The combined effect of microstructure and crystallographic texture on the toughness anisotropy of API 5L X46 steel was studied. The steel exhibited microstructural banding along the 0° direction and near random texture in the material. The mechanical properties were characterized by the use of tensile and impact toughness samples oriented along the rolling direction (0°, 45°, and 90°). For impact toughness, anisotropic behavior was observed, where the 0° oriented samples showed lower impact toughness than the 90° and 45° directions, with differences of 25% and 31%, respectively, while the yield strength (YS) showed the lowest values in the 0° oriented samples. The relative variation coefficient (rrv) indicates significant correlation between microstructure, texture, and toughness anisotropy. It was observed that the banding degree and the amount of {110} and {111} oriented grains are directly related to the degree of anisotropy, which high values are the cause of lower mechanical properties.
Nowadays, an increasing number of oil and gas transmission pipes are constructed with high-strength low alloy steels (HSLA); however, many of these pipelines suffer from different types of hydrogen damages, such as hydrogen-induced cracking (HIC). So many research efforts are being carried out to reduce the detrimental effects of hydrogen damage in HSLA steel pipes. The thermomechanical control process (TMCP) is a microstructural control technique that is able to eliminate the conventional heat treatment after hot rolling. Recent research demonstrated that TMCP provides high HIC resistance without adding high amounts of alloying elements or expensive heat treatments. However, once these HSLA steel pipes are put into service, they experience HIC damage, and the prediction of its kinetics is a necessary condition to perform Fitness-For-Service assessments. To develop a reliable predictive model for the kinetics of HIC, the relations among the microstructural features, environmental parameters, and mechanical properties have to be fully understood. This paper presents a review of the key metallurgical and processing factors to develop HSLA steel pipes, as well as a review of the phenomenological and empirical models of HIC kinetics in order to identify specific research directions for further investigations aimed to establish a reliable and sound model of HIC kinetics.
This paper presents the results of experimental and in-service observations of the nucleation and growth of hydrogen-induced cracking (HIC) in hydrocarbon transport pipelines made of type API 5L steel. The experimental work was done by inducing HIC on steel plates by electrochemical cathodic hydrogen charging and using a straight beam ultrasonic inspection technique to observe the crack growth behavior. Scanning electron microscopy was also used to observe the crack nucleation and propagation mechanisms. The study was complemented by the fractographic analysis of a pipe segment removed from a sour gas pipeline after an in-service rupture caused by HIC, so the pipe segment contained a significant group of blisters and laminations caused by HIC. The results of the cathodic charging indicated that HIC cracks nucleated in less than one hour of hydrogen charging at specific non-metallic inclusions and not necessarily the largest ones as commonly thought. It is observed that the HIC cracks propagated by a quasi-cleavage mechanism in transgranular paths, linking to other cracks by ductile tearing. However, after a few hours of hydrogen charging, the crack growth rate dropped to almost zero, and the overall HIC growth was due almost solely to the interconnection of previously formed individual cracks. The examination of the in-service failed pipe showed similar fractographic and growth characteristics as compared to the laboratory-induced ones. It showed that HIC was little affected by the primary stresses and the proximity of other defects and structural discontinuities.
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