Patent Application
20210278043
The present invention relates to a pressure vessel for hydrogen that can perform storage, transport, and accumulation of hydrogen, and can be used for a hydrogen station or the like, and a method of manufacturing the same.
High strength low alloy steel is frequently used as a pressure vessel for hydrogen, storing hydrogen. It is known that the high strength low alloy steel exhibits hydrogen embrittlement behavior such as decrease in elongation and drawing, decrease in low cycle fatigue life, acceleration of fatigue crack propagation rate, and the like, by being in contact with high pressure hydrogen gas in a region where plastic deformation occurs.
In addition, in the related art, for the pressure vessel, an inner surface side of the vessel is plastically deformed so that compressive residual stress is applied to the inner surface side of the vessel by elastic restraint on an outer surface side of the vessel, a so-called autofrettage treatment is performed. As an effect on the autofrettage treated pressure vessel, it is known to inhibit occurrence of fatigue crack and a propagation of fatigue crack.
In PTLS 1 to 3, it is described that an autofrettage treatment is performed on a high pressure vessel. In addition, in PTLS 4 and 5, it is described that an autofrettage treatment is performed on members. In these PTLS, it is not described to perform the autofrettage treatment on a pressure vessel for hydrogen in any cases.
In the related art, there is no example illustrating the effect of autofrettage treatment of a vessel for high pressure hydrogen gas using high strength steel illustrating brittle behavior in the high pressure hydrogen gas, and the effect is unknown. In a case of subjecting the pressure vessel to autofrettage treatment, the autofrettage treatment plastically deforms the inner surface serving as a portion in contact with the high pressure hydrogen gas, so that there is concern on the effect due to hydrogen.
In addition, in PTLS 6 and 7, there are described that a fiber reinforced resin layer is wrapped around a vessel body, and the autofrettage treatment is performed on the vessel body. In these PTLs, the fiber reinforced resin layer is indispensable.
PTL 1: JP-A-2004-28120
PTL 2: JP-A-2007-239853
PTL 3: JP-A-2007-198531
PTL 4: JP-T-2009-529113
PTL 5: JP-A-2011-99435
PTL 6: JP-A-2012-52588
PTL 7: WO 2004/51138 A1
In high strength low alloy steel such as Cr—Mo steel and Ni—Cr—Mo steel normally used for the pressure vessel, in a case where a crack is present on an inner front surface, hydrogen environment embrittlement occurs by contacting hydrogen with a high stress crack tip end, and as illustrated in
In addition, the high strength low alloy steel is known to exhibit embrittlement behavior in a high pressure hydrogen gas environment, and it is a well-known fact that there is a contribution of plastic deformation to the feature of fracture in hydrogen embrittlement. The embrittlement behavior appears more remarkably, especially in a large plastic deformation region.
In order to use the pressure vessel for hydrogen safely and in an economically superior state as facilities, it is necessary to inhibit crack propagation even in a high pressure hydrogen gas environment. As a technique for inhibiting the occurrence of crack and crack propagation, the above-described autofrettage treatment is generally known.
However, since the autofrettage treatment plastically deforms the inner surface side by applying a large internal pressure to the pressure vessel, in an accumulator using high strength low alloy steel used in high pressure hydrogen gas environment, the effect of inhibiting crack propagation is not always obtained in the same manner as in the atmospheric environment due to the effect of residual strain.
In addition, in
Therefore, the present inventors understand the effect of strain in the high pressure hydrogen gas environment, and check the crack propagation behavior in the high pressure hydrogen gas environment in the vessel actually subjected to the autofrettage treatment, accordingly they accomplish the present invention.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a pressure vessel for hydrogen which is capable of inhibiting crack propagation and is excellent in safety and durability, and a method of manufacturing the same.
Therefore, among pressure vessels for hydrogen of the present invention, the first illustrative aspect is a pressure vessel for hydrogen which is made of steel, wherein a compressive residual stress is present on an inner surface side of a pressure vessel body for hydrogen, and wherein an elastic region is provided on an outer surface side of the pressure vessel body for hydrogen.
The invention of the pressure vessel for hydrogen according to another illustrative aspect, in the present invention according to another illustrative aspect, wherein an equivalent plastic strain remaining on the inner front surface of the pressure vessel body for hydrogen is 1% or less.
The invention of the pressure vessel for hydrogen according to another illustrative aspect, in the present invention according to another illustrative aspect, wherein in a radial direction of the pressure vessel body for hydrogen, a plastic region on the inner surface side is 50% or less of a wall thickness.
The invention of the pressure vessel for hydrogen according to another illustrative aspect, in the present invention according to another illustrative aspect, wherein the steel used for the pressure vessel for hydrogen has a tensile strength of 725 MPa or more.
The invention of the pressure vessel for hydrogen according to another illustrative aspect, in the present invention according to another illustrative aspect, wherein the steel is a Cr—Mo steel, a Ni—Cr—Mo steel, or a Ni—Cr—Mo—V steel.
Among pressure vessels for hydrogen of the present invention, the first illustrative aspect is a method of manufacturing a pressure vessel for hydrogen which is made of steel, performing an autofrettage treatment by applying a pressure of an internal yield stress or higher of a pressure vessel body for hydrogen to an inside thereof; and applying a compressive residual stress to an inner surface of the pressure vessel body for hydrogen.
The method for manufacturing a pressure vessel for hydrogen according to another illustrative aspect, in the present invention according to another illustrative aspect, wherein after the pressure is unloaded, pressure is applied so that an equivalent plastic strain remaining on the inner front surface of the pressure vessel body for hydrogen is 1% or less.
The method for manufacturing a pressure vessel for hydrogen according to another illustrative aspect, in the present invention according to another illustrative aspect, wherein after the pressure is unloaded, the pressure is applied so that a plastic region on the inner surface side is 50% or less of a wall thickness in a radial direction of the pressure vessel body for hydrogen.
The technical matters defined in the present invention will be described below.
In the present invention, in an accumulator using a high strength low alloy steel illustrating a hydrogen embrittlement behavior by the effect of plastic deformation and illustrating a large crack propagation rate under high pressure hydrogen gas environment, by understanding the effect of the equivalent plastic strain in the high pressure hydrogen gas environment, the autofrettage treatment condition for the pressure vessel for hydrogen of the present invention is limited.
In the present invention, it is preferable that the equivalent plastic strain is 1% or less. The reason why the equivalent plastic strain is 1% or less is to set the pressure range which does not re-yield due to Bauschinger effect as autofrettage tension. If a material with a tensile strength of 1,046 MPa is used in the atmosphere (other than hydrogen), even if the equivalent plastic strain is approximately 4%, it is considered that an autofrettage effect can be obtained without decreasing the fracture toughness value. However, for hydrogen, if the equivalent plastic strain is 1% or less, it is considered that the autofrettage effect can be obtained without decreasing the fracture toughness value.
The equivalent plastic strain is obtained by measuring residual stress and converting the residual stress into the strain. The method of measuring the residual stress is not particularly limited, and typical examples thereof include an X-ray diffraction method and a magnetostriction method. In addition, it is possible to calculate the magnitude of the equivalent plastic strain by measuring the strain on an outer surface and the dimension of the inner and outer surfaces before and after the autofrettage treatment and by analyzing them.
When the crack is artificially introduced to the inner front surface of an accumulator vessel that stores high pressure hydrogen inside and a fatigue test is repeated, in a case of small crack, the crack propagation was observed by the repeat in the vessel not subjected to the autofrettage treatment, and the crack propagation was not observed in the vessel subjected to the autofrettage treatment. Furthermore, when the fatigue test was performed by introducing the crack large enough to ensure that the crack propagated, it was described that the number of times of the vessel subjected to the autofrettage treatment until the crack propagated and penetrated was several times longer than that of the vessel not subjected to the autofrettage treatment. From these results, it was proved by the demonstration test using the simulated specimen that an inhibiting effect of crack propagation can be obtained by performing the autofrettage treatment.
Since the autofrettage effect can be exerted on the pressure vessel for hydrogen, in the present invention, it is preferable that the autofrettage treatment condition is a plasticity rate of 50% or less. In the present invention, the plasticity rate refers to a wall thickness ratio of a plastic region to the pressure vessel for hydrogen in a standard in the radial direction of the pressure vessel for hydrogen. That is, the plastic region on the inner surface side in the radial direction of the pressure vessel body for hydrogen is set to 50% or less of the wall thickness.
If the plasticity rate exceeds 50% and the elastic region narrows, the tightening from the outside is weak and it is unlikely to generate sufficient compressive residual stress. The plasticity rate is preferably 10% or more for the purpose of applying effective compressive residual stress.
The steel used for the pressure vessel for hydrogen of the present invention (for example, Cr—Mo steel, Ni—Cr—Mo steel, Ni—Cr—Mo—V steel) preferably has a tensile strength of 725 MPa or higher. If the strength decreases, since the effect of hydrogen embrittlement decreases, even if the plastic strain increases, it is considered that the decrease in fracture toughness value due to the effect of hydrogen is reduced, and the difference between the atmosphere and hydrogen in the autofrettage effect also is reduced. Since the effect of hydrogen on steel is brittle as the strength is higher, it is preferable to provide a minimum value of tensile strength. The tensile strength can be measured based on JIS Z2241 (2011) “Metal material tensile test method”.
That is, according to the present invention, it is possible to provide the pressure vessel for hydrogen in which the compressive residual stress is generated on the inner surface by providing the plastic region on the inner surface side and the elastic region on the outer surface side of the pressure vessel body for hydrogen, the crack propagation on the inner surface of the pressure vessel body for hydrogen can be effectively inhibited in the high pressure hydrogen gas, and which is excellent in safety, reliability, and durability. In addition, by using the high strength low alloy steel as the material of the pressure vessel, the wall thickness can be reduced and the weight of the pressure vessel can be reduced.
The above plastic region and elastic region can be determined by performing a known analysis in the related art, or determined by, for example, a Sachs method measuring the strain value released while scraping the vessel.
In addition, the method of measuring the compressive residual stress is not particularly limited similarly to the measurement of the residual stress, and typical examples thereof include the X-ray diffraction method and the magnetostriction method.
The present invention relates to a pressure vessel for hydrogen, which is a steel pressure vessel for hydrogen, and has a compressive residual stress on an inner surface side of a pressure vessel body for hydrogen and an elastic region on an outer surface side. Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
As illustrated in
The material of the pressure vessel body 1 and the lid 3 is not particularly limited, and as the material of the pressure vessel body 1, manganese steel, chromium molybdenum steel (Cr—Mo steel), nickel chromium molybdenum steel (Ni—Cr—Mo steel), nickel chromium molybdenum vanadium steel (Ni—Cr—Mo—V steel), other low alloy steel, or the like can be used. The Cr—Mo steel, the Ni—Cr—Mo steel, or the Ni—Cr—Mo—V steel is preferable.
Cr is an element which improves hardenability, strength, abrasion resistance, corrosion resistance, and the like. Mo is an element which improves hardenability, strength, corrosion resistance, and inhibits grain coarsening and temper embrittlement. Ni is an element which improves hardenability, toughness, and corrosion resistance. V is an element which improves hardenability and refines grain size to improve strength, toughness, and the like. Alloy steel containing these elements have high strength. When the strength of the alloy steel is high, the wall thickness of the pressure vessel can be made thin, so that a lightweight and compact pressure vessel can be obtained.
It is preferable that a tensile strength of the above steel is 725 MPa or higher. It is more preferably 795 MPa or higher, and still more preferably 825 MPa or higher. In addition, it is preferably 1,205 MPa or less, and more preferably 1,100 MPa or less.
The material of the lid 3 may be the same material as that of the pressure vessel body 1 or may be made of other materials. Furthermore, members made of different materials can be used for each member of the lid 3. The materials of the pressure vessel body 1 and the lid 3 are not limited to the above.
The pressure vessel body 1 is formed in a tubular shape with steel. A manufacturing method thereof is not particularly limited, a processing method with fewer defects is preferable, and for example, it is integrally formed by forging or extrusion. The size of the pressure vessel body 1 is not particularly limited, and it is preferably one having an internal volume of 20 liters or more, and the total length is preferably 6,000 mm or less.
By having the internal volume of 20 liters or more, a sufficient amount of hydrogen can be accumulated. In addition, the total length of the pressure vessel body 1 is preferably not too long for reasons such as installation in a narrow area such as an urban area. Therefore, it is preferable to set the total length to 6,000 mm or less.
In addition, since the inner diameter of the pressure vessel body 1 varies depending on the inner volume and the total length, it is not limited to a specific range, and the inner diameter is preferably, for example, 180 mm or higher. In a case of performing a penetration treatment or development treatment of a penetrant flaw detection method, for example, at the time of direct inspection of the inner surface, the reason is that such an opening diameter is required. In addition, the wall thickness of the pressure vessel body 1 is not particularly limited. The wall thickness can be determined in consideration of the load shared by the pressure vessel body 1.
In the pressure vessel body 1, it is preferable that an inner surface 1a is mirror-finished so as to be free of defects. A hollow portion of the pressure vessel body 1 is formed in a straight cylindrical shape at the portion having the inner surface 1a, and mirror finishing can be easily performed. The inner surface 1a is a portion to which the pressure of accumulated hydrogen is loaded.
It is preferable to prevent the crack propagation due to hydrogen embrittlement by the mirror finishing to have a surface shape not having defects of 0.5 mm in depth and 1.6 mm or higher in surface length in the wall thickness direction. If a defect exceeding this size is left on the inner surface of the pressure vessel body 1, a crack due to hydrogen embrittlement is likely to propagate and fatigue crack life is reduced. However, depending on the shape of the pressure vessel body or the like, the pressure vessel body having the above defect or more may be permitted.
The present invention relates to a method of manufacturing a pressure vessel for hydrogen, which is a method of manufacturing a steel pressure vessel for hydrogen, in which a pressure of an internal yield stress or higher of the pressure vessel body for hydrogen is applied to the inside to perform an autofrettage treatment, and the compressive residual stress is applied to the inner surface of the pressure vessel body for hydrogen.
For the pressure vessel body 1, the pressure is applied from the inner surface side to perform the autofrettage treatment. In the autofrettage treatment, the pressure vessel body 1 expands in the outer peripheral direction and plastically deforms only on the inner surface side, so that residual stress remains on the inner surface side after depressurization, and the strength increases.
In the autofrettage treatment, the pressure is applied to the inside to such an extent that the inner surface of the pressure vessel body 1 is partially yielded (internal yield stress or higher). Internal pressure loading is normally performed by water pressure, and the medium of pressure is not particularly limited.
In the autofrettage treatment, as illustrated in
In the standard in the radial direction of the pressure vessel for hydrogen, using the wall thickness ratio of the plastic region 2a to the pressure vessel for hydrogen as a plasticity rate, it is preferable that the plasticity rate after the pressure is unloaded is 50% or less, and the elastic region 2b is 50% or more of the wall thickness. The plasticity rate is more preferably set to 40% or less. In addition, it is preferably 10% or more, and more preferably 20% or more. The plasticity rate is adjusted by internal pressure in the autofrettage treatment. The internal pressure is set so as to obtain the above characteristics in consideration of material characteristics, pressure vessel size, and the like.
The pressure vessel for hydrogen of the embodiment can be used for applications such as an automobile using hydrogen as a hydrogen station.
For example, assuming that 54 fuel cells are filled in fuel cell hydrogen vehicles, an accumulator for 82 MPa hydrogen station (compressed hydrogen stand) that supplies hydrogen is subjected to repeated internal pressure of 54 per day, 19,710 times per year, and 295,650 times in 15 years. In order to ensure such durability, the pressure vessel for hydrogen of the present embodiment realizes high strength and light weight, and can provide absolute safety and high reliability in a hydrogen station installed in an urban area or the like.
Using a high strength low alloy steel Ni—Cr—Mo—V steel (C: 0.27%, Si: 0.06%, Mn: 0.30%, Ni: 3.6%, Cr: 1.7%, Mo: 0.5%, V: 0.08%, with a balance being Fe and unavoidable impurities, a cylindrical specimen of the example subjected to the autofrettage treatment and a cylindrical specimen not subjected to the autofrettage treatment as a comparative example were prepared, and cycle tests in a high pressure hydrogen gas environment simulating an accumulator were performed. As a result of performing the autofrettage treatment at a pressure of 380 MPa with an inner and outer diameter ratio of 1.6, the circumferential strain generated on an inner front surface was approximately 0.3%, and approximately 40% of the wall thickness from the inner front surface became the plastic region (plasticity rate: approximately 40%).
As illustrated in
The crack propagation length was confirmed by periodically performing an ultrasonic flaw detection test by TOFD method. It was confirmed that the specimen not subjected to the autofrettage treatment showed large crack propagation at 40,154 times. On the other hand, in the specimen subjected to the autofrettage treatment, no crack propagation was observed at 40,154 times, and thereafter repetition cycles of approximately 24,000 more times were applied, and the ultrasonic flaw detection test could not catch cracks even when it reached 64,064 times. The fracture surface of specimen not subjected to the autofrettage treatment was opened after 40,154 times of cycle tests, and that of specimen subjected to the autofrettage treatment was opened after 64,064 times of cycle tests. The results of observation on crack propagation using an electron microscope are also illustrated in
In the specimen subjected to the autofrettage treatment, if the size of the artificial defect introduced at the beginning is small, it is necessary to apply a significantly enormous cycle in order to propagate the crack. Therefore, even in the specimen subjected to the autofrettage treatment, the cycle test was performed by increasing the size of the initial artificial defect to be introduced so that the crack propagation was observed, and the results of confirming the behavior of fatigue crack propagation in a 90 MPa hydrogen gas environment by fracture surface observation is illustrated in
In the specimen not subjected to the autofrettage treatment, the specimen is elongated in a semi-elliptical shape, whereas in the specimen subjected to the autofrettage treatment, the crack propagation on the inner front surface is inhibited by the effect of the compressive residual stress on the inner front surface, and the specimen only propagated in the depth direction. The semi-elliptical dashed line on the fracture surface in
Hereinbefore, although the present invention has been described based on the above embodiments and examples, the present invention is not limited to the description in the above embodiments and examples, and appropriate modifications can be made in the above embodiments and examples without departing from the scope of the present invention.
While the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications and variations are possible without departing from the spirit and scope of the present invention. This application is based on Japanese Patent Application (Japanese Patent Application No. 2016-140774) filed on Jul. 15, 2016, the entirety of which is incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-140774 | Jul 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/025811 | 7/14/2017 | WO | 00 |
