An automated, computer-controlled K-decreasing technique was used to determine the threshold, ΔKth, and low-rate fatigue crack growth of a NiMoV rotor steel. A more conventional K-increasing technique was also used. Excellent agreement between results obtained from both techniques was observed. For the material and environment studied, no crack arrest was observed for crack growth rate down to 2.5 × 10−8 mm/cycle (10−9 in./cycle). As such, an operational definition of ΔKth was defined as the stress intensity factor range corresponding to a crack growth rate of 2.5 × 10−8 mm/cycle (10−9 in./cycle). In room temperature air environment, ΔKth was found to be 6.2 and 4.0 MPa$m$ (5.6 and 3.6 ksi$in.$) for R = 0.1 and R = 0.5, respectively. At the same ΔK level, crack growth rate was found to increase with increasing stress ratio. The influence of stress ratio on crack growth rate, however, decreases with increasing ΔK. By raising temperature to 93° C (200°F), ΔKth was found to be suppressed to 4.4 and 2.9 MPa$m$ (4.0 and 2.6 ksi$in.$) for R = 0.1 and R = 0.5, respectively. Stress ratio effect on crack growth rate is the same as at room temperature, but is less significant. Temperature was found to influence crack growth rate in the threshold region for both stress ratio studied, with higher crack growth rate at 93°C (200°F) than at room temperature. Temperature sensitivity was found to be less for R = 0.5 than R = 0.1. The existence of hydrogen was found to have little effect on ΔKth and low-rate fatigue crack growth behavior of this NiMoV rotor steel.

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