It’s an unspoken rule, but a wind turbine really must be environmentally-friendly, highly-efficient and cost-effective. And because the massive wind machines rise into the sky on enormous feet, they also need to have a functional life of at least 20 years or more so they can generate electricity.
How well the structure itself and the components are manufactured has taken center-stage because of the increasing demands and size of some of the turbines in operation today. This means it will be up to the iron processing industry to manufacture the massive components in a stable, resource-saving and yet cost-effective way.
When iron processors are casting components for the wind industry, it is almost unavoidable to end up with some material inclusions from slag, known as dross in the cast iron product. And for wind turbines, these defects are considered undesirable because they greatly reduce the load-bearing capacity of the turbine’s mainframe and rotor hubs containing spheroidal graphite.
Yes, you read that right – spheroidal graphite iron (SG iron), or nodular cast iron, is an iron that was discovered in 1943. The big difference between SG iron and most varieties of cast iron is that it isn’t brittle, meaning it is much more impact and fatigue resistant. With this kind of iron, the graphite is in the form of rounded nodules rather than flakes.
Since dross is usually found on the surface of the products, or a few centimeters below the surface, they have to be ground out by hand, a labor-intensive job. And, as yet, there is no reliable way of dealing with dross, and this problem has now become the focus of research by the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt, Germany, Europe’s largest application-oriented research organization.
Dr. Christoph Bleicher, since 2015, has been the consortium leader of the “unverDROSSen” project, which aims to move away from the customary requirements for dross-free products and thus dispense with the time-consuming post-production work.
“To be able to do this, we have to provide manufacturers and users with a sound measurement concept so they can evaluate the degree and type of dross. That’s why, together with the Fraunhofer Institute for Nondestructive Testing IZFP in Saarbrücken, we’re developing an experimentally proven dross strength classification system,” says Bleicher.
Using mechanized ultrasound testing equipment, the researchers were able to display and measure the distribution of dross in a three-dimensional format. They also tested the component parts using magnetic and electromagnetic methods. “Using our nondestructive testing technology, we measured cuboids each measuring 500 x 500 x 200 millimeters. We found that the dross distribution in the test pieces varied extremely. Sometimes the material defect extends across a very large surface area, and it can range in depth from a few millimeters to several centimeters,” reports Fabian Weber from Fraunhofer IZFP.
Weber added that with this method of testing they have developed, each component will have to be tested individually, but in the long run, it will cut down on unnecessary post-production work. It is a known fact that dross leads to crack formation, which greatly reduces the component’s cyclic load bearing capacity.
“However, such cast-iron components are completely adequate for other purposes,” says Bleicher. “In the future, we will offer a concept for reliably handling material defects in component design, manufacture and release of large cast components made of cast iron with spheroidal graphite. This will apply not just to wind farms but to all plant and mechanical engineering sector,” concludes Bleicher with certainty.
