Investigation on the precipitation behavior of M3C phase in T91 ferritic steels

As traditional 9–12% Cr heat-resistant ferritic steels, T91 steels have been considered as candidate reduced-activation materials for nuclear engineering applying due to its excellent creep resistance and high resistance to void formation during neutron irradiations at elevated temperature. Needle-like M3C precipitates are produced during the routine normalizing process before tempering. Differential scanning calorimetry and infrared radiation thermometer have been employed to study the precipitation behavior of the secondary M3C particles upon subsequent cooling process after austenization. Various austenization conditions (holding time, temperature and the subsequent cooling rate) were carried out to clarify effect of normalizing condition on the formation of the M3C phase. In spite of various austenization conditions applied, it is found that the precipitation of M3C phase is depends only on the cooling rate applied. Furthermore, the precipitation of M3C phase occurs before the onset of the martensite transformation, which is contrary to the previous statement that it takes place during the auto-tempering stage after martensitic transformation. The above observation points out that the precipitation of M3C would produce an effect on the subsequent martensitic transformation behavior, leading to the formation of wide martensite laths with a low dislocation density.