Precipitate-Induced Fracture Mechanisms of API X60 Linepipe Steel Suitable for Sour Conditions

Abstract

Sour service environments, characterized by high levels of hydrogen sulfide (H₂S), pose significant risks of material failure due to cracking. These conditions are commonly encountered in oil and gas production wells, transmission pipelines, and refinery equipment. To ensure steel suitability for these challenging conditions, it must meet the API 5 L Annex H requirements. One critical assessment is the hydrogen-induced cracking (HIC) test, which evaluates the steel's resistance to crack formation in sour environments. Herein, HIC in pipeline steel is thoroughly examined, with an emphasis on mitigating failure through tailored microstructures and alloying strategies. HIC tests are conducted on high-strength low-alloy steels with varying alloy compositions. These tests adhere to the National Association of Corrosion Engineers TM0284-2016 standard, involving exposure to a sour environment where hydrogen sulfide is continuously bubbled through the test solution for 96 h. The findings reveal that precipitates play a crucial role in increasing the steel's susceptibility to HIC, leading to intergranular fractures. Specifically, large precipitates such as NbTi carbonitride are identified as key contributors to crack initiation. This detailed analysis highlights the importance of understanding precipitate behavior to enhance the performance of pipeline steels in sour service environments, guiding the development of more resilient materials.