformation through austempering. As a result, PAGB coherency can be significantly improved while achieving an acceptable hardness loss.