PSI - Issue 37

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

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Mostert et al/ Structural Integrity Procedia 00 (2021) 000 – 000

For the part of the curve beyond the initial incubation period (175 hours), a four parametric sigmoidal curve was fitted to the experimental strain - time data obtained. This equation accurately described the HTHA strain development over time for all the HTHA strain curves evaluated:

( με) = + ( − ) + −

(1)

with R 2 ≥ 0.983 for all regions. Where: A 2 is the maximum level of HTHA strain attained by the specific region at operating conditions which are above the carbon steel in the Nelson Curve. A 1 is the minimum reading on the strain gauge before the sigmoidal behaviour, which approaches zero. t is exposure time in hours. x o is the exposure time when the HTHA damage rate is at maximum. dx is “Hill slope” of the sigmoidal curve. The rate of HTHA cavity growth and coalescence is believed to be closely linked to the process of creep (Welding Research Council 2021a). Creep is currently adequately described by the MPC Omega method, which makes use of the concept of a damage fraction (American Petroleum Institute, Division of Refining 2016b) . As the creep damage progresses with time, the damage fraction progresses from zero to 1.0. The principle of damage fraction development was applied to the HTHA strain-time curves developed for the C 0.5 Mo weld zones, as well as for a carbon steel curve obtained from literature (Munsterman, Seijas, and Williams 2010), and are presented in Figure 3. The test conditions for the carbon steel differed from that of the current study, with the temperature being higher (545 °C) and the hydrogen pressure being lower (100 bar). The results normalise the C - 0.5 Mo three weld zone damage results and allow direct comparison of the three zones of the weldment, as well as a comparison between the carbon steel and C - 0.5 Mo base metal HTHA behaviour. The resulting sigmoidal equation is accordingly simplified, with only two constants, x o and dx, as described before. The simplified HTHA damage fraction sigmoidal equation could be expressed as in equation 2 with the constants as given in Table 2:

HTHA Damage Fraction = − + −

(2)

Figure 2: Microstructure of HAZ sample (coarse-grained region) run to completion of HTHA damage, as shown in Figure 1.