Study the microstructure and hardness of FSW of API 70 steel at the presence of TiO2 particles

In this study, the microstructural properties and hardness of Friction Stir Welded (FSWed) of high strength low alloy API X70 steel at the presence of titanium dioxide (TiO2) particles were investigated. In this regard, a homogeneous mixture of TiO2 and X70 steel powders were inserted into the weld groove before applying FSW. The FSW method was applied on HSLA X70 with and without addition of titanium oxide (TiO2) particles. The optical microscopy and Vickers microhardness measurements were employed to evaluate the microstructure and hardness of the different weldments zones. The results showed that the hardness of various zones in the weldment are strongly depended on the microstructure which is affected by heat input and stiring action. In addition, the TiO2 particles were homogenously dispersed in the stir zone of TiO2-doped weldment and subsequently has changed the microstructure and hardness. In particular, a transition from an acicular ferrite (AF) dominant microstructure with a hardness value of 300 HV to a polygonal ferrite (PF) dominant microstructure with a hardness value of 180 HV was observed by moving from top surface region to near root region.