Effect of pre-existing martensite within the microstructure of nano bainitic steel on its mechanical properties

Partial quenching below the martensite start temperature accelerates the subsequent bainite transformation in nanostructured bainitic steels. However, it is essential to clarify how the microstructural variations affect the resultant mechanical properties in these materials. This article aims to study the strength-ductility combinations in nanobain after introducing partial martensite within the microstructure. Two series of heat treatments were designed after preliminary dilatometry tests. First set of samples were directly transformed to bainite at 250 and 300 degrees C and for the second set, bainite transformation has been implemented after partial quenching at 150, 140 and 120 degrees C to obtain different amounts of pre-existing martensite. Mechanical properties have been described according to the microstructural evolutions by optical, field-emission gun scanning electron microscopy and XRD tests. It has been shown that pre-existing martensite changed the amount of high carbon retained austenite and increased both carbon content in solid solution and dislocation density in austenite. Interfacial autocatalytic transformation of bainite approved during which bainite started to form from pre-existing martensite/austenite interfaces. Microstructural changes gave rise to strength promotion and reduction of the elongation and impact energy even if still an acceptable balance of strength-elongation-impact energy could be obtained.