NDT-based assessment of shrinkages and dross in heavy nodular cast iron components of wind energy turbines

Material defects like shrinkage defects, dross, pores and chunky graphite are likely to occur in thick-walled castings and are a challenge for the foundries and their customers. Large and heavy nodular cast iron components are an essential part of wind energy turbines (WET) now and also in the future. The renewable energy market is also a growing area where reliable components and efficiency as an economic market aspect are core values. The further increase of the size of WET will require components of high strength, ductility and fatigue resistance. Due to the size and weight of thick-walled castings full-scale fatigue tests under realistic conditions are not possible in most cases. Therefore, flaw detection and materials characterization methods need to be improved for the assessment of WET components. With the help of X-ray analysis and the ultrasonic technique Sampling Phased Array (SPA) as well as conventional automated ultrasound techniques it is possible to get information about geometry and density as base for a numerical analysis of shrinkage defects in thick-walled castings concerning fatigue. For assessing the effect of shrinkage defects on the fatigue behavior, a method was developed for manufacturing hourglass specimens based on ultrasound results. Dross as another relevant material defect in thick-walled castings can be analyzed and characterized by non-destructive methods, too. Localization of dross and assessing its distribution and characteristic are the major tasks. Magnetic flux leakage and micromagnetic methods are used in order to localize and assess the type of dross close to the surface. Ultrasound techniques allow the assessment of the spatial structure of dross. Within the frame of this contribution the results of the non-destructive investigations will be presented and discussed. Special attention will be given to the assessment of size, location and severity of shrinkage defects and dross in thick-walled castings.