A Comparative Study of the Non-Metallic Inclusions in C-Mn Steel Weld Metals Containing Titanium or Aluminium

This contribution describes a comparative study on the effect of additions of individual amounts of aluminium or titanium to C-Mn steel weld metal and the implications of these additions for weld-metal microstructures and mechanical properties. The experiments involved the evaluation of the metallic component of the weld-metal microstructures by optical microscopy and the determination of the overall chemistries and phases present in the non-metallic inclusions in the weld metals by analytical TEM. The inclusion size distributions and volume fractions were also determined by SEM. Inclusions in essentially micro-alloy free weld metals, containing less than 6 ppmw of both elements, were found to contain small (less than 3.0 mol.%) amounts of Mn(Cu)S and mainly MnO.SiO₂ + SiO₂, with no Ti oxides and traces of Al₂O₃; these inclusions nucleated only about 10 % acicular ferrite (AF). Additions of Ti to similar weld metals, containing less than 6 ppmw Al, reduced the sulphide contents of the inclusions and the MnO.SiO₂ + SiO₂ was initially partially replaced with MnO.Ti₂O₃ (28 and 120 ppmw Ti) and subsequently with predominant proportions of TiO (410 ppmw Ti). These weld metals contained significant amounts of acicular ferrite (from 58 to 72%) and had improved strengths and notch toughness. In contrast, additions of Al proved less effective at increasing AF content. Indeed, it wasn’t increased significantly until about 250 ppmw Al or more was present in the weld metal so that γ-Al₂O₃ was formed in the inclusions. In this case between 17 and 22 % AF was nucleated with some increase in strength and toughness.