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S79:E15
https://doi.org/10.3221/IGF-ESIS.79.10
The optimization of mechanical properties in superduplex stainless steel forgings requires precise control of phase balance and precipitation kinetics. This study investigates the effect of different heat treatment routes, including single- and double-step solution annealing and varying cooling rates, on Cr nitride precipitation and secondary austenite formation in S32760 superduplex stainless steel. Mechanical and corrosion properties were assessed through Charpy impact, tensile, Vickers hardness, and pitting corrosion tests, complemented by optical and SEM microstructural characterization. Unlike the heat treatment, the radial sampling position within the investigated forged component did not significantly affect the measured properties. While pitting corrosion resistance remained unaffected, impact toughness was highly sensitive to the thermal treatment. A single-step treatment at 1120 °C caused severe embrittlement due to extensive Cr nitride precipitation, whereas a second solution annealing step at 1060 °C was necessary to restore toughness. Its effectiveness, however, depended strongly on the cooling rate applied after the first step. Microstructural analysis revealed that slower cooling after the first holding phase promotes coarser nitrides, which act as nucleation sites for secondary austenite during the second annealing stage, enhancing N absorption and limiting subsequent nitride precipitation. Conversely, rapid cooling reduces both nitride and secondary austenite formation, favoring renewed nitride precipitation during final cooling. These results highlight the initial cooling rate as the key parameter governing the effectiveness and reliability of two-step heat treatment for large-scale superduplex components.