Owing to the existence of both high residual stresses and ferrite decomposition phases in a wide range of temperatures, the forging and quenching processes of heavy section pieces made of X2CrNiMoCuN 25.06.03 superduplex steel may lead to cracks, failures and loss of properties. Investigations were conducted on the influence of the cooling rates on microstructure and impact strength to define a possible retreatment of a heavy section piece (U-shaped piece 610 mm diameter, 350 mm high, 150 mm thick) presenting impact strength unconformities.

Various cooling rates (denoted as V₇₀₀, cooling down rate at 700°C) between 200°C/h and 52 000°C/h were applied to blocks originated from the piece. The impact strength is shown to drop by more than 180 J and the transition temperature at 50% crystallinity to rise by more than 160°C when V₇₀₀ is varied in the range 52 000°C/h to 200°C/h.

At 52 000°C/h and 23 000°C/h, only Cr₂N precipitates are observed in TEM and the Charpy V-notch rupture feature observed in SEM is mainly dimpled ductile transgranular with rare ruptures at the a /g interfaces. Nevertheless, at V₇₀₀ = 23 000°C/h, some ferrite grains present a brittle transgranular rupture with cleavages or pseudo-cleavages. Microstructural investigations conducted with the Tomographic Atom Probe revealed the presence of Cu rich clusters containing about 30 at.% Cu, and smaller than 3nm in diameter. Some Cr rich domains containing 50 to 60 at.% Cr and smaller than 2nm were also detected. These chemical heterogeneities can most probably be associated with pre-precipitation stage of e Cu and s phases. At 1650°C/h, e Cu precipitates and the Charpy V-notch energy at room temperature is strongly affected, the rupture feature being mainly brittle transgranular with cleavages and ductile intergranular with small dimples (intergranular precipitation of both Cr₂N nitrides and M₂₃C₆ carbides). For lower cooling rates, s and c phases precipitate and grow and the CVN energy at + 20°C becomes lower than 10 Joules.

The embrittlement is caused by the pre-precipitation stage and the precipitation of the intermetallic phases e Cu and s and not by a '_Cr phase as often written. The chromium nitrides and the Cr-Mo carbides seem to have a minor effect. The pre-precipitation and precipitation sequences are clearly related to the cooling rate and may possibly explain different behaviours of thick products, equivalent in chemistry and thermal states.

The embrittlement is reversible and a satisfactory retreatment is possible on pieces as close their final dimensions as possible up to 150mm thickness in the same way that for super duplex X2CrNiMoN 25.07.04 heavy section forgings.