Determination of novel assessment criteria for the Evaluation of welding-related micro cracks using high resolution synchrotron-refraction-computer-tomography

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The welding processing of innovative materials is always connected with high requirements. By minor imperfections a safe integrity of welding constructions can not be guaranteed when subjected to stresses. In this context, micro-cracks in µm-range present a serious danger because a detection by non-destructive testing may not be possible or is limited because the size of the cracks is mostly smaller than the resolution of conventional X-ray techniques. Due to the high safety significance and the very high costs for purchasing and processing, specifically for nickel-base alloys, it is of high interest to analyse reasons for micro-crack formation and to create corrective actions for avoiding cracks.Nickel-base alloys tend to form micro-cracks of the type ductility dip cracking (DDC) during welding. Below recrystallization during cooling of the weld seam a decrease of deformability appears. When critical strains are exceeded intergranular cracks occur in the solid state. The micro-cracks are formed in the filler metal or in the heat affected zone (HAZ). Causes for crack formation are currently unknown, but in literature grain size and precipitation condition on grain boundaries are referred to as main influences. Previous investigations concentrated on filler metal and HAZ of the base material. However, analyses on real structures of a HAZ with different precipitation conditions and, therefore, different grain structures, have not taken place yet.Therefore, the research objective is the determination of innovative evaluation criteria of micro-cracking in the HAZ of welds. Micro-cracks are characterised and quantified by means of high-resolution synchrotron-refraction-computer-tomography. The strain-to-fracture (STF) test is used to create cracks during thermo-mechanical loads of the base material. Two nickel-base alloys with the same alloy matrix but different precipitation conditions are compared. Moreover, physical simulated structures of a HAZ enable the correlation between microstructure, precipitation kinetics and grain structure. Based on these results temperature-strain curves will be developed for different areas of the HAZ.Finally, it is the goal of the studies to create an extended model for micro-crack formation, which includes both thermo-mechanical and microstructure specific influence factors as well as evaluation criteria including a voluminous network of cracks. With given material factors a statement about the susceptibility against micro-cracking in the HAZ should be made. In a renewal proposal the results will be verified on multi-layer welds under restraint.