Microscopic Observation of Precipitation Behavior at Friction Stirring Zone of Super Duplex Stainless Steel

摘要

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, sigma, was observed in the heat affected zone (HAZ). The sigma phase precipitated at the interfaces between delta-Fe (ferrite) and gamma-Fe (austenite) grains 1-2mm away from the stir zone (SZ)/HAZ boundary. On the other hand, the Cr2N was observed together with the belt-like gamma-Fe grain aggregates in the vicinity of the advancing side (AS) of the SZ. The other intermetallic phase, chi, was also observed at a triple junction of gamma-Fe grains. This demonstrates that the precipitation of the Cr2N and chi phases correlates with the transformation of delta-Fe to gamma '-Fe (secondary austenite).