Neutron diffraction analysis of martensite ageing in high-carbon FeCMnSi steel

A high-carbon metastable austenite phase was quenched in liquid nitrogen to obtain fresh athermal martensite. Different ageing treatments were performed in order to study the transformation of the martensite phase and the formation of carbides. Neutron diffraction experiments give detailed information on the transformations during the annealing treatment itself. The tetragonal athermal martensite transformed immediately, i. e. during the heating to the ageing temperature, to cubic martensite. Ageing at 400°C resulted in the decomposition of the austenite in ferrite and carbides and the formation of bainite. Those carbides could be determined as η-carbides transforming in χ-carbides after extended annealing times. As was expected, ageing at 170°C resulted in the formation of a small amount of η carbides while the austenite phase remained stable.