As the name implies, low temperature steel pipe is the steel pipe with a good low temperature resistance and used for in low temperature applications, generally refers to the alloy steel pipe used below -10℃ and has enough toughness. Usually, we refer the steel pipe used at -10 ~ -196℃ as low temperature steel and below -196℃ as cryogenic steel pipe or ultra-low temperature steel pipe. It is generally believed that manganese, nickel and other elements can improve the toughness at low temperature. Experiments showed that with the increase of nickel content by 1%, the critical transition temperature of brittleness can be reduced by about 20℃.
According to the crystal lattice structure, low temp steel tubes can be divided into body-centered cubic Ferrite low temperature steel and face-centered cubic Austenite low temperature steel. Ferritic steel at low temperature is obvious ductile to the brittle transition temperature when the temperature reduced sharply to a critical value (or range), the toughness decline suddenly, so the steel pipe should not be used below the transition temperature, by adding alloy elements such as Mn, Ni to reduce the interstitial impurity, refine the grain size, control the size, shape and distribution of the second phase in steel, etc., making the ferritic ductile to brittle transition temperature reduced (see metal reinforcement).
Low-temp steel pipe has good strength and toughness at low temperatures, excellent processing performance and corrosion resistance, etc. There is no brittle transition temperature in the low temperature range of Austenitic cryogenic pipe, and it can be used generally without verifying its low temperature impact toughness. Ferritic low temperature steel is the most commonly used pipe material. The heat treatment process affects the microstructure and grain size of low temperature steel, especially the low temperature toughness, which is improved obviously after quenching and tempering. It is divided according to the composition:
Low carbon Mn steel
It contains 0.05 ~ 0.28% C and 0.6 ~ 2%Mn. Making Mn/C≈10 to reduce O, N, S, P and other harmful impurities, or added a small amount of Al, Nb, Ti, V and other elements to refine the grain. The lowest service temperature of this kind of steel is about -60℃.
Low alloy steel
It mainly includes low Nickel steel(2-4% Ni), Mn-Ni-Mo steel( 0.6 ~ 1.5%Mn, Ni0.2 ~ 1.0%, 0.4 ~ 0.6%Mo ,C≤ 0.25%, Ni-Cr-Mo steel(0.7 ~ 3.0%Ni, 0.4 ~ 2.0%Cr, 0.2 ~ 0.6%Mo, C≤ 0.25%). The lowest working temperature can be low to -110℃ or so.
Medium (high) alloy steel
There are mainly 6% Ni steel, 9% Ni steel, 36% Ni steel, of which 9% Ni steel is widely used for cryogenic steel. Its operating temperature can be as low as -196℃.
ASTM A333, JISG3460, EN10216-4 and other standards specified the details of Ferritic cryogenic steel tubes, of which ASTM A333 Gr.3, Gr.6, Gr.8 and Gr.11 are the most commonly used cryogenic tubes grades. Low-temperature steel pipe has wide uses, like production of ethylene, propylene, urea, ammonia, compound fertilizers such as washing, purification, desulfurization, degreasing and other process equipment and for the pharmaceutical industry, and cryogenic equipment manufacturing, ultra-low temperature cold storage, the transmission of cryogenic liquefied gas pipe and tube components, liquefied petroleum gas (LPG) cryogenic separation equipment, air separation equipment, etc.
In the cryogenic separation equipment of LPG, the lowest service temperature of most cryogenic tubes is -110℃, and some even up to -150℃. Low alloy steel, 3 ~ 6% nickel steel or 9% nickel steel can be respectively used. For the air separation equipment with the lowest working temperature of -196℃, generally used 9% nickel steel or Austenitic low temperature steel. For example, Austenitic cryogenic steels with stable structure should be used for liquid hydrogen production/storage equipment at -253 °C and liquid helium equipment at -269 °C.
Although the temperature limit of steel pipe is slightly different worldwide, such as the temperature limit defined by the United Kingdom, Germany/Japan, China, the United States/Russia is 0, -10, -20, -30℃ respectively, but the actual industrial environment can be as low as -253℃ (liquid hydrogen) or even higher.Therefore, a certain value of impact toughness is usually specified at the lowest temperature.