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

The main goal of the present study was to identify the chemistry of the pha ses on the surface of inclusions found in steel welds, to enable a better u nderstanding of their role in the formation of acicular ferrite. Transmissi on electron microscopy and parallel electron energy loss spectrum imaging o f specimens prepared by ultramicrotomy have been used to characterise inclu sions found in C-Mn welds having different titanium concentrations. The res ults indicate that, for a weld containing 28 ppm titanium, which is the tou ghest 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 120ppm, the MnTi2O4 phase is found more towards the centre of the inclus ions. It is partially enveloped by a slag of MnO.SiO4. The presence of TiO, often cited as a phase promoting the formation of acicular ferrite, was fo und only in the specimen that contained 410 ppm titanium. Thermodynamic cal culations using databases for oxide solutions corroborate the experimental findings. They show that, below 112 ppm titanium, the first phase to solidi fy in the liquid inclusions is MnO.SiO2, followed by MnTi2O4. Above a conce ntration of 112 ppm titanium, the calculations indicate that there should b e a reversal in the solidification sequence, i.e. MnTi2O4 should solidify f irst, followed by MnO.SiO2. Furthermore, these calculations indicate that t he inclusions are still liquid after the solidification of the steel matrix .