The Influence of Ageing at Intermediate Temperatures on the Monotonic Stress‐strain Behaviour and Fracture Toughness of a Duplex Stainless Steel

Abstract

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

THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MECHANICAL BEHAVIOUR OF A DUPLEX STAINLESS STEEL: Part II. FATIGUE LIFE AND FATIGUE CRACK GROWTH

“…The effect of ageing on fatigue behaviour of duplex stainless steels should be also related to this change in ferrite, which increases the yield strength by a factor as high as 2. At the same time, ageing tends to change the mechanism of fracture from nucleation and growth of ductile voids to cleavage with a strong reduction in fracture toughness (see [ 3 ] ) .…”

“…This change makes the steel brittle and limits severely its use in many potential applications. The influence of ageing on the mechanical behaviour under monotonic loading has been the subject of the first part of this work [3].…”

Cyclic deformation of superduplex stainless steels at intermediate temperatures

“…As previously commented, at this temperature, the kinetics of spinodal decomposition is quite fast and the long duration of the test induces a prominent aging effect. [11,14] Finally, at De t /2 5 9 3 10 À3 , secondary hardening was never observed. The reason is probably that fatigue lives at this high strain amplitude were too short to reach this stage.…”

“…At 475°C, the kinetics of spinodal decomposition of ferrite is quite fast. [11,14] It has been proven that this microstructural change interferes with dislocation glide, inhibiting cross-slip, and, as a consequence, high imposed strain values are required to induce plastic activity in aged ferrite. [12,[39][40][41] Llanes et al [12] performed room-temperature tests on DSS that had been previously aged during 25 and 200 hours at 475°C and showed that, at plastic strains around 10 À3 , substructural scenarios were very different for as-received and thermally aged steels.…”

“…This phenomenon leads to increments in mechanical strength and hardness, together with a reduction of ductility and toughness, the latter being a serious restriction on the maximum temperature for safe operation. From this perspective, although thermal aging effects on the cyclic response of DSSs at room temperature have been extensively explored, [11][12][13][14] studies on their cyclic properties at intermediate temperatures are rather scarce. [15,16,17] Regarding physical mechanisms, monotonic and cyclic mechanical responses exhibited for single-phase stainless steels at intermediate temperatures are usually related with the operation of dynamic strain aging (DSA) phenomena.…”