High-Temperature Fatigue Behaviour of Austenitic Stainless Steel : Influence of Ageing on Thermomechanical Fatigue and Creep-Fatigue Interaction

Abstract

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. CO 2. 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 shutdowns. 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.

Stress-Controlled Creep–Fatigue of an Advanced Austenitic Stainless Steel at Elevated Temperatures

“…One of softening manifestations is the sigmoidal hysteresis loops ( Figure 7 ), rather than symmetric loops [ 31 ], where de-twining is most probably due to the activity of partial dislocations. Additionally, the unique orientation relationship between the parent grains and the twin grains favors de-twining during loading.…”

“…In short, in hold time tests such as the 60 s hold time ( Figure 7 a), more than one cycle was needed to cause material softening and shift to the negative strain regime, as mentioned above. Wärner [ 31 ] studied the high-temperature fatigue of austenitic stainless steels, and found that dynamic recrystallization can occur if a certain critical strain is reached. The number of cycles needed to reach this critical strain depends on the dwell time applied in each test.…”

High Temperature Fatigue Behaviour of Austenitic Stainless Steel : Microstructural Evolution during Dwell-Fatigue and Thermomechanical Fatigue

“…For safety reasons, the effect of prolonged microstructural degradation is also investigated to evaluate the durability of the materials. More specifically, the following research questions (RQ) have been addressed: [28] which was presented in December 2018. However, the content has been significantly modified and extended since then.…”

The state-of-art of microstructural evolution of bearing materials under Rolling Contact Fatigue

“…First, the toughness of the material should be improved to resist the impact load, such as using carburised steel [34]. Second, the surface damage and the probability of crack occurrence can be reduced by coatings [94,95], especially in the case with roll-slip phenomenon. Finally, reducing inclusions in steel, especially those that induce WECs easily, can inhibit the occurrence of early fatigue.…”