Material technology is crucial for meeting the requirements-such as high strength, high toughness, and corrosion resistance-of casing used in ultra-deep and super-deep wells. Research and development efforts have employed techniques like clean steel smelting and controlled rolling to optimize the material's microstructure, ensuring a balance between good low-temperature impact toughness and high-temperature mechanical properties. For instance, a series of high-grade materials ranging from 125 ksi to 165 ksi has been successfully developed; the composition and processing of the 165 ksi grade, in particular, were perfected through extensive experimental research.
Threaded connections are a critical component of casing assembly, and their manufacturing process directly impacts the safety of the casing string. To address the increased torque and pressure encountered during the rotating run-in of ultra-deep wells, the manufacturing process for threaded connections was improved. Modifications included switching to a hooked thread profile and optimizing the buttress thread geometry to enhance torsional resistance, prevent thread disengagement, and withstand bending and tensile loads, thereby improving the overall reliability of the casing string.
27MnCrV is a new steel grade used to produce TP110T-grade oil country tubular goods (OCTG), whereas conventional production of TP110T-grade casing typically relies on 29CrMo44 and 26CrMo4 steels. Compared to the latter two, 27MnCrV contains less molybdenum (Mo), which helps reduce production costs. However, when produced using standard austenitizing and quenching processes, 27MnCrV exhibits significant high-temperature temper brittleness, resulting in low and inconsistent impact toughness.
Two methods are commonly used to address this issue: first, rapid cooling after tempering to avoid high-temperature brittleness and ensure toughness; and second, intercritical quenching (sub-critical quenching), which utilizes incomplete austenitization to effectively mitigate the adverse effects of harmful elements and impurities, thereby enhancing toughness. The first method imposes relatively strict requirements on heat treatment equipment and entails additional costs.
For 27MnCrV steel, the Ac1 temperature is 736°C and the Ac3 temperature is 810°C; the heating temperature for intercritical quenching is selected within the 740–810°C range. For the intercritical quenching process, a heating temperature of 780°C with a holding time of 15 minutes was selected, followed by tempering at 630°C for 50 minutes. Since intercritical quenching involves heating within the α+γ dual-phase region-quenching while retaining some undissolved ferrite-it enhances toughness while maintaining high strength.
Additionally, the lower quenching temperature (compared to conventional methods) reduces quenching stress and deformation, thereby ensuring smooth heat treatment operations and providing high-quality raw material for subsequent processes such as threading.
This process has been implemented in manufacturing plants; quality assurance data indicate that the heat-treated steel pipes exhibit a yield strength (Rt0.6) of 820–860 MPa, a tensile strength (Rm) of 910–940 MPa, and an impact toughness (Akv) of 65–85 J, with a 100% pass rate for damage resistance. These results demonstrate that the properties of 27MnCrV steel pipes meet the requirements for high-grade petroleum casing and confirm that intercritical quenching is an effective method for avoiding high-temperature brittleness in steel product manufacturing.
These advanced material and manufacturing technologies have been successfully applied to the development of high-end domestic casing. For instance, high-strength, high-toughness petroleum casing developed using these technologies shows significantly improved performance metrics compared to conventional products and has been utilized in my country's first 10,000-meter scientific exploration well. Furthermore, the series of high-end casing products designed for ultra-deep, complex wells has increased the domestic content of casing used in deep and ultra-deep wells, thereby supporting deep-earth drilling projects.
