Achieve batch localization of small-diameter ultra-long seamless steel pipes made of 3.5Ni steel (ASTM A334 Gr.3) for use at -101℃

Since the United States first successfully developed -101℃ grade 3.5Ni steel in the 1930s and included it in the ASTM standard, Europe and Japan have successively developed their own 3.5Ni steel, which has been successfully used in the manufacture of low-temperature equipment in projects such as ethylene, fertilizer, rubber, liquefied petroleum gas, and coal gasification. In China, steel plates and forgings with a nickel content of 3.5% have achieved mature and stable industrial production, and domestically produced 3.5Ni steel exhibits good technical performance and economy; however, the production of seamless steel pipes, especially small-diameter products, has been limited.

Addressing the industry pain point of the long-term dependence on imports and low localization rate of domestically produced small-diameter 3.5Ni steel seamless pipes, our company, leveraging our technical expertise in low-temperature pipes, small-diameter precision pipes, and ultra-long pipes, has successfully developed 25×2min×22m cold-drawn small-diameter ultra-long 3.5Ni steel seamless pipes. These pipes were successfully delivered to a certain equipment manufacturer in April, and the final products will be applied to a project within the Arctic Circle in Russia. After inspection by a national authoritative testing agency, the products met the ASTM A334 Grade3 standard requirements in terms of key indicators such as low-temperature impact toughness at -101℃, room temperature tensile properties, and dimensional accuracy, reaching the advanced level of similar international products. These pipes can be widely used in fields such as ethylene cryogenic pipelines, liquefied gas transportation, and air separation equipment.

Based on customers' technical requirements for low-temperature seamless steel pipes, and taking into account the production process and cost control of steel pipes, the composition design of low-temperature seamless steel pipes must ensure that the product possesses appropriate strength, high low-temperature toughness, convenient heat treatment process, excellent welding performance, and resistance to hydrogen-induced cracking. Drawing on the successful experience of 3.5Ni steel plates and hot-rolled seamless steel pipes, our company has established a strict control range for the chemical composition of cold-drawn seamless low-temperature steel pipes. Please refer to the table below for details:

In the smelting process, the process route of electric arc furnace steelmaking + refining + vacuum degassing is adopted. Subsequently, the required tube billets are produced through die casting and rolling. This ensures that the content of harmful elements and gases is extremely low from the source, and the surface quality of the tube billets is excellent. After the tube billets are heated in a step-by-step furnace, pierced by two-stick cross-rolling, and then cold-rolled and cold-drawn through multiple processes to become finished products, they undergo heat treatment and finishing processes to obtain finished steel pipes with an outer diameter of 24.96~25.05mm, a wall thickness of 2.04~2.16mm, and a length of 22 meters. The roughness of both the inner and outer surfaces is less than 1.4um, meeting the quality requirements of Grade T1 heat exchange tubes specified in NB/T47019.1-2021 standard.

Due to the uneven thickness of the finished pipe wall, direct sampling for impact testing is not feasible. Instead, the parent pipe and finished pipe undergo the same heat treatment in the same furnace, resulting in a metallographic structure composed of ferrite and bainite, with a grain size of approximately grade 10. Samples measuring 2.5×10×55mm were tested for impact at -117℃, with measured impact energies of 32J, 33J, and 35J, significantly exceeding the standard requirements of an average value ≥5J and an individual value ≥4J.