Most steel is prone to crack at low temperatures. This is due to something called the brittle-ductile transition; a phenomenon that causes materials at low temperatures to shatter, rather than deform, on impact. However, steel produced with 9% Nickel is different: it has an especially low brittle-ductile transition temperature, meaning that it remains ductile even when extremely cold.
These properties, combined with 9% nickel steel’s high toughness and tensile strength, makes it perfect for the production, containment and storage of liquefied natural gas (LNG) and other cryogenic substances.1
The Problem with the Brittle-Ductile Transition
On the 25th of January 1959, a recently-rebuilt cargo set sail on a transatlantic voyage from Louisiana with an unusual cargo.2 The ship, aptly named Methane Pioneer, carried the first ever shipment of liquefied natural gas to ever be transported by boat.
It had long been known that liquifying natural gas using low temperature condensation drastically reduces its volume. Compared to uncompressed gas, liquid natural gas takes up 600 times less volume.3 In fact, the energy density of LNG is around 2.4 times greater than that of compressed natural gas, making it by far the most efficient form for transporting natural gas. However, before the Methane Pioneer, transporting this condensate was especially difficult. Steel, the material of choice for constructing pressure vessels, had the unfortunate habit of becoming brittle at low temperatures. This loss of ductility rendered it dangerous to use for transporting LNG, as any impact could cause the cylinder to crack and explode.
The loss of the characteristic ductility of most steels at low temperatures is due to a phenomenon known as the brittle-ductile transition. This transition only occurs in certain crystal structures – including that of carbon steel.4 Brittle failure of steel can have catastrophic effects. In fact, it was the brittle-ductile transition of the steel hull of the Titanic in the icy waters of the North Atlantic that effectively sealed its fate.5,6 This, and a series of lower-profile brittle failures of steel were enough to turn away commercial interest in any kind of steel based cryogenic systems for some time. However, this changed in the 1940’s when advancements in the nature of ductile-brittle transformation at low temperatures lead to the development of a new material: one that exhibited all the acclaimed strength and ductility of “normal” carbon-steel, but at low temperatures – 9% nickel-steel.7
9% Nickel Steel: A Tough and Ductile Low-Temperature Alloy
9% Nickel Steel’s unique properties stem from its crystal structure. Theoretically, it can be shown that materials exhibit ductile behaviour if their crystal structure exhibits 5 distinct “slip systems” – meaning that the atoms forming the solid can be easily dislocated in 5 different ways.
Typical steel has only 2 or 3 slip systems at low temperatures. However, Nickel exhibits a large number of slip systems, which persist even at low temperatures. In effect Nickel never undergoes a ductile-brittle transformation. What’s more, these slip systems persist even when Nickel is alloyed with other metals. The magic number turns out to be 9%: when steel is made with this proportion of Nickel, the metal contains a small proportion of “austenitic” steel grains which are inherently ductile. Though these grains make up only around 4% of the steel’s volume, they are are enough to imbue it with a resistance to the weakening effects of low temperatures, while retaining as much toughness as possible.8
By reducing the carbon content of steel and increasing the proportion of Nickel, the crystal structure of the alloy is altered so that it can behave in a ductile manner at very low temperatures. After heat treatment, 9% Nickel steel can meet requirements for ductility at temperatures as low as -196°C.1 Incidentally, LNG can be stored at temperatures below -162°C.9
Although the Methane Pioneer actually used aluminium tanks, the invention of 9% Nickel and other high-Nickel steels with low ductile-brittle transition temperatures revolutionised the LNG industry by making production, storage and transport of LNG vastly safer and more affordable.10 Making tanks with 9% Nickel steel ensured that LNG transport became a major part of the international energy industry; while on land the metal has found use in static storage tanks and components for LNG plants.11
Today, LNG storage tanks are often made from high-Nickel steel tanks, surrounded by a layer of insulation and encased in a prestressed concrete outer wall.12
UK Steel distributor Masteel supplies 9% Nickel steel to both commonly used specifications: ASTM A353, which comes in plate thicknesses up to 100mm, and ASTM A553 Type 1, which is generally limited to a plate thickness of 50mm.13 This high purity 9% Nickel steel is ideal for applications in the storage and production of LNG, oxygen, nitrogen and other cryogenic applications. Due to its resistance to brittle fracture, 9% Nickel steel plate can also be used for other pressure vessel applications.
References and Further Reading
- ArcelorMittal USA Plate 9% Nickel Steel: For use at cryogenic temperatures.
- A Short History of LNG Shipping. Noble, P. G. (1959).
- Growing Demand for Natural Gas. https://www.energy.gov/sites/prod/files/2013/04/f0/LNG_primerupd.pdf
- DoITPoMS – TLP Library The Ductile-Brittle Transition. Available at: https://www.doitpoms.ac.uk/tlplib/BD6/ductile-to-brittle.php. (Accessed: 17th March 2018)
- Metallurgy of the RMS Titanic. Foecke, T. J. NIST Interagency/Internal Rep. – 6118 (1998).
- JOM Article on The Titanic: Did a Metallurgical Failure Cause a Night to Remember? Available at: http://www.tms.org/pubs/journals/JOM/9801/Felkins-9801.html. (Accessed: 19th March 2018)
- History of Cryogenics: A Cryo Central resource from the CSA. Available at: https://www.cryogenicsociety.org/resources/cryo_central/history_of_cryogenics/. (Accessed: 17th March 2018)
- Properties of Metallic Materials for LNG Service. Craig, B. http://www.intetech.com/images/downloads/Paper57.pdf
- CH•IV | LNG Engineering & Consulting Services | A Clough Ltd. Member. Available at: http://www.ch-iv.com/all-about-lng/. (Accessed: 19th March 2018)
- Methane Pioneer: The First LNG Ship in the World. Available at: https://www.marineinsight.com/types-of-ships/methane-pioneer-the-first-lng-ship-in-the-world/. (Accessed: 19th March 2018)
- Nickel steel plates. Grobblech. https://www.voestalpine.com/stahl/content/download/4678/file/Folder_Grobblech_Nickel_0911_EN.pdf
- Liquefied natural gas | Hamina LNG. Available at: http://www.haminalng.fi/liquefied-natural-gas/. (Accessed: 19th March 2018)
- Masteel Supply 9% Nickel Alloy Steel Plates. Available at: https://masteel.co.uk/news/9-nickel-alloy-steel-plates/. (Accessed: 19th March 2018)