Abstract
A comparison of the crack opening and fatigue crack growth characteristics of three tempered martensitic steels; a modified 4135, 2 1/4Cr-1Mo and a modified 9Cr-1Mo; in ambient air and in vacuum (3 × 10 −5 torr) has been carried out. In vacuum, roughness-induced closure is responsible for closure in the near-threshold region and the level of closure is independent of ΔK. The presence of oxygen in the ambient environment (50% relative humidity) can increase the overall closure level by as much as 50% in the case of 4135 and 2 1/4Cr-1Mo steels, but does not contribute significantly to closure in the case of the 9Cr-1Mo steel. Roughness-induced closure increases with increase in tempering temperature, but the extent of the environmental effect on closure in a given alloy is much less dependent on the tempering temperature. Rewelding is not responsible for the high threshold levels found in vacuum. Oxide film rupture as well as hydrogen embrittlement increase the rate of crack growth in air over that observed in vacuum.
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