This paper presents results of the research on impact of microstructure of austenitic-ferritic steel of duplex type on its mechanical properties and susceptibility to stress corrosion cracking. As showed, improper processing technologies more and more often used in shipbuilding industry for plates and other half-finished products made of duplex steel may cause significant lowering their properties, which frequently makes their replacing necessary. Results of the tests on stress corrosion under tension with low strain rate (SSRT) conducted in an inert and corrosion (boiling magnesium chloride) environment, are presented. It was proved that even minor structural transformations taking place in 500°C ageing temperature lower corrosion resistance of the steel. Structural transformations occurring in 700°C temperature to a smaller extent influence susceptibility to stress corrosion of the steel, however they cause drastic drop in its plasticity.