Evidence of pre-precipitation stages during cooling of X2CrNiMoCuN 25.06.03 superduplex steel

Owing to the existence of both high residual stresses and ferrite decomposition phases over a wide range of temperatures, the forging and quenching processes of a heavy section piece made of X2CrNiMoCuN 25.06.03 superduplex steel may produce cracks, failures and loss of mechanical properties. Investigations were conducted on the influence of various cooling rates (denoted as V700, cooling down rate at 700°C) between 200 and 52 000°C h−1 on microstructure and impact strength. The impact strength is shown to drop by more than 180 J when V700 is varied through 52 000–200°C h−1. As shown by TEM, conventional and 3D atom probes, the embrittlement is caused by the pre-precipitation stage and the precipitation of the intermetallic phases εCu and σ and not by the α′Cr phase as often reported. The chromium nitrides and the Cr–Mo carbides also observed, seem to have a minor effect. The pre-precipitation and precipitation sequences are clearly correlated to the cooling rate and may account for the variations in the mechanical integrity of thick products, equivalent in chemistry and thermal states.