Tetsu-to-Hagane
Online ISSN : 1883-2954
Print ISSN : 0021-1575
ISSN-L : 0021-1575
Welding and Joining
Friction Stir Welding of High Phosphorus Weathering Steel– Weldabilities, Microstructural Evolution and Mechanical Properties
Takumi KawakuboTomoya NagiraKohsaku UshiodaHidetoshi Fujii
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2020 Volume 106 Issue 12 Pages 892-901

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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 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, 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.

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© 2020 The Iron and Steel Institute of Japan

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