Evolution of compressive strains in retained austenite during sub-zero Celsius martensite formation and tempering

The development of martensite during sub-zero Celsius treatment of a 1 wt.% C, 1.6 wt.% Cr steel was investigated by in situ and ex situ (synchrotron) X-ray diffraction at the synchrotron facility HZB-BESSY II in order: (i) to quantitatively assess the fractions of retained austenite and martensite; (ii) to measure the evolution of the lattice strain in retained austenite; and (iii) to identify the different stages of tempering. This work shows for the first time that the compressive strains built up in austenite upon martensite formation during sub-zero Celsius treatment are retained after tempering. Furthermore, it is demonstrated that sub-zero Celsius treatment after tempering leads to compressive strain in austenite. Finally, it is reported that no compressive strain builds up in austenite when the martensite formation occurs below a certain critical temperature.