Blade Lock Ring of 4th Stage Compressor Rotor: Failure Investigation

Abstract

Present work describes the failure investigation of blade lock ring of 4^th stage compressor rotor. The lock ring is fabricated from martensitic stainless steel. The microstructure of failed lock ring is tempered martensite. It shows non-metallic inclusions with three distinct shapes namely, elongated (MnS), globular (Al₂O₃) and complex shaped (oxy-sulphide). The corrosion pits and corrosion debris are observed in un-etched microstructure and fracture surface, respectively. The tree like branching cracks has observed at several places near surface. These cracks have initiated from the corrosion pits and then propagated inside the material. The interface of inclusions and matrix has also acted as crack initiator.  The lock ring has initially suffered pitting corrosion in service and then cracks have propagated inside the material by stress corrosion cracking.