Twinning and Precipitation Processes in Hardened and Tempered 54SiCr6 Spring Steel

Using positron annihilation spectroscopy (PAS), x-ray diffraction (XRD), dilatometry and transmission electron microscopy (TEM) methods, the formation and evolution of the twins, carbides and dislocation density in medium carbon spring steel 54SiCr6 after quenching and subsequent tempering were studied. Combined measurements of positron lifetimes and coincidence Doppler broadening spectroscopy revealed at tempering temperatures of 350 and 400 °C; the formation of the Cu-rich defects characterized by single component with positron lifetime τ₂ ~ 130 ps, which is attributed to positron trapped on the defects that, obviously, are not dislocations or vacancies. The relative fraction of screw dislocations assessed using PAS was used to estimate the dislocation density by the modified Williamson–Hall method from XRD data. The change in the average distance between twins with increasing tempering temperature was studied using TEM. The evolution of retained austenite, carbides and cementite was characterized by XRD, TEM and dilatometry. A conclusion was drawn about the nature of defects characterized by a positron lifetime τ₂ ~ 130 ps.