二相ステンレス鋼レーザ溶接部におけるオーステナイト相の析出挙動に及ぼす溶接速度と溶接後熱処理の影響

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  • Effects of Welding Velocity and Short Time Post Weld Heat Treatment on Precipitation of Austenite in Laser Beam Welds of Duplex Stainless Steels

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When conventional arc welding methods welded various kinds of duplex stainless steels, it is well known that the ferrite phase was enriched in welds. Notably, in the case of laser beam welding (LBW), due to high energy density and welding velocities, the phase balance might be remarkably changed from that in the base material. Also, it is concerned that such phase unbalances in welds because of the degradation of corrosion resistance, mechanical properties, and so on. Therefore, it is crucial to clarify the effect of alloy composition and welding heat cycle on microstructure change. Also, the morphology of phases formed in welds and mechanism of phase transformation during LBW thermal cycles should be clear, to predict the phases formed in LBW welds of duplex stainless steels and recovering method to maintain the phase balance. This experimental, detailed microstructure of austenite precipitated at ferrite grains in LB welds was investigated in this study. The morphology of widmanstätten austenite was observed by a scanning electron microscopy (SEM) after an immersion test using a sulfuric acid aqueous solution. The austenite was precipitated from grain boundaries growth with plate shape into the ferrite matrix. The elemental distributions in the welds were examined by an electron probe microanalyzer (EPMA). It was revealed that nitrogen was concentrated in austenite. Based on the result by electron backscattered diffraction pattern analysis (EBSD), it suggested that austenite precipitates at high-angle grain boundaries and widmanstätten austenite growth were maintaining some kinds of orientation relationship with ferrite matrix.

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