On High-Temperature Behaviours of Heat Resistant Austenitic Alloys

Advanced heat resistant materials are important to achieve the transition to long term sustainable power generation. The global increase in energy consumption and the global warming from greenhouse gas emissions create the need for more sustainable power generation processes. Biomass-fired power plants with higher efficiency could generate more power but also reduce the emission of greenhouse gases, e.g. CO2. Biomass is the largest global contributor to renewable energy and offers no net contribution of CO2 to the atmosphere. To obtain greater efficiency of power plants, one option is to increase the temperature and the pressure in the boiler section of the power plant. Raised temperature and pressure increase the demands of the operating materials of the future high-efficient biomass-fired power plants. This requires improved properties, such as higher yield strength, creep strength and high-temperature corrosion resistance, as well as structural integrity and safety. Also, the number of start-and-stop cycles will increase, leading to demands on increased material performance under cyclic loading, both from thermal and mechanical loads. Heat resistant austenitic alloys, such as austenitic stainless steels and nickelbased alloys, possess excellent mechanical and chemical properties at the elevated temperatures and cyclic loading conditions of today’s biomass-fired power plants. Today, some austenitic stainless steels are design to withstand temperatures up to 650 ◦C in tough environments. Nickel-based alloys are designed to withstand even higher temperatures. Austenitic stainless steels are more cost effective than nickel-based alloys due to a lower amount of expensive alloying elements. However, the performance of austenitic stainless steels at the elevated temperatures of future operation conditions in biomassfired power plants is not yet fully understood. This thesis presents research on the influence of long term high-temperature ageing on mechanical properties, the influence of very slow deformation rates at high-temperature on deformation, damage and fracture, and the influence