Cast iron for wind energy
Metals such as steel and iron are know for their high density. Due to their stability these materials have a large scope of application. Every application has spefic requirements which have to be fulfiled by the materials. The Institute for Materials Applications in Mechanical Engineering of RWTH Aachen University, headed by JARA-ENERGY member Prof. Christoph Broeckmann, work (among other subjects) on the challenge to adjust cast iron perfectly to the use for wind turbins.
Cast iron with nodular graphite is a well-established construction material for the purpose of wind energy. The mechanical properties, adjustable in a broad way, as well as the flexibility in design make it attractive for most differing components with individual requirements. Against the background of continuous wind energy plants’ advancement the material is faced with multilateral challenges for which innovative solutions have to be developed in future.
Structural components made of cast iron
Nowadays approximately 20 t cast iron, mainly for the structural components within the nacelle, are used per megawatt installed system performance . Classical representatives are for instance rotor hub, main shaft, bearing boxes and main frame as well as torque arm and planet carrier as part of the main gear unit. The actually used cast iron qualities can be roughly classified into ductile ones (e.g. GJS-400-18LT) and less ductile but high-strength ones (e.g. GJS-700-2) . Figure 1 shows exemplarily the microstructure of the latter cast iron quality whose strength originates from the almost pure pearlitic matrix.
Abbildung 1: Lichtmikroskopische Aufnahme des Gefüges von GJS-700-2 - perlitische Matrix mit darin eingebettetem Kugelgraphit
Fracture mechanical strength assessment for cast iron components
The conventional components‘ requirements, ductility and strength, constitute largely on the collective stresses typical for wind power plants. These are characterized by a moderate load spectrum accomplished by a few extreme overloads. Depending on the installation site there is further the requirement for low-temperature toughness which high-strength cast iron qualities commonly do not meet. In this case a fracture mechanics strength assessment often has to be performed additionally to the regular one during the phase of constructive design .
This is also the case if there are microstructural discontinuities whose presence can never be excluded due to the manufacturing process. Especially for the application in wind energy a VDMA task force – consisting of industrial representatives and the IWM of RWTH Aachen University – developed the guideline 23902 . It outlines the procedure for a fracture mechanical strength assessment of high strength cast iron components and summarizes the characteristic boundary conditions. Actually the task force works on an extension concerning elastic-plastic fracture mechanics so that the guideline will be thoroughly applicable to more ductile cast iron qualities.
Challenges in the construction with cast iron
Consequently, opposite to the advantage of design flexibility during the casting process there are also challenges when designing with cast iron. For example the locally varying cooling conditions in large-volume cast iron components result in distinctive property gradients over the wall-thickness. The international competition – also among the renewable energy sources – supports the observed tendency to higher performance levels  resulting in a continuous enlargement of power plants and components installed therein. The increasing weight of cast iron parts implies manufacturing and logistic problems so that design optimization and light weight construction come more and more to the fore.
Tasks in future
On the one hand, the challenges described can be met, today and in future, by newer cast iron qualities which have to be qualified for the purpose of application in wind energy plants. For instant, in line with a publically promoted research project, the IWM of RWTH Aachen University investigates the potential of highly Silicon alloyed, solid solution hardening cast iron the wind industry is discovering increasingly for its purpose. On the other hand, there is in particular the need for innovative and material specific design concepts making accessible cast iron’s potential in consideration of its specifics. Thereby a close cooperation between user, manufacturer and research is highly expedient and effective in developing creative problem solutions. At IWM of RWTH Aachen University, this confirms in various research projects on cast iron which are always characterized by a close cooperation between research and industry oft dealing with actual questions – especially of the latter ones.
 Bundesverband der Deutschen Gießerei-Industrie: Die Gießerei-Industrie. Eine starke Branche in Zahlen. URL: http://www.bdguss.de/fileadmin/content_bdguss/BDG-Service/Infothek/Broschueren/BDG_EinestarkeBranche.pdf. Abrufdatum 29.02.2016.
 VDMA 23901: Komponenten und Systeme für Windenergieanlagen in kalter Umgebung (2015). Berlin.
 VDMA 23902: Leitlinie für den bruchmechanischen Nachweis von Planetenträgern aus EN-GJS-700-2 für Getriebe von Windenergieanlagen 21.200 (2014). Berlin.
 Frauenhofer-Institut für Windenergie und Energiesystemtechnik: Anlagenzubau nach Leistungsklasse (1990-2015). URL: http://windmonitor.iwes.fraunhofer.de/windmonitor_de/3_Onshore/2_technik/2_leistungsklasse/. Abrufdatum 29.02.2016.