Ferrite transformation from oxide–steel interface in HAZ-simulated C–Mn steel

Abstract

Abstract In steel weld metals and their heat affected zones, acicular ferrite or intragranular ferrite developed on selected oxide inclusions within prior austenite is a key to refining the microstructure and hence improving the toughness. In this study, to obtain a better understanding of the mechanism of the intragranular ferrite formation, different oxides were deposited onto C – Mn steel by a physical vapor deposition process, and ferrite formation was examined at the different oxide – steel interfaces after an HAZ-simulated thermal cycle. TiO, MgO, CaO and Al2O3 were deposited on C – Mn steel containing 0.16% C and 1.4% Mn and then they were heated up to 1623 K and cooled down at a cooling rate of 20 K s−1. Allotriomorphic ferrite was found to develop from TiO and MgO, while no allotriomorphic ferrite but Widmanstätten ferrite formed from CaO and Al2O3. Electron backscatter diffraction pattern analysis indicates that the allotriomorphic ferrite growing from the oxide has a specific orientation relationship either with the oxide or with prior-austenite. In the former case, the ferrite seems to be aligned so as to minimize the misfit against the oxide, implying that the likelihood of ferrite formation can be increased by the oxides having a good lattice match with ferrite. Manganese depletion was identified in the vicinity of TiO, but it is concluded that, although it encourages ferrite formation from the oxide, it is not a critical factor for the ferrite formation because of the fact that the ferrite formation was even found from TiO in manganese-free steel. First principles calculation indicates that the interfacial energy between oxide and steel is reduced when the magnesium in MgO is replaced by titanium, which suggests that the ferrite formation from TiO is also affected by the chemical effect of titanium.