Solid Expandable Tubular (SET) technology is a proven method in oil and gas drilling, enabling water shutoff, zonal isolation, and life extension of wells through in situ plastic expansion of steel casing. The expansion mandrel, a precision-engineered conical tool, performs this cold expansion under demanding downhole conditions. D6 steel (a high-carbon, high-chromium cold-work tool steel) was selected for its manufacture. This tool steel has exceptional wear and abrasion resistance, high compressive strength, dimensional stability, and heat-treatment versatility. These properties make it well suited for repeated contact, resistance to plastic deformation, and consistent expansion performance.

This study investigates the influence of heat treatment on D6 steel to determine an optimum sequence for SET mandrel applications. The process includes annealing, austenitizing, air and oil quenching, and single and double tempering at six temperatures (100-600°C). Mechanical testing (following ASTM standards) included Rockwell hardness, Charpy V-notch impact toughness, and tensile properties, supported by microstructural and fractographic analyses. Single tempering produced variable properties and reduced strengths, while double tempering with air cooling (DTA) or oil quenching (DTO) yielded superior results. DTO at 400°C achieved maximum hardness (57 HRC), whereas DTA at 400°C offered slightly lower hardness (52 HRC) but higher yield and tensile strengths with improved ductility. Considering overall performance, DTA at 400 °C is recommended for optimum combination of hardness, toughness, and ductility, enhancing mandrel life and tubular expansion accuracy. The findings provide practical heat treatment guidelines for D6 steel in SET applications, improving durability, dimensional precision, and operational efficiency in harsh oilfield environments.