Elevated Temperature Creep-Fatigue-Oxydation Interactions in 9–12 % Cr Steels: Effects of Tensile/Compressive Hold Times on Fatigue Life

The 9–12%Cr martensitic steels are candidate materials for several components of the generation IV and fusion nuclear reactors. In these future applications, in addition to long holding periods, cyclic loadings corresponding to start and stopoperations and maintenance must also be taken into account. Creep-fatigue interactions must therefore be considered to design these components. A broad literature review showed that between 20°C and 650°C the fatigue lifetime of these materials followed a unique Manson-Coffin law. Adding a stress-relaxation holding period significantly reduces the fatigue lifetime for total strain lower than Δε_t = 0.7%. For higher strain ranges, no significant effect of holding period exists anymore. Moreover, several studies reported a more deleterious effect of compressive hold times compared to tensile holding periods. Additional tests and detailed observations of the damage mechanisms responsible for fracture of pure fatigue, relaxation-fatigue and creep-fatigue tests were carried out on a 9 Cr − 1 Mo modified steel tested at 550°C in air. This material showed a strong work softening effect. The cyclic plastic behavior of the material was studied using an enhanced stress partitioning method to evaluate the kinematic, isotropic and viscous parts of the cyclic stress. It was concluded that in all the cases the observed softening effect was mainly due to the kinematic stress decrease [1]. The effect of a tensile or compressive hold time on fatigue life was also investigated [2, 3]. The deleterious effect of compressive hold times was thus confirmed. No creep cavitation was observed and the fracture was due to the propagation of transgranular fatigue cracks. Two distinct damage mechanisms were identified, depending on the strain range and the hold time : (i) crack initiation occurred due to usual Stage I extrusions/intrusions mechanisms leading to the propagation of a bifurcated crack; (ii) multiple cracks were initiated from the brittle fracture of the oxide layer formed at the free surface of the specimens. It was shown that this oxide failure leads to a penetration of oxygen along the microstructural boundaries enabling the cracks to propagate. Oxide layers grown during tensile (compressive) holding periods are mainly loaded in compression (tension) during the fatigue cycle. The critical strains necessary to crack oxide layers are lower for tensile loading (i.e. compressive holding periods) as shown by finite element and analytical calculations. Therefore compressive holding periods leads more easily to the second and more severe damage mechanism [2, 3]. A model, identified on short crack propagation tests and from experimental endurance curves, gives excellent predictions in pure-fatigue [4]. In creep-fatigue the predicted lifetimes are in the usual range [N_exp/2, 2N_exp] for all strain amplitudes and hold times. In addition, complex phenomena, such as the deleterious effect of compressive holding periods are also reproduced. An attempt is made to show how this model can be extrapolated in temperature to longer hold times.