Development and Performance of X60M-Grade HFW Steel Pipe for Supercritical Carbon Dioxide Transportation

Abstract

An X60M-grade hot-rolled coil with outstanding low-temperature performance was developed using a low-carbon, medium-manganese microalloy design, clean steelmaking processes, and TMCP-controlled rolling and cooling technologies. The microstructure of the coil primarily consists of elongated, flat polygonal ferrite (PF), with a small amount of pearlite (P). By optimizing the HFW pipe forming and welding process, an X60M-grade HFW steel pipe of 323.9 mm × 10 mm was created for the transportation of supercritical carbon dioxide. The pipe underwent rigorous testing, including low-temperature impact toughness, tensile strength, hardness, flattening, and hydrostatic burst tests. The results demonstrate that the pipe exhibits high strength, ductility, and toughness, with weld seam strength equal to or exceeding that of the base material. At -45°C, the impact energy of the weld seam ranges from 293 to 351 J, the heat-affected zone from 219 to 315 J, and the pipe body from 248 to 342 J, with a 100% shear area in all samples. At -75°C, the impact energy of the weld seam, heat-affected zone, and pipe body ranges from 204 to 350 J, with shear areas ranging from 85% to 100%, demonstrating the pipe's excellent low-temperature toughness. This pipe meets all design and safety requirements for supercritical carbon dioxide transmission pipelines.

Introduction

Globally, tens of thousands of kilometers of carbon dioxide transmission pipelines have been constructed, with nearly 8,000 km located in the United States, and the remainder primarily in Canada, Norway, Turkey, and other countries. Carbon dioxide is mainly transported in its supercritical state, which occurs when the pressure exceeds 7.38 MPa and the temperature exceeds 31.1°C. Supercritical carbon dioxide combines the high density of a liquid with the low viscosity of a gas, making it the most economical and efficient option for large-scale pipeline transport. The design pressure of international carbon dioxide pipelines typically ranges from 8 to 20 MPa, with small- and medium-diameter pipes often utilizing cost-effective high-frequency welded (HFW) steel pipes. However, China’s limited carbon dioxide pipeline infrastructure currently relies on gas-phase transport, with supercritical carbon dioxide transport not yet implemented. To support the domestic CCUS demonstration project, a steel pipe company developed a low-carbon, medium-manganese microalloyed X60M-grade coil with excellent low-temperature performance. The successful application of the HFW high-frequency welding process resulted in the development of X60M-grade HFW steel pipes for supercritical carbon dioxide transportation, achieving high performance, especially in low-temperature toughness, thereby providing a strong material foundation for CCUS pipeline projects.

1. Coil Composition and Microstructural Design

The composition of HFW steel pipes for supercritical carbon dioxide transportation is designed using a low-carbon, medium-manganese microalloying strategy, with tight control over sulfur and phosphorus content to minimize compositional and microstructural segregation. To facilitate oxide removal during HFW welding, the Mn/Si ratio is maintained between 5 and 7 to minimize the formation of high-melting-point manganese-silicon-oxygen inclusions. The addition of microalloying elements such as niobium, vanadium, and titanium refines the microstructure, improving low-temperature toughness and strength, while preventing grain coarsening in the heat-affected zone. Chromium and nickel further enhance the plate's strength. The chemical composition of the hot-rolled coil for the X60M-grade HFW steel pipe used in supercritical carbon dioxide transportation is shown in Table 1, with an Mn/Si ratio of 6.4.

Table 1: Chemical Composition of X60M Steel Grade Hot-Rolled Coil (%)

A comprehensive clean smelting process is employed, incorporating LF and RH refining, electromagnetic stirring, and large-tonnage pressing to minimize inclusion content and ingot segregation. The microstructure consists of fine, flattened polygonal ferrite (PF) and pearlite (P), with no notable banded segregation. This uniform and refined microstructure improves crack propagation resistance and reduces further crack growth. Figure 1 shows the microstructure of the X60M plate. The grain size is approximately 10.5, and the non-metallic inclusion rating does not exceed 0.5. Ultra-low sulfur and phosphorus content, along with controlled inclusion levels, significantly enhance low-temperature toughness, prevent crack initiation and rapid propagation under impact loads, and ensure pipeline safety.

Figure 1: X60M steel grade hot-rolled coil metallographic structure

2. Manufacturing Process Research of HFW Steel Pipes 

The HFW pipe manufacturing process relies on the proximity and skin effects of high-frequency currents to rapidly heat the pipe edges to approximately 1300°C after the hot-rolled coil has been roll-formed. The weld seam is then fused under the pressure of the extrusion roller. Factors such as the geometry of the plate edges, pipe forming, and welding processes influence the pipe’s overall quality and performance. Given the high pressure in supercritical carbon dioxide transmission pipelines, further research and optimization of the forming, extrusion, welding, and heat treatment processes are essential to improve low-temperature performance. This study aims to develop a high-performance Φ323.9 mm × 10 mm straight-seam HFW steel pipe made from X60M steel grade for supercritical carbon dioxide transportation.

2.1 Forming Process Research

For the X60M steel grade Φ323.9 mm × 10 mm steel pipe designed for supercritical carbon dioxide transportation, a high-precision milling process is applied to the edges of the steel strip to ensure uniformity and a clean cut. The pipe is formed using the roller downhill forming method, with the extrusion force of the roller adjusted according to the pipe’s curvature and material strength. The shape of the external weld seam burr directly reflects the degree of forming extrusion and the quality of the HFW weld seam. Figure 2 shows the weld seam’s cross-section and external burr morphology of the X60M-grade Φ323.9 mm × 10 mm HFW steel pipe under different extrusion conditions. In Figure 2(a), at an extrusion amount of 4.5 mm, the extruded metal burr appears flat, pressing against the weld seam metal surface, with large cavities and reflux inclusions present in the burr. The weld seam and heat-affected zone (HAZ) are excessively wide, indicating insufficient extrusion, which prevents the complete expulsion of oxides and inclusions. In Figure 2(b), at an extrusion amount of 5.5 mm, the extruded metal burrs form a mushroom-shaped profile, covering the upper portion of the weld seam metal and partially fusing with it. The burrs are relatively uniform, and the weld seam and HAZ widths are well-balanced, indicating an optimal extrusion amount and resulting in high weld quality. In Figure 2(c), at an extrusion amount of 5.75 mm, the weld seam metal burrs widen, reducing the HAZ width at the weld seam center while increasing it at the inner and outer surfaces. This condition can lead to incomplete extrusion of oxides and inclusions. Therefore, for this steel grade and specification, an extrusion amount of 5.5 mm ensures a high level of cleanliness in the HFW weld seam metal.

(a) 4.5 mm extrusion (b) 5.5 mm extrusion (c) 5.75 mm extrusion

Figure 2: Weld seam and Burr Morphology at Different Extrusion Amounts

2.2 Welding Process Research

For the X60M-grade Φ323.9 mm × 10 mm HFW steel pipe used in supercritical carbon dioxide transportation, process tests and optimizations were carried out to minimize oxide inclusions in the weld seam and enhance weld toughness. The welding bevel angle was reduced and maintained within a range of 3°–5° to improve the extrusion effect. Under the same extrusion force, the welding speed was maximized, and the welding frequency was adjusted to 415–435 kHz based on the pipe’s wall thickness.

Three welding processes (Table 2) were selected to analyze variations in weld toughness at low temperatures. Figure 3 presents the variation in weld impact energy with temperature for the different welding processes. Process 1 exhibited the highest weld  seam toughness, with the weld seam’s Charpy impact energy remaining at 200 J at -75°C. To further improve weld seam toughness, austenitizing heat treatment at 930°C was applied to eliminate hard-phase structures and improve the structural consistency between the weld seam and the base material. Figure 4 shows the microstructure of the weld seam after heat treatment in Process 1. The heat-affected zone (HAZ) and weld seam microstructure consist of F+PF+P phases, with a grain size of 10.5. The grains are fine; the microstructure is uniform, and it closely matches that of the base material. No significant inclusions were observed in the weld seam, ensuring excellent low-temperature toughness of the weld joint.

Table 2: Research on Welding Process of Supercritical Carbon Dioxide HFW Welded Pipe

Figure 3: Curve of impact energy of weld seam with temperature in different welding processes

(a) Weld seam (b) Heat affected zone

Figure 4: Metallographic structure of HFW welded joint

3. Mechanical Properties of Steel Pipes

X60M-grade Φ323.9 mm × 10 mm HFW steel pipes for supercritical carbon dioxide transportation were produced in batches using a German 16-inch straight-seam high-frequency resistance welding (HFW) production line, following the specified welding process.

3.1 Low-Temperature Impact Toughness

Charpy impact tests were performed on the base material, weld seam, and heat-affected zone of the X60M-grade Φ323.9 mm × 10 mm steel pipe at various temperatures using a ZWICK PSW750J oscilloscope impact tester. The test results are shown in Figure 5. Over the temperature range of -75°C to 0°C, the base material, weld seam, and heat-affected zone demonstrate high impact toughness, with impact energy exceeding 204 J. The shear area of the base material, weld seam, and heat-affected zone remains above 85%.

Table 3 presents the Charpy impact toughness and shear area for different locations on the supercritical carbon dioxide HFW steel pipe at -45°C. At -45°C, the average impact energy of the weld seam, heat-affected zone, and base material is at least 237 J, indicating excellent low-temperature toughness.

(a) Transverse section of the tube body (b) Weld seam (c) HAZ

Figure 5: Ductile-brittle transition curve with temperature for the HFW steel pipe series used in supercritical carbon dioxide transportation

Table 3: Impact Test Results of HFW Steel Pipe at -45°C for Supercritical Carbon Dioxide Transport

3.2 Tensile Strength

Tensile testing was performed using a ZWICK Z1200KN universal testing machine, with the results presented in Table 4. The tensile strength of the base material ranges from 580 to 620 MPa, while the tensile strength of the weld seam ranges from 570 to 620 MPa. The tensile strength of the weld seam is comparable to that of the base material, with both values exceeding the minimum requirement of 520 MPa for X60 steel grade. Additionally, the yield strength ratio is ≤ 0.89, fully meeting the standard requirements and providing a safety margin to ensure the steel pipe's reliable performance during service.

Table 4 Transverse and weld tensile test results of HFW steel pipe for supercritical carbon dioxide transportation

3.3 Hardness Test

Figure 6 presents the hardness distribution of the welded joint in the X60M-grade Φ323.9 mm × 10 mm HFW steel pipe for supercritical carbon dioxide transportation. The hardness distribution across the weld seam, parent material, and heat-affected zone is relatively uniform, with all values below 220 HV10. The maximum hardness of the parent material is 213 HV10, while the maximum hardness of the weld seam is 207 HV10. The average hardness of the welded joint is 193 HV10. These lower hardness values help mitigate the risk of stress corrosion cracking in the pipeline.

Figure 6: Hardness distribution of HFW steel pipe welded joint for supercritical carbon dioxide transportation

3.4 Flattening Test

The flattening test evaluates both the weld seam quality and the overall mechanical properties of HFW welded pipes. A test section 250 mm in length is used for the full-pipe flattening test. In accordance with ASTM A370-22a standards, the YH41-100C flattening test machine is used, with the weld seam positioned at both the 12 o'clock and 3 o'clock positions during testing. The plates are first pressed to 0.33D and 0.50D, respectively, to check for cracks, and then pressed to zero plate spacing (0 mm) for further crack inspection, as shown in Figure 7. No cracks were observed in either case. The results of the flattening test, presented in Table 5, exceed the requirements specified in the API SPEC 5L standard.

(a) The weld seam is at the 12 o'clock position. (b) The weld seam is at the 3 o'clock position.

Figure 7: Morphology of HFW steel pipe after flattening test

Table 5 Flattening test results of HFW steel pipe for supercritical carbon dioxide transportation

3.5 Hydrostatic Burst Test

To further verify the pressure-bearing capacity of the X60M-grade Φ323.9 mm × 10 mm HFW steel pipe for supercritical carbon dioxide transportation, an 8-meter-long full-pipe sample was selected for the hydrostatic burst test. The test was conducted in accordance with the SY/T 5992 standard. The reference standard predicted a burst pressure of 32.11 MPa, while the actual test measured a burst pressure of 39.31 MPa, surpassing the calculated value. The burst morphology, shown in Figure 8, indicates that the failure point occurred in the steel pipe's parent material. This demonstrates not only the high internal pressure-bearing capacity of the pipe but also that the weld seam strength is comparable to that of the parent material.

Figure 8: Morphology of steel pipe for supercritical carbon dioxide HFW transportation after burst test

4. Conclusion

(1) The X60M steel grade hot-rolled coil, developed for supercritical carbon dioxide transportation, is produced using a low-carbon, medium-manganese microalloying design, clean steelmaking processes, and TMCP-controlled rolling and cooling technologies. Its metallographic structure is primarily composed of slender, flat polygonal ferrite (PF) with a small amount of pearlite (P), providing excellent low-temperature toughness.

(2) By optimizing the HFW steel pipe forming and welding process, the X60M steel grade Φ323.9 mm × 10 mm HFW steel pipe for supercritical carbon dioxide transportation was developed with the following parameters: extrusion volume of 5.5 mm, welding power of approximately 400 kW, welding speed of 17 m/min, welding frequency of 425 kHz, and a weld seam heat treatment temperature of 930°C. The mechanical properties fully meet the GB/T 9711 PSL2 standards, and both the base material and weld seam exhibit high strength, excellent low-temperature toughness, and superior plasticity.

(3) At a test temperature of -45°C, the impact energy of the weld seam in the X60M steel grade Φ323.9 mm × 10 mm HFW steel pipe for supercritical carbon dioxide transportation ranges from 293-351 J, the impact energy of the heat-affected zone ranges from 219-315 J, and the impact energy of the pipe body ranges from 248-342 J. The shear area for all specimens is 100%. When the test temperature is lowered to -75°C, the impact energy of the pipe body, weld seam, and heat-affected zone remains between 204-350 J, and the shear area ranges from 85% to 100%. These results demonstrate the pipe's excellent low-temperature toughness, enhancing the pipeline's ability to resist crack initiation, extension, and arrest at low temperatures. This performance fully meets the design requirements for supercritical carbon dioxide pipelines, ensuring the pipeline's safe operation.