Physical and kinetic simulation of nitrogen loss in high temperature heat affected zone of duplex stainless steels

Abstract

High temperature heat affected zone (HTHAZ) of duplex stainless steels is prone to local corrosion attack due to a high ferrite fraction and nitride formation. Literature commonly attributes formation of this undesirable microstructure to rapid cooling from high peak temperatures. However, this study investigated the possible role of nitrogen loss in HTHAZ using a combination of physical and kinetics simulation. Applying a stationary gas-tungsten arc (GTA) on a water-cooled plate, a technique known as arc heat treatment, showed that considerable nitrogen loss occurred already after 0.5 min up to 150 µm from the fusion boundary. This zone was extended to 1300 µm after 600 min arc heat treatment. The results of bead-on-plate GTA welding and Gleeble testing replicating the thermal cycle in HTHAZ showed that the ferrite fraction of the real HTHAZ was 7% higher than that for Gleeble samples. This agrees with results from arc heat treatment, where ferrite fraction was found to increase due to nitrogen loss. Numerical and Dictra approaches were developed to simulate the kinetics of nitrogen loss in HTHAZ considering ferrite as the nitrogen rapid diffusion path towards the weld pool. Simulation showed good agreement with both welding and physical simulation. A combination of thermodynamic and kinetics simulations properly predicted the ferrite fraction at 1100 °C for different arc heat treatment times. In conclusion, the experiments (physical simulations and GTA welding) and kinetics simulation showed that nitrogen was lost from HTHAZ to the weld pool.