Effect of Stress Transients on the Crack Propagation Rate in Steam Turbine Disc Steel

An evaluation has been made of the effect of stress transients on the growth rate of long cracks in fracture mechanics specimens of a 3NiCrMoV steam turbine disc steel in deaerated 300 ppb Cl^- + 300 ppb $SO42−$ solution at 90°C. Trapezoidal loading simulating two-shfting in service (cycling from on-load to off-load conditions on a daily basis and off-load for the weekend) gave a high cyclic crack growth rate of about 10^-6 m/cycle. With typically one cycle per day in service except weekends, and accounting for any stress corrosion cracking component at maximum load, a crack growth rate of 0.4 mm/y is estimated. Under constant load for the same environment no crack growth occurred at exposure times up to 9 months but a temporary uncontrolled increase in oxygen and chloride for just 1 h was sufficient to induce crack growth with rates as high as 1.1 × 10^-11 m/s, with crack growth then sustained for thousands of hours.