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Fatigue and fracture performance of UNS N07718 (718) and UNS N07716 (625+) in sour environments over a range of temperatures (300 to 400°F) and chloride concentrations (0.15wt% to 25wt% NaCl). Rising displacement fracture toughness test performed in a range of environments.
Fatigue and fracture performance of UNS N07718 (718) and UNS N07716 (625+) in sour environments over a range of temperatures (300 to 400°F) and chloride concentrations (0.15wt% to 25wt% NaCl). Rising displacement fracture toughness test performed in a range of environments indicated that the fracture toughness was a strong function of chloride concentration and temperature.
The initiation fracture toughness values were lower in environment compared to the in-air values. The initiation toughness decreased with increasing chloride concentration and temperature. At a given chloride concentration, the initiation fracture toughness at 400_F was significantly lower than at 300_F or 350_F. Increasing chloride concentration at 400_F led to a sharp decrease in the initiation toughness. Decreasing initiation toughness was associated with increasing intergranular cracking. Increasing temperature and chloride concentration resulted in a decrease in the repassivation potential of IN718. The decrease in repassivation potential was sharp at 400_F. Increasing chloride concentration at 400_F also led to a sharp decrease in the repassivation potential. Similar behavior was observed for 625+. There was no significant effect of the chloride concentration on the corrosion potential. The decrease in fracture toughness coincided with a decrease in the repassivation potential (in particular when the repassivation potential was below the corrosion potential), which suggest that in sour environments at elevated temperatures, the factors that are responsible for stabilizing pit growth also contribute to stabilizing local environments that favor environmentally assisted cracking.
Keywords: Downloadable, Fractures, Fatigue, alloys, sour environment, temperature, repassivation,
Test results for four alloys in six different sour environments are presented. Alloys 625, 825, 316L and carbon steel were testing in sour gas with varying exposure to moisture at 280°C. Corrosion rates for each alloy over a 30 day period are measured from mass and thickness changes.
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Sour service behavior of a 110ksi material was investigated in a range of production environments. Slow strain rate tests were performed at a strain rate of 510-7/s, in sweet as well as in sour production environments. The strain to failure in sweet environments is lower than the in-air values and is substantially lower in the presence of H2S.
Multiple leaks occurred with a carbon steel pipe carrying hot condensate water (300F and 140 psi). A root cause analysis was performed, including visual and metallographic examination of two failed samples chemical, analysis of weld metal and base metal and chemical analysis of water samples. Results show that flow accelerated corrosion (FAC) was the responsible mechanism leading to the failures.