Observations reveal phase changes in super duplex stainless steel welds, suggesting significant microstructural effects.
To investigate the precipitation behavior of super duplex stainless steel in its weld by friction stir welding (FSW) at a low welding speed, we carried out microstructural observation and analysis. An intermetallic compound phase, σ, was observed in the heat affected zone (HAZ). The σ phase precipitated at the interfaces between δ-Fe (ferrite) and γ-Fe (austenite) grains 1 - 2 mm away from the stir zone (SZ) / HAZ boundary. On the other hand, the Cr2N phase was observed together with the belt-like γ-Fe grain aggregates in the vicinity of the advancing side (AS) of the SZ. The other intermetallic phase, χ, was also observed at a triple point of γ-Fe grains. One can interpret that the precipitation of the Cr2N and χ phases correlates with the transformation of δ-Fe to γ’-Fe (secondary austenite).
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Sugimoto et al. (2018) studied this question.
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