Abstract
In 2013, a Nickel based alloy - UNS-N07725 rubber cutter was found with scattered pitting after only 18 months in service. In addition, PT inspection discovered cracking at other locations of the blade. The cracking was thought to be Chloride Stress Corrosion Cracking (SCC); however, this failure was the first of its kind experienced within the circuit after more than 10 years of experience with this alloy in this service. During the initial material selection process, UNS-N07725 was chosen due to its high pitting and SCC resistance combined with excellent mechanical properties, for which experiencing a failure in such a short time in service was unexpected. The branched and transgranular cracking found during the metallurgical analysis by a third party laboratory, combined with the operating environment, led the investigation team to propose SCC as the damage mechanism; however, since this was not consistent with previous circuit experience the other half of the fracture was requested for additional analysis. Further evaluation confirmed the same branched and transgranular cracking. Consistent with SCC however a fatigue propagation morphology was also noticed during fractography evaluation, suggesting corrosion assisted fatigue as the damage mechanism. The focus of the investigation was to determine the initiation mechanism of the cracking. Subsequent discussions with the subsidiary about the fabrication steps highlighted a fundamental difference with other blades in the circuit. The failed component had been machined using Electric Discharge Machining (EDM) with no subsequent finishing step after the EDM. Another section of the blade with scattered pitting was then evaluated metallographically, finding evidence of re-cast structure normally observed on parts subjected to EDM. The reduced corrosion resistance of the re-cast structure allowed the localized attack/pitting. The pitting acted as a stress riser and allowed the nucleation of cracks that were propagated by the cyclic loads on the blade.
