Effect of Cooling Medium on Microstructure and Properties of Ultrahigh Strength Martensitic Quenched and Tempered Steel

In deep and ultra‐deep wells, tubing and casing face increasingly severe service conditions requiring higher performance materials. High‐grade steel tubing and casing (HGTC), made from highly ductile materials, are widely used for their reliability under extreme pressure. Although ultra‐high yield strength is essential, it often leads to reduced impact toughness. This study modifies the cooling method after quenching in the conventional Q&T process using water, oil, or air cooling after austenitization. Results demonstrate that oil‐quenched steel achieved the most balanced properties: after tempering, it exhibits a Charpy impact energy of 61.0 J—significantly higher than water‐quenched (50.5 J) and air‐cooled (38.0 J) samples—and a tensile strength of 1392 MPa, outperforming the other methods. Key microstructural features such as precipitate morphology, dislocation density, and grain size play a decisive role in mechanical performance. Controlled optimization of these parameters enables an optimal balance of strength, toughness, and ductility. The superior performance of oil‐quenched steel underscores the importance of microstructure‐mechanical property relationships, providing valuable insights for developing advanced high‐strength steels.