PNNL

Abstract

High cycle corrosion fatigue (HCCF) life S-N tests were conducted on wrought 17-4PH in various water environments under accelerated testing conditions. For tests conducted in aerated Columbia River water and in aerated ASTM Type IV water, at ~1e06 cycles, the HCCF stress amplitude was ~70% of the value in air. For deaerated water, the value was ~80%. These values are consist with the small amount of literature data on HCCF life of 17-4PH in water environments. The measured fatigue life both in air and water are much better than the ASME trend curve of high cycle fatigue life of moderate and high strength steels. Measured values were also much better than found in a hydropower-relevant study in the literature on cast 17-4PH. The effect of Amersite 61W on HCCF life was evaluated and found to cause very low times to specimen failure compared to all other water environments. The potential effects of microbial-induced corrosion and crevice corrosion were reviewed as part of literature study, but no experiments were conducted to evaluate these. Post-test examination of the specimens tested in water revealed no signs of pitting or staining anywhere on the specimens, including at the site of primary crack initiation. For both water and air tests, the location of the site of crack initiation was often at a particle, likely an inclusion, intersecting the surface of the specimen. Hydrogen embrittlement is the likely environmental degradation mechanism driving reduced time to crack initiation and specimen failure during the HCCF tests. This is consistent with conclusions made by other researchers who have investigated HCCF of 17-4PH and similar materials.