Alloys used in cryogenic mechanical engineering

Improvement of machines, mechanisms and devices necessitates development of materials with unique properties.

For example, advances in computer technologies, lasers, magneto hydrodynamic generators (MHD generators) initiated development of superconducting materials of a new type which have relatively high temperatures of normal-superconducting transition, sufficient raw materials sources, affordable production and processing technology.

Materials traditionally used in engineering such as semiconductor, high-resistance and magnetic materials as well as materials with specified thermal linear expansion coefficient are constantly improving. Non-traditional approach to metal and alloy processing enabled the development of high-speed crystallization methods.

Many technical appliances and constructions such as gas and oil pipelines, bridges, rails, motor vehicles, aircrafts, etc are subject to the impact of negative temperatures in the process of operation. In the Extreme North regions the temperature can reach -60°. Hulls of planes and spacecrafts are cooled to the temperature of liquid oxygen (-183°). Some units of refrigerating and cryogenic equipment operate at the liquid-helium temperature (-269°).

At the negative temperatures metals lose plasticity and viscosity, show heightened liability to brittle fracture that is they become cold-brittle (cold brittleness is the increase of brittleness with the decrease of temperature). At low temperatures interatomic distances in lattices of metals reduce and the value σ 0.2 increases (up to the temperature 77K). Then the increase of σ 0,2 parameter slows down at the temperature close to the absolute zero, in many metals it becomes temperature-independent (Picture 1).

Metals are divided into four main groups according to cold resistance criterion:

• 1.    Metals and alloys used at the temperatures up to 210K. These materials are employed for production of so called "north" versions of goods. This group includes ferrite and pearlitic fine low-alloyed carbon steels with bcc lattice.

• 2.    Materials preserving preset values of viscosity and plasticity at the temperatures up to 170K. These are the steels additionally alloyed with Ni, Cr, Ni, Mo as well as low-carbon ferrite steels with 2 – 5% Ni content.

• 3.    Constructional alloys that can operate at the temperatures up to 77K (boiling temperature of liquid nitrogen). Here belong the steels 12Х18Н10Т, 0Н9А, alloys based on Al, Ti, Cu. The alloys Cr–Mn and Cr–Ni–Mn as well as steels of the brands 10Х14Г14Н4Т (ЭИ711), 03Х13АГ19 (ЧС36), 07Х21Г7АН5 (ЭП222) are used for unstressed structures. Strength properties of such steels are given in the Table 1.

Picture 1. Dependence of metal yield point on testing temperature

Table 1. Mechanical properties of 03Х13АГ19 steel flats at low temperatures

Constructional alloys operating at the lowest (below 77K) temperatures. These are the materials used in space engineering, hydrogen production and consumption, in vacuum technology - high-alloy corrosion-resistant steels of the brands 03Х20Н16АГ6, 10Х11Н23Т3МР (ЭП33), some bronzes, magnesium-alloyed nickel and aluminium alloys, and titan alloys.

Phosphorus causes embrittlement of steels due to heavy liquation and formation of stress concentrators such as phosphide eutectic. Embrittling influence of phosphorus grows with the increase of carbon content. The growth of phosphorus content by 0,01% for cast steel 35 L increases critical brittle temperature by approximately 20K. Cargo bodies for the Extreme North are made of 03Г4АФ steel. After hardening and tempering it has mechanical properties and cold resistance similar to the properties of more expensive molybdenum steel 14Х2ГМ for north versions of pipe headers. In the USA and Canada high-strength weld steels Mn–Mo–Nb containing 1.6 – 2.2% Mn, 0.25 – 0.4% Мо, 0.04 – 0.10% Nb are used.

Metastable austenite steels containing not more than 0.06% С, 13-17% Cr, 8 – 10% Ni, 6 – 10% Mn have the optimal combination of plastic and tough properties at low temperatures. Fortification of these steels is achieved by the introduction of nitrogen which generates solid interstitial solutions.

Austenite steels alloyed with Cr, Ni and Mn are quite promising for application in cryogenics. Such steels include corrosion-resistant steel 10Х14Г14Н4Т economically alloyed with nickel which is satisfactorily processed with pressure and cutting, and have good welding characteristics. It can replace the steel 12Х18Н10Т at low temperatures by its mechanical properties and corrosion resistance in atmospheric conditions. The steel 10Х14Г14Н4Т is used for production of welded elements of vessels (rings, bottoms, flanges, branch pipes) and pipelines operating at the temperature range 77 - 773 К (-196 - 500%).

Conclusion. High-alloy invars are used to produce units of cryogenics with stable sizes. They are used in rocket engine pipelines, some fittings of cryogenic devices operating at temperatures up to 20K. Aluminium, titan and copper alloys are used in low-temperature equipment along with steels.