In-service welding of X70 steel gas pipeline: Numerical and experimental investigations

Abstract

The in-service welding process is widely used to repair defective gas pipelines, which allows the damaged section to be repaired without interrupting gas flow. In this way, consumers and gas transportation companies will be able to minimize losses. In this study, a thermomechanical finite element analysis model was developed to examine the effects of pipe pressure, thickness, diameters, and welding heat input on temperature history and residual stress. The results were analyzed using the design of the experiment method (DOE). According to the results, these parameters significantly affect the thermal distribution and pipe yield stress at the welding point by 48.8%, 29.4%, 15.1 and 6.9%, respectively. On the basis of optimal conditions, in-service welding is conducted. Investigations were conducted on the microstructure, temperature distribution, and residual stresses induced during the in-service welding of API 5L X70 steel pipelines. As a result, it was determined that the HAZ microstructure consists primarily of ferrite, bainite, and M/A constituents, whereas the pipe microstructure undergoes fewer phase transformations. In order to validate the numerical model, mechanical tests, as well as hole drilling, were conducted to gather data, and a combination of equivalent carbon and T8/5 was used to determine hardness at various locations in the HAZ in order to determine which areas are susceptible to cold cracking. The numerical results and the experimental results were highly concordant.