Stress Corrosion Cracking of Conventional and Super-Clean 3.5-NiCrMoV Rotor Steels in Simulated Condensates

The stress corrosion cracking (SCC) behavior of two bainitic 3.5-NiCrMoV low-pressure (LP) rotor steels has been studied at 95 C in environments containing species that are possible contaminants in steam condensates. The steels were a conventional modern rotor material of regular chemical composition and a super-clean steel with low levels of Mn, Si, S, and P. The environments were 3.5 M NaOH, 1 M Na₂CO₃ + 1 M NaHCO₃ and a saturated CO₂/H₂O solution. A combination of slow strain rate tensile testing methods and fracture mechanics techniques were used to produce SCC. A unique specimen configuration, based on a modification of the notched double cantilever beam geometry, was developed to improve the ease of testing by fracture mechanics methods. Cracking was observed only in the NaOH and carbonate/bicarbonate environments and the SCC behavior of the two steels was reasonably similar, with the super-clean steel having slightly lower crack propagation rates in the NaOH solution. Based on 500-h exposure tests, the data showed that K_ISCC was 35 to 40 MPa.m^1/2 and Region II crack propagation rates were near ~2 × 10⁻¹⁰ m/s. The significance of the results was discussed in relation to residual elements in steels. The SCC behavior was compared and contrasted with other studies on rotor steels and the effect of exposure time was considered.