Effect of hydrogen on the mechanical behavior of API X70 ageing

The present work reports on the effect of hydrogen on the mechanical behavior of API 5L X70 ageing. Bending test specimens underwent solution treatment in an electric induction furnace at 1100°C for 30 min under argon atmosphere and water quenching and then ageing process for five temperatures in the range between 204 to 650°C for 30 min of time exposition and water quenching, respectively. The microstructural characterization was examined by scanning electron microscopy (SEM) and matrix samples ageing showed microstructures, acicular ferritic, polygonal ferrite, bainitic-ferrite, degenerate perlite and retained austenite and energy dispersive spectroscopy (EDS) evidencing the presence of carbonitrures type Nb, Ti, V (N, C). Hydrogen was introduced in a sample of bending test, through cathodic charging, applying a current density of 50 mA/cm2. Three point bend test was carry out on specimen on API X70 with and without hydrogen and the result showed a reduction of ductility and 65% of index of embrittlement (IH). Also, samples undergoing ageing with hydrogen showed an increase in bending stress compared to a sample undergoing ageing without hydrogen, and the parameter (IH) decreased in the range of 21 to 29% due to the formation of traps. The fracture surface of bend tested samples were examined using a SEM.