Steels for Cryogenic and Low-Temperature Service

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Key Considerations for Low-Temperature Steel Selection

Carbon and alloy steels designed for low-temperature service must deliver high strength, ductility, and toughness for applications at –45°C and below. Some steels are engineered to perform at even lower temperatures, around –100°C. Selecting the right material requires a thorough understanding of the application and the specific mechanical properties each grade provides.

Temperature Effects on Steel Behavior

At sub-ambient temperatures, the behavior of metals is influenced by their crystalline structure:

• Body-Centered Cubic (BCC) Structures: Found in iron, molybdenum, vanadium, and chromium, these metals exhibit a loss of ductility in a narrow temperature range below room temperature.
• Face-Centered Cubic (FCC) Structures: Found in aluminum, copper, nickel, and austenitic stainless steel, these metals often gain ductility as temperatures decrease. Their tensile strength is more temperature-sensitive than their yield strength.

Metastable compositions, like certain stainless steels, can undergo transformations at cryogenic temperatures, impairing their toughness and ductility. For instance, grades like 301 and 304 partially transform to BCC structures under these conditions, while fully stable grades like 310 remain unaffected.

300 Series Stainless Steels

The 300 series offers an excellent combination of toughness and weldability, making it suitable for cryogenic applications. However, their strength in the annealed condition may not suffice for lightweight structures. Cold-working enhances their strength for aerospace use, but this process introduces challenges like reduced weld-joint efficiencies and martensitic transformations. Fully stable grades like 310 and precipitation-hardening A286 stainless steel overcome these issues.

Alloy Steels for Cryogenic Service

• 9% Nickel Steel: Recommended for cryogenic service down to -195°C, it is commonly used for transporting and storing cryogens due to its low cost and ease of fabrication.
• Other Alloy Steels: A201 and T-1 steels are suitable down to -45°C, while nickel steels with 2.25% Ni and 3.5% Ni perform at -59°C and -101°C, respectively.

Example Steel Composition for Cryogenic Use

A specific grade with the following composition offers excellent mechanical properties at cryogenic temperatures:

• C: 0.072%
• Mn: 16%
• P: 0.02%
• S: 0.008%
• Si: 0.41%
• Ni: 5.85%
• Cr: 17.8%
• N: 0.36%
• Fe: Remainder

Steels for Low-Temperature Applications

For low-temperature systems, notch toughness is a critical consideration. Structural members subjected to dynamic loads or cyclic stresses, such as vessels experiencing pressure changes or equipment in harsh weather, require materials with excellent fatigue limits.

Carbon Steels

Carbon steels offer superior weldability, toughness, and strength compared to alloy steels. A516 grades are widely used, with tensile strengths ranging from 379 MPa to 586 MPa. They are cost-effective and easy to fabricate.

A537 grades provide higher strength and notch toughness and can be normalized or quenched and tempered for enhanced performance. Table 1 summarizes the mechanical properties of some common ASTM carbon steels.

Designation	Lowest usual service temperature, (°C)	Min Yield Strength (MPa)	Tensile Strength (MPa)	Min Elongation, L0= 50 mm (%)	Uses
A442 Gr. 55	-45	221	379 - 448	26	Welded pressure vessels and storage tanks; refrigeration; transport equipment
A442 Gr. 60	-45	221	414 - 496	23
A516 Gr. 55	-45	207	379 - 448	27
A516 Gr. 60	-45	221	414 - 496	25
A516 Gr. 65	-45	241	448 - 531	23
A516 Gr. 70	-45	262	483 - 586	21
A517 Gr. F	-45	690	792 - 931	16	Highly stressed vessels
A537 Gr. A	-60	345	483 - 620	22	Offshore drilling platforms, storage tanks, earthmoving equipment
A537 Gr. B	-60	414	551 - 690	22
A203 Gr. A	-60	255	448 - 531	23	Piping for liquid propane, vessels, tanks
A203 Gr. B	-60	276	482 - 586	21
A203 Gr. D	-101	255	448 - 531	23	Land-based storage for liquid propane, carbon dioxide, acetylene, ethane and ethylene
A203 Gr. E	-101	276	482 - 586	21
A533 Gr. 1	-73	345	552 - 690	18	Nuclear reactor vessels where low ambient toughness required for hydrostatic testing; some chemical and petroleum equipment
A533 Gr. 2	-73	482	620 - 793	16
A533 Gr. 3	-73	569	690 - 862	16
A543 Gr. 1	-107	586	724 - 862	14	Candidate material with high notch toughness for heavy-wall pressure vessels
A543 Gr. 2	-107	690	793 - 931	14

Table 1. Specifications for Low-temperature Steels

Pipeline Steels for Low Temperatures

Pipeline steels for natural gas must withstand crack growth and brittleness in Arctic environments (-25°C to -4°C). Steels alloyed with molybdenum, manganese, and columbium achieve low-temperature toughness, fine-grained structures, and excellent weldability. Modern pipeline steels, such as API 5L X grades, incorporate microalloying and controlled rolling processes to enhance strength and resistance to ductile fracture propagation. These steels are particularly suitable for extreme environments and offer improved safety margins for high-pressure pipelines.

In addition to their toughness, pipeline steels must meet stringent industry standards to resist hydrogen-induced cracking, an issue common in sour gas conditions. Advanced metallurgical techniques, such as thermomechanical control processing (TMCP), have further optimized these materials for demanding conditions.

Pipeline steels are also employed in renewable energy projects, such as offshore wind turbine foundations, where resistance to dynamic stresses and low temperatures is crucial.