Development of L450M HFW pipes for supercritical CO₂ transmission

Objective This study aims at fulfilling the steel requirements for supercritical CO₂ transmission.

Methods L450M (X65) hot rolled coils for pipes used in supercritical CO₂ transmission were developed, with a microstructure mainly consisting of fine and flat polygonal ferrite + ferrite + a small amount of pearlite, boasting a Low-C medium-Mn micro-Ni alloying design, and leveraging the high-purification steelmaking and large-tonnage reduction rolling technologies. Numerous calculations were performed, focusing on OD219.1 mm×10 mm (pipe diameter×wall thickness) High Frequency Welding (HFW) pipes for supercritical transmission with a design pressure of 16 MPa, to ensure their resistance to cracking initiation and fracture propagation during operation, yielding the following results. The Charpy impact absorbed energy in the base metal at －45 ℃ was not less than 88 J for single values or not less than 117 J for averages. The Charpy impact absorbed energy in the welds and heat-affected zones at －45 ℃ was not less than 42 J for single values or not less than 56 J for averages.

Results Based on the initial investigation into the forming, welding, and heat treatment processes, L450M HFW pipes for supercritical transmission with excellent cryogenic properties were developed under the following conditions: forming extrusion of 5.25 mm, welding rate at 17 m/min, and weld heat treatment at 930 ℃. The third-party testing validated the performance of the developed welded pipes in full compliance with the Line Pipe (API Spec 5L, 46th edition), and cryogenic toughness requirement. During the flattening experiment, the flattened pipes showcased no cracks within the base metal and welds. This observation underscored the robust plasticity and exceptional weld quality of both the base metal and welds. The yield strength of the base metal was 534 MPa, and the tensile strengths of the base metal and welds closely matched at 619 MPa and 620 MPa respectively, meeting the standard requirement of not less than 535 MPa and suggesting a sufficient strength margin. The hardness levels of the base metal, welds, and heat-affected zones did not surpass 220 HV10 and exhibited uniform profiles. At －45 ℃, the impact absorbed energy ranged from 177 J to 401 J for welds, from 200 J to 415 J for the heat-affected zones, and from 248 J to 416 J for the base metal. Furthermore, the ductile-brittle transition temperatures of the base metal, welds, and heat-affected zones remained below －60 ℃.

Conclusion The developed HFW pipes demonstrate exceptional cryogenic toughness, high ductile crack arrest capabilities still under low temperatures, and impressive internal and external pressure-bearing capacities of 62.29 MPa and 38.9 MPa respectively. These properties are fully aligned with the operational requirements for HFW pipes utilized in supercritical CO₂ transmission.