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51315-6122-Corrosion Assisted Fatigue of an Alloy 725 (UNS-N07725) Electric Discharge Machined (EDM) Rubber Cutting Blade

Picture for Corrosion Assisted Fatigue of an Alloy 725 (UNS-N07725) Electric Discharge Machined (EDM) Rubber Cut
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Product Number: 51315-6122-SG
ISBN: 6122 2015 CP
Author: Ivan Morales
Publication Date: 2015
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Ivan A MoralesExxonMobil Research and Engineering2800 Decker DriveBaytown TX 77520USAABSTRACTIn 2013 an Alloy 725 (UNS-N07725) rubber cutter was found with scattered pitting after only 18 months in service which prompted additional PT inspection noticing additional cracking in section of the blade. The cracking was thought to be Chloride Stress corrosion cracking; however this failure was the first of its kind experienced within the circuit after more than 10 years of experience with this alloy in similar service. The initial material selection chose this alloy 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 completely 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 root cause of the failure; however since this was not consistent with our experience the other half of the fracture was requested for additional in-house analysis. Our evaluation confirmed the same branched and transgranular cracking consistent with SCC but also noticed a fatigue propagation morphology during fractography evaluation for which corrosion assisted fatigue was considered as the failure mode but the initiation of the cracking was still not fully understood. 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 EDM’d parts. The reduced corrosion resistance of the re-cast structure allowed the localized attack/pitting observed. The pitting acted as stress riser and allowed the nucleation of cracks that were propagated by the cyclic loads on the blade.Key words: Corrosion assisted fatigue Electric Discharge Machining EDM
Ivan A MoralesExxonMobil Research and Engineering2800 Decker DriveBaytown TX 77520USAABSTRACTIn 2013 an Alloy 725 (UNS-N07725) rubber cutter was found with scattered pitting after only 18 months in service which prompted additional PT inspection noticing additional cracking in section of the blade. The cracking was thought to be Chloride Stress corrosion cracking; however this failure was the first of its kind experienced within the circuit after more than 10 years of experience with this alloy in similar service. The initial material selection chose this alloy 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 completely 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 root cause of the failure; however since this was not consistent with our experience the other half of the fracture was requested for additional in-house analysis. Our evaluation confirmed the same branched and transgranular cracking consistent with SCC but also noticed a fatigue propagation morphology during fractography evaluation for which corrosion assisted fatigue was considered as the failure mode but the initiation of the cracking was still not fully understood. 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 EDM’d parts. The reduced corrosion resistance of the re-cast structure allowed the localized attack/pitting observed. The pitting acted as stress riser and allowed the nucleation of cracks that were propagated by the cyclic loads on the blade.Key words: Corrosion assisted fatigue Electric Discharge Machining EDM
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