Fatigue behaviour of HIC damaged line pipe steel

The fatigue behavior of hydrogen damaged pipeline steels is essential to provide useful and necessary background information for performing any Fitness-For-Service study on pipelines in the oil industry. Corrosion of steel in wet sour environment produces atomic hydrogen which is absorbed by the steel. Hydrogen atoms accumulate at non-metallic inclusions and other metallic defects such as grain boundaries. At inclusions hydrogen atoms combine to form molecular hydrogen inside the steel. As the concentration of hydrogen increases, the pressure builds up to produce microscopic blisters. Hydrogen-induced cracking (HIC) occurs when short blisters at varying depths within the steel link together to form a series of steps which may reach the surface and cause the metal to fail.;The present study investigates the influence of hydrogen damage on the fatigue behavior of line pipe steel. The specimens used for the experimental work are designed in compliance with ASTM-E-466-96 "Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials". A standard test method was used to simulate the service environment of hydrogen absorption from aqueous sulfide corrosion. The hydrogen charged samples were then fatigue tested. Metallography and fractography examinations of failed samples were performed for fracture surface analyses to determine the relationship of the fracture mode to the microstructure, and evaluate the materials' response to mechanical and chemical environments. Samples prepared transverse to the rolling direction of the steel and exposed to HIC were found to have a better fatigue life than HIC exposed samples that were prepared along the rolling direction of the steel.