Precipitation sequence and its effect on age hardening of alumina-forming austenitic stainless steel

The precipitation sequence during ageing of Fe-14Cr-20Ni-0.9Nb-2.5Al based alumina-forming austenitic (AFA) steel was explored through a transmission electron microscopy analysis and a small angle neutron scattering experiment. The samples were aged at 700 °C for up to 504 h. Particles of NbC, M23C6 and Ni3Al-type L12 were observed in the early stage of ageing. Metastable L12 particles were formed both in grain interior and along grain boundary. M23C6 carbides precipitated along grain boundary accompanied with precipitation of L12 particles. After ageing for longer than 48 h, particles of B2-NiAl and Laves-Fe2Nb were newly formed. We suggest the possibility of phase transition from L12 to B2 with increase in ageing time. Finally, this study examined the change of mechanical properties during ageing through a Gleeble hot tension test and a Vickers hardness test, and then the relationship between precipitation behavior and mechanical properties was carefully investigated and discussed in terms of precipitation behavior.