Effect of Quenching Post-Intercritical Austenitizing on the Microstructure and Tensile Properties of an K55 Grade Steel for Oil and Gas Industry

Abstract

The API K55 grade steel is widely utilized in seamless pipes for oil and gas exploration, especially as casing pipes for wellbores. Traditionally, this steel is processed using hot rolling followed by quenching and tempering to achieve the desired dimensional and microstructural characteristics, balancing high strength with ductility. This article introduces an alternative method to attaining the required tensile properties for API K55 grade steel by employing a biphasic microstructure (ferrite/martensite) achieved through quenching post-intercritical austenitizing heat treatment to high-strength-low-alloy steel. Thermodynamic simulations and dilatometric experiments revealed that increasing the austenitizing temperature enhances austenite formation, decreasing significantly its carbon content, which facilitates martensitic transformation and increases the M_s and M_f temperatures. A complete phase transformation mapping was presented, highlighting how the austenitizing temperature influences martensitic transformation kinetics during the quenching heat treatment. It was concluded that austenitizing at 750 °C, followed by quenching and short tempering at 650 °C, produced a biphasic microstructure with 30% ferrite and 70% martensite, providing a favorable balance between mechanical strength and ductility that meets the API K55 grade requirements, surpassing traditional methods in the industry.