This article reports on the failure analysis of the geothermal steam pipe. Macro and micro examination of failed pipelines was studied using optical and scanning electron microscopy. The energy-dispersive spectroscopy studied the elemental composition of the corroded surface. Further, the study measured the pipe thickness on the failed pipeline section. Visual examination showed a significant thinning of the outer section of the failed pipe to 3 mm from 12.7 mm of the original pipe. The chemical composition results show that the steel meets the minimum requirements for API 5L Grade B steels used in steam pipelines for geothermal power plants. The microstructural analysis of the investigated steel shows that the steel had pearlite and ferrite phases. The steel failure mechanism was due to erosion-corrosion, which caused localised wall thinning near the drain port and elbow section. The study recommends creep-resistant steel for the drain port and elbow for geothermal power plant application.

This study reports the flow stress behavior of three P92 steels with different compositions. Uniaxial compression tests were conducted in the deformation temperature range of 575 °C to 650 °C and strain rate range of 0.001-0.5 s-1 using a Gleeble® 3500 thermo-mechanical simulator. A simple physically-based constitutive model was used to analyse the effects of deformation conditions (temperature and strain rate) on the metal flow stress behavior during the deformation process. The method accounts for the temperature dependence of Young’s modulus and the lattice self-diffusion coefficient of Fe in the ferrite. Constitutive equations describing the flow stress behavior of the three P92 steels were developed. From the results, the stress exponent n of 26.13(steel A), 21.61(steel B) and 27.55 (steel C) were obtained using the self-diffusion activation energy in the physically-based constitutive equation. From the results, the three steels had variation in the stress exponent values, which was attributed to differences in elemental content, such as chromium and tungsten. The developed constitutive equations were verified using statistical parameters: Pearson’s correlation coefficient (R) and average absolute relative errors (AARE). Statistical analysis showed that the three steels had the same R of 0.98, while AARE was: 1.68 (steel A), 1.72 (steel B), and 1.82 (steel C). The constitutive equations developed showed a good correlation between the experimental and predicted flow stress data. Hence, the method is applicable in describing flow stress behavior in the metalworking process in the industry.