Kinetics and microstructural evolutions during the tempering of martensitic and nano-bainitic low alloyed steel : in situ experimental study and modelling

Nano-bainitic steels represent a new class of alloys, whose microstructure consists of nanostructured bainitic ferrite formed at low temperature with a high amount of retained austeniteleading to a high ductility and high tensile strength of the steel. Formation of nanobainite has been studied thoroughly in literature as well as tempering of nano-bainitic steels. More recently it has been shown that adding carbide forming elements such as V and Mo increases the resistance to softening and hence the mechanical properties of nano-bainite at moderate temperature. Investigating the secondary carbide precipitation inside a nanobainitic microstructure is thus necessary to optimize the thermal treatments for this promising new class of steels. Three initial microstructures of the same steel composition are investigated: martensite, martensite + retained austenite and nano-bainite. Studying the more conventional case of martensite has served as a basis to better understand the microstructure evolutions inside the nano-bainitic steel. The microstructural evolutions during the tempering were followed by complementary experimental techniques including dilatometry, in situ high energy synchrotron X-ray diffraction (HEXRD), conventional and high resolution TEM. The sequence of carbides precipitation and dissolution (transition-iron-carbides, cementite, and alloyed carbides) both during heating and holding is shown similar for the three initial microstructures. The kinetics are similar as well as cementite chemical composition and size distributions of cementite and alloyed carbides. It has been shown too that the three microstructures present a high retained austenite stability. Moreover, the analyses of the lattice parameters evolutions all along the tempering treatment associated with carbon mass balances have allowed to better understand the carbon distributions between carbides and matrix phases (martensite, bainitic ferrite, retained austenite). The nucleation and growth model from a previous work was upgraded to take into account secondary precipitation and different new features (e.g. para-equilibrium interface condition for first stage of cementite growth, dislocation recovery kinetics based on HEXRD experiments, etc.). This model predicts the kinetics of precipitation, the particle densities and size distributions as well as matrix and carbides mean composition for different tempering conditions. Apart from the comparison with the experimental results that is discussed, it allowed to interpret the similar tempering behaviour for the three initial microstructures