FUEL TANK FOR A FUEL CELL SYSTEM AND METHOD FOR PRODUCING A FUEL TANK

Abstract

The invention relates to a fuel tank (1), in particular a hydrogen tank, for a fuel cell system, having a monolithic base body (10) made of a metal alloy, wherein the base body (10) has a first inner layer (11) having a first inner structure and a second outer layer (12) having a second inner structure, which differs from the first inner structure, and wherein the first inner structure is formed from a metastable austenite and the second inner structure is formed from a martensite.

Claims

1. A fuel tank (1) for a fuel cell system, the fuel tank having a monolithic base body (10) made from a metal alloy, wherein the base body (10) comprises a first inner layer (11) with a first inner structure and a second outer layer (12) with a second inner structure, different from the first inner structure, and wherein the first inner structure is formed from a metastable austenite and the second inner structure is formed from a martensite.

2. The fuel tank (1) as claimed in claim 1, characterized in that the base body (10) has a substantially circular or elliptical cross section (1.1), or a substantially square cross section (1.2), or a cross section (1.3) with at least one inwardly curved side wall.

3. The fuel tank (1) as claimed in claim 1, characterized in that the base body (10) is made from an austenitic steel.

4. A method for producing a fuel tank (1) for a fuel cell system, the method comprising the following steps: a) producing a monolithic base body (10) having a first inner structure made from a metastable austenite, and b) producing a second outer layer (12) having a second inner structure, different from the first inner structure, by a martensitic transformation on the outside of the base body (10).

5. The method as claimed in claim 4, characterized in that the method involves at least one further step: a1) treatment of a first inner layer (11) of the base body (10) by nitriding of the base body (10) from an inside to an outside.

6. The method as claimed in claim 4, characterized in that the method involves at least one further step: a2) treatment of the second outer layer (12) of the base body (10) by denitriding of the base body (10) from an outside to an inside.

7. The method as claimed in claim 4, characterized in that the method involves at least one further step: a3) treatment of the second outer layer (12) of the base body (10) by carburizing of the base body (10) from an outside to an inside.

8. The method as claimed in claim 4, characterized in that the base body (10) is produced in step a) by a deep drawing.

9. The method as claimed in claim 4, characterized in that at least one desired pressure in the fuel tank (1) or a desired size of the fuel tank (1) is taken into account in step a).

10. (canceled)

11. The fuel tank (1) as claimed in claim 1, characterized in that the first inner layer (11) is made from an austenitic steel.

12. The fuel tank (1) as claimed in claim 11, wherein the second outer layer (12) is produced by a martensitic transformation on an outside of the base body (10).

13. The fuel tank (1) as claimed in claim 1, characterized in that the first inner layer (11) is made from an austenitic steel with a nickel fraction of 7 to 9% and/or a nitrogen fraction up to 1%, wherein the second outer layer (12) is produced by a martensitic transformation on the outside of the base body (10) down to a defined second penetration depth (h2).

14. The method as claimed in claim 4, characterized in that the method involves at least one further step: a1) treatment of a first inner layer (11) of the base body (10) by nitriding of the base body (10) from an inside to an outside up to a defined first penetration depth (h1).

15. The method as claimed in claim 4, characterized in that the method involves at least one further step: a2) treatment of the second outer layer (12) of the base body (10) by denitriding of the base body (10) from an outside to an inside up to a defined second penetration depth (h2).

16. The method as claimed in claim 4, characterized in that the method involves at least one further step: a3) treatment of the second outer layer (12) of the base body (10) by carburizing of the base body (10) from an outside to an inside up to a defined second penetration depth (h2).

17. The method as claimed in claim 4, characterized in that at least one desired pressure in the fuel tank (1) or a desired size of the fuel tank (1) is taken into account in step a), wherein at least one material thickness of the fuel tank (1), a first inner layer (11) or a second outer layer (12) of the base body (10) is chosen in dependence on a desired pressure in the fuel tank (1) or a desired size of the fuel tank (1).

18. The fuel tank (1) as claimed in claim 1, characterized in that the base body (10) has a substantially circular or elliptical cross section (1.1).

19. The fuel tank (1) as claimed in claim 1, characterized in that the base body (10) has a substantially square cross section (1.2).

20. The fuel tank (1) as claimed in claim 1, characterized in that the base body (10) has a cross section (1.3) with at least one inwardly curved side wall.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The filter system according to the invention and its modifications as well as its benefits and the method according to the invention and its modifications as well as its benefits shall be explained more closely below with the aid of drawings. There are shown each time schematically:

[0029] in FIG. 1, a schematic representation of a fuel tank according to the invention,

[0030] in FIG. 2, a further schematic representation of a fuel tank according to the invention, and

[0031] in FIG. 3, different geometries of a fuel tank according to the invention.

[0032] In the different figures, the same parts of the fuel tank 1 are always given the same reference numbers, so that in general they will only be described once.

DETAILED DESCRIPTION

[0033] FIGS. 1 and 2 show a fuel tank 1 for a fuel cell system, which is not represented in the interests of simplicity. The fuel tank 1 can be used in fuel cell systems both for mobile applications, such as in motor vehicles, and for stationary applications, such as in emergency power supply and/or a generator or the like.

[0034] The fuel tank 1 is formed with a monolithic base body 10 made from a metal alloy, wherein the base body 10 comprises a first inner layer 11 with a first inner structure and a second outer layer 12 with a second inner structure, different from the first inner structure, and wherein the first inner structure is formed from a metastable austenite and the second inner structure is formed from a martensite.

[0035] By a fuel tank 1 in the sense of the invention can be meant a hydrogen tank or a tank for a hydrogen-containing fuel. The monolithic base body 10 in the sense of the invention is made as a single piece of a continuous material. The first inner layer 11 and the second outer layer 12 of the base body 10 are produced here by a phase transformation or layer formation in the very same monolithic base body 10, and not for example by gluing or welding of separate bodies to form a multi-part or multilayered body.

[0036] The invention is based on the knowledge that the hydrogen resistance of steels depends critically on the inner structure. Thus, mechanically high-strength martensitic materials have great vulnerability to hydrogen embrittlement, whereas austenitic steels exhibit almost no hydrogen influence.

[0037] According to the invention, at first in step a) a monolithic base body 10 is fabricated from a metastable austenite. In a following optional nitriding process in step a1), nitrogen N can be introduced into the inner wall of the fuel tank 1 down to a defined first penetration depth h1. In a subsequent martensitic transformation, for example by appropriate heat treatment of the base body 10, the second outer layer 12 of mechanically high-strength steel with a mechanically stable martensitic structure is formed on the outside of the base body 10. On the inside of the base body 10 there remains the first inner layer 11 with a chemically stable austenitic structure, having a high resistance to the harmful influences of hydrogen, especially a high corrosion resistance. The first inner layer 11 thus serves as a diffusion and permeation barrier for hydrogen H2 to protect the surrounding martensite in the second outer layer 12. This accomplishes a separation of the functions in the two layers 11, 12. The first inner layer 11 serves as an austenitic diffusion barrier for hydrogen H2 and the second outer layer 12 of the base body 10 serves as a strength-optimized martensitic outer shell for the fuel tank 1.

[0038] Hence, a weight-optimized, cost effective, mechanically high-strength and chemically stable fuel tank 1 can be provided, which is easy to fabricate. Moreover, metal alloys can be easily shaped, for example by drawing, so that the configuration and design freedom is enlarged for an optimal packaging in the fuel tank 1 according to the invention.

[0039] The first inner layer 11 of the base body 10 can be produced from an alloy which is enriched with austenite-stabilizing alloy elements, such as nickel, carbon, manganese, nitrogen and cobalt, preferably with a nickel fraction of 7 to 9% and/or a nitrogen fraction of up to 1%.

[0040] As indicated by FIG. 1, the first inner layer 11 of the base body 10 can take up so much additional nitrogen N inside the fuel tank 1 by an annealing treatment under a nitrogen atmosphere in the interior of the fuel tank 1 (nitriding, optional step (a1)) that its austenitic properties are stabilized and/or enlarged over a broad temperature range, such as from 70 C. to +150 C. The nitriding can occur from the inside to the outside as far as a preferably defined or adjustably regulated first penetration depth h1.

[0041] As further indicated by FIG. 2, the second outer layer 12 of the base body 10 can give off so much nitrogen N on the outer surface of the fuel tank 1 without a nitrogen atmosphere (denitriding, optional step (a2)) and/or take up so much carbon K by a carbon donor gas (carburizing, optional step (a3)) that, under a sufficiently rapid cooldown, the outer region of the fuel tank 1 is martensitically hardened and the inner region of the fuel tank 1 remains austenitic due to the additional nitrogen N. The denitriding and/or carburizing may be achieved from the outside to the inside as far as a preferably defined or adjustably regulated second penetration depth h2.

[0042] As further indicated by FIG. 3, the invention may provide for a fuel tank 1 that the base body 10 can be produced with different cross sections. This is advantageously possible in that the base body 10 is made from a malleable material, such as a metal alloy, for example by deep drawing. Different cross sections are conceivable, such as a substantially circular or elliptical cross section 1.1, shown on the left in FIG. 3, or a substantially square cross section 1.2, for example with rounded corners, shown in the middle of FIG. 3, or a cross section 1.3 with at least one inwardly curved side wall, shown on the right in FIG. 3. The advantage of a substantially circular or elliptical cross section 1.1 may be the resultant achieving of an improved ratio between surface and volume content of the fuel tank 1. Furthermore, in this way an improved pressure distribution can be accomplished over the surface of the fuel tank 1, such as a uniform distribution. A fuel tank 1 with a substantially square cross section 1.2, in turn, can be better stowed and/or stacked. A fuel tank 1 with a cross section 1.3 having at least one inwardly curved side wall may provide the benefit that no tensile stresses will be present, but only compressive stresses, in the highly stressed regions of the fuel tank 1. In this way, the mechanical strength of the fuel tank 1 can be increased.

[0043] The preceding description of FIGS. 1 to 3 describes the present invention solely in the context of examples. Of course, individual features of the embodiments, so far as is technically meaningful, may be combined with each other at will, without leaving the scope of the invention.

[0044] Furthermore, it is conceivable when producing the base body 10 in step a) to employ different proven methods for the mechanical forming of steel plates, such as deep drawing, rolling, or the like, which may further simplify the production of the fuel tank 1.

[0045] Furthermore, at least the total material thickness h of the fuel tank 1, or the first penetration depth h1 or the material thickness of the first inner layer 11 or the second penetration depth h2 or the material thickness of the second outer layer 12 of the base body 10 can be adjusted in dependence on a desired pressure in the fuel tank 1 or a desired size of the fuel tank 1. The first penetration depth h1 and the second penetration depth 2 may be adjusted individually, in order to adapt different properties of the fuel tank 1 in a flexible manner. Alternatively, it is conceivable that a ratio of 1 to 1, especially 50% each of the total material thickness h of the fuel tank 1, may be advantageous for the first penetration depth h1 and the second penetration depth 2 in order to be able to adjust the desired properties of the fuel tank 1 easily through the choice of a suitable total material thickness h of the fuel tank 1.