The mechanical behaviour of the duplex stainless steel AISI 329 has been investigated for ageing times up to 15,000 h at 475,425, 375, 325 and 275°C. The study has concentrated on changes in the monotonic stressstrain behaviour and fracture toughness as a function of ageing temperature and time. It is shown that the tensile behaviour of the steel changes strongly due to ageing. A large increase in yield strength and reductions in ductility and fracture toughness are observed. The deformation hardening behaviour of the aged steel is explained by using a model based on a modified rule of mixtures. Finally it is shown that the higher toughness of aged duplex stainless steels, in comparison with ferritic stainless steels aged under the same ageing conditions, may be associated with the increase in crack growth resistance induced by ductile ligaments of austenite which bridge the crack faces. NOMENCLATURE a =crack length a. = average radius of ductile ligaments b = Burgers vector E = Young's modulus G = shear modulus HV= Vickers hardness J,, =critical value of J integral for crack extension K,c = plane strain fracture toughness E, E,, = thickness and net thickness of WOL specimens K , = fracture toughness measured by the equivalent energy method KO, K , = fracture toughness of aged ferrite, and of duplex stainless steel k = constant n = strain hardening exponent P = load t =length of ductile ligaments w = width of ductile ligaments W = width of WOL specimens LY =ferrite V,, V, = volume fraction of ferrite and of austenite AG, = work of fractureAu; = strain hardening in the austenite phase when the femte starts to deform plastically y = austenite E, ~p = total and plastic deformation v = crack opening displacement u = applied stress ug = back stress us = stress due to geometrically necessary dislocations us = stress induced by statistically stored dislocations utr = load transfer stress a , , = yield strength uo = stress at the beginning of plastic deformation of austenite do,, Au, = strain hardening in ferrite and in austenite 645