13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Damage and fracture behaviours in advanced heat resistant materials during slow strain rate test at high temperature Mattias Calmunger1,*, Guocai Chai1, 2, Sten Johansson1, Johan Moverare1 1 Division of Engineering Materials, Department of Management and Engineering, Linköping University, SE-58183 Linköping, Sweden 2 AB Sandvik Material Technology R&D center, SE-81181 Sandviken, Sweden * Corresponding author: mattias.calmunger@liu.se Abstract As a renewable energy resource, biomass or biomass co-firing in coal-fired power plants with high efficiency are desired which corresponding to elevated temperature and high pressure. An upgrade of the material performance to austenitic stainless steels is therefore required in order to meet the increased demands due to the higher temperature and the more corrosive environment. These materials suffer from creep and fatigue damage during the service. In this study, these behaviours are evaluated using slow strain rate testing (SSRT) with strain rate down to 1*10-6/s at temperature up to 700°C. The influence of temperature and strain rate on strength and ductility in one austenitic stainless steel and one nickel base alloys are investigated. The damage and fracture due to the interaction between moving dislocations and precipitates are studied using electron channelling contrast imaging (ECCI) and electron backscattering diffraction (EBSD). The deformation and damage mechanisms active during SSRT are essentially the same as under creep. The influence of dynamic strain ageing (DSA) phenomena that appears in the tested temperature and strain rate regime is also discussed, DSA is intensified by increased temperature and decreased strain rate. Keywords Elevated temperature, slow strain rate, biomass power plant, austenitic stainless steel, nickel base alloy. 1. Introduction Biomass or biomass co-firing in coal-fired power plants is a contribution to renewable energy resources for sustainable energy production. Also high efficiency which requires higher temperature and pressure is highly desired. However, the materials used in those power plants with increasing temperature and pressure also need to be resistant to the corrosive atmosphere connected to combustion of biomass fuel [1, 2]. In the operating temperature range of biomass or biomass co-firing coal-fired power plants, austenitic materials often show dynamic strain ageing (DSA). The temperature regime of DSA is from 200 to 800°C for austenitic materials [3, 4] and arises during plastic deformation from interaction between solute atoms related to bulk or pipe diffusion and mobile dislocations or forest junctions[5, 6]. Temperature and strain rate directly influence diffusing solute atoms and the mobility of dislocations respectively, which effects DSA [6]. At elevated temperatures (400°C to 650°C) nitrogen and/or substitutional chromium atoms are responsible for DSA [7, 8]. Portevin-Le Châtelier (PLC) effect which is serrated yielding in the stress-strain curve, characterizes DSA. The PLC effect is due to the pinning and unpinning of dislocations or formation of new dislocations [9]. These materials can undertake low deformation rate from 10-5/s (low cycle fatigue) to 10-7/s (creep) during service. This study was focused on damage and fracture mechanisms related to low strain rate and high-temperature in two austenitic materials. Using uniaxial slow strain rate tensile testing (SSRT) at high-temperatures, the influence of low strain rates on these mechanisms could be investigated. Also precipitation due to high-temperature and deformation could be coupled to the damage and fracture behaviour.
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