Many industries require materials that can withstand extreme cold. We do not only refer to installations in polar regions, which easily reach -40 °C. Much lower temperatures are also often reached by equipment and installations in sectors such as petrochemicals, refrigeration, and industry:
Certain materials, ductile at room temperature, abruptly lose their ductility when a given threshold is exceeded. Common construction steels, ferritic or martensitic stainless steels (400 series), but also iron, chromium and tungsten, become brittle even at relatively low temperatures. Metals such as copper, silver, gold, aluminium and nickel, on the other hand, remain ductile even at very low temperatures. Austenitic steels can also be added to the list.
The different properties of these materials at low temperatures depend on various factors, such as crystalline structure, grain size, the propensity to absorb contaminants from the atmosphere, heat treatments, the inclusion of slag, and processes such as melting, welding, chip removal, deformation.
This first threshold is important because, besides being typically the lower limit of the temperatures naturally reached on the planet, it is also the temperature at which some industrial operations and some chemical processes are carried out.
Unfortunately, common construction steels are no longer usable at this level, either because of their intrinsic characteristics or because they are not usually tested for hardness and resistance to low temperatures. Some steelworks, however, have special carbon steels for these applications. These are mainly quenched and tempered low alloy steels.
Almost all aluminium alloys can be used at temperatures down to -45 °C, except series such as 7075-T6 and 7178-T6, and titanium alloys 13V-11Cr-3Al or 8Mn. Copper and nickel alloys can generally all be used at these temperatures. PH stainless steels, i.e. precipitation hardening stainless steels, are not suitable for temperatures below -20 °C because of embrittlement and cracks.
Some steels can be used at these temperatures, such as low alloy, quenched and tempered steels or ferritic nickel steels. Most low carbon (0.20-0.35%) martensitic steels can be used with sufficient reliability. Many of these alloys contain manganese, nickel, chromium, molybdenum and vanadium, and some zirconium and boron.
Low carbon, 3.5%-nickel steels are often used in liquid gas storage tanks at temperatures down to -100°C. Many aluminium, nickel, and titanium alloys are also suitable for these temperatures. Aluminium 7076-T6 can also be used up to -128 °C, but not for critical applications.
The austenitic stainless steels of the 300 series are all suitable for working in this temperature range. Maraging steels with nickel content between 20% and 25% and the addition of cobalt, molybdenum, titanium, aluminium, and niobium are also suitable. Maraging steels have excellent malleability, toughness and hardness characteristics, and must be hardened at a temperature of just 400 °C.
Many aluminium alloys, such as 2024-T6, 7039-T6 and 5456-H343 have excellent fracture resistance at -196 °C; also 2014-T6 but with the exception of welds. Other alloys resistant to even lower temperatures are the 5000 series aluminium-magnesium alloys, the 2219-T87 and the 6061-T6.
The nickel-based materials are almost all resistant to -196 °C. Titanium alloys such as 5Al-2.5Sn-Ti, 6A1-4V-Ti and 8Al-2Cb-1Ta-TiY are also suitable, but should be kept free of impurities such as oxygen, nitrogen, carbon and iron as they cause embrittlement.
These very low temperatures are of great interest to industry, as they correspond to the temperature at which helium (-270 °C) and especially hydrogen (-253 °C), a promising element for energy storage and nuclear fusion projects, liquefy.
Among steels, only high-alloy austenitic stainless steels are suitable for these temperatures, such as 304 and 310. If welds are required, the use of low carbon variants is recommended. These alloys generally contain between 18% and 21% chromium and between 9% and 14% nickel.
The aluminium alloys that can be used at the temperatures involved are typically in the 2000 and 5000 series, or the 6061-T6 alloy. In particular, welds on 2219-T87 have demonstrated excellent fracture resistance, while 5052-H38 and 5083-1138 have high crack resistance. The same applies to Monel, K-Monel, electroformed nickel, hardened nickel for thorium dispersion, and nickel alloys such as Inconel X, Inconel 718, René 41, and Hastelloy B. At these temperatures, only Ti45A and 5Al-2.5Sn-Ti titanium alloys can be used, both as base metal and welded.
Copper alloys are generally also used in contact with liquid hydrogen and helium, such as 70-30 brass, copper-beryllium, iron-silicon and aluminium bronzes. Magnesium alloys, on the other hand, tend to become brittle but can be used in low stress applications with careful design.
A. Hurlich, Low temperature metals (General Dynamics/Astronautics, San Diego – California)