Quenching and partitioning-induced bainitic ferrite in API X65 steel: Impact on mechanical and corrosion integrity

Low-carbon microalloyed bainitic ferrite steels are highly sought for their excellent combination of strength, toughness, and weldability, making them suitable for applications like pipelines, pressure vessels, and structural components. In this study, a quenching and partitioning (Q&P) heat treatment was applied to an API 5L X65 pipeline steel to develop a bainitic ferrite microstructure containing film and block-type retained austenite. The effect of this transformation on mechanical properties and corrosion resistance was systematically investigated. Mechanical testing revealed a significant improvement in yield and tensile strength (630 and 950 MPa, respectively), while preserving reasonable toughness. The transformation also led to continuous yielding behavior and high strain hardening due to suppressed Cottrell atmospheres and the presence of retained austenite. Electrochemical analysis demonstrated superior corrosion resistance compared to the traditional ferrite-pearlite microstructure, characterized by a 78% reduction in corrosion current density and improved passivation behavior. Immersion testing and surface analysis further confirmed the stability of retained austenite and the uniform corrosion morphology. These findings demonstrate that the Q&P process is an effective and novel route for enhancing both mechanical and corrosion performance of low-carbon pipeline steels, particularly for applications in harsh and corrosive environments such as oil and gas transmission.