Composition-Structure-Properties and Process Comprehensive Control Technology of 125ksi Super Martensitic Oil Well Pipe Material

The 125ksi grade 15Cr super martensite stainless steel exhibits excellent high strength and high toughness matching， and corrosion resistance to CO₂， which making it a promising material for oil well pipes exceeding a depth of 7 000 m. This paper comprehensively discusses the control technology for achieving high strength and high toughness in 15Cr super martensite stainless steel tubing through composition design， production process optimization， and microstructure regulation. In terms of composition control， a δ ferrite-free composition design is employed with a chromium content （<italic>w</italic>［Cr］） of 15%， nickel content （<italic>w</italic>［Ni］） ranging from 6.5% to 7%， carbon content （<italic>w</italic>［C］） between 0.01% and 0.03%， and copper content （<italic>w</italic>［Cu］） varying from 1.25% to 1.5%. The appropriate combination of chromium and nickel elements plays a crucial role in reducing the δ ferrite content， while the optimal balance between carbon and copper contents is key to obtaining superior mechanical properties characterized by high strength and high toughness. Regarding the smelting process， this study investigates the influence of impurity elements on electric furnace steel. The results show that vanadium （V）， nitrogen （N）， and aluminum （Al） impurities increase the hardness of the 15Cr steel alloy.and raise the tempering temperature to 550 ℃-575 ℃， which can reduce the hardness and ensure the toughness. Concerning hot processing and molding techniques， the dynamic recrystallization behavior are obtained through hot deformation tests conducted on the15Cr steel alloy. The optimum hot perforation deformation temperature for seamless pipe manufacturing falls within 1 100°C to 1 150°C as it facilitates grain refinement via fine recrystallized grains formation. Microstructure regulation involves determining an appropriate normalizing temperature during heat treatment processes. The research results suggest that normalizing at temperatures ranging from 950°C to 980°C helps maintain grain size without significantly grown， while the match of high strength and high toughness can be obtained.