High Temperature Fatigue

About 35 years ago, thermal fatigue was identified as an important phenomenon which limited the lifetime of high temperature plant. In the intervening years many investigations have been carried out, primarily to give guidance on likely endurance (especially in the presence of time dependent deformation) but latterly, with the introduction of sophisticated testing machines, to provide knowledge of the underlying mechanisms of failure. A previous edited book (Fatigue at High Temperature, Elsevier Applied Science Publishers, 1983) summarised the state-of-the-art of high temperature fatigue testing and examined the factors influencing life, such as stress state, environment and microstructural effects. It also considered, in some detail, cyclic crack growth as a more rigorous approach to life limitation. The aim of the present volume (which in style and format follows exactly the same lines as its predecessor) is once again to pursue the desire to translate detailed laboratory knowledge into engineering design and assessment. There is, for example, a need to consider the limitations of the laboratory specimen and its relationship with engineering features. Many design procedures still rely on a simple endurance approach based on failure of a smooth specimen, and this is taken to indicate crack initiation in the component. In this volume, therefore, crack propagation is covered only incidentally, emphasis being placed instead on basic cyclic stress strain properties, non-isothermal behaviour, metallography, failure criteria and the need for agreed testing procedures.

The global energy consumption is increasing and together with global warming from greenhouse gas emission, create the need for more environmental friendly energy production processes. Higher efficiency of biomass power plants can be achieved by increasing temperature and pressure in the boiler section and this would increase the generation of electricity along with the reduction in emission of greenhouse gases e.g. CO2. The power generation must also be flexible to be able to follow the demands of the energy market, this results in a need for cyclic operating conditions with alternating output and multiple start-ups and shut-downs. Because of the demands of flexibility, higher temperature and higher pressure in the boiler section of future biomass power plants, the demands on improved mechanical properties of the materials of these components are also increased. Properties like creep strength, thermomechanical fatigue resistance and high temperature corrosion resistance are critical for materials used in the next generation biomass power plants. Austenitic stainless steels are known to possess such good high temperature properties and are relatively cheap compared to the nickel-base alloys, which are already operating at high temperature cyclic conditions in other applications. The behaviour of austenitic stainless steels during these widened operating conditions are not yet fully understood. The aim of this licentiate thesis is to increase the knowledge of the mechanical behaviour at high temperature cyclic conditions for austenitic stainless steels. This is done by the use of thermomechanical fatigue- and creepfatigue testing at elevated temperatures. For safety reasons, the effect of prolonged service degradation is investigated by pre-ageing before mechanical testing. Microscopy is used to investigate the microstructural development and resulting damage behaviour of the austenitic stainless steels after testing. The results show that creep-fatigue interaction damage, creep damage and oxidation assisted cracking are present at high temperature cyclic conditions. In addition, simulated service degradation resulted in a detrimental embrittling effect due to the deterioration by the microstructural evolution.

From concept to application, this book describes the method of strain-range partitioning for analyzing time-dependent fatigue. Creep (time-dependent) deformation is first introduced for monotonic and cyclic loading. Multiple chapters then discuss strain-range partitioning in details for multi-axial loading conditions and how different loading permutations can lead to different micro-mechanistic effects. Notably, the total-strain method of strain-range partitioning (SRP) is described, which is a methodology that sees use in several industries. Examples from aerospace illustrate applications, and methods for predicting time-dependent metal fatigue are critiqued.