PSI - Issue 37

R.J. Mostert et al. / Procedia Structural Integrity 37 (2022) 763–770

769

7

Author name / Structural Integrity Procedia 00 (2021) 000 – 000

1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

200 400 600 800 HTHA strain rate derivatives ( με /h, με /h 2 , με /h 3 )

First derivative

Second derivative

-800 -600 -400 -200 0

Third derivative

4

1

2

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0

5

10

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20

25

30

Exposure time (h)

Figure 5: First, second and third derivatives of the strain – time plot of the carbon steel HTHA (Pretorius et al. 2021) exposed to hydrogen at 98 bar and 545 °C.

4. Discussion The study demonstrated that the extent and nature of the HTHA damage evolution could be accurately described for the various zones of a C-0.5 Mo welded joint, as well as for a carbon steel base metal, by using a sigmoidal constitutive equation plotting the HTHA strain versus exposure time. The HTHA strain-time curves could also be accurately transformed into damage fraction and damage rate versus time plots and corresponding sigmoidal equations. The latter observation paves the way for a relatively simple experimental set- up for the determination of the α -parameter of the WRC 585 and 586 documents. The experimental determination of creep damage fraction vs time plots at 500 °C (for the C-0.5 Mo steel), in a neutral environment and at different levels of applied stress, will yield similar damage fraction plots to that presented in Figure 3. The variation of applied stress levels will cause some of the curves to be faster and some to be slower, and the applied stress level which will match the HTHA curves can be determined experimentally or by calculation. This applied stress value will represent the term of the “α - Ω HTHA model” (Welding Research Council 2021a, 2021b) as presented in equation 3. This term represents the “additional stress” due to the internal methane pressure and will facilitate the determination of the α -parameter of WRC 585/ 586. = (3) The determination of the static methane pressure in the cavities, 4 , can be readily calculated using the WRC 585/ 586 methodologies. If both the additional stress and static methane terms are known for a given alloy/ microstructure/ service exposure history, the α -parameter can be obtained, leading to accurate assessments of structural integrity and remaining life using the WRC 585/586 methodologies. The finding that the HTHA kinetics can be described by a sigmoidal equation is significant in terms of life monitoring of