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

Abstract 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 V 700 , 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 V 700 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 e Cu 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.