Patent Application
20100193083
The present invention relates to a hydrogen-resistant high strength material and a method for producing the same.
In recent years, prevention of global warming is noted. Hydrogen energy is attracting attention since it does not emit carbon dioxide (CO2) having a high greenhouse effect responsible for global warming. Hydrogen energy results in discharging only water during usage thereof (after combustion). Accordingly, hydrogen has been studied as alternative energy of fossil fuels such as conventional gasoline or natural gas.
In order to promote usage of hydrogen energy, studies and developments for putting fuel cell vehicles and hydrogen vehicles into practical use are under way. As well, developments for building infrastructures such as hydrogen stations are in progress in order to make them widely available.
Metal materials are generally used for high-pressure containers, pipings, reactors and so on for storage, transportation and usage of the hydrogen gas. However, metal materials are in general likely to be subjected to embrittlement under an environment that is exposed to hydrogen. The embrittlement problem is more remarkable as the material becomes to have higher strength.
For example, an on-vehicle container of a high-pressure hydrogen container-mounted vehicle has been designed until recently to have a filling pressure of about 35 MPa. However, the filling pressure is being attempted to increase up to about 70 MPa because of its short running distance. In this case, the used container and piping are necessarily increased in thickness to obtain necessary strength, resulting in such a demerit that a total weight increases.
Furthermore, a Coriolis flowmeter is used as an example of a high-pressure hydrogen flowmeter of a hydrogen dispenser that is used in a hydrogen station. In the Coriolis flowmeter, a flow rate is measured by detecting a vibration caused when hydrogen gas flows in a piping bent in a U-shape. The thinner the piping is formed, the higher the accuracy is. Accordingly, it is impossible to secure the strength by making the thickness larger.
From these requirements, it is necessary that the material itself has high strength and excellent hydrogen embrittlement resistance.
At present, JIS SUS316L (stainless steel) is used as a material for high-pressure hydrogen containers or pipings. Furthermore, JIS SUH660 (superalloy generally referred to as A286) is used for components necessary to have tensile strength in particular. These materials are superior to other alloys in the mechanical characteristics such as the hydrogen embrittlement resistance in hydrogen or when hydrogen is absorbed. Still furthermore, it is studied to adopt high strength martensitic stainless steel such as JIS SCM435 in a storage tank.
However, the tensile strength is about 600 MPa for JIS SUS316L and about 1000 MPa for JIS SUH660. Accordingly, these materials cannot be applied to members requiring greater strength. An outer circumference of a storage tank that uses martensitic stainless steel is reinforced with carbon fiber to deal with the embrittlement.
JP-A-2007-126688 discloses an austenitic high-Mn contained stainless steel that is superior to JIS SUS316L in hydrogen embrittlement resistance susceptibility and has low concentrations of Mo and Ni. The austenitic high-Mn contained stainless steel can reduce amounts of Mo and Ni in comparison with JIS SUS316L and is advantageous in production at a low cost. However, the tensile strength is substantially same as that of JIS SUS316L.
JP-A-2008-69435 di closes a high-strength steel excellent in the hydrogen embrittlement resistance, which contains a strain-induced martensitic structure therein and a part of or an entirety of a surface of which is made mainly art austenitic structure. It also discloses a method for producing the high-strength steel by induction-heating a surface of a steel having a strain-induced martensitic structure to cause a reverse transformation of the strain-induced martensitic structure on the surface into an austenitic structure. However, since these high-strength steels are based on an austenitic stainless steel, it is considered that the tensile strength is at most about 1000 MPa and applications to materials having a shape other than a round bar are difficult.
SP-A-7-278768 discloses a method for reducing the hydrogen embrittlement in order to effectively reduce or inhibit hydrogen embrittlement of alloy members such as a rocket engine that uses hydrogen as a fuel. The method includes a step of irradiating a laser beam on a whole surface, which has been aged, to heat the irradiated surface to a temperature higher than the solvus temperature and dissolve precipitates generated in the alloy member by aging treatment, followed by quenching the irradiated portion to form a hydrogen embrittlement resistant portion that is devoid of precipitates on a surface of the irradiated portion and homogeneous in a structure. This is a method where a γ′-phase that is a precipitation strengthening phase is locally dissolved with heating. However, there is fear that a saluted layer devoid of the γ′-phase may have decreased tensile strength.
JP-A-2006-9982 discloses a high-pressure container for high-pressure hydrogen, which includes a container body and a cap hermetically disposed to one end or both ends of the container body. The container body is made of a steel and a metal film is coated on an inner surface of the container for inhibiting hydrogen from intruding. However, there is a concern that when the metal film for inhibiting hydrogen from intruding is spalled, the performance thereof is rapidly deteriorated.
As mentioned above, a material having high hydrogen embrittlement resistance has not been developed at present, and there are problems when a surface treatment or coating is applied since surface characteristics are deteriorated and rapid functional deterioration is caused by spalling of the coating.
The objective of the present invention is to provide a hydrogen embrittlement resistant piping having both hydrogen embrittlement resistance and high tensile strength, and inhibiting rapid deterioration caused by deterioration of surface characteristics and by spalling of a coating, and a method for producing the same.
A hydrogen-resistant high strength material according to the invention is made of a Ni-based alloy or an Fe—Ni-based alloy and includes an aged portion and a hydrogen embrittlement suppressing layer that is to be exposed to hydrogen. The hydrogen embrittlement suppressing layer has a hydrogen embrittlement index of not less than 0.9. The hydrogen embrittlement index is defined as a ratio of an elongation after hydrogen charging in relation to an elongation before hydrogen charging. The aged portion has a tensile strength exceeding 1000 MPa.
A method for producing a hydrogen-resistant high strength material according to the invention includes the steps of applying an aging heat treatment to an entirety of a material made of a Ni-based alloy or an Fe—Ni-based alloy to form an aged portion; and applying a solution treatment locally to the aging-heat-treated material to form a hydrogen embrittlement suppressing layer.
A method for producing a hydrogen-resistant high strength material according to the invention includes the steps of applying a solution treatment to an entirety of a material made of a Ni-based alloy or an Fe—Ni-based alloy; and after the solution treatment step, while keeping a portion where a hydrogen embrittlement suppressing layer is formed cooled, applying an aging heat treatment to other portion to form an aging-treated portion.
According to the invention, an inner layer of a hydrogen embrittlement resistant piping bears hydrogen embrittlement resistance and an outer layer thereof bears tensile strength. Accordingly, the hydrogen embrittlement resistance and high tensile strength can be obtained.
Furthermore, according to the invention, there is no concern about spelling of the coating and also strength deterioration caused by completely solution treating can be inhibited.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.
The present invention relates to a hydrogen embrittlement resistant piping (metal piping for hydrogen infrastructure) and a method for producing the same.
A γ′-phase precipitation strengthened Ni-based alloy or Fe—Ni-based alloy is used for a material of a hydrogen embrittlement resistant piping of the invention. In this case, the Ni-based alloy or Fe—Ni-based alloy has a face-centered cubic lattice structure (referred to as FCC). Here, the γ′-phase is an L12 ordered phase that precipitates in a γ-matrix of FCC after the alloy is subjected to the aging heat treatment. That is, the γ′-phase is an FCC ordered phase finely precipitated in a γ-phase having an FCC solid solution phase as a matrix.
A hydrogen embrittlement resistant piping is simply referred to as a piping in some cases.
According to a study of the inventors, tensile tests are carried out according to a thermal precharging method in high-pressure hydrogen gas with the γ′-phase precipitation strengthened Ni-based alloy. While the ductility was remarkably deteriorated in a material having high strength by a predetermined heat treatment, the ductility was remarkably inhibited from deteriorating when an amount of precipitated γ′-phase was reduced by applying the aging heat treatment at a temperature higher than the predetermined heat treatment. Accordingly, it was studied to apply this to a piping for high-pressure hydrogen gas.
A crack that becomes a starting point of fatigue or fracture starts from a surface of a material. The hydrogen embrittlement is considered to progress with a fine crack initially formed as a starting point. Accordingly, it is considered effective to inhibit the initial fine crack from occurring.
Accordingly, it is considered that the crack can be inhibited from generating and progressing by imparting the ductility and hydrogen embrittlement resistance only to a surface (the inside of a piping exposed to hydrogen as) to be in contact is gas.
A inner surface (inner layer) of a hydrogen embrittlement resistant piping 101 to be exposed to hydrogen is constituted of a hydrogen embrittlement suppressing layer 1 to be exposed to hydrogen and the outside (outer layer) thereof is constituted of an aging-treated phase 2. By considering applications other than the piping, in general, the aging-treated phase 2 may be referred to as an aging-treated portion.
The ductility can be maintained when an amount of the γ′-phase (precipitation amount) of the hydrogen embrittlement suppressing layer 1 is reduced to less than that of the aging-treated layer 2, as is shown in the drawing.
The amount of precipitated γ′-phase is highest in the aging-treated layer 2, that is, the outer phase in
A horizontal axis represents a precipitation amount of the γ′-phase and a vertical axis represents a hydrogen embrittlement index and a relative tensile strength. Herein, the hydrogen embrittlement index is calculated from a formula (1) shown below. The relative tensile strength is a relative value of tensile strength of each of the respective materials represented in relation to that of a solution treated material (also referred to as ST (solution treatment) material).
From the drawing, it is found that the tensile strength of the material can be heightened by increasing an amount of precipitated γ′-phase, while the hydrogen embrittlement index tends decrease.
(Hydrogen embrittlement index)=(elongation after hydrogen charging)/(elongation before hydrogen charging) (1)
Here, the elongation and the tensile strength of the were measured based on JIS Z2241.
As the formula (1) shows, the larger the numerical value of the index is, the smaller or zero a decrease of elongation is even when the hydrogen charging is performed. In other words, a material having a larger hydrogen embrittlement index is more difficult to be embrittled by hydrogen.
That is, the drawing shows that the γ′ concentration is not necessarily require to be zero.
From the drawing, it is found that the hydrogen embrittlement index has a higher value as the γ′ amount is smaller. That is, it is found that the γ′ amount has a permissible amount.
A practically usable piping desirably has the hydrogen embrittlement index shown by the formula (1) of not less than 0.9. Accordingly, when the limit γ′-amount (that is the limit value of amount of precipitated γ′-phase) where the hydrogen embrittlement index is not less than 0.9 is obtained for every materials, a range where both the tensile strength and the elongation (ductility) of the material are maintained can be set.
Furthermore, through a study of a method for producing the hydrogen embrittlement resistant piping, it is found that there are two methods.
In the first method, a material processed previously into a necessary shape is subjected to a solution treatment, by cooling to render an entirety of the piping a solution treated structure that is difficult to cause hydrogen embrittlement. Next, cooling air or cooling water is flowed inside of the piping, and, at the same time therewith, a heat treatment is externally applied, thereby an outer layer of the piping is rendered an aging-treated layer without changing the solution treated structure of an inner layer of the piping.
In the second method, an entire piping is subjected to an aging heat treatment to render the entire piping an aging-treated layer, and an inner surface of the piping is locally heated by laser to dissolve the γ′-phase to make an amount of precipitated γ′-phase smaller than that of an outer layer.
According to the two methods, a hydrogen embrittlement resistant piping having high tensile strength can be provided.
Exemplary embodiments of the invention will be specifically described below.
It is desirable that a material used in a hydrogen embrittlement resistant piping of the invention is a γ′-phase precipitation strengthening alloy and has the tensile strength exceeding 1000 MPa owing to a predetermined solution treatment and aging heat treatment of the alloy. This is because an A286 alloy can be used alternatively if the tensile strength is sufficient to be 1000 MPa or less.
Furthermore, the tensile strength is desirable to exceed 1200 MPa and more desirable to exceed 1400 MPa. It is appropriate that a hydrogen embrittlement suppressing layer is applied to an inner surface of the piping, that is directly exposed to a hydrogen gas. It is necessary that an amount of precipitated γ′-phase of the hydrogen embrittlement suppressing layer is larger than 0% and lower than a predetermined amount of the precipitated γ′-phase in the respective materials. In a completely solution treated structure (0%), the tensile strength is drastically lowered.
The upper limit of the amount of precipitated γ′-phase of the hydrogen embrittlement suppressing layer is preferably that represented by the hydrogen embrittlement index of not less than 0.9 and more preferably not less than 0.98 that shows a degree of embrittlement caused by hydrogen (that is, elongation after hydrogen charging/elongation before hydrogen charging). Here, 0.9 is a value of the hydrogen embrittlement index in A286 and this is because A786 is assumed to correspond to the lower limit of the hydrogen embrittlement index.
A thickness of the hydrogen embrittlement suppressing layer (inner layer) defined above is desirably 5 to 30% or less of a thickness of the piping. The thickness the piping is a value obtained by total thicknesses of the inner layer and the outer layer. It can be also said that the thickness of the hydrogen embrittlement suppressing layer is from 5 to 30% of an total thickness obtained by adding a thickness of an aged portion (aging-treated phase) and a thickness of the hydrogen embrittlement suppressing layer. This is because in the case where the thickness of the hydrogen embrittlement suppressing layer is less than 5%, an generated initial crack immediately reaches an outer layer where a γ′-phase precipitates. When the thickness of the hydrogen embrittlement suppressing layer exceeds 30%, the tensile strength as the piping is largely deteriorated. A range from 10 to 20% is still more preferable.
Next, a method for producing a hydrogen embrittlement resistant piping will be described below.
Firstly, a material to be used is processed into a necessary shape of a piping. The method therefor is not particularly restricted.
Then, according to predetermined processes suitable for the material, a solution treatment and an aging heat treatment are carried out.
Thereafter, a local heating device insertable into the inside of the piping such as laser is used to locally heat an inner surface of the piping to form a solid solution so that a γ′-phase having the predetermined concentration may remain. When the laser is used to form a solid solution, it is desirable to previously set laser irradiation conditions, such as an output, a focus distance or a scanning speed, to a used material to study conditions capable of forming the hydrogen embrittlement suppressing layer such as mentioned above.
Furthermore, another method of the invention for producing a piping will be described.
Firstly, a material to be used is processed into a necessary shape of a piping. The processing method is not particularly restricted. Thereafter, the material processed into a piping shape is subjected to a solution treatment, followed by rapidly cooling to impart a solution treated structure to an entire material.
Then, cooling air or cooling water as a cooling medium is flowed inside of the piping and the piping is externally heated to apply the aging heat treatment. As the cooling air, for example, compressed air is used and the cooling water is supplied by use of a chiller unit or the like.
As a heating unit, an electric furnace having capacity capable of uniformly heating a prepared piping raw material is desirable. Furthermore, electromagnetic induction heating or laser may be used.
In any of the producing methods, it is desirable to determine in advance a temperature, a flow rate of the cooling medium and an externally heating temperature while measuring temperatures of an inner surface and an outer surface of the piping.
The present invention will be described with reference to Examples.
Table 1 shows compositions of samples prepared as Examples.
As is shown in the table, γ′-phase precipitation strengthening Ni-based alloys A-1 to A-3 and Fe—Ni-based alloy B were prepared. An alloy A-0 is a material having the same composition as that of the alloy A-1 and is obtained by solution treating the alloy A-1. Furthermore, the alloy A-3 is a material having the same composition as that of JIS SUH660 (A286). Still furthermore, an alloy B is a material which is improved in heat resistance. The alloys A-1 to A-3 and the alloy B were assigned to Examples 1 to 4 respectively.
For the Ni-based alloys A-1 to A-3 (Examples 1 to 3), three kinds of alloys A-1 to A-3 were prepared by varying compositions of Al and Ti to vary amounts of precipitated γ′-phase.
The heat treatment of the Ni-based alloys A-1 to A-3 was performed at 980° C. for 2 hours and an aging heat treatment was performed at 720° C. for 8 hours.
For an Fe—Ni-based alloy B (Example 4), amounts of precipitated γ′-phase were varied by varying a final aging heat treatment temperature (referred to as the final aging temperature) in a range of 625 to 910° C. The solution treatment is performed for 2 hours and the aging time is 8 hours.
Thereafter, tensile test pieces prepared from these alloys were subjected to the hydrogen charging in a hydrogen gas at 450° C. and under 20 MPa, followed by performing the tensile test at room temperature and in atmosphere, further followed by measuring elongation and tensile strength each of the test pieces. The tensile speed at this time was set at 0.3 mm/min with use of a displacement gauge. Based on measurements, hydrogen embrittlement indices (elongation after hydrogen charging/elongation before hydrogen charging) were calculated.
The tensile strengths are represented by relative values or relative strengths where the tensile strength as solution treatment is assigned to 1.
It is found that the relative tensile strength increases depending on an amount of precipitated γ′-phase that increases strength. On the other hand, it is found that the hydrogen embrittlement index decreases depending on an amount of precipitated γ′-phase. Furthermore, it is found that the embrittlement can be suppressed when the amount of precipitated γ′-phase is lower than a predetermined value. The upper limit of the amount of precipitated γ′-phase, which can suppress the embrittlement, is about 10% for the alloy A and about 7% for the alloy B. These were defined as the limit values of amounts of precipitated γ′-phase in the respective alloys.
Piping materials having an outer diameter of 10 mm and a thickness of 1.5 mm were prepared with the alloy A-1 and the alloy B. The piping from the alloy A-1 was subjected to a solution treatment at 980° C. for 2 hours and an aging heat treatment at 720° C. for 8 hours. The piping from the alloy B was subjected to a solution treatment at 980° C. for 2 hours and an aging heat treatment at 740° C. for 8 hours. When an aging heat treatment is applied at 740° C. for 8 hours, the tensile strength of 1300 MPa class can be imparted for those subjected to an ordinary heat treatment.
Each of these pipings was heat-treated by irradiating laser on an inner surface of the piping. The piping of which surface was heat-treated by irradiating laser was sliced into rings to observe a cross-section thereof, and an amount of precipitated γ′-phase was calculated by image analysis.
As the results, an amount of precipitated γ′-phase was 9.6% by average at a position about 300 μm from an inner surface (referred to as “inside surface”) in a piping of the alloy A-1, and an amount of precipitated γ′-phase was 1.1% by average in the vicinity of the inner surface of the piping. On an outer surface (referred to as “outside surface”) of the piping, and an amount of precipitated γ′-phase was 12% that is substantially equal to a value obtained by thermodynamic equilibrium calculation.
Similarly, an amount of precipitated γ′-phase was 6.2% by average at a position about 300 μm from the inner surface of a piping of the alloy B, and an amount of precipitated γ′-phase was 0.9% by average in the vicinity of the inner surface of the piping. On an outside surface of the piping, an amount of precipitated γ′-phase was about 20% that is substantially equal to a value obtained by thermodynamic equilibrium calculation.
Piping materials having an outer diameter of 10 mm and a thickness of 1.5 mm were prepared from the alloy A-1 and the alloy B.
Both of the alloy A-1 and the alloy B were solution treated at 980° C. for 2 hours and thereafter cooled with water to maintain the solution-treated state.
Thereafter, the aging heat treatment was performed with cooling the inside of the piping with water as a cooling medium. A piping from the alloy A-1 was subjected to the aging heat treatment in an electric furnace at 730° C. for 8 hours. A piping from the alloy B was subjected to the aging heat treatment in an electric furnace at 755° C. for 8 hours. Thermocouples were attached to inner and outer surfaces of the pipings of both materials, and surface temperatures during the heat treatment were monitored.
As the results, it was found that inner surface temperature, of the alloys A-1 and B were about 583° C. and about 609° C. respectively, and outer surface temperatures thereof were 720° C. and 740° C. respectively.
After the heat treatments, each of the pipings was sliced in rings to observing a structure thereof, followed by calculating amounts of precipitated γ′-phase by image analysis.
As the results, an amount of precipitated γ′-phase was 9% by average at a position about 400 μm from an inner surface of the piping of the alloy A-1, and an amount of precipitated γ′-phase was 1.5% by average in the vicinity of the inner surface of the piping. On an outer surface of the piping, an amount of precipitated γ′-phase was 12%. This is substantially same value as that obtained by thermodynamic equilibrium calculation.
On the other hand, an amount of precipitated γ′ phase was 6% by average at a position about 350 μm from an inner surface of the piping of the alloy B, and an amount of precipitated γ′-phase was 2% by average in the vicinity of the inner surface of the piping. On an outside surface of the piping, an amount of precipitated γ′-phase was about 20%. This is substantially same value as that obtained by thermodynamic equilibrium calculation.
As mentioned above, the heat treatments of the pipings were mainly described. However, a produced hydrogen-resistant high strength material is not restricted thereto and a kind of a product is not questioned as long as a hydrogen embrittlement suppressing layer is formed in a portion that is exposed to hydrogen and an aging-treated phase is formed in other portion.
A range of application of the hydrogen embrittlement resistant technology of the invention is not restricted to hydrogen embrittlement resistant pipings. The technology can be applied also to hydrogen producing equipments (reactors) at hydrogen stations, holders (hydrogen tanks (high-pressure containers)), compressors, accumulators, hydrogen dispensers (hydrogen supply equipments), high-pressure hydrogen flowmeter for hydrogen dispenser (such as Coriolis flowmeter), hydrogen vehicles and so on.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-021195 | Feb 2009 | JP | national |
