Hollow-forged AHD steel rotor shafts for wind turbines – a case study on power density, costs and GWP

Abstract. Hollow-forging and air-hardening ductile (AHD) forging steels are a novel manufacturing process and steel grade for the wind energy sector. Together they enable new rotor shaft design possibilities for wind turbines. Hollow forging combines the high material strength of a solid-forged shaft with direct inner contour manufacturing similar to casting. To compare an AHD steel hollow-forged rotor shaft to a state-of-the-art cast rotor shaft, a case study is carried out, focusing on power density, manufacturing costs and (manufacturing) global warming potential (GWP). To ensure comparability between the hollow-forged and cast rotor shaft, two predesigns of a main bearing unit (MBU), consisting of the rotor shaft, main bearings and bearing housings, are generated via a structural integrity assessment and calculation of the bearing lifetime according to ISO 76 / 281. The resulting hollow rotor shaft has 37 % less mass than the cast rotor shaft, corresponding to a 16.5 % lower MBU mass. For the hollow-forged rotor shaft to be comparable to casting regarding manufacturing costs, the forging surcharges need to be less than 50 % of the shaft mass. Due to the shortened heat treatment of AHD steels and the use of green steel, the manufacturing GWP of hollow forging is comparable to casting and makes up around one-third of the GWP of the entire MBU.