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

structures which are being degraded by HTHA . In this way, the progress of the damage can be continuously tracked, and future damage accumulation can be predicted. Therefore, the use of this type of strain gauge in conjunction with the use of the established sigmoidal equations in remaining life evaluations holds significant promise. The shape of the strain rate vs. time curve, provides insight into the kinetics of the HTHA damage evolution process. Initially, the rate of the development of damage and its associated swelling is slow as micro voids are formed adjacent to grain boundary carbides. Microcracks with minimal face separation develop, joining adjacent grain boundary carbides, with low swelling resulting. This stage can be regarded as incipient attack or the “incubation period”. As the micro voids grow and the microcr ack face separation increases due to the internal methane pressure, the swelling rate increases and grain boundary carbides along with the free carbon are consumed by the hydrogen present. This process becomes increasingly rapid up to a specific time, represented by x o which is the time associated with the peak strain rate. At this point, micro voids and microcracks have formed at many locations within the microstructure, and rapid growth is fuelled by the abundance of damage sites and the internal methane pressure resulting in pore growth. Beyond this peak, the swelling rate decreases as the availability of carbides to feed the methane reaction diminishes. With further increases in time, the swelling rate increasingly diminishes as full decarburisation, and the absence of carbides and free carbon are approached. 5. Conclusions 1. The application of encapsulated strain gauges to laboratory samples in an autoclave can be used to experimentally determine the methane proportionality factor ( α ) for service-exposed samples in various microstructural states, facilitating accurate HTHA assessments using the WRC 585/586 methodologies. 2. The kinetics of the HTHA damage process can be accurately described using sigmoidal constitutive equations. 3. The evolution of HTHA swelling strain on high-risk structural members can be monitored in real time using the encapsulated high-temperature strain gauges which can be attached with a view to establish the stage of HTHA degradation for structural integrity evaluation. The higher order derivatives of the constitutive equations have been shown to provide guidance regarding the stage of degradation experienced, regarding a specific service life. Acknowledgements The authors are grateful to Sasol (Pty) Ltd for financial support and as well as providing the material used in this work. References Ali, Murad, Anwar Ul- Hamid, Luai M. Alhems, and Aamer Saeed. 2020. “Review of Common Failures in Heat Exchangers – Part I: Mechanical and Elevated Temperature Failures.” Engineering Failure Analysis 109: 104396. https://doi.org/10.1016/j.engfailanal.2020.104396. American Petroleum Institute, Division of Refining. 2016a. API 571: Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants, American Petroleum Institute, Washington, D.C . ——— . 2016b. Fitness for Service API 579 - 1/ASME FFS - 1 . Askari, A., and S. Das. 2006. “Practical Numerical Analysis of a Crack near a Weld Subjected to Primary Loading and Hydrogen Embrittlement.” Journal of Materials Processing Technology 173 (1): 1 – 13. https://doi.org/10.1016/j.jmatprotec.2004.12.005. Defteraios, N., C. Kyranoudis, Z. Nivolianitou, and O. Aneziris. 2020. “Hydrogen Explosion Incident Mitigation in Steam Refor ming Units through Enhanced Inspection and Forecasting Corrosion Tools Implem entation.” Journal of Loss Prevention in the Process Industries 63: 104016. https://doi.org/10.1016/j.jlp.2019.104016. Manna, G., P. Castello, F. Harskamp, R. Hurst, and B. Wilshire. 2007. “Testing of Welded 2.25CrMo Steel, in Hot, High -Pressure Hydrogen under Creep Conditions.” Engineering Fracture Mechanics 74 (6): 956 – 68. https://doi.org/10.1016/j.engfracmech.2006.08.021. Munsterman, Tim, Antonio Seijas, and Dana G Williams. 2010. “High Temperature Hydrogen Attack Resistance Using Autoclave Testing of Scoop Samples.” In IPEIA 14th Annual Conference in Banff, Alberta. Pretorius, C. C. E., Roelf J. Mostert, Tulani W. Mukar ati, and V. M Mathoho. 2021. “Microstructural Influences on Damage Evolution and Kinetics of High Temperature Hydrogen Attack in a C- 0.5 Mo Welded Joint.” In Proceedings of Conference of the South African Advanced Materials Initiative (CoSAAMI 2021) . Saba, Brent Matthew. 2003. “Evaluation of Mechanical Fitness for Service of High Temperature Hydrogen Attacked Steels.” Louisiana S tate University, Masters Theses. Wahab, M. A., B. M. Saba, and A. Raman. 2004. “Fracture Mechanics Evaluation of a 0.5Mo Carbon Steel Subjected to High Temperature Hydrogen Attack.” Journal of Materials Processing Technology 153 – 154: 938 – 44. https://doi.org/10.1016/j.jmatprotec.2004.04.149. Welding Research Council. 2021a. WRC Bulletin 585: The α - Ω HTHA Model and the Time Dependent Prager Curves, . ISSN 2372-1057. ——— . 2021b. WRC Bulletin 586: The α - Ω Loss of Containment Model for HTHA with Guidelines for FFS, . ISSN 2372-1057.