Improvement of toughness and strength of thick structural steel weld by friction stir welding conditions

Structural steel plates with tensile strength of 490 MPa and 6 mm in thickness were successfully joined with microstructural control by single sided one-pass butt friction stir welding (FSW). The microstructure in the stir zone (SZ) could be changed to bainite, bainite with proeutectoid ferrite, and fine equiaxed ferrite based on the welding temperature. The bainite increased the hardness and tensile strength, and the ferrite gave the joint similar mechanical properties similar to the base metal. The welding temperature was controlled between below Ac1 and above Ac3 by the rotating rate of the tool and tool materials; i.e. Ir–Re alloy and sintered c-BN with W–Re.