Relationship between ferrite–austenite phase transformation and precipitation behavior of sigma phase in super duplex stainless steel weldment

The presence of sigma phase in duplex stainless steel weldments can cause embrittlement. The precipitation takes place at a temperature range in which austenite is also formed. The aim of this study was to investigate the relationship between the ferrite–austenite phase transformation and sigma phase precipitation in super duplex stainless steel welds. Different microstructures created by autogenous gas tungsten arc welding (weld metal) and induction heating (heat-affected zone) were aged at 800–950 °C. For all investigated microstructures, there was a gradual increase in austenite content with the annealing time and the sigma phase precipitation did not begin until the ferrite ratio had reached approximately 50%. The change in the concentration of solute elements in the ferrite phase upon aging was investigated using energy dispersive X-ray spectroscopy (EDS). The nickel content decreased with an increase in the austenite content, whereas the chromium and molybdenum contents increased with a lower ferrite ratio. The chromium and molybdenum contents decreased after the sigma phase precipitated. It was found that the sigma phase occurs after the concentration of the solute elements reaches an arbitrary threshold, which for this alloy was obtained at approximately 50% ferrite. The start temperature of sigma phase precipitation was correlated with the ferrite-austenitic phase balance in all microstructures using the Johnson–Mehl–Avrami-Kormogolov (JMAK) model.