Thermal, magnetic and composition analyses of the reverse transformation of intermetallic sigma phase to ferrite

The reverse transformation of the sigma phase to ferrite in a duplex stainless steel upon heating has been studied by using differential thermal analysis (DTA), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), quantitative metallography and magnetic susceptibility measurement. It has been demonstrated that the reverse transformation of the sigma phase to ferrite is sensitively affected by the morphology of the sigma phase. However, EDS has shown that the composition of the sigma phase does not significantly affect its reverse transformation behaviour. Magnetic susceptibility measurements and TEM have revealed that untransformed, residual ferrite is present even after prolonged annealing between 873 and 1173 K and hence it does play a role in the sigma-to-ferrite reverse transformation. Quantitative metallography was employed to follow the coarsening of the sigma phase upon annealing and DTA was used to estimate its enthalpies of dissolution. As a potential application, a method for temperature monitoring using duplex stainless steels and DTA is suggested.