Internal Stress Generation in Austentic Stainless Steels During Creep Deformation

Abstract

It has been well established in materials such as austenitic steels and aluminium alloys that plastic strain leads to generation of internal stresses. A number of intergranular factors such as the anisotropic stiffness and yield behaviour of a single crystal, orientation of grain families to the loading direction, and the constraint each grain places on its neighbours are responsible for creating these stresses. The presence of these accumulated internal stresses in power plant components is important because they interact with the applied external stress and play a critical role in the initiation and development of material degradation that may lead to eventual failure. This study focuses on measuring the generation of internal (intergranular) strains and stresses in austenitic stainless steels subjected to creep deformation. Creep processes increase the overall inelastic strain in a material and this correspondingly alters the internal strain state. A combination of in-situ and static neutron diffraction measurements was conducted to assess the internal strain generation through a material’s creep life. These experiments have revealed similarities between internal strain generation during primary creep deformation and that during monotonic tensile deformation leading to the conclusion that common mechanisms may be responsible. Such studies are important in the current context when a number of power plants are being life extended. Components in high temperature service applications undergo a number of creep-fatigue cycles during their operation. It is vital to have accurate and robust life assessment procedures that can take account of long-term internal strain evolution effects to maintain economic but safe operation and avoid costly repairs or replacement.