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Fatigue Crack Growth Rate Behavior of Welded Line Pipe Steel in Sour Environments

Frequency scan fatigue crack growth rate tests were performed at a fixed stress intensity factor range to determine the effect of frequency in two different sour environments. Both sour environments had the same partial pressure of H2S (0.21psia) but different pH values.

Product Number: 51317--9200-SG
ISBN: 9200 2017 CP
Author: Thodla Ramgopal
Publication Date: 2017
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Sour fatigue crack growth rate (FCGR) behavior of welded line pipe steel was investigated in a range of conditions. Frequency scan FCGR tests were performed at a fixed stress intensity factor range (DK) to determine the effect of frequency in two different sour environments. Both sour environments had the same partial pressure of H2S but with different pH values and inhibitor concentration. A limited number of frequency scan experiments were also performed to evaluate the influence of two different H2S levels (0psia and 0.21psia) on the FCGR performance. The FCGR increased with decreasing frequency and in most cases a plateau value was reached at low frequency.The maximum FCGR (i.e. at the plateau frequency) was higher at lower pH (with high inhibitor concentration) when compared to the higher pH environment (with lower inhibitor concentration). The plateau FCGR in the absence of H2S was about two times lower than in the presence of H2S.The effect of microstructure was evaluated in the two different sour environments by comparing the FCGR performance of parent pipe (PP) heat affected zone (HAZ) and weld center line (WCL) samples. The effect of straining and aging was also investigated. The results of frequency scan tests indicated that straining and aging did not have a significant effect on the FCGR behavior in the sour environments.A small set of tests were performed to investigate the effect of various mechanical variables on the frequency scan response in the sour environments including the maximum stress intensity factor (Kmax) and waveform. The results suggested that high Kmax values tended to trigger KISSC type behavior (i.e. static crack growth cause by hydrogen embrittlement) in WCL where the FCGR is independent of DK. The effect of DK was also explored by performing Paris curve tests at two different frequencies (0.1Hz and the plateau frequency) in the two different sour environments. Lower FCGRs were observed in the Paris curve tests at 0.1Hz than at the plateau frequency which is consistent with the data observed in the frequency scan tests.

Key words: Sour Service, Fatigue Crack Growth Rate, Static Crack Growth, Frequency Scans, Paris Curves.

Sour fatigue crack growth rate (FCGR) behavior of welded line pipe steel was investigated in a range of conditions. Frequency scan FCGR tests were performed at a fixed stress intensity factor range (DK) to determine the effect of frequency in two different sour environments. Both sour environments had the same partial pressure of H2S but with different pH values and inhibitor concentration. A limited number of frequency scan experiments were also performed to evaluate the influence of two different H2S levels (0psia and 0.21psia) on the FCGR performance. The FCGR increased with decreasing frequency and in most cases a plateau value was reached at low frequency.The maximum FCGR (i.e. at the plateau frequency) was higher at lower pH (with high inhibitor concentration) when compared to the higher pH environment (with lower inhibitor concentration). The plateau FCGR in the absence of H2S was about two times lower than in the presence of H2S.The effect of microstructure was evaluated in the two different sour environments by comparing the FCGR performance of parent pipe (PP) heat affected zone (HAZ) and weld center line (WCL) samples. The effect of straining and aging was also investigated. The results of frequency scan tests indicated that straining and aging did not have a significant effect on the FCGR behavior in the sour environments.A small set of tests were performed to investigate the effect of various mechanical variables on the frequency scan response in the sour environments including the maximum stress intensity factor (Kmax) and waveform. The results suggested that high Kmax values tended to trigger KISSC type behavior (i.e. static crack growth cause by hydrogen embrittlement) in WCL where the FCGR is independent of DK. The effect of DK was also explored by performing Paris curve tests at two different frequencies (0.1Hz and the plateau frequency) in the two different sour environments. Lower FCGRs were observed in the Paris curve tests at 0.1Hz than at the plateau frequency which is consistent with the data observed in the frequency scan tests.

Key words: Sour Service, Fatigue Crack Growth Rate, Static Crack Growth, Frequency Scans, Paris Curves.

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