Laves phase precipitation and its transformation kinetics in the ferritic stainless steel type AISI 441

Electrolytic extraction followed by XRD analysis of precipitates after isothermal annealing of an AISI 441 ferritic stainless steel between 600 and 850 °C produced the time–temperature–precipitation (TTP) diagram for the Fe2Nb-Laves phase. The TTP diagram shows two classical C noses, the first one between 750 and 825 °C and the second one, estimated to be close to 650–675 °C. TEM analyses show two independent nucleation mechanisms, i.e. at about 600 °C nucleation takes place on dislocations but above 750 °C, grain boundary nucleation is dominant. The measured solvus temperature of 875 °C for Fe2Nb in this steel is 50 °C higher than predicted by Thermo-Calc®. Kinetic modelling at 800 °C agreed well with the experimental results for a surface energy of 0.435 J m−2 and an initial particle center-to-center separation distance on grain boundaries of about 0.2 μm.