Early Stages of Deleterious Phases in Super and Hyper Duplex Stainless Steel and Their Effect on Toughness

Duplex stainless steels combine excellent mechanical properties with good corrosion resistance. Consequently, they are in frequent use in various highly demanding applications, like Oil & Gas, chemical processing, pulp and paper, and desalination plants. Due to their high alloying contents, super and hyper duplex are susceptible to the formation of detrimental secondary phases such as chromium nitrides, chi, and sigma phase. The presented investigation deals with the precipitation of those phases in super and hyper duplex by employing rapid isothermal heat treatments on a Gleeble 3800 thermo-mechanical simulator. The Charpy-V impact toughness was measured as a function of the sensitization time and temperature in order to characterise their effect on low temperature toughness. Both alloys suffer from severe embrittlement at the early stages of sensitization. In super duplex at 850 °C for 20 s, chromium nitrides are precipitated, a chi-phase is formed after 200 s, and small amounts of sigma after 500 s. In hyper duplex, the precipitation sequence is the same but significantly faster. In this alloy, the sigma-phase is encountered after merely 50 s and ferrite is completely consumed after 500 s. © 2019, The Author(s).