Stress corrosion cracking of CrNi martensitic stainless steels for steam turbine blades

Abstract

Stress corrosion cracking (SCC) is a delayed failure process that can occur in susceptible materials that are both under stress and in a corrosive environment. Microscopic cracks may be initiated at surface flaws, such as pits, and propagate by the combined influence of stress and corrosive attack until eventually the stress in the metal ahead of the crack exceeds that of its fracture strength, whereby component failure could occur. Steam power plants generate electricity through transforming the energy contained in steam to the rotation of a generator. The presence of condensing steam, in combination with high centrifugal stresses, provides the potential for SCC in low pressure cylinders.Laboratory samples of 12 wt% Cr martensitic stainless steels, used for low-pressure steam turbine blades, previously exposed to condensing steam at 95°C for various times, have been examined using optical and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). The corrosion products formed within stress corrosion cracks have been analysed with the objective to devise a method of dating stress corrosion cracks by establishing links between the scale thickness and morphology and variables such as exposure time, crack length and initial stress intensity.