Metallurgical Failure Investigation of Cracked Hook Nuts Used for Upending Gas Turbine Rotors on Site

Abstract Two types of non-metallic inclusion stringers were found in so-called upending hook nuts, in the vicinity of an indication on the bore wall's surface, first, manganese sulfides that can be expected in this material, and second, endogeneous oxidic inclusions from the steelmaking process. The latter ones, too, can be expected to an extend in the structural steel that was used to make the hook nuts from. However, they are present in an excessive amount and were causative for the decohesions that produced the reported MT indications. The forging process which was employed to produce the semifinished product used to machine the hook nuts from was unable to fragment and disperse the non-metallic inclusions sufficiently as is usually the case.