High strength alloy field failures and successes

High-strength, age-hardeneable Ni-Cr-Fe alloys X-750, A286 and 718 are used by the nuclear power industry because these austenitic alloys provide a combination of good mechanical properties and high corrosion resistance. Alloys X-750 and 718 are nickel-base alloys and A-286 is a stainless steel. Age-hardeneable Ni-Cr-Fe alloys are complex austenitic materials with properties that can be controlled by heat treatment. These alloys have a variety of applications in both boiling water reactors (BWRs) and pressurized water reactors (PWRs), including use in such components as core internals bolting, springs, guide pins, and other structural members. In general, these alloys have been used successfully in nuclear system applications, although there have been reported component failures due to fatigue, corrosion fatigue and intergranular stress corrosion cracking. Most of the applications involve components that are subject to high operational stresses in aqueous media. Although these alloys are not used as extensively as some other alloys in nuclear plants, failures in certain components that are made from these alloys can affect, and have affected, plant availability. Usually, a crevice environment and local stresses near or above the yield strength of the material have been associated with failures. Localized stress raisers such as notches or threads are also believed to have contributed to some failures. Intergranular stress corrosion cracking (IGSCC) and fatigue or corrosion fatigue have been identified as the primary failure mechanisms. Failures of these precipitation-hardened austenitic alloys have been reported in both BWR and PWR plants. Most of the reported failures or damage instances have been found and repairs implemented during routine in service inspections, repair outages or refueling outages. Consequently, plant safety has not been affected, and the impact on plant availability has been minimized