An experimental and computational investigation of the high temperature behaviour of MarBN steel with application to effects of manufacturing
O'Hara, Eimear Marie
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The key step for next generation power plants is the development of advanced materials capable of achieving high flexibility and efficiency at increased steam temperatures and pressures. Such operating conditions will cause increased fatigue and creep degradation of plant components, where a key limitation of operating under such conditions is the capability of the current generation of materials. Consequently, multi-scale characterisation via experimental and computational methods is necessary to both characterise and predict next generation material behaviour under flexible plant operating conditions at increased temperatures. Attention is focussed here on MarBN, a new precipitate-strengthened 9-12Cr martensitic steel, with improved strength and microstructure stabilisation under long-term loading via increased tungsten solute strengthening and boron enriched grain boundary precipitates. High temperature low cycle fatigue, creep-fatigue, oxidation and corrosion testing is performed on cast MarBN, and compared to both a forged equivalent, and current state-of-the-art material, P91. Microstructural analysis allows identification of micro- and nano-scale strengthening and degradation mechanisms, as a result of high temperature exposure. Quantification of the shapes and volume fractions of defects in cast and forged MarBN, before and after high temperature low cycle fatigue testing, is performed via 3D X-ray micro-computed tomography. Based on these experimental methods, uniaxial and multi-axial life prediction and damage models, in conjunction with a unified cyclic viscoplastic material model, are developed and applied to cast MarBN at 600 °C and 650 °C. 3D X-ray micro-computed tomography, in conjunction with microstructural analysis, has identified manufacturing defects, in both cast and forged MarBN, as a primary source of fatigue crack initiation. Both idealised (spherical) and measured (complex) defects, acting as regions of localised stress and strain accumulation, are predicted to cause fatigue crack initiation at less than 12% of total fatigue life in all cases. However, forging is found to significantly reduce the volume fraction and complexity of manufacturing defects, compared to the cast material, and as a result, increases fatigue life by approximately double.