Microstructural and electrochemical characterisation of heat-treated 347 stainless steel with different phases

Type 347 stainless steel was solution annealed at three different temperatures (1100, 1250 and 1300 °C) followed by ageing at 650 and 750 °C. This was done to study the phase transformations that may occur in stainless steel 347 during welding and/or subsequent ageing. The microstructural examination showed chromium carbide and σ phase formation at the grain boundaries after ageing. The electrochemical behaviour was evaluated by double-loop electrochemical potentiokinetic reactivation test and anodic polarisation in HCl solution. Chromium carbide precipitation caused large increase in the degree of sensitisation, while σ phase had the major effect in decreasing the pitting resistance.

► Electrochemical behaviour correlated with different phases in stainless steel 347.
► Formation of both Cr23C6 carbide and σ phase at the grain boundaries at 650 °C.
► Only σ phase formation at the grain boundaries at 750 °C.
► As opposed to σ phase, large Cr depletion due to carbide formation.
► Large decrease in pitting resistance due to σ phase as opposed to carbide formation.