The stochastic nature of the thickness of boride layers formed on carbon steel is described in this paper. Additionally, the probability distribution of the layer thickness is studied to determine the best-fit probability distribution. The study combines the use of an empirical model (power-law) and the Markov chain principles, with the purpose of demonstrating that it is feasible to develop a model that represents the non-uniformity of the thickness of boride layers that form on carbon steel. The results indicate that the mean and variance tend to increase when the time or temperature is increased. The findings of this paper demonstrate that an analytical solution to the Kolmogorov’s system differential equation can adequately represent the behavior of non-uniform boride layer formed on low-carbon steel, regardless of the temperature or duration of treatment.

Although the use and promotion of renewable energies have increased in recent years, it is evident that the use of fossil fuels such as oil and gas continues to be of great importance. Likewise, pipelines are widely recognized as the most reliable and profitable means of transportation for liquid and gaseous hydrocarbons. Nevertheless, due to the nature of hydrocarbons, oil and gas pipelines are continually exposed to deterioration by corrosion and mechanical damage. In this context, this research focuses on the improvement of the surface properties of API 5L grade B pipeline steel by applying a surface hardening process. Samples of an API 5L grade B pipeline steel were exposed to boriding to form a layer of high hardness (from 2.60 GPa for the non-treated material to 14.12 GPa for the samples exposed to 1000 °C for 6 h). The treatment time was set at 2, 4, and 6 h, at temperatures of 850, 900, 950, and 1000 °C. Due to the saw-tooth morphology of the layers and the random nature of the process, it was possible to fit their thicknesses to a probability density function in all the experimental conditions. The crystalline structure of the layers was analyzed by X-ray diffraction and the morphology was observed using SEM and optical microscopy. The layer’s thickness ranged between 26.6 µm to 213.9 µm showing a close relationship with the experimental parameters of time and temperature. Finally, it is studied the changes undergone in the pipeline steel after the thermochemical process, observing an increase in the grain size as a function of the temperature.

Fixed platforms employed in oil & gas shallow waters industry are subjected to dynamic loads created by winds, waves and currents. However, such loads can produce fatigue damage, like localized cracking in the weld toes of T-welded connections of the submerged structures of fixed platforms. Therefore, to repair localized cracking, a technique that combines grinding and wet welding can be applied at the weld toes of submerged structural connections. Accordingly, in the present work it is presented a methodology to restore the fatigue life of T-welded connections fabricated with A36 plate steel. Consequently, T-welded connections samples were prepared and tested under intact, grinding and grinding-wet-welding repair conditions. For the repair conditions, the weld toes were grinded 6 mm and 10 mm, and wet welding was deposited to fill the grinded material in a hyperbaric chamber, which simulated 50 m, 70 m and 100 m water depths. Afterwards, the repaired connections were subjected to fatigue loads, and the S-N curves were measured for intact and repair conditions. Fatigue results showed that the combined repair technique was able to restore the fatigue life of T-welded connections for a maximum water depth of 50 m. Nonetheless, for water depths of 70 m and 100 m the fatigue life was smaller than for 50 m, but higher compared to the only grinding repair condition.