In situ observations of ferrite - Austenite transformations in duplex stainless steel weldments using synchrotron radiation

Ferrite (δ)-austenite (γ) transformations in the heat affected zone (HAZ) of a gas tungsten arc weld in 2205 duplex stainless steel are observed in real time using spatially resolved X-ray diffraction (SRXRD) with high intensity synchrotron radiation. A map showing the locations of the δ and γ phases with respect to the calculated weld pool dimensions has been constructed from a series of SRXRD scans. Regions of liquid, completely transformed γ, a combination of partially transformed γ with untransformed δ, and untransformed γ + δ are identified. Analysis of each SRXRD pattern provides a semiquantitative definition of both the δ/γ phase balance and the extent of annealing, which are mapped for the first time with respect to the calculated weld pool size and shape. A combination of these analyses provides a unique real time description of the progression of phase transformations in the HAZ. Results show that during heating, δ and γ both show signs of annealing as temperatures approach 550°C. The δ phase then starts to transform to γ as temperatures approach 700°C. Although supported by thermodynamic calculations, this δ→γ transformation during heating has not been directly observed until now. Following this reaction, the HAZ microstructure evolves in three different ways. For peak temperatures less than approximately 1100°C, δ retransforms, reverting to its original base metal fraction on cooling. When peak temperatures exceed approximately 1375°C, the microstructure completely transforms to δ before retransforming to a mixture of δ and γ during weld cooling. For peak temperatures between 1100 and 1375°C, γ is only partially transformed during both heating and cooling. Using these real time observations, important kinetic information about the transformations occurring in duplex stainless steels during both non-isothermal heating and cooling of welding can be determined.