FIELD OF THE INVENTION

The present invention generally relates to austenitic alloys. More particularly the invention is concerned with a stable austenitic, corrosion resistant alloy for cryogenic applications, but not exclusively.

BACKGROUND OF THE INVENTION

Nowadays, there is an increasing demand for materials capable of with- standing uses at very low temperatures (and especially at cryogenic temperatures, i.e. down to about 77 K and as low as 4 K) such as the materials of LNG tanks and pipes, liquid hydrogen fuel containers for rockets, superconductive magnets which have to be used at liquid helium temperature, and so forth. In view of the current changing of energy resources, it is believed that the use of such materials will be drastically increased and spread in the near future to cope with demands in various apparatuses.

The structural materials for use at very low temperatures have to meet a number of requirements. First of all, it is essential that the materials do not exhibit brittle fracture at a very low temperature at which the materials are used. High strength and high proof stress are also important requisites. When the materials are used for superconductive magnets, they must be non-magnetic.

Austenitic stainless steels, the most employed and renowned stainless steels, have been used for decades in cryogenic applications due to their good ductility even at low temperatures. In order to improve strength and toughness at low temperatures, stainless steels of low carbon content with addition of nitrogen have been developed. The stability of the austenitic phase in such steels was insufficient. A portion of the austenitic phase was transformed into a ferromagnetic martensitic phase at low temperature, resulting in lowering of the toughness. Another consequence of the presence of ferromagnetic phase is that it is not possible to sufficiently lower the magnetic permeability at cryogenic temperatures, which is problematic for some applications.

Subsequently, austenitic stainless steels with further increased nickel content have been developed; but there are problems of increased costs and high thermal expansion coefficient as the structural material for cryogenic tempera- ture use.

Alternatively, high manganese non-magnetic steels have been proposed for cryogenic temperature use, especially in techniques utilising super conductivity such as nuclear fusion, particle accelerators, superconductive power storage and superconductive magnets. Indeed, in such applications large electromagnetic forces are induced in the superconductive magnet — usually cooled to a cryogenic temperature about 4 K by liquid helium — structural materials supporting the superconductive magnet require high strength capable of withstanding the large forces under cryogenic temp.

Despite previous works in the field that have permitted to increase the strength of austenitic alloys at cryogenic temperatures (77 - 4 K), the known alloys generally suffer from insufficiently high yield strength at room temperature. This leads to an increase in design weight, in connection with an increase in the charge of alloys. This also brings about problems of reliability and durability. An example of austenitic steel with high Mn content is e.g. described in

Ref. [1]: E. A. Ulianin and N. A. Sorokina, Steels and alloys for cryogenic techniques, Moscow "METALLURGY' 1984, p. 79-80. The steel composition is, by weight percent: carbon < 0.05; silicon < 0.6; manganese 18 - 21 ; chromium 12 - 14; nickel 8 - 10; molybdenum 2 - 3; nitrogen 0.3 - 0.4; sulphur < 0.025 and phosphorus < 0.030.

OBJECT OF THE INVENTION

The object of the present invention is thus to provide an alloy suitable for use at cryogenic temperatures that exhibits a high yield strength at room temperature. This object is achieved by an alloy as claimed in claim 1.

SUMMARY OF THE INVENTION

According to the present invention, a stable austenitic alloy comprises iron and, by weight per cent: carbon: 0.023 to 0.050; chromium: 9.0 to 10.0; manganese: 30.0 to 35.0; silicon: 1.3 to 2.5; nickel: 4.0 to 6.0; nitrogen: 0.15 to 0.25; molybdenum: 2.5 to 3.5; and vanadium: 0.4 to 0.6.

The present alloy has been designed to be suitable for cryogenic use, and thus has high yield strength at cryogenic temperatures, typically of about 1200 Mpa or more at 77 K and preferably also min. 1500 MPa at 4.2K. It shall be appreciated that the alloy according to the present invention also has good yield strength at room temperature, specifically at least 500 MPa, over the whole composition range, which is of great advantage in terms of design and durability. These advantages are also observed on welded designs, which makes the present alloy suitable for welded applications.

In particular, as it will be explained in more detail below, the present alloy has a yield strength at room temperature which is 10 to 22% greater than the alloy of Ref. [1].

It may further be appreciated that the austenitic structure of the present alloy is very stable both before and after destroying deformation up to cryogenic temperatures. Accordingly, the present alloy exhibits at least 95% austenite before and after destructive deformation, and generally the austenitic content is about 100%.

In addition to the high yield strength at room temperature, the present alloy also exhibits high plasticity, typically elongation of at least 25%, and ductility, typically Kcv up to 200 J/cm2, at cryogenic temperatures (below 77 K). The present alloy can be used in various fields, namely in nuclear, space and rocket-building technologies as well as for the manufacture of LNG tanks and pipes, liquid hydrogen fuel containers, superconductive magnet structures, auxiliary space equipment, barochambers and generally vessels for storing and transporting cryogenic liquid, or any other applications involving use at cryogenic temperatures.

As it will be appreciated by those skilled in the art, the present invention results from a careful selection of alloying elements in adequate proportions. Without willing to be bound by theory, a description is given hereunder as to the reasons of limitation of the amount of the respective elements.

The alloy has been developed on a Fe-Cr-Mn basis in order to achieve the desired mechanical properties.

Manganese, added in the amount of 30 to 35 wt.%, is thus a basic con- stituent of the present alloy, which is useful for stabilising the austenitic phase and attaining extremely low permeability at cryogenic temperatures.

Chromium is another basic constituent of this alloy and provides resistance to corrosion. However chromium is known as ferrite former and its addition must be controlled. For obtaining an appreciable effect of addition of chromium and maintaining the austenitic structure a concentration of 9 to 10 wt.% chromium was defined.

Carbon and nitrogen are interstitial solute elements, which are effective for increasing the strength of steels by solid solution hardening. For enhanced yield strength, especially at cryogenic temperatures, it has been decided to contain C below 0.05 wt.%. This is also favourable as regards weldability, workability, and austenite stability. The minimum carbon content is set at 0.023, and may typically be about 0.03 wt.%. Further, N is an addition element also useful for stabilizing the austenitic phase. Its content must however be controlled to avoid deterioration of weldability and permeability. Accordingly, N is defined within a range from 0.15 to 0.25 wt.%.

Silicon is typically used as deoxidizer in the course of steel making. It also provides for strength, but is a known ferrite stabilizer. Therefore, Si is defined within a range from 1.3 to 2.5 wt.%. Preferably, in view of the influence of Mo and V the quantity of silicon may be in the range of 1 .3 to 2.0 wt.%. Nickel is advantageous in that it contributes to the stabilization of the aus- tenitic phase; it also improves toughness at cryogenic temperatures as well as corrosion resistance. High amounts of Ni are however industrially not desirable due to costs. An appropriate amount of nickel was found to be between 4.0 to 6.0 wt.%. As for carbon, molybdenum and vanadium have a strengthening effect.

Molybdenum is defined between 2.5 to 3.5 wt.% and vanadium between 0.4 and 0.6 wt.%.

Preferably, the balance other than the chemical compositions described above is Fe and incidental and/or unavoidable impurities. The nature and amount of such impurities depend on the steel making route. As unavoidable impurities, S: 0.025 wt.% or less and P: 0.03 wt.% or less are permissible with a viewpoint of the industrial economy.

Optionally, the present alloy may comprise from 0.001 to 0.003 wt.% of boron for a further strengthening effect. A reduction in the sulphur content can be achieved by addition of calcium, cerium or lanthanum. Accordingly, the present alloy may comprise from 0.001 to 0.01 wt.% calcium; and/or from 0.005 to 0.05 wt.% cerium; and/or 0.005 to 0.05 wt.% lanthanum.

The present alloy can be manufactured by conventional processes. It can also be processed by conventional transformation techniques to form finished of semi-finished products/components, especially for use at cryogenic temperatures.

EXAMPLES

In order to illustrate the present invention, several exemplary compositions in accordance with the present steel have been produced and tested in the laboratory. In table 1 , samples # 6, 7, 10 and 1 1 have a composition in accordance with the present steel; the remainder is iron and incidental and unavoidable impurities (S not more than 0.025 wt.% and P not more than 0.03 wt.%). Test samples (in the form of 12 mm sheets) were produced with the prescribed composition and quenched (hardened) in water from 1273 K.

Mechanical tests of the samples were then carried out. Table 2 summarises the mechanical properties of the samples of Table 1. Sample C1 is cited as comparative example and corresponds to the composition of Ref. [1]. These data were calculated in result of tensile tests in accordance with International Standard ISO 7801-84. Table 2 indicates for each sample the test temperature as well as: Yield Strength (σo. ₂ , MPa) ; Tensile Strength (σ _B , MPa); Specific Breaking Elongation (δ, %); and Area Reduction (ψ, %). The impact strength (Kv, J/cm ² ), carried out according to ISO 442 (1965), is also indicated for tests at 293 and 77 K.

As can be seen, the samples from the present alloy exhibit a yield strength that is 10 to 22% superior to the prior art sample C1. Metallographic, fractographic and magnetometric were performed before destroying deformation. It was established that in the temperature range 293 to 4.2 K, the samples kept a stable austenitic structure.

To determine the properties of the present steel in welding applications, welded connections of samples were produced from 12 mm sheets after thermal processing (quenching in water at 1273 K).

Welded connections were executed by manual argon-arc welding with covered electrodes of the basic type and with following parameters:

- built-up metal type 04X19H 18;

- electrode diameter of 3 mm);

- welding regime: I _CB = 100 A; U _C B = 16 B.

Samples for mechanical testing on tension and shock bend were made from welded connections. Table 3 shows the test results. Sample # C Si Cr Mn Ni N Mo V

6 0.023 1.4 10.0 34.0 5.6 0.17 2.5 0.4

7 0.023 1.6 10.0 33.8 5.7 0.16 2.5 0.6

10 0.025 1.9 10.0 33.9 5.6 0.18 3.5 0.4

11 0.024 1.6 9.9 33.9 5.7 0.20 3.5 0.6

Table 1 - alloying elements in weight per cent

Table 2 - Mechanical properties

Table 3 - Results of welding tests

The microstructure of welded seams was investigated and radiographic searches were performed. It was established that both in initial and after destroying deformation the alloy was single-phase austenite.