HYDROGEN-CARRYING PIPE COMPONENT

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of European Patent Application, Serial No. 24 179 799.2, filed Jun. 4, 2024, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference in its entirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

The present invention relates to a hydrogen-carrying pipe component.

The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.

Hydrogen-carrying pipe components are in particular high-pressure lines or also hydrogen tanks in order to store hydrogen in stationary or mobile applications. Hydrogen as an energy carrier for fuel cell vehicles is emission-free, but has a low volumetric mass density at room temperature. The low volumetric energy density can be increased by compressing hydrogen, which substantially facilitates the storage, the transport, and the use of hydrogen. Suitable high-pressure line systems are necessary to transport compressed hydrogen from a tank to a fuel-cell stack or to an internal combustion engine. In trucks and passenger vehicles, these systems are typically operated at a hydrogen pressure of 35 MPa or 70 MPa. The requirements for high-pressure line systems in hydrogen-operated vehicles and also in peripheral systems or comparable applications are high. They have to withstand the high hydrogen pressure safely and reliably. In addition, they have to be protected against corrosion by the hydrogen and against corrosion by other media.

Stainless steels are normally used for hydrogen-carrying components. Stainless steels have a minimum proportion of 16% chromium and 10% nickel. They are relatively costly due to the high content of chromium and nickel. Moreover, the material has a low strength, so that components which are designed for relatively high pressure resistances have to have relatively high wall thicknesses. These in turn increase the weight and therefore the energy consumption in mobile applications. Moreover, the higher material use results in higher costs.

Approaches to develop cost-effective, hydrogen-resistant types of steel involve the use of higher-strength steels. Conventional, low-alloyed carbon steels having tensile strengths between 350 MPa and 630 MPa could theoretically represent an optimum alternative, however, the hydrogen compatibility of low-alloyed carbon steels is lower than that of stainless steels.

It would therefore be desirable and advantageous to optimize an alloy composition of low-alloyed carbon steels with respect to use for hydrogen-carrying pipe components and with respect to material costs.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a hydrogen-carrying, cold-drawn pipe component made of a steel alloy can advantageously be a line use of a hydrogen pressurized tank system of a motor vehicle and includes a steel alloy, which, in addition to iron and smelting-related impurities, includes the following elements in mass-% as the remainder:

• • C: 0.07-0.22
  • Si: 0.10-0.55
  • Mn: 0.30-1.60
  • P: ≤0.025, in particular 0.005-0.025,
  • S: ≤0.015, in particular 0.002-0.015,
  • Ti: 0.010-0.030
  • V: 0.003-0.30
  • N: 0.008-0.025
  • Al: 0.010-0.025
  • Ca <0.10, in particular 0.0005-0.10.
    Optionally, the alloy comprises the following elements in mass-%:
  • Nb: ≤0.10
  • Mo: ≤0.50
  • Cr: ≤0.50
  • Ni: ≤1.0
  • Cu: ≤0.20.

Carbide-forming elements are deliberately added, such as vanadium. The addition of carbide-forming elements results in an improvement of the HE resistance (HE=hydrogen embrittlement). The addition of carbide-forming elements results in increased strength of the material due to precipitation hardening. Nanoscale vanadium carbon nitrides V(C,N) play a large role here. They are used as hydrogen traps. They capture the hydrogen strongly and thus reduce the hydrogen diffusion speed. They also refine the previous austenite grain size, by which the HE resistance is significantly improved.

Coarse vanadium carbon nitrides lose their property as strong hydrogen traps and function as crack initiation points, which reduces the HE resistance. Therefore, only vanadium carbon nitride nano-precipitates having sizes below 60 nm represent effective hydrogen traps.

Specific upper limits for phosphorus and sulfur are important features for the resistance to high-pressure hydrogen gas. Calcium modifies the form and distribution of MnS inclusions, due to which they become smaller and less deformable. This results in a more homogeneous microstructure, which is less susceptible to the hydrogen diffusion, by which the hydrogen resistance is increased.

In carbon steels, identical chemical analyses do not necessarily result in identical properties of vanadium carbon nitride precipitates. This is because the properties of vanadium carbon nitride precipitates are influenced not only by the chemical composition, but also by other factors, such as heat treatments, presence of other alloy elements, which modify the formation and properties of these precipitates, and the microstructure, including the grain size and the distribution of other phases. The cold drawing also plays a role.

Thermokinetic calculations have shown that for the alloy composition according to the invention, the number of vanadium carbon nitride precipitates per volume unit is, from a vanadium content of 0.003 wt. %, greater than 1×10{circumflex over ( )}18/m³ and therefore above the TEM detection limit (TEM=transmission electron microscopy). The average size of the vanadium carbon nitride precipitates is 10 nm in diameter. The equivalence diameter is determined. The number of nanoscale vanadium carbon nitride precipitates increases with increasing vanadium content. At the same time, the hydrogen diffusion coefficient decreases by an order of magnitude. The maximum value of 0.3 wt. % vanadium is not to be exceeded, since otherwise the HE resistance decreases again with increasing vanadium contents.

HRTEM (High Resolution Transition Electron Microscopy) represents a suitable measuring method for determining the size of V(C, N) precipitates. Resolutions of up to 0.1 nm can be achieved using this method. Moreover, the number of V(C,N)-precipitates per unit of area or volume (number density) can also be determined thereby.

Titanium and titanium nitrides have a grain-refining effect due to recrystallization obstruction and obstruction of austenite grain growth. Moreover, TiN precipitates are used as nucleation points for vanadium carbon nitrides, by which their number and therefore the number of hydrogen traps is increased.

Vanadium results in the formation of vanadium carbon nitrides V(C,N), which have a grain-refining effect. The grain-refining effect increases the strength and ductility of the material.

The pipe component can be in particular produced without weld seams, advantageously from a hot-rolled microstructure normalized once or twice. The pipe component can be heated above the Ac3 temperature, held at the target temperature, and slowly cooled with the goal of producing a fine-grained microstructure for the normalization. A homogeneous distribution of the mechanical properties is achieved.

The pipe component according to the invention includes a ferrite-perlite steel having high HE resistance due to the addition of vanadium. The pipe component according to the invention is easily weldable and has a CEV (carbon equivalent value) less than or equal to 0.42%. The carbon equivalent value is a measure to assess the suitability for welding of unalloyed and low-alloyed steels. The carbon content and the weighted proportion of the elements which similarly influence the suitability of the steel for welding as would be expected from carbon are summarized in a numeric value in the carbon equivalent value. Values less than 0.45% imply a good suitability for welding. Higher values require preheating. From 0.65%, the workpiece is only suitable for welding with increased effort, because of martensite formation, which can result in cold cracks or hardening cracks.

The CEV can be determined for a carbon content from 0.18 mass-% according to the following equation:

CEV = C + Mn / 6 + ( Cu + Ni ) / 15 + ( Cr + Mo + V ) / 5.

Copper, nickel, chromium, and molybdenum can be optional alloy components.

As a result, the microstructure includes more than 95% ferrite and perlite. The remaining components of the microstructure include martensite, bainite, and residual austenite. The microstructure is distinctively fine-grained corresponding to a grain size of the grain size class 9 or finer according to ASTM 112-13 (2021). Advantageously, the grain size class is at least 10.

The grain size class 9 is achieved after cold drawing without normalization. The microstructure becomes even more homogeneous by cold drawing and subsequent single or double normalization. It becomes finer and therefore better, so that grain size class 10 is also achieved.

The steel alloy can be formed into a cold-drawn pipe component, wherein the pipe component has a tensile strength of at least 340 MPa. The tensile strength is advantageously 400 MPa to 850 MPa. The yield strength Rp0.2 is advantageously at least 220 MPa. The elongation at fracture A is at least 25%.

According to another advantageous feature of the invention, the steel alloy can include, in addition to iron and smelting-related contaminants, the following elements in mass-%:

• • C: 0.07-0.22
  • Si: 0.10-0.55
  • Mn: 0.30-0.7
  • P: 0.005-0.025
  • S: 0.002-0.015
  • Ti: 0.010-0.030
  • V: 0.003-0.30
  • N: 0.008-0.025
  • Al: 0.010-0.025
  • Ca 0.0005-0.050
    and optionally:
  • Nb: ≤0.050
  • Mo: ≤0.30
  • Cr: ≤0.35.

A reduced manganese content has the technical effect that the ductility is improved and the susceptibility to hydrogen embrittlement is reduced. This results in a higher toughness and an improved resistance to hydrogen-induced cracks, which is advantageous in particular in hydrogen-carrying applications. At the same time, the strength is kept at a sufficiently high level.

According to another advantageous feature of the invention, the steel alloy can include, in addition to iron and smelting-related contaminants, the following elements in mass-%:

• • C: 0.09-0.22
  • Si: 0.10-0.55
  • Mn: 0.30-0.70
  • P: 0.005-0.025
  • S: 0.002-0.015
  • Ti: 0.010-0.030
  • V: 0.030-0.30
  • N: 0.008-0.025
  • Al: 0.010-0.025
  • Ca: 0.0005-0.050
    and optionally
  • Nb: ≤0.050
  • Mo: ≤0.30
  • Cr: ≤0.350.

According to another advantageous feature of the invention, the steel alloy can include, in addition to iron and smelting-related contaminants, the following elements in mass-%:

• • C: 0.09-0.22
  • Si: 0.10-0.55
  • Mn: 0.70-1.60
  • P: 0.005-0.025
  • S: 0.002-0.015
  • Ti: 0.010-0.030
  • V: 0.050-0.30
  • N: 0.008-0.025
  • Al: 0.015-0.025
  • Ca: 0.0005-0.050
    and optionally
  • Nb: ≤0.050
  • Mo: ≤0.30
  • Cr: ≤0.50
  • Ni: ≤0.50
  • Cu: ≤0.20.

The increased concentration of manganese and vanadium reinforces the strength and hardness of the pipe component, which increases the resistance to mechanical strains. This composition is particularly advantageous for applications which require a high mechanical carrying capacity. This composition furthermore ensures good hydrogen resistance and is particularly advantageous for applications which require a high mechanical carrying capacity, for example, fueling procedures at 700 bar H2 pressure.

According to another advantageous feature of the invention, the steel alloy can include, in addition to iron and smelting-related contaminants, the following elements in mass-%:

• • C: 0.09-0.19
  • Si: 0.20-0.55
  • Mn: 0.90-1.50
  • P: 0.005-0.025
  • S: 0.003-0.015
  • Ti: 0.010-0.030
  • V: 0.110-0.30
  • N: 0.008-0.025
  • Al: 0.015-0.025
  • Ca: 0.0010-0.010
    and optionally
  • Nb: ≤0.050
  • Mo: ≤0.30
  • Cr: ≤0.50
  • Ni: ≤0.50
  • Cu: ≤0.180,

The increased concentration of manganese and vanadium reinforces the strength and hardness of the pipe component, which increases the resistance to mechanical strains. This composition is particularly advantageous for applications which require a high mechanical carrying capacity. This composition furthermore ensures good hydrogen resistance and is particularly advantageous for applications which require a high mechanical carrying capacity, for example, fueling procedures at 700 bar H2 pressure.

According to another advantageous feature of the invention, the steel alloy can include, in addition to iron and smelting-related contaminants, the following elements in mass-%:

• • C: 0.07-0.17
  • Si: 0.10-0.55
  • Mn: 0.70-1.60
  • P: 0.005-0.025
  • S: 0.002-0.015
  • Ti: 0.010-0.030
  • V: 0.003-0.30
  • N: 0.008-0.025
  • Al: 0.010-0.025
  • Ca: 0.0005-0.050
    and optionally
  • Nb: ≤0.050
  • Mo: ≤0.30
  • Cr: ≤0.35.

The reduced carbon upper limit and the increased manganese content contribute to improved ductility and toughness. This composition reduces the susceptibility to hydrogen embrittlement and increases the ability to deform and weld the pipe component, which is advantageous in particular in hydrogen-carrying environments. The strength remains here at a level which is suitable for many applications.

The respective above-mentioned compositions were carefully developed to meet the specific requirements with respect to strength and ductility. These are balanced combinations of mechanical properties and hydrogen resistance which enable the hydrogen-carrying pipe components to be used optimally in various applications and under different operating conditions. This flexibility ensures that different strength requirements can be served and contributes to the versatility and robustness of the pipe components.

The use of the pipe component as a hydrogen-carrying, cold-drawn pipe component in mobile applications requires effective corrosion protection. For this purpose, the hydrogen-carrying pipe component can be coated on an outer surface, advantageously only on one side. This can be a zinc or zinc alloy coating which was applied by a dip bath, optionally in combination with an organic coating, for example, a powder lacquering. Suitable coatings can be applied galvanically or by electrophoretic deposition methods, in particular by cathodic dip lacquering. The coating is in particular multilayered and is used for the corrosion protection of the outer side of the hydrogen-carrying, cold-drawn pipe component, in particular in its function as a pipeline of a hydrogen pressurized tank system, namely as a line of a system or as a tank itself.

The specific application of the hydrogen-carrying, cold-drawn pipe component presumes a high static and cyclic carrying capacity.

The burst pressure test according to ISO 11114-4:2017 (method A) is a testing method for assessing the susceptibility of metals to hydrogen embrittlement. A small flat disk of the material to be tested is placed between two stainless steel flanges. The pressure is increased from one side of the disk at different pressure increase rates until a fracture occurs. The burst pressures are determined. The test is carried out both using helium gas and using hydrogen gas. The burst pressures thus determined are compared. The ratio between these burst pressures is an indicator of the hydrogen compatibility. The lower the ratio of the burst pressure of helium gas is in comparison to the burst pressure with hydrogen gas, the less susceptible the steel is to hydrogen embrittlement. The helium burst pressure is to be equal to or only somewhat greater than the hydrogen burst pressure. Helium burst pressure/hydrogen burst pressure factor is to be at most 2, advantageously at most 1.75, particularly advantageously at most 1.50, in particular at most 1.25.

For ratios less than or equal to 2.0, steels are considered to be hydrogen compatible. Values close to 1.0 are desirable for hydrogen high-pressure line systems. The steel alloy according to the invention reaches values of at most 1.2, in particular values of at most 1.16. It is therefore a steel alloy which is suitable for the production of hydrogen-carrying, cold-drawn pipe components, and in particular for the use for hydrogen high-pressure line systems.

A hydrogen-carrying, cold-drawn pipe component according to the invention also meets high requirements for a cyclic pressure load, however.

For this purpose, a cyclic pressure test is carried out using hydrogen gas over up to 50,000 pressure cycles at a defined testing temperature. The cyclic pressure test is suitable in particular in order to assess the effects of cyclic aging under compressed hydrogen influence. At a temperature of −60° C., maximum induced hydrogen damage results. A pressure cycle comprises 20 to 30 seconds, wherein within this period of time the pressure is increased from 0 to 87.5 MPa and is reduced back to 0 MPa. After the up to 50,000 pressure cycles, a burst pressure test is carried out with the proviso that an unloaded pipe component, i.e. a pipe component without cyclic pressure test, is used as a reference pipe and is compared to a pipe sample after the cyclic pressure test. The ratio between the determined burst pressures is an indicator of the hydrogen compatibility after dynamic load, wherein the determined burst pressure of the pipe sample in comparison to the reference pipe is at most 40% lower, advantageously at most 30% lower, particularly advantageously at most 20% lower, in particular at most 10% lower.

A pipe component according to the invention is distinguished in that the determined burst pressure of the pipe sample is lower in comparison to the reference pipe.

It is to be noted that with the alloy compositions according to the invention, no significant disadvantageous difference was detected between these two burst pressures, i.e. the burst pressure of the unloaded pipe component and the pipe component which was subjected to the cyclic pressure test. This proves excellent HE resistance.

Hydrogen is stored at a pressure level of 70 MPa. With a work pressure of 70 MPa and a safety factor of 1.25, 87.5 MPa results, corresponding to the test pressure during the cyclic pressure test. The experiments have thus confirmed that a burst pressure of at least 80 MPa was determined. The cyclic pressure test was carried out on a pipe component according to the invention having the dimensions 6.35 mm diameter and at a wall thickness of 1.675 mm. The test was carried out using hydrogen gas having a purity of at least 99.999%. It is to be added with respect to the test conditions during the cyclic pressure test that with a stainless-steel pipe in identical test conditions, a reduction of the burst pressure by 1.9% in comparison to the starting pipe took place in each case.

BRIEF DESCRIPTION OF THE DRAWING

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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

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While the invention has been described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.