Since the United States first successfully developed 3.5% nickel steel rated for -101°C in the 1930s and incorporated it into ASTM standards, Europe and Japan have subsequently developed their own 3.5% nickel steels, all of which have been successfully used in the manufacture of low-temperature equipment for 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 has demonstrated excellent technical performance and cost-effectiveness; however, the production of seamless steel pipes, especially small-diameter products, has remained relatively limited.

      Addressing the industry’s long-standing challenges—namely, the heavy reliance on imports and low domestic production rates for small-diameter 3.5Ni steel seamless pipes—our company leveraged its technical expertise in low-temperature pipes, small-diameter precision pipes, and ultra-long pipes to successfully develop 25×2 mm × 22-meter cold-drawn, small-diameter, ultra-long 3.5Ni steel seamless pipes. These were successfully delivered to an equipment manufacturer in April, and the final products will be used in a project within the Arctic Circle in Russia. Following testing by a national authoritative inspection agency, the product’s key indicators—including low-temperature impact toughness at -101°C, tensile properties at room temperature, and dimensional accuracy—all meet the requirements of the ASTM A334 Grade 3 standard. It has reached the advanced international level for similar products and can be widely applied in fields such as ethylene cryogenic pipelines, liquefied gas transmission, and air separation equipment.

       Based on the customer’s technical requirements for low-temperature seamless steel pipes, and taking into comprehensive consideration the production process and cost control, the chemical composition design of the low-temperature seamless steel pipes must ensure the product possesses appropriate strength, high low-temperature toughness, a convenient heat treatment process, excellent weldability, and resistance to hydrogen-induced cracking. Drawing on the successful experience with 3.5Ni steel plates and hot-rolled seamless steel pipes, our company has established strict chemical composition control ranges for cold-drawn seamless low-temperature steel pipes, as detailed in the table below:

       During the smelting process, we employ an electric arc furnace steelmaking process followed by refining and vacuum degassing. The molten steel is then cast into ingots and rolled into the required tube blanks, ensuring extremely low levels of harmful elements and gases from the source and excellent surface quality of the tube blanks. After heating the tube blanks in a step furnace and piercing them via two-roll oblique rolling, they undergo multiple stages of cold rolling and cold drawing to produce the finished product. Through heat treatment and finishing processes, finished steel tubes with an outer diameter of 24.96–25.05 mm, a wall thickness of 2.04–2.16 mm, and a length of 22 meters are obtained. with both inner and outer surface roughness below 1.4 μm, meeting the quality requirements for Grade T1 heat exchanger tubes as specified in the NB/T 47019.1-2021 standard.

      Due to the thin wall thickness of the finished tubes, direct sampling for impact testing was not feasible. Instead, after the parent tube and finished tubes underwent the same heat treatment in the same furnace, the microstructure was found to be ferrite + bainite, with a grain size of approximately Grade 10; A 2.5 × 10 × 55 mm specimen was tested for impact strength at -117°C, yielding measured impact energies of 32 J, 33 J, and 35 J, which far exceed the standard requirements of an average value ≥ 5 J and a single value ≥ 4 J.