In-situ Observations of Acicular Ferrite Growth Behavior in the Simulated Coarse-grained Heat-affected Zone of High-strength Low-alloy Steels

The formation of acicular ferrite in the simulated heat-affected zone of high-strength low-alloy steels was investigated by means of in-situ observation. The acicular ferrite laths or plates were nucleated on intragranular Zr–Ti complex oxides in big austenite grains and then lengthened very fast. The growth of individual acicular ferrite lath or plate was completed in a very narrow temperature range, especially at low temperatures. The measured lengthening rates of ferrite laths or plates were ranged from 33.3 μm/s to 190.5 μm/s, which fell between limit calculated assuming para-equilibrium and negligible-partition local equilibrium modes from the semi-empirical equation proposed by Hillert.