DE102011010316B4 - Austenitic steel with high resistance to hydrogen-induced embrittlement - Google Patents

Abstract

Austenitic steel for use in an at least partially direct contact area with a hydrogen-containing fluid, in particular for a hydrogen tank, which has embrittlement-resistant properties with respect to the hydrogen, consisting of an alloy in mass% of - chromium (Cr) 10.0-23, 0 - Manganese (Mn) 3.0-9.0 - Nickel (Ni) 3.0-8.0 - Carbon (C) 0.05-0.15 - Nitrogen (N) 0.001-0.03 - Aluminum ( Al) 0.005-0.15 and silicon (Si) and a balance iron (Fe) and impurities caused by melting, wherein the alloy is substantially free of molybdenum (Mo), characterized in that the proportion of silicon (Si) 1.1 -3.0 mass%.

Description

The invention relates to austenitic steel having a high resistance to hydrogen-induced embrittlement according to the features of patent claim 1.

The design of modern drive systems increasingly relies on the utmost independence from fossil fuels. As a suitable alternative electric drives are considered, which dispense not only the direct combustion and the indirect use of such energy sources. In particular, their use in non-stationary applications requires, in addition to suitable accumulators especially mobile power generation systems.

Fuel cells proposed for this purpose are intended, for example, to serve on board a motor vehicle for the required range within the supply network. The hydrogen necessary for power generation must be carried along, so that suitable containers are needed for its storage.

The required hydrogen storage places high economic and safety requirements.

Due to the low particle density of hydrogen is in principle its high compression necessary to enable volume-related economic storage. In addition to the cooled liquid gas storage, the most common methods include highly compressed compressed gas storage.

From a safety point of view, a highly explosive detonating gas is also produced in combination with oxygen. In particular, by its gaseous nature, the premature detection of leakage of hydrogen is difficult, so that high demands are placed on the tanks to be used. The correspondingly high tightness must also be maintained in the event of a crash.

In order to meet the high requirements, appropriate steel alloys are used as material for the production of such tanks. This austenitic stainless steels are used, which in addition to a high weight-related strength also have a high resistance to hydrogen-induced embrittlement. Another advantage is due to the low thermal conductivity, which is about 15 W / (mK) lower than that of low alloy ferritic steel of about 42 W / (mK). As a result, any thermal losses can be reduced. In particular, compared to the metallic alternative material aluminum austenitic stainless steels have significantly lower thermal losses, since their thermal conductivity is only 1/16 of that of aluminum with about 240 W / (mK). Although the thermal conductivity of aluminum in alloys is compensated, it always remains above austenitic stainless steels.

The hydrogen embrittlement in steel is determined by a complex interaction of numerous effects. The high diffusivity of hydrogen is due to its very small atomic diameter, so that a high diffusion rate is achieved.

The atoms which have penetrated into the lattice structure remain under high pressure at the site of their local agglomeration. The associated stress state within the steel reduces its ductility and leads to a brittle behavior. When reaching the separation strength cracks can form, which can lead to breakage.

The preferred use of austenitic stainless steels is based on the fact that they have, on the one hand, a high corrosion resistance and, moreover, a low diffusion rate of hydrogen, which is lower by several orders of magnitude than in ferrite. Due to the already difficult diffusion of hydrogen into austenitic steels, it can not penetrate deep into the material at room temperature and below. However, within the known alloys, ferrite suddenly forming as a result of deformation and / or temperature reduction, and more particularly ferritic martensite, is one of the main causes of hydrogen-induced embrittlement.

The DE 10 2007 022 611 A1 discloses an austenitic stainless steel which has high resistance to hydrogen embrittlement and / or low temperature embrittlement. The steel is essentially free of molybdenum (Mo) and has a nitrogen (N) content of more than 0.16 mass%.

The EP 1 645 649 A1 shows an austenitic stainless steel for hydrogen gas, which has high resistance to hydrogen-induced embrittlement in addition to high mechanical strength properties. The alloy contains a proportion of silicon (Si) of at most 1.0% by mass, with no further dependencies between the individual elements being defined.

The steels proposed for use in contact with hydrogen sometimes have high levels of costly elements. The mass-produced tanks and their components, however, require cost-effective solutions with at least the same quality. Thus, the composition of a hydrogen-induced embrittlement-resistant austenitic steel with respect to its economic production still offers room for improvement.

The object of the present invention is to provide an austenitic steel which is suitable for the above-described use and which has greater resistance to hydrogen embrittlement and, moreover, is more economical to produce.

The solution to this problem consists according to the invention in an austenitic steel for use in an at least partially direct contact area to a hydrogen-containing fluid according to the features of claim 1.

The proposed austenitic steel, which has embrittlement-resistant properties against hydrogen, is used in particular for the production of hydrogen tanks and their components. In addition, this steel is also used to make other hydrogen-contacting metallic components, such as pipes or valves. The austenitic steel consists of an alloy in mass% of - Chrome (Cr) 10.0 to 23.0 - manganese (Mn) 3.0-9.0 - nickel (Ni) 3.0-8.0 - carbon (C) 0.05-0.15 Nitrogen (N) 0.001-0.03 - aluminum (Al) 0.005-0.15 and silicon (Si) and a balance of iron (Fe) and impurities caused by melting, the alloy being substantially free of molybdenum (Mo). According to the invention, the proportion of silicon (Si) is 1.1-3.0% by mass. The content of silicon (Si) is chosen as high as possible to increase the enthalpy for migration to hydrogen (H). This counteracts possible embrittlement of the steel.

The small amount of nickel (Ni) reduces the overall manufacturing cost of the alloy. In addition, the desired resistance to hydrogen embrittlement is positively influenced. Thus, a high proportion of nickel (Ni), for example of 30.0% by mass, facilitates the diffusion of hydrogen (H). The high proportion of nickel (Ni) additionally reduces the migration enthalpy for the atomic hydrogen (H), so that it can penetrate into the lattice structure of the steel more easily and interact with dislocations.

On the other hand, the observed positive effect on the resistance to hydrogen embrittlement with a slight increase in the proportion of nickel (Ni) within a total considered to be low overall content of, for example, 8.0 to 10.0-12.0 mass% alone on resulting increase in austenite stability against strain-induced transformation into α-martensite.

The proportion of nitrogen (N), in particular of nitrogen (N) in solution, is chosen as small as possible. This is against the background that the influence of hydrogen (H) or nitrogen (N) on the structure of austenitic steels is similarly negative, since both hydrogen (H) and nitrogen (N) increase the electron state density at the Fermi level. The metallic binding character is strengthened, resulting in a local weakening of the steel according to the electronic interpretation of the HELP mechanism ("hydrogen enhanced localized plasticity"). The localization of plasticity leads to a decrease in ductility, which manifests macroscopically in a brittle failure.

Especially at low temperatures, both hydrogen (H) and nitrogen (N) have a negative effect on the macroscopic mechanical properties of the steel. At temperatures between -30 ° C and -100 ° C, this effect manifests itself, inter alia, by the occurrence of a brittle-ductile transition of stable austenitic, nitrogen-alloyed materials.

Furthermore, nitrogen (N) may be incorporated in the form of an interstitial atom on a non-regular lattice site of the atomic lattice, where it represents a zero-dimensional lattice defect. Within the cubic face-centered atomic lattice of the proposed alloy, such an interstitial element promotes hydrogen embrittlement. A corresponding setting of the dissolved nitrogen (N) is carried out according to the invention by the addition of aluminum (Al), which leads to the formation of aluminum nitride (AlN). This is necessary because the presence of nitrogen (N) can not be completely avoided due to production. The content of aluminum nitride (AlN) in the solution-annealed state of the material is from 0.01% by mass to 0.5% by mass.

In order to obtain the desired material properties, a heat treatment in the form of solution annealing is performed. As a result, unwanted carbides, especially chromium carbides, dissolved in order to ensure the corrosion resistance of the steel. The typical temperature for the solution heat treatment is in a range of 1020 ° C and 1080 ° C, preferably at 1050 ° C.

Overall, stainless steels are characterized by a chromium (Cr) content of more than 10.5% by mass. The content of chromium (Cr) according to the invention in the alloy is preferably 15.0-18.0% by mass. The proportion of chromium (Cr) forms a protective and dense passive layer of chromium oxide (CrO) on the surface.

The proportion of manganese (Mn) is preferably 6.0-9.0 mass%.

For an economical production of the austenitic steel, the content of nickel (Ni) is preferably 5.0-8.0 mass%.

The proportion of carbon (C) should be alloyed as high as possible taking into account the technological property "corrosion resistance". The content of carbon (C) is preferably 0.1-0.15 mass%.

A proportion of nickel (Ni) which falls below 8.0% by mass is compensated by an increase in the proportions of carbon (C) and manganese (Mn). This establishes a direct dependence of the proportions of nickel (Ni), carbon (C) and manganese (Mn).

The aluminum (Al) serves to bond dissolved nitrogen to aluminum nitride (AlN). The proportion of aluminum (Al) is chosen so large that the forming aluminum nitride (AlN) has a proportion of 0.01% by mass to 0.5% by mass. By setting the nitrogen (N) with aluminum (Al) to aluminum nitride (AlN), the lowest possible proportion of nitrogen (N) in solution is achieved in an advantageous manner.

The substitution of nickel (Ni) can basically be done by the elements manganese (Mn) and carbon (C). The corresponding nickel equivalent Nickel (Ni) + 0.5 × manganese (Mn) + 30 × [carbon (C) + nitrogen (N)] the alloy is at least 12, with a chromium equivalent Chromium (Cr) + 1.4 × molybdenum (Mo) + 1.5 × silicon (Si) + 0.5 × niobium (Nb) 2 × titanium (Ti) from 18.

Only the elements nickel (Ni), manganese (Mn) and carbon (C) contribute to the nickel equivalent of the proposed alloy, while the chromium equivalent results from the elements chromium (Cr) and silicon (Si). The nickel equivalent of the proposed alloy follows, taking into account the Schaeffler diagram, as follows: Nickel (Ni) + 30 × carbon (C) + 0.5 × manganese (Mn) × 12.

As a result, the desired microstructural proportions are adjustable to an austenitic steel.

The chromium equivalent results from the Schaeffler diagram as follows: 15 <chromium (Cr) + 1.5 × silicon (Si) <25.

As a result, the desired microstructural proportions are to be selected to an austenitic steel.

Thus, according to the invention, chromium (Cr) is proportionally replaced by silicon (Si) according to the chromium equivalent. In this case, 1.0% by mass of chromium (Cr) is replaced by 2/3% by mass of silicon (Si). Thus, with respect to the proposed alloy, 3.0% by mass of chromium (Cr) is replaced by 2.0% by mass of silicon (Si). As a result, the precipitation temperature for chromium carbides remains below 1,000 ° C. Possible alloy variants A to F in% by mass below: Carbon (C) Nitrogen (N) Silicon (Si) Chrome (Cr) Nickel (Ni) Manganese (Mg) Iron (Fe) incl. Contamination A 0.1 <0.005 2.0 15.0 8.0 6.0 rest B 0.1 <0.005 0.2 18.0 8.0 6.0 rest C 0.15 <0.005 2.0 15.0 8.0 3.0 rest D 0.15 <0.005 0.2 18.0 8.0 3.0 rest e 0.05 <0.005 2.0 15.0 8.0 9.0 rest F 0.05 <0.005 0.2 18.0 8.0 9.0 rest

In particular, the values for nitrogen (N) given in alloy variants A to F relate to the nitrogen (N) dissolved in the metal lattice of austenite, which according to the invention should be as low as possible. In practice, lowest contents of nitrogen (N) of 0.005% by mass and less can hardly be realized industrially. This is particularly due to the steel mill open melt compared to the laboratory, where steels with high levels of chromium are alloyed with nitrogen (N) in the air. For the invention of relevance is therefore dissolved in the metal grid of austenite amount of nitrogen (N) and not its chemical amount, which is bonded, for example by the aluminum (Al) in aluminum nitride (AlN). The proportion of nitrogen (N) indicated for alloy variants A to F can in principle also be ≦ 0.005% by mass.

As already mentioned, the atomic lattice of the material in the solution-annealed state has the cubic face-centered crystal structure of austenite. Due to the production, the crystal structure contains small amounts of impurities which are formed by the aluminum nitride (AlN) formed when binding the dissolved nitrogen (N) and also sulfides, such as manganese (II) sulfide (MnS), and oxides, such as silicon dioxide (SiO ₂ ).

The respective particle size in the solution-annealed state is preferably d <50 μm. In a particular embodiment, the grain size may also be d <10 microns.

The resulting microstructure in the solution-annealed state is free of chromium carbides to ensure a correspondingly high corrosion resistance. The respective alloy contents, in particular chromium (Cr), carbon (C) and silicon (Si) are thereby designed so that the chromium carbides can only precipitate at temperatures of more than 50 K below the typical solution annealing temperature of 1050 ° C. The respective alloy contents are preferably designed so that the temperature at which chromium carbides form is as low as possible, in particular as far below the solution annealing temperature as possible. A rapid cooling of the solution annealing temperature prevents this precipitation.

Preferably, the alloy is solution-annealed used. If a higher yield strength is to be set by strain hardening, it must be ensured by the respective process control that the content of deformation-induced martensite remains below 1.0% by volume. Only in this way can it be ensured that the high resistance to hydrogen embrittlement in the contact area to hydrogen-containing atmospheres is maintained.

The self-adjusting material properties were determined in corresponding experiments, wherein the yield strength in the uniaxial tensile test at room temperature in the solution-annealed condition Rpo. 2> 230 MPa. The resulting tensile strength was determined to be Rm> 700 MPa. Furthermore, the relative elongation at break A _{5_} H ₂ / A _{5_} air measured in the uniaxial tensile test at room temperature in the solution-annealed state> 90%.

The stability of the austenitic structure, more specifically the tendency to form martensite when deformed, can be expressed by the Md ₃₀ temperature. At the Md ₃₀ temperature, a true plastic strain of 30% results in a volume content of martensite of 50% by volume.

• Md₃₀ = 551 – 462 × Kohlenstoff (C) – 9,2 × Silizium (Si) – 8,1 × Mangan (Mn) – 13,7 × Chrom (Cr) – 29 × Nickel (Ni) – 18,5 × Molybdän (Mo) – 29 × Kupfer (Cu) – 68 × Niob (Nb) – 462 × Stickstoff (N) – 1,42 × (Korngröße [ASTM] – 8,0)

The Md ₃₀ temperature of the alloy according to the invention is lower than -20 ° C and was calculated according to the following formula (all figures in% by mass):

• Md ₃₀ = 551 - 462 × carbon (C) - 9.2 × silicon (Si) - 8.1 × manganese (Mn) - 13.7 × chromium (Cr) - 29 × nickel (Ni) - 18.5 × Molybdenum (Mo) - 29 × copper (Cu) - 68 × niobium (Nb) - 462 × nitrogen (N) - 1.42 × (grain size [ASTM] - 8.0)

Alloying copper (Cu), which also stabilizes austenite, was largely avoided, as the steelworks, in particular, did not want to find any copper (Cu) in their steel, if possible. In addition, copper (Cu) is among the expensive elements, so that its extensive absence makes the material according to the invention even more economical.

The material created has an overall high austenite stability. This is important in terms of the required embrittlement resistance, since even during a deformation at low temperatures little or no strain-induced martensite can form. The ferrite suddenly forming in known alloys as a result of deformation and / or temperature reduction, more precisely ferritic martensite, is the cause of hydrogen-induced embrittlement. Since these austenitic steels are so-called metastable structures, a corresponding formation of martensite occurs in the known alloys in the prior art. The chemical composition described here achieves high metastability, by which martensite formation is largely prevented.

Claims (8)

Austenitic steel for use in an at least partially direct contact area with a hydrogen-containing fluid, in particular for a hydrogen tank, which has embrittlement-resistant properties with respect to the hydrogen, consisting of an alloy in mass% of - chromium (Cr) 10.0-23, 0 - Manganese (Mn) 3.0-9.0 - Nickel (Ni) 3.0-8.0 - Carbon (C) 0.05-0.15 - Nitrogen (N) 0.001-0.03 - Aluminum ( Al) 0.005-0.15 and silicon (Si) and a balance iron (Fe) and impurities caused by melting, wherein the alloy is substantially free of molybdenum (Mo), characterized in that the proportion of silicon (Si) 1.1 -3.0 mass%. Austenitic steel according to claim 1, characterized in that the proportion of chromium (Cr) is 15.0-18.0 mass%. Austenitic steel according to claim 1 or 2, characterized in that the proportion of manganese (Mn) is 6.0-9.0 mass%. Austenitic steel according to one of claims 1 to 3, characterized in that the proportion of nickel (Ni) is 5.0-8.0 mass%. Austenitic steel according to one of claims 1 to 4, characterized in that the proportion of silicon (Si) is 1.5-2.5 mass%. Austenitic steel according to one of claims 1 to 5, characterized in that the proportion of carbon (C) is 0.1-0.15 mass%. Austenitic steel according to one of Claims 1 to 6, characterized in that a proportion of nickel (Ni) which falls below 8.0% by mass is compensated by an increase in the proportions of carbon (C) and manganese (Mn). Austenitic steel according to one of Claims 1 to 7, characterized in that the aluminum (Al) serves to bond dissolved nitrogen (N) to aluminum nitride (AlN), the aluminum nitride (AlN) having a proportion of 0.01% by mass 0.5% by mass.