Classification of Low-Temperature Steels

Low-temperature steels are generally classified into two major categories based on their crystal lattice structure: ferritic low-temperature steels (body-centered cubic, BCC) and austenitic low-temperature steels (face-centered cubic, FCC).

1.Ferritic Low-Temperature Steels

Ferritic low-temperature steels exhibit a significant ductile-to-brittle transition temperature (DBTT). When the temperature drops below a critical point, the toughness decreases drastically. For example, carbon steel with 0.2% carbon has a transition temperature of around -20°C, making it unsuitable for use at lower temperatures.

To improve low-temperature performance, elements such as manganese (Mn) and nickel (Ni) are added to refine the grain structure, reduce impurities, and enhance the toughness of ferritic steels. Based on their composition, ferritic low-temperature steels can be categorized into three types:

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(1) Low-Carbon Manganese Steels

Composition: C (0.05–0.28%), Mn (0.6–2%)

Features:

Mn/C ratio of approximately 10 to improve toughness.

Reduced levels of harmful impurities such as oxygen, nitrogen, sulfur, and phosphorus.

Small amounts of aluminum (Al), niobium (Nb), titanium (Ti), and vanadium (V) may be added for grain refinement.

Minimum operating temperature: -60°C

(2) Low-Alloy Steels

Includes:

Low-nickel steels (Ni 2–4%)

Manganese-nickel-molybdenum steels (Mn 0.6–1.5%, Ni 0.2–1.0%, Mo 0.4–0.6%, C ≤ 0.25%)

Nickel-chromium-molybdenum steels (Ni 0.7–3.0%, Cr 0.4–2.0%, Mo 0.2–0.6%, C ≤ 0.25%)

Features:

Higher strength than low-carbon steels

Suitable for temperatures as low as -110°C

China has developed nickel-saving low-temperature steels, such as 09Mn2V

(3) Medium and High-Alloy Steels

Includes:

6% Ni Steel

9% Ni Steel → The most widely used deep cryogenic steel.

36% Ni Steel

Features:

9% Ni steel is the most common choice for cryogenic applications.

These steels can be used at extremely low temperatures, down to -196°C.

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2.Austenitic Low-Temperature Steels

Austenitic low-temperature steels have excellent low-temperature toughness and generally do not exhibit a ductile-to-brittle transition temperature. They are classified into three main series based on their alloy composition:

(1) Fe-Cr-Ni Series

The 18-8 stainless steel series (18% Cr, 8% Ni) is the most common.

Features:

Excellent low-temperature toughness, corrosion resistance, and workability.

Widely used in deep cryogenic applications from -150°C to -269°C.

(2) Fe-Cr-Ni-Mn and Fe-Cr-Ni-Mn-N Series

Uses manganese (Mn) and nitrogen (N) to partially replace nickel (Ni) for a stable austenitic structure.

Features:

Nitrogen strengthens the steel, improves toughness, and reduces magnetic permeability, making it ideal for ultra-low-temperature, non-magnetic applications.

Examples: 0Cr21Ni6Mn9N, 0Cr16Ni22Mn9Mo2, used for non-magnetic structural components at -269°C.

(3) Fe-Mn-Al Series (Austenitic Low-Temperature Non-Magnetic Steel)

A newly developed nickel- and chromium-saving steel in China.

Example: 15Mn26Al4

Features:

Suitable for temperatures below -196°C.

If its corrosion resistance can be improved, it has potential for broader applications.

As an expert in the field of piping systems, Haihao Group has many piping system products that are used in various low-temperature environments. According to needs, we will select suitable raw materials to produce piping system supporting products that meet specific needs. If you have any needs, please feel free to contact us. Email:sales@haihaogroup.com