Quantitative assessment of intermetallic phase precipitation in a super duplex stainless steel weld metal using automatic image analysis

The microstructure of weld metal of the type 25%Cr-10%Ni-4%Mo-0.28%N in both as-welded and isothermally heat treated (temperature range: 700-1050 degrees C; time range: 10s-72h) conditions has been investigated. Multipass welding was performed in Ar+2%N-2 atmosphere using GTAW. By means of the electron diffraction technique: a-phase and;C-phase were detected and investigated;C-phase precipitated more readily than a-phase and was found to be a precursor to a-phase by providing suitable nucleation sites. Quantitative image analysis of ferrite and intermetallic phases was performed as well as manual point counting (ISO 9042). Automatic image analysis was found to be more accurate. The results were used to assess the TTT-diagram with respect to intermetallic phase formation. On the basis of these results a CCT-diagram was computed, considering the intermetallic phase formation described by an Avrami type equation and adopting the additivity rule.