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Large components in the wind energy sector are made using cast iron with spheroidal graphite. The image shows a nacelle in which such components are used.
© Fotolia/Kruwt
Large castings in the wind energy sector
15.12.2016

The dross spots glow yellow in the fluorescent magnetic particle inspection carried out on the cast surface.
© Fraunhofer LBF

Impact of defects can be calculated

Large wind turbine components are made of cast iron with spheroidal graphite. During their production, however, chemical reactions can cause defects. These casting defects are called dross and their impact on component lifetime has, to date, only been evaluated using assumed material characteristics. There is a measure of uncertainty involved in providing evidence of their occurance. New non-destructive test methods should now be able to evaluate dross more reliably.

Cast iron with spheroidal graphite has proven itself a versatile and reliable material for castings in large machine construction and wind turbines. In the case of large components made of this material, however, the casting process is known to produce dross. These are material defects, also called discontinuities, which are formed by oxidic inclusions and impurities. Dross is difficult to manage within the limitations of casting technology. This is a frequent issue for casters during destruction-free component inspection, and for users of the cast components when assessing the effects of discontinuities on overall component lifetime. In particular for the wind energy sector, components with discontinuities are only approved for use following increased efforts in calculation and testing.

This has economic consequences in particular. To date, foundries have been working with material additions that are intended to "trap" dross. The material addition must then be removed, which requires enormous efforts in terms of personnel, machinery and financial resources. In addition, the defects in hard-to-reach areas can not be removed mechanically, only manually. If the permissible material or wall thickness is undershot, the operational strength of the component must be recalculated and the relevant spots will require welding. Only then can the component be given final approval.

Reliably proving material discontinuities

Currently, there are no methods that can reliably detect dross and take it into account for calculations. In practice, one gets by with assumed material characteristics. "The cyclic loadability or service life of a component can not be estimated reliably without non-destructively determined dross characteristics," Dr. Christoph Bleicher explained. Therefore, he and his team at the Fraunhofer Institute for Structural Durability and System Reliability LBF are currently developing a new concept as part of the project "unverDROSSen". It relies on measured values gleaned from non-destructive tests (NDT) to produce evidence of component lifetime. This is to help safely manage dross as early as the design phase, and the manufacture and approval, of large cast components for wind turbines and the plant and mechanical engineering sectors. "So dross can finally be characterised, assessed and component lifetime estimations can be improved," Dr. Christoph Bleicher added. This includes the integration of NDT, such as the magnetic particle test (left figure), ultrasound or micro-magnetic techniques for dross detection. These methods help use measured variables gained from the non-destructive testing of dross spots for a dimensioning concept for large castings, thereby making components with dross fit for use.

Assessment criteria and certification

The scientists carry out destruction-free tests on thick-walled test specimens as well as on components made of cast iron with spheroidal graphite and dross. In order to be able to carry out a numerical loadability analysis of the components and to develop a dimensioning concept, the different characteristics of the dross must then be characterised quasi-statically as well as cyclically.

The researchers derive a test and measurement method with the aid of measured data gained from NDT and findings from tensile strength and vibration stability tests carried out on specimens with dross. The research results are intended to make dimensioning and approval of large-size castings with dross possible. In order to implement the results as quickly as possible, the project team continuously discusses the results obtained with the relevant certification bodies – especially those responsible for the wind energy sector.

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