Grain interaction in local plastic deformation of welded structural steel - Influence of length scale on sub-grain deformation behaviour for polycrystalline BCC material

Polycrystalline body-centred cubic (BCC) steel is the most commonly used structural material in the transportation industry. In order to optimize sustainable use of steel materials in welded structures such as ships, better fundamental understanding of the factors affecting material behaviour is required in different length scales. This thesis studies the length scale interface between microstructural features and continuum scale deformation in BCC steel materials. Special attention is paid on characterization of microstructure and local plastic deformation in heterogeneous steel weld metals. In this work, microstructural characteristics are correlated with plastic deformation response, and the fundamental deformation mechanisms are resolved using electron backscatter diffraction. The local deformation process is characterized for welded steel using instrumented indentation testing in different length scales, ranging from a fraction of one grain to tens of grains. A serial sectioning procedure is developed in order to consider the stochastics of the plastic deformation process in heterogeneous microstructures. Furthermore, advanced orientation data post-processing and misorientation analysis are utilized to identify the plastic deformation zone and formation of dislocation cells beneath hardness indentations. The results of the thesis reveal the importance of grain size dispersion to mechanical properties, and the significance of grain interactions for the plastic deformation process in polycrystalline BCC steel. Comparison between base and weld metal revealed that the size of the plastic deformation zone is proportional to the average grain size. A transition region was defined between continuum and single crystal material behaviour, where the interaction of grains of different size controls the local plastic deformation of polycrystalline steel. The strongest influence grain interaction on hardness variation was found to take place at indentation diagonal lengths 0.1 – 2dv, when slip transmission primarily occurs between two grains. When the size of the plastic deformation zone is considerably larger than the average grain size, spatial hardness variation decreases significantly. In this regime, hardness and strength are affected by average grain size and grain size dispersion. To consider these aspects, a modified Hall-Petch relationship is introduced utilizing the volume-weighted average grain size based on the rule of mixtures. The modified Hall-Petch relationship is validated with literature data, showing its applicability to a wide range of materials that show grain size dependent mechanical properties.