Atom probe field ion microscopy study of commercial and experimental structural steels with fire resistant microstructures

The atomic scale microstructure of four structural steels designed to have fire resistant microstructures has been characterised using atom probe field ion microscopy (APFIM). These steels are microalloyed with molybdenum and niobium. Results show that the good high temperature strength of these steels is due to the very fine distribution of MC and Laves phase precipitates, molybdenum clusters, and molybdenum and niobium in solid solution. There is strong precipitation at approximately 650°C. These probably coherent precipitates have the effect of producing a very fine and stable dispersion.