Microstructure and Mechanical Properties of Friction Stir Welded Q345 Steel

Abstract

The Q345 steel sheet of 2.2 mm thickness was welded by friction stir welding, and the microstructure and mechanical properties of the joints were analyzed. The experimental results demonstrate that a perfect joint, without internal defects, can be obtained using a stir-pin rotation speed of 800 rpm, a weld speed of 50 mm·min⁻¹, and water-sprayed cooling. The macroscopic morphology of the cross-section of the weld seam is bowl-like. Phase transition occurred in part of the heat-affected zone, and austenite grains turned to fine ferrite and pearlite grains during cooling: these co-existed with the non-phase transition base metal. The strongest dynamic recrystallisation occurred in the stir zone, where proeutectoidferrite exhibited slightly overheated characteristics and the matrix was an acicular distribution of sorbite. The hardness of the weld zone was evenly distributed and 12% greater than the base material. The tensile strength of the welded joints was higher than that of the base material. The enhanced mechanical properties were mainly due to the fine grain size in the heat-affected zone and the sorbite matrix in its stir zone.