Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Cyclic material behavior of repair welded thick-walled nodular cast iron for wind energy
application
Christoph Bleicher, Steffen Schönborn, Heinz Kaufmann
Fraunhofer Institute for Structural Durability and System Reliability LBF
Darmstadt, Germany

ABSTRACT

technology, especially in the wind energy sector, the unit weight of theGJS components used here for rotor hubs, machine carriers, gearcomponents and torque arms is in some cases well over 20 tons. Thesecomponents have to meet high requirements, especially in terms offatigue strength, fracture resistance and stiffness, which can often only
be achieved with defect-free material structures. Driven by the demandsof increasing performance, the improvement of the power-to-weightratio (t/kW) and the implementation of lightweight construction
measures for weight reduction with retention of the stressability, thecomplexity of modern cast iron components is increasing. To meet thesedemands, minimization of component tolerances and wall thickness
optimization are carried out, which, coupled with the increasing use ofcores to form cavities and undercuts as well as changing local wall
thicknesses, exert an influence on the mold filling and solidification
processes and represent a manufacturing challenge. The resulting
probabilities for the occurrence of mold-filling and shrinkage-related
discontinuities as well as irregularities in the microstructure, such as
dross, blowholes and chunky graphite, increase. In most cases, this leads
to the rejection of the components due to a lack of acceptance by thecasting user and ambiguous recommendations in the relevant standards
regarding the effect of these defects, especially on the component
strength. Due to the high costs associated with the rejection of large
castings with imperfections, there is increasing interest in repair methods
such as welding to repair surface imperfections or insufficient moldfilling. In the context of repair welding for wind turbines, DNV requires
in its "Guideline for the certification of wind turbines" a special
qualification of the welding company as well as tested weldingprocedure approval (WPA or welding process specification WPS) and aregistered test report (welding procedure approval record - WPAR)(DNV 2016). According to (DNV 2016), dissimilar welding is not
permitted for cyclically stressed components, which is due to the lack ofknowledge about the effect of repair welding on the load-bearing
capacity.
To assess the weldability and the effect of welded zones on fatigue
strength of thick-walled EN-GJS-400-18LT various cast blocks withrepair welded areas and a thickness of nearly 200 mm were investigatedwithin the research project “nodularWELD” (Schönborn, 2017). During
the investigations stress- and strain-controlled fatigue tests wereLarge cast components made of nodular cast iron for wind energy
turbines and heavy industry are usually not completely free of local
material defects. Nevertheless, only the position, size and typecharacterize the defects and the reduction in cyclic material behavior andlifetime to be expected. Especially local defects at the components
surface or near the surface lead to negative effects on lifetime, when highsurface stresses and sharp notches or stress gradients are present in thecomponent. But in any case, foundries strive for reducing theircomponent rejects especially when it comes to wind energy components
with masses up to several tens tons. Thus, even components with small
surface or near-surface defects need to be brought into usage. One
measure is to determine the defect characteristics and remove the defectby a defined manufacturing and a subsequent welding process. This
procedure is common for steel castings and part of the production
process but usually not accepted by certification bodies for wind energy
components made of nodular cast iron.
Within a research project called “nodularWELD” the possibilities of
welding of EN-GJS-400-18LT were investigated. After a welding
process with different welding electrodes on large cast blocks with premanufactured defects the quasi-static and cyclic material behavior of thebase material and the welding filler material was determined.
Additionally integral samples containing base material, the heat transfer
zone and the welding filler material were investigated. The results areused to determine the cyclic material behavior of the cast component andto obtain a lifetime assessment concept for practical application.

KEY WORDS: Nodular cast iron; wind energy; fatigue; cyclic material
behavior; repair welding; thick-walled

INTRODUCTION
Nodular cast iron (GJS) (DIN EN 1563 2019) is used in many areas ofmechanical and plant engineering as well as in automotive and energy
technology. Due to its good strength properties and the design freedom
associated with casting, GJS is used for a wide range of highly stressedcomponents of the most varied dimensions. In the field of energy2984