Heterogeneous Ferritic Microstructure in Low-Carbon Micro-alloyed Pipeline Steel: Impacts on Mechanical and Corrosion Properties

Micro-alloyed low-carbon steels are integral to oil and gas pipeline applications, requiring both strength and durability to endure harsh conditions. Composition and microstructure play a critical role in the ultimate behavior of steel. This study investigated influence of heterogeneous microstructure on the mechanical and corrosion characteristics of a micro-alloyed low carbon API 5L X65 steel. A relatively new quenching and partitioning heat-treatment process was used to develop a heterogeneous ferritic microstructure. The experimental procedure involved characterizing the heat-treated steel's microstructure, mechanical behavior, and corrosion resistance. The heat treatment produced a heterogenous ferritic microstructure with a fine-grained network surrounding a dispersed coarse-grained core. The presence of carbon-rich retained austenite was also identified in the microstructure. The heterogeneous ferrite microstructure results in improved tensile strength, ductility, and toughness. Corrosion resistance experienced significant improvement in the heterogeneous ferrite microstructure, attributed to ferrite size and distribution pattern and retained austenite stabilization. Electrochemical assessments, including open-circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy, corroborated the superior corrosion resistance of the heterogeneous ferrite microstructure. Overall, the heterogeneous ferritic microstructure effectively improved the mechanical properties and corrosion resistance, highlighting its potential in enhancing pipeline steel longevity.