ASTM A334 Gr.3 Small-Diameter Ultra-Long Seamless Steel Pipes(Chinese-made )
 
Mass Production of -101℃ 3.5Ni Steel (ASTM A334 Gr.3) Small-Diameter Ultra-Long Seamless Steel Pipes Achieved
 
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 steels, which have been successfully used in the manufacture of cryogenic equipment in ethylene, fertilizer, rubber, liquefied petroleum gas, and coal gasification projects. In my country, 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 good technical performance and economic efficiency; however, the production of seamless steel pipes, especially small-diameter products, has remained relatively limited.
 
Addressing the industry pain point of long-term reliance on imports and low domestic production rate for small-diameter 3.5Ni seamless steel pipes, our company, leveraging its technological accumulation in cryogenic pipes, small-diameter precision pipes, and ultra-long pipes, has successfully developed a 25×2min×22m cold-drawn small-diameter ultra-long 3.5Ni seamless steel pipe. This product was successfully delivered to an equipment manufacturer in April and will ultimately be used in a project within the Arctic Circle in Russia. Tested by a national authoritative testing institution, the product's key indicators, including low-temperature impact toughness at -101℃, room-temperature tensile properties, and dimensional accuracy, all meet the requirements of ASTM A334 Grade 3 standards, reaching the advanced level of similar international products. It can be widely used in cryogenic pipelines for ethylene, liquefied gas transportation, and air separation equipment.
 
Based on the customer's technical requirements for cryogenic seamless steel pipes, and considering the steel pipe's production process and cost control, the composition design of cryogenic seamless steel pipes must ensure that the product possesses appropriate strength, high low-temperature toughness, convenient heat treatment processes, excellent weldability, and resistance to hydrogen-induced cracking. Drawing on successful experience with 3.5Ni steel plates and hot-rolled seamless steel pipes, our company has established stringent chemical composition control ranges for cold-drawn seamless low-temperature steel pipes, as detailed in the table below:

Chemical Composition (%)
	C	Si	Mn	P	S	Ni
Agreement Requirements	≤0.19	0.18~0.37	0.31~0.64	≤0.025	≤0.025	3.18~3.82
Design Values	0.06~0.09	0.18~0.33	0.45~0.60	≤0.010	≤0.005	3.30~3.70
Cast Analysis	0.07	0.24	0.51	0.004	0.001	3.40

In the smelting process, an electric arc furnace steelmaking + refining + vacuum degassing process is adopted. The pipe blanks are then cast and rolled into the required tube blanks, ensuring extremely low levels of harmful elements and gases from the source, resulting in excellent surface quality. After heating in a walking beam furnace and piercing by two-roll skew rolling, the tube blanks undergo multiple cold rolling and cold drawing processes to reach the finished product. Through heat treatment and finishing, finished steel pipes 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. The surface roughness of both the inner and outer surfaces is less than 1.4 μm, meeting the quality requirements of T1 grade heat exchanger tubes in the NB/T47019.1-2021 standard.
 
Due to the thinness of the finished tube wall, it was impossible to directly sample for impact testing. After the mother tube and the finished tube underwent the same heat treatment in the furnace, the metallographic structure was ferrite + bainite with a grain size of about 10. A 2.5×10×55mm sample was taken and impact tested at -117℃. The measured impact energy was 32J, 33J, and 35J, which is far better than the standard requirement of an average value ≥5J and a single value ≥4J.