ESSC & DUPLEX 2019

Duplex stainless steels combine excellent mechanical properties and corrosion resistance, while simultaneously maintaining lower alloying costs than comparable types of stainless steels. Consequently, they are in frequent use in various types of highly demanding applications, like Oil&Gas, chemical processing and desalination plants. An increasing amount of alloying elements, as used in Super or-Hyper Duplex Stainless Steels (SDSS and HDSS), improves mechanical strength and corrosion resistance; likewise, the susceptibility to the formation of deleterious phases increases. Avoiding these unwanted phases is crucial during heat-treating and welding, so proper cooling conditions are necessary since small fractions already massively deteriorate toughness and corrosion resistance. Consequently, this paper deals with the isothermal sensitisation between 700 and 1000°C in SDSS and HDSS and highlights possible cooling conditions to minimize the formation of unwanted phases. Conductive heating by a Gleeble enables rapid and reproducible heat-cycles, with varying sensitisation times from 20 to 1000 seconds, within the aforementioned temperature span. Both SDSS and HDSS suffer from sensitisation during most of the examined time-temperature combinations, with HDSS being remarkably prone. TTT-Diagrams result from the degradation of charpy-v impact toughness and the relative phase fractions of Sigma-, Chi-Phase and chromium nitrides. The degree of sensitisation is characterised by optical and electrical microscopy and charpy-v impact toughness at -46°C. Chi- and Sigma-Phase both result in severe embrittlement. Nitrides also precipitate rapidly, however are significantly less detrimental to toughness.