Characterisation of inclusions found in C–Mn steel welds containing titanium

Abstract

The main goal of the present study was to identify the chemistry of the phases on the surface of inclusions found in steel welds, to enable a better understanding of their role in the formation of acicular ferrite. Transmission electron microscopy and parallel electron energy loss spectrum imaging of specimens prepared by ultramicrotomy have been used to characterise inclusions found in C–Mn welds having different titanium concentrations. The results indicate that, for a weld containing 28 ppm titanium, which is the tough est weld studied, a phase comprising MnTi₂O₄ is present on the surface of the inclusions. As the concentration of titanium in the weld is increased to 120 ppm, the MnTi₂O₄ phase is found more towards the centre of the inclusions. It is partially enveloped by a slag of MnO. SiO₂. The presence of TiO, often cited as a phase promoting the formation of acicular ferrite, was found only in the specimen that contained 410 ppm titanium. Thermodynamic calculations using databases for oxide solutions corroborate the experimental findings. They show that, below 112 ppm titanium, the first phase to solidify in the liquid inclusions is MnO. SiO₂, followed by MnTi₂O₄. Above a concentration of 112 ppm titanium, the calculations indicate that there should be a reversal in the solidification sequence, i.e. MnTi₂O₄ should solidify first, followed by MnO. SiO₂. Further more, these calculations indicate that the inclusions are still liquid after the solidification of the steel matrix.