Effect of isothermal heat treatment at 650, 750 and 850°C on the microstructures of lean duplex stainless steel S32101 welds

The present paper deals with the influence of post welding heat treatment on the microstructure of particular zones in welded joints of the lean duplex stainless steel S32101. The joints were produced by a MAG welding method using single-pass technique. The welded joints were then treated by isothermal ageing. The phase composition of each welded joint was studied and the ferrite content in parent metal and in the heat affected zone (HAZ) was evaluated. Detailed microstructural analyses were made using both optical and scanning electron microscopy. The influence of isothermal heat treatment 650, 750 and 850°C on the microstructural changes and formation of precipitates in different zones of welded joints of the lean duplex stainless steel S32101 were studied. The stainless steel in the as-welded condition shows nitrides precipitation only in the HAZ. Isothermal heat treatment at 650°C for a short time (15min) causes the formation of numerous precipitates of nitrides, both in the parent metal, HAZ and the weld. Extension of holding time to 90 min for all test temperatures promotes the formation of carbides and nitrides at the grain boundaries and inside grains in each zone of the welded joints. The presence of inter-metallic σ-phase was not revealed in either parent metal or welded joint even after heat treatment at 850°C for 90 min.