Corrosion Fatigue Crack Propagation Rates for Steam Turbine Blade 13% Cr Steels

Abstract

Studies of stress corrosion in X12Cr13 and X20Cr13 steels used for rotor blades of steam turbines have been conducted for determining the average rate of corrosion cracking development da / dt. Specimens were exposed to the salt mist environment of 1,2 ppm NaCl concentration in temperature of 60° C. The research environment represents the initial condensate of humidity from 3÷4%. Average rate of corrosion cracking development for specimens of the X12Cr13 is by 0,005 µm/h higher from specimens of the X20Cr13 steel. Course of the corrosion cracking initiation depends to a high degree on metal structure, and probably the close value of cracking propagation rate depends on the similar structure of those steels – high-tempered martensite with carbides at grain boundaries.

Analysis on corrosion fatigue cracking mechanism of 17-4PH blade of low pressure rotor of steam turbine

“…As a result of water erosion, a sharp groove which changes the stress state of the blade surface is formed at the edge of the blade, and makes the crack appear and expand more easily at this position. Pitting is considered to be the precursor of corrosion fatigue damage again and again, which has been proved by many blade failure analysis [10][11][12][13][14][15] . Pitting not only reduces the actual strength of blade, but also leads to stress concentration, which can become the core of crack in the range of low cycle stress.…”