Study of the effect of hot forging and continuous cooling on the precipitation of secondary phases and corrosion properties of UNS J93380 duplex stainless steel

Owing to their dual-phase morphology, duplex stainless steels offer an excellent compromise in terms of corrosion resistance and mechanical properties. Upon thermo-mechanical processing, these alloys are subject to the precipitation of intermetallic phases such as sigma (σ), chi (χ) and chromium nitrides, which might severely affect their overall performance. The objective of the present work is to evaluate the effect of hot forging on secondary phases in cast UNS J93380 super duplex stainless steels, characterized by the addition of copper and tungsten, which make them particularly suitable when employed in environments containing high amounts of chlorides. The hot forging simulations were carried out at a solution annealing temperature of 1100 °C, followed by continuous cooling at various rates to determine how they affect the kinetics of precipitation. Secondary phases were quantified and investigated by means of both microstructural analysis and corrosion testing. Specifically, DL-EPR technique was employed to test the material's behaviour towards intergranular corrosion and obtain the degree of sensitization of the specimens, which was then compared to the volume fraction of secondary phase precipitates to assess how the two variables might relate to each other. Pitting corrosion resistance of the steel was also evaluated by means of potentiodynamic polarization and critical pitting temperature measurements, to investigate the correlation between pitting and sensitization. The study revealed that hot forging and slow cooling rates always cause an increase in the precipitation of σ and χ as well as a decay in the corrosion properties, also confirmed by macroindentation measurements. On the other hand, rapid cooling favours the precipitation of quenched-in nitrides, which were demonstrated to aggravate the corrosion performance in terms of both IGC and pitting. To overcome such precipitation issues during forging, a possibility would be to perform an intermediate cooling step at low rate and quenching from lower temperatures. In addition to this, heating to higher solubilization temperatures and applying a higher degree of deformation should also be considered to enhance recrystallization.