Fracture toughness properties of HIC susceptible carbon steels in sour service conditions

Abstract

Hydrogen Induced Cracking (HIC) in carbon steels is a well-studied mechanism, where diffusing hydrogen atoms accumulates at the steel imperfections/laminations to create gaseous hydrogen with very high pressure, leading to initiation and growth of internal cavities, so-called HIC. Measurements of relevant fracture toughness properties of non-HIC resistant steels in hydrogen environment is critical to predict and assess the initiation and growth of HIC. The present work attempts to quantify the effect of hydrogen on the fracture toughness properties (KQ and CTOD) of an API X42 pipeline steel under simulated H2S in-service conditions. The fracture toughness properties are measured in TL and SL directions: perpendicular and parallel to the pipeline wall thickness, respectively, following ASTM E1820, standard. Since the X42 is a non-HIC resistant steel, the measurement of the fracture toughness properties in the SL direction is more relevant in terms of HIC initiation and growth than fracture toughness properties in the TL direction. Indeed, parallel to the thickness of the pipeline wall, X42 steel shows microstructural features prone to HIC formation and growth. Steady state H2S in-service conditions were simulated by charging the specimen for 48 h using a special electrolytic solution and then tested (ex-situ) to evaluate the fracture toughness properties. The steady state H2S environment was obtained by measuring the Hydrogen Concentration (CH) in the bulk of the specimen, using Thermal desorption Spectroscopy at three levels of CH. It was observed that the KQ was not affected in the SL direction, while it was reduced in the TL direction for 1.5 ppmw of CH. The CTOD showed mixed results in the TL direction while it was significantly reduced in the SL direction reaching a saturation at 1 ppmw of CH. Besides, microstructural analyses showed that the presence of inclusions coalescence in form of dimples promote the early failure, which is more pronounced in the hydrogen environment especially at higher levels of CH.