RETARDATION OF STRESS CORROSION CRACKING IN HTH ALLOY X-750 BY PRESTRAIN

摘要

Application of prestrain has been shown to retard stress corrosion cracking (SCC) of Alloy X-750. Prestrain, defined as the intentional application of an initial displacement that is greater than the final displacement, lowers the local stresses ahead of notches or cracks, which retards cracking. In this study, tests of precracked 0.4T compact tension specimens of Condition HTH Alloy X-750 were conducted in 360°C hydrogenated water to determine the effects of prestrain and load path history on SCC. These tests evaluated unload-reload cycles, shallow cracks and thermally induced unloading and uploading effects. In addition, the frequent cooldowns that were necessary for periodic inspections were conducted in high hydrogen water to provide an opportunity to evaluate prestrain effects on rapid low temperature crack propagation to which this alloy is subject. Prestrain levels of 20% or greater either eliminate or significantly retard low and high temperature SCC for both long and shallow cracks. However, the prestrain operation was only effective when the load (displacement) was reduced directly from the prestrain level to the final level. While it was much less effective when the load was totally removed and returned directly to the test load, the benefit was recovered by repeating the prestrain process. These results are consistent with the expected effect of the prestrain cycle in reducing the local stresses at the crack tip. Thermally induced prestrain, introduced by reducing the displacement during heatup, retards high temperature SCC but offers no protection against low temperature cracking because of the uploading on cooldown. Combined mechanical and thermally induced prestrains are effective in retarding SCC, whereas uploading during heatup significantly diminishes the mechanical prestrain benefit.