Inclusion Characteristics and Acicular Ferrite Nucleation in Ti‐Containing Weld Metals of X70 Pipeline Steel

The X70 steel weld metals with Ti content of 0.003–0.13% are prepared by single pass submerged‐arc welding process. The effects of Ti content in weld metals on the constituent phases of inclusions and potential of acicular ferrite nucleation on the inclusions are in detail investigated by means of electron probe micro‐analysis, scanning transmission electron microscopy, and thermodynamic calculation. The obtained results show that with an increase of Ti content, primary constituent phases of inclusions are changed from (Mn–Al–Si–O) compound to a mixture of spinel and pseudobrookite solid solutions, eventually to pseudobrookite. The spinel solid solution is characterized by MnTi2O4 constituent. The proportion of Ti3O5 in pseudobrookite is increased remarkably with an increase in Ti content at the expense of MnTi2O5 constituent. Compared with pseudobrookite, spinel has lower Ti concentration, but significantly higher Mn content regardless of Ti content in the weld metals. In the case of presence of a considerable amount of spinel primarily composed of MnTi2O4, Mn element is enriched strongly in the inclusions, resulting in the development of Mn‐depleted zone in matrix around the inclusions, which enhances the capability of inclusions nucleating intragranular acicular ferrite.