Microstructure investigation of duplex stainless steel welds using arc heat treatment technique

A heat treatment technique was applied on standard duplex stainless steel welds in order to investigate the influence of thermal treatments, e.g., multi-pass welding, on the microstructure. By using a stationary arc, a spatial steady-state temperature field ranging from liquidus to room temperature evolves within a single sample and results in a graded microstructure. The arc heat treatment was applied for 10 and 60 min respectively, and the experimental results were compared against thermodynamic calculations. Metallographic investigations revealed the formation of secondary phases within distinct zones. For the 10 min arc heat treated sample, the formation of sigma phase was observed in a temperature range of 730–1000 °C and chi phase was found above 700 °C. For the 60 min sample, sigma phase formed between 675 and 1025 °C, while chi phase formed above 600 °C. In both samples, transformation of chi phase to sigma phase as well as the formation of secondary austenite at 575–1100 °C was observed. Hardness measurements identified brittle regions, which correlated with the regions enriched in sigma phase and decomposition of ferrite. Compared to the initial microstructure, regions with secondary phases showed increased sensitization to local corrosion, when tested according to ASTM A262-Practice A.