Alloying a topmost steel-plate layer with WC-tool constituent elements during friction stir processing

Friction stir processing (FSP) on weld toes of arc-welded high-strength steel joints was found to significantly increase the fatigue strength. This can be explained by an accompanied increase in the radius of curvature at the weld toes and grain refining, solid solution hardening in the topmost layer. Solid solution hardening seemed to be accompanied with the WC tool wear during FSP. The topmost layer contained the supersaturated W and C, leading to martensitic transformation. To clarify the alloying mechanism in the topmost layer, the stop action technique was employed during FSP, immediately followed by water-cooling the tool and the steel plate. The interface between the WC tool and the topmost steel-plate layer in the obtained keyhole samples was investigated. As tool rotational speed increased, Fe atoms diffused from the steel plate into the WC-tool Ni-based binder regions. Subsequently, some of the WC particles in the tool reacted with the Fe atoms to form Fe4W2C. The Fe4W2C particles agglomerated at the interface, followed by formation of a thin layer. The intermediate layer was fragmented and decomposed by FSP shear stress and friction heat, and alloying the topmost steel-plate layer with the WC-tool constituent elements.