Creep and anelasticity of conventional and ODS steels

Abstract

Materials for the components of advanced nuclear reactors are expected to undergo harsh operational conditions, in which high temperature levels (of the order of 650°C or higher) and severe mechanical stresses provide conditions for creep deformation to be significant as to become a key factor limiting the lifetime of the material. At the same time, components in nuclear power plants are subject to cycles of start-up and shut-down, due to operations of maintenance, refuelling, variations in demand and emergency stops. These conditions, characterized by removal of mechanical loads only or both mechanical loads and temperature, trigger a time-dependent recovery of the plastic strain accumulated during creep deformation, known as anelasticity. The studies herein presented were aimed at investigations on the creep performance and on the impact of stress transients on the creep behaviour of the materials, due to the anelastic response. These transient tests, carried out by imposing stages of load removal, were concentrated at simulating the in-service conditions. Specific goals were set for each material. For the 316H, the anelastic response under partial unloading conditions was investigated, in an effort to determine the magnitude of the back stress, which is the driving force for anelastic recovery, stemming from basic physical mechanisms within the crystalline domains. The Oxide Dispersion Strengthened (ODS) 316L steel had its mechanical properties characterised, with particular focus on the creep response, analysed under the scope of performance requirements established by structural integrity code for materials in nuclear applications. And, finally, the behaviour in full load removal transients was characterised for the MA956 ferritic ODS steel, in order to study its anelastic response. It was observed that, for the 316H, partial unloading stages produce the same recovery effect on the microstructure, provided that the drop in stress is accompanied by change in the dominant creep mechanism from dislocation-based to diffusion-controlled. Moreover, the heterogeneous dislocation arrangement model of back stress was successfully applied for calculating the intragranular stresses and, along with neutron diffraction measurements of intergranular stresses, found good correspondence with the anelastic behaviour of the material. As for the ODS 316L, creep properties at 650°C were found to comply with the RCC MR code for the stress levels tested, but its creep performance fell short of a conventional 316L. HR TEM surveys on the oxides and a pilot diffusion-bonding study showed the effects of particle growth and how they influence the mechanical response of this alloy. And, in the case of the MA956, the investigations suggested that anelasticity is absent in this ODS steel. Although a more detailed approach is required, all the results obtained provided evidence that most of the grains in this material are resisting creep deformation. The outcomes of the investigations are the deepening of the understanding of the potential for recovery of steels subject to creep deformation, in terms of the basic microstructural features, and how they influence the creep response. From these studies, better materials assessment procedures may be envisaged.