Duplex stainless steels (DSSs), with a balanced fraction of ferrite and austenite, provide a great combination of superior mechanical properties and corrosion resistance. Super duplex stainless steels (SDSSs), containing a higher content of Cr, Mo, N, and Ni compared to standard grades, provide even higher corrosion resistance and better mechanical properties [
1‐
3]. Phase separation in the ferrite may, however, occur during intermediate temperature applications by the formation of alpha (
α)-alpha prime (
α’) domains, Cu-rich particles (CRPs), G-phase particles, etc., causing a toughness loss, known as “475 °C-embrittlement”. The separation occurs on a nanoscale in the temperature range of about 300–500 °C [
4‐
8]. The
α-
α’ separation during quenching and different solutionizing treatments has been the subject of different studies. In two studies [
9,
10], the degree of this phase separation has been calculated theoretically. Both showed that spinodal decomposition can occur during quenching but that, depending on the composition, it might be avoided by faster cooling. Zhou et al. [
11] showed that Cr atoms clustered in the solutionized condition of a Fe-46.6%Cr alloy, where this clustering decreased with increasing solutionizing temperature. Xu et al. [
12] also studied binary Fe-(20–40)% Cr alloys in furnace-cooled and brine-quenched conditions and showed that the initial state of phase separation may have a significant effect on the outcome of subsequent aging. Lemoine et al. [
13] also mentioned the progression of spinodal decomposition during quenching of a cast DSS alloy. Some recent studies also showed that noticeable phase separation can occur already after 3 min aging in SDSSs, with material datasheets indicating up to 50% toughness loss [
4,
14]. It, therefore, raises the concern of what the level of this phase separation is in as-fabricated thick SDSS sections and how it impacts the properties. Particularly, in previous studies of hot isostatically pressed (HIPed SDSS products, Smuk et al. [
15,
16] reported a toughness loss during cooling; however, its main cause remained unrevealed.
The aim of the present study is to investigate the state of phase separations in industrially fabricated thick SDSS plates and copper-bearing HIPed SDSS products using atom probe tomography (APT), arc heat treatment, scanning electron microscopy, and thermodynamic calculations. Low-temperature impact toughness and microhardness tests were employed to study how detrimental these phase separations are for the mechanical properties. The paper also shows the complex effect of Cu on phase separations.