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, rotor shaft, main bearings, 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 greatly reduced. Due to the shortened heat treatment of AHD steels and the use of green steel, the GWP of hollow forging is comparable to casting.