TOUGHNESS RECOVERY OF F55 GRADE STEEL AFTER 475 °C EMBRITTLEMENT

Super Duplex Stainless Steels are well-known alloys in the industrial world, mainly in the oil and gas field, for their excellent combination of mechanical and corrosion properties. Because of the increasing application to lower and lower temperatures, the toughness optimization is hence justified. Many detrimental phases can nucleate from approximately 300 °C because of their complex chemical composition. The 475 °C embrittlement phenomenon is one of the most known causes of toughness loss. The spinodal decomposition of ferrite induces a selective increase in ferrite hardness and a dramatic drop in impact energy. Such phenomenon can occur in both isothermal and non-isothermal conditions, especially during the cooling of large components from the solution-annealing temperature and during welding operations. Similar drawbacks can be also associated to mistakes during processing and service. The modification induced by this phenomenon will be investigated by macro-hardness and micro-hardness tests on the constituent phases and by Charpy impact tests varying the soaking time at 475 °C. The microstructural constituents of the crack path and its length will be studied by optical microscopy looking for correlations with the impact energy. Since the detrimental effects of the embrittlement can be reversed by a suitable heat treatment, the toughness recovery will be investigated varying the soaking time at different temperatures. Finally, the fracture surfaces will be analyzed by SEM to study the ductile and brittle mechanisms.