Wind turbines are designed for a calculated lifespan of 20 years, which means that ideally, the machine is supposed to work without interruption throughout these 20 years. The fatigue strength of the material is therefore of great significance since large-volume, thin-walled cast parts are also generally needed here to keep the weight of the wind turbine to a minimum.
It is imperative to ensure that the unfinished surface areas are absolutely free of any kind of cavities in the material that could cause a notch effect. Typical examples of this type of defect include cavities that result from adhering residual mold material, slag components adhering to the surface, grooves from pattern stripping, scores caused by improper application of the coating, metal slag and similar defects. Particular attention must be paid to the elimination or rather the prevention of metal slag since this can weaken the elasticity modulus of the material by up to 50%.
The notch effect that results from such occurrences must generally be eliminated. This is usually accomplished through grinding while adhering to the required minimum wall thicknesses.
The liquid material, which consists of special pig iron, selected scrap steel and portions of cleaned scrap return, should be melted in a high-performance medium frequency induction furnace.
It is only possible to set the required metallurgical parameters in a way that is ideal for casting if all process steps are monitored continuously during the melting process and if the batch receives special inoculation treatment while the quality requirements for all feedstock are ensured. This is followed by deslagging the melt after the magnesium treatment and further process steps, such as sampling and primary inoculation. A secondary inoculation is usually performed in the gate basin when casting.
Reliable casting quality/homogeneity of the material is assumed under consideration of a confidence factor when the computational design of the cast parts is done. The caster must verify that this planned quality grade is fulfilled throughout all phases of cast manufacturing, from planning (simulations of mold filling and solidification) to production (verification of process reliability) and the outgoing goods check (ultrasonic examination, X-ray inspection, structural examination, etc.). The mechanical properties must be determined for every cast part by means of a tensile test and a notched bar impact test that are performed on samples from cast-on test specimens.
Solutions for wind turbine casting
Together with its partners, ASK Chemicals is developing system solutions especially for large-scale cast parts for wind turbines. These solutions deal with the special requirements for binder systems, such as high dimensional consistency and high thermal stability, and also with low-sulfur furan resin systems. Tailor-made products for ensuring the purity requirements for the melts for these special applications are just as much a focal point as special wire treatments, inoculants and inoculation methods.
The ferritic material EN-GJS-400-18U-LT was used for these cast components in particular because of its high ductility, i.e. the ability to reduce overloading by means of plastic deformation and to compensate sudden loads without rupture. The material also proves exceptionally successful when used at low or strongly fluctuating temperatures due to the ensured notched bar impact value of 12 joules at −20°C. The breaking strain is 18%. The required cast quality is achieved by applying the simultaneous engineering methods that begin with a simulation of the mold filling, followed by a simulation of the solidification and a simulation of the structural formation.
The wind turbine designs whose performance ranges up to 5 MW require cast parts made of cast iron with nodular graphite (EN-GJS-400-18U-LT) and a weight of up to 50 metric tons. They entail new dimensions in terms of sizes, mold material requirements, metallurgy, especially magnesium treatment and inoculation technique, and moving masses. Foundry suppliers who work in the wind power industry are thus challenged to face up to this developmental trends and to adapt accordingly with expertise, verified material properties of their products, equipment, and their capacities.
The cooperation between the supplier and the foundry specialist on site is accompanied by extraordinary synergy effects. A quick exchange of information is conducive to the foundry- and production-related optimization and the necessary further development of the ASK Chemicals products. Numeric simulation techniques for mold filling and solidification help to solve problems that would be very difficult to handle in terms of feeding, for example. Tests are conducted to gain insight into how the functionality of the components can be further optimized by means of casting-related changes. Weak points in the design or oversizing are thus identified at an early stage without the cast part having to be manufactured. ASK Chemicals’ expertise regarding their products is complemented in an optimum way by the caster’s comprehensive knowledge of mold material, material and metallurgy.