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{sup}- + 300 ppb (SO{sub}4){sup}(2-) solution at 90℃. Trapezoidal loading simulating two-shifting in service (cycling from onload to off-load conditions on a daily basis and off-load for the weekend) gave a high cyclic crack growth rate of about 10{sup}(-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{sup}(-11) m/s, with crack growth then sustained for thousands of hours.
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