Friction stir welding of high phosphorus weathering steel —weldabilities, microstructural evolution and mechanical properties

Abstract

Phosphorus (P) addition is expected to simultaneously increase the strength and corrosion resistance of weathering steels. However, P causes solidification cracking in the fusion welding process and reduces the toughness of the steel. To avoid these problems, the P content of weldable SMA490AW weathering steel is currently limited to below 0.035 mass%. High P steels which are impossible to be joined by the fusion welding process, can be joined by a solid-state joining process, i.e., friction stir welding (FSW). Because the stir zone obtained by FSW contained very fine grains, its toughness was expected to improve. This study applies FSW to high-P weathering steels and examines the weldability of the product. The microstructural evolution and mechanical properties of the stir zone were investigated at different welding temperatures. The macroscopic cross-sectional observations of the FSW joints revealed crack-free structures even in steel containing 0.3 mass% P. Moreover, FSW significantly refined the grain structure in the stir zone. Consequently, the ductile-to-brittle transition temperature of the stir zone was approximately 150°C lower in the steel containing 0.3 mass% P and welded below A1 (average grain size = 2.5 μm) than in the base material (average grain size = 23 μm). It appears that the grain refinement by FSW overcomes the embrittlement caused by excessive P content.