Duplex stainless steels (DSSs) are important engineering materials due to their combination of good mechanical properties and corrosion resistance. However, as a consequence of their ferrite content, DSSs are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation, namely, the ferrite decomposed into Fe-rich ferrite (α) and Cr-rich ferrite (α'), respectively. The phase separation is accompanied with a severe loss of toughness. Thus, the ‘475°C embrittlement’ phenomenon limits DSSs’ upper service temperature to around 250°C.

In the present work, Fe-Cr binary model alloys and commercial DSSs from weldments were investigated for the study of phase separation in ferrite. Different techniques were employed to study the phase separation in model alloys and commercial DSSs, including atom probe tomography, transmission electron microscopy and micro-hardness test.

Three different model alloys, Fe-25Cr, Fe-30Cr and Fe-35Cr (wt. %) were analyzed by atom probe tomography after different aging times. A new method based on radial distribution function was developed to evaluate the wavelength and amplitude of phase separation in these Fe-Cr binary alloys. The results were compared with the wavelengths obtained from 1D auto-correlation function and amplitudes from Langer-Bar-On-Miller method. It was found that the wavelengths from 1D auto-correlation function cannot reflect the 3D nano-scaled structures as accurate as those obtained by radial distribution function. Furthermore, the Langer-Bar-On-Miller method underestimates the amplitudes of phase separation.

Commercial DSSs of SAF2205, 2304, 2507 and 25.10.4L were employed to investigate the connections between phase separation and mechanical properties from different microstructures (base metal, heat-affected-zone and welding bead) in welding. Moreover, the effect of external tensile stress during aging on phase separation of ferrite was also investigated. It was found that atom probe tomography is very useful for the analysis of phase separation in ferrite and the radial distribution function (RDF) is an effective method to compare the extent of phase separation at the very early stages. RDF is even more sensitive than frequency diagrams. In addition, the results indicate that the mechanical properties are highly connected with the phase separation in ferrite and other phenomena, such as Ni-Mn-Si-Cu clusters, that can also deteriorate the mechanical properties.

Stainless steels are very important engineering materials in a variety of applications such as in the food industry and nuclear power plants due to their combination of good mechanical properties and high corrosion resistance. However, ferrite-containing stainless steels are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation of the ferrite phase, where it decomposes into Fe-rich ferrite (α) and Cr-rich ferrite (α'). The phase separation is accompanied with a severe loss of toughness. Therefore, the upper service temperature of ferrite-containing stainless steels in industrial applications has been limited to around 250°.

In the present work, Fe-Cr based steels were mainly investigated by atom probe tomography. A new method based on the radial distribution function (RDF) was proposed to quantitatively evaluate both the wavelength and amplitude of phase separation in Fe-Cr alloys from the atom probe tomography data. Moreover, a simplified equation was derived to calculate the amplitude of phase separation. The wavelength and amplitude was compared with evaluations using the auto-correlation function (ACF) and Langer-Bar-on-Miller (LBM) method, respectively. The results show that the commonly used LBM method underestimates the amplitude of phase separation and the wavelengths obtained by RDF shows a good exponential relation with aging time which is expected from the theory. The RDF is also an effective method in detecting the phenomena of clustering and elemental partitioning.

Furthermore, atom probe tomography and the developed quantitative analysis method have been applied to investigate the influence of different factors on the phase separation in Fe-Cr based alloys by the help of mainly mechanical property tests and atom probe tomography analysis. The study shows that: (1) the external tensile stress during aging enhances the phase separation in ferrite. (2) Phase separation in weld bead metals decomposes more rapidly than both the heat-affected-zone metals and the base metals mainly due to the high density of dislocations in the welding bead metals which could facilitate the diffusion. (3) The results show that Ni and Mn can enhance the phase separation comparing to the binary Fe-Cr alloy whereas Cu forms clusters during aging. (4) Initial clustering of Cr atoms was found after homogenization. Two factors, namely, clustering of Cr above the miscibility gap and clustering during quenching was suggested as the two responsible mechanisms. (5) The homogenization temperatures significantly influence the evolution of phase separation in Fe-46.5at.%Cr.