METHOD FOR PRODUCING A PRESSURE VESSEL

Abstract

The present invention relates to a method of manufacturing a pressure vessel.

Description

TECHNICAL FIELD

The invention relates to a method of manufacturing a pressure vessel.

BACKGROUND ART

For reasons of cost and because of weight restrictions, pressures in pressure vessels in vehicle construction are becoming ever greater. FRP hybrid vessels are an option for such pressures, these consisting of a multilayer material having a gas-tight inner layer of a stainless steel and an outer layer of a carbon steel, where the multilayer material composed of steel is pressure rolled to the corresponding shape from a blank or pipe and then ensheathed with a CFRP laminate; cf. DE 10 2014 101 972 B4. The costs for this type of pressure vessel are very high. In addition, DE 10 2015 113 869 A1 discloses flow forming of a rotationally symmetric shaped article composed of at least two blanks made of different materials, with intermetallic bonding between the different materials before or during the flow forming.

Additionally known is flow forming of monolithic vessels made of a tubular stainless steel into appropriate shape; cf. WO 2021/040133 A1. Vessels of this kind are also costly because of the material used.

SUMMARY OF INVENTION

It is thus an object of the invention to specify a method of producing a pressure vessel that meets the demands made and can be manufactured with favorable materials and at more favorable manufacturing costs.

This object is achieved by a method of producing a pressure vessel having the features of claim 1.

The method of manufacturing a pressure vessel having a base at one end of the pressure vessel, a wall section, and a neck section which is at the opposite end of the pressure vessel from the base and has an opening comprises the following steps: —providing at least a first blank, where the first blank consists of a carbon steel; —creating the wall section from the at least one blank by flow forming to give a pressure vessel preform; —creating the neck section from the pressure vessel preform by swivel forming to give a pressure vessel. According to the invention, the pressure vessel, after the swivel forming, is heated at least partly to a temperature Ac1 at which the microstructure of the carbon steel is transformed at least partly to austenite and then cooled down at least in sections by active cooling such that the microstructure is at least partly transformed to martensite and/or bainite, and hence a tensile strength R_m of at least 1000 MPa is established at least in sections of the carbon steel of the pressure vessel.

A carbon steel, which is more preferably hardenable and/or heat-treatable in order to provide corresponding strengths in the carbon steel or in the finished pressure vessel and hence to meet the demands made, is relatively inexpensive compared to the materials disclosed in the prior art and correspondingly inexpensive to process.

Depending on the composition of the carbon steel, the tensile strength R_m can be adjusted individually, i.e. by suitable choice of the carbon steel, such that a tensile strength R_m especially of at least 1100 MPa, preferably of at least 1200 MPa, more preferably of at least 1300 MPa, especially preferably of at least 1400 MPa and further preferably of at least 1900 MPa is also possible at least in sections. Favorable unalloyed carbon steels that are hardenable and/or heat-treatable are, for example, C qualities, for example C15, C22, C45 etc., or low-alloyed steels, especially manganese-boron steels, for example 22MnB5, 37MnB4, 39MnCrB6-2, 40MnB4 etc.

Flow forming is understood to mean a method of shaping, without removal of material, of rotationally symmetric hollow bodies. This involves clamping and/or fixing a blank on a mandrel and setting it in rotation. At least one pressure disk/roll or another corresponding means is moved against the rotating blank, such that forming is effected partially by compressive stresses that are introduced into the blank material via the radial flow forming. The material flows and adopts the outline of the internal mandrel from one end to the other of the blank in an axial processing run. The mandrel is basically circular, such that the “flow-formed” pressure vessel preform takes on a circular cylindrical internal geometry. In the flow forming operation, the at least one pressure disk/roll is plastically deformed as a result of the force acting directly on the material, a defined axial movement of the at least one pressure disk/roll can have the effect that the starting wall thickness of the blank is reduced to an adjustable (end wall) thickness or minimum thickness. Flow forming is prior art.

In swivel forming, the pressure vessel preform is set in rotation and a pressure disk/roll acts on the open end of the pressure vessel preform at the opposite end from the base in such a way that the neck section is shaped to the corresponding form, in particular without a mandrel. The opening in the neck section which is required for the pressure vessel can be introduced in the course of swivel forming or subsequently after swivel forming. Swivel forming is also prior art.

The microstructural transformation to austenite commences at Ac1, and the microstructure is essentially completely austenitic with attainment of Ac3 or above. After the heating, the hot (partly) austenitized carbon steel of the pressure vessel is actively cooled by suitable means such that the microstructure is transformed to a microstructure composed of martensite and/or bainite. This can be effected, for example, in a corresponding mold or in an oil bath. Heating and cooling curves for adjustment of the required microstructure are dependent on the chemical composition of the hardenable and/or heat-treatable carbon steel used and can be inferred or derived from what are called TTA or TTT diagrams. An essentially martensitic microstructure can thus achieve the highest (tensile) strength of the carbon steel used.

The thickness of the first blank may, for example, be between 6 and 16 mm. The thickness is especially at least 6.5 mm, preferably at least 7 mm and, and is especially limited to a maximum of 15 mm, preferably a maximum of 14 mm. The diameter of the blank may vary according to the size of the pressure vessel to be manufactured, especially between 150 and 800 mm.

Depending on the execution, the carbon steel, at least in the base and in the wall section of the pressure vessel, may have a tensile strength R_m of at least 1000 MPa, especially of at least 1100 MPa, preferably of at least 1200 MPa, more preferably of at least 1300 MPa, especially preferably of at least 1400 MPa, further preferably of at least 1900 MPa. In a preferred configuration, the carbon steel in the pressure vessel has a tensile strength R_m throughout of at least 1000 MPa, especially of at least 1100 MPa, preferably of at least 1200 MPa, more preferably of at least 1300 MPa, especially preferably of at least 1400 MPa, further preferably of at least 1900 MPa, in order to provide uniform characteristics throughout.

In particular, the carbon steel of the pressure vessel, in the regions having a tensile strength R_m of at least 1000 MPa, especially of at least 1100 MPa, preferably of at least 1200 MPa, more preferably of at least 1300 MPa, especially preferably of at least 1400 MPa, further preferably of at least 1900 MPa, has a microstructure of martensite and/or bainite. In order to establish the desired property in the pressure vessel, a hard microstructure in the carbon steel is thus required, comprising at least 70% martensite and/or bainite, especially at least 80% comprising martensite and/or bainite, preferably at least 90% comprising martensite and/or bainite, where remaining microstructure constituents in the form of ferrite, perlite, cementite, austenite and/or residual austenite may be present. Preference is given to a hard microstructure having at least 70% martensite, especially at least 80% martensite, preferably at least 90% martensite, where remaining microstructure constituents in the form of ferrite, perlite, bainite, cementite, austenite and/or residual austenite may be present.

Further advantageous configurations and developments will be apparent from the description that follows. One or more features from the claims, the description or else the drawing may be combined with one or more other features therefrom to give further configurations of the invention. It is also possible to combine one or more features from the independent claims with one or more other features.

If the base of the finished pressure vessel is not to be planar, in one configuration, the creation of the pressure vessel preform is preceded by forming of a base in the at least first blank in a deep drawing step. The base in the finished pressure vessel may be shaped outward, such that the deep drawing step provides for convex shaping of the base, especially in the middle, in the blank, or, alternatively, if permitted by the later installation space, the base in the finished pressure vessel may be shaped inward, such that the deep drawing step provides for concave shaping of the base, especially in the middle, in the blank. In both cases, the shaping of the base can especially serve as fixing on the mandrel, by comparison with a planar execution. The deep drawing step can be effected in the cold state or alternatively also in the hot state.

In order to reduce the flow resistance of the carbon steel and hence the forces in the flow forming and/or swivel forming, in one configuration, active heating is conducted before and/or during the creation of the pressure vessel preform. Alternatively or additionally, active heating is conducted before and/or during the creation of the neck section. The active heating is effected at least in some regions, meaning that at least the regions that (still) have to be formed are heated. Alternatively, the blank may be heated in its entirety before the creation of the pressure vessel preform or, for example, only the region of the wall section to be completed may be heated. The heating can thus also be effected by way of assistance during the creation of the pressure vessel preform. In addition, it is also possible to heat only the region of the neck section to be completed before the swivel forming and optionally also to heat it by way of assistance during the swivel forming.

The active heating is effected especially at a temperature of at least 300° C., meaning that the carbon steel is heated to that temperature. The temperature in the active heating is in particular 400 to 1100° C., preferably 700 to 1100° C. Means of heating used may include furnaces through which the corresponding forms (blank, pressure vessel preform) are conducted and then sent to the appropriate step (optional thermoforming, flow forming and/or swivel forming). Alternatively and preferably, means such as inductor(s), for example, which may be designed in order solely to selectively heat particular regions, or (a) burner(s) with an open flame may be used. Inductors and also burners may be integrated in the corresponding apparatuses for conduction of flow forming and/or swivel forming in order to enable heating in situ either before and/or during the performance of the respective step.

In one configuration of the method of the invention, the carbon steel, as well as Fe and unavoidable impurities from the production, contains the following chemical elements in % by weight:

C:  0.01 to 0.7%,
Si: 0.01 to 3.0%,
Mn: 0.01 to 3.0%,
N:  up to 0.1%,
P:  up to 0.1%,
S:  up to 0.1%,

• • optionally at least one or more than one element from the group of (Al, Cr, Cu, Mo, Ni, Nb, Ti, V, B, Sn, Ca, REM):

AI:  up to 1.0%,
Cr:  up to 1.0%,
Cu:  up to 1.0%,
Mo:  up to 1.0%,
Ni:  up to 1.0%,
Nb:  up to 0.2%,
Ti:  up to 0.2%,
V:   up to 0.2%,
B:   up to 0.01%,
Sn:  up to 0.1%,
Ca:  up to 0.1%,
REM: up to 0.2%.

In one configuration of the method of the invention, a second blank is provided, where the second blank consists of an austenitic steel. Austenitic steels, especially CrNi steels, have the advantage that they are impervious to gases, especially atomic hydrogen, and hence effectively have a barrier effect and are preferably of particularly good suitability as inner layer of a pressure vessel. Furthermore, austenitic steels are thermally stable, meaning that they do not undergo any changes and retain their properties in the course of heat treatment of the carbon steel of the pressure vessel in order to establish the required properties.

The thickness of the second blank is less than that of the first blank and may be between 0.2 and 4 mm. The thickness is especially at least 0.3 mm, preferably at least 0.5 mm, and is especially limited to at most 3.5 mm, preferably at most 3 mm. The diameter of the blank may vary depending on the size of the pressure vessel to be manufactured, especially between 150 and 800 mm.

In one configuration of the method of the invention, the austenitic steel, as well as, Fe and unavoidable impurities from the production, contains the following chemical elements in % by weight:

Cr: 11.0 to 22.0%,
Ni: 5.0 to 15.0%,
C:  up to 0.2%,
Si: up to 1.5%,
Mn: up to 3.0%,
N:  up to 0.2%,
P:  up to 0.1%,
S:  up to 0.1%.

Alternatively, the austenitic steel, as well as Fe and unavoidable impurities from the production, may contain the following chemical elements in % by weight:

C:  up to 0.6%, especially 0.1 to 0.6%,
Si: up to 1.5%,
Mn: 4.0 to 25.0%, especially 10.0 to 25.0%,
N:  up to 0.2%,
P:  up to 0.1%,
S:  up to 0.1%,

• • optionally at least one or more than one element from the group of (Al, Cr, Cu, Mo, Ni, Nb, Ti, V, B, Sn, Ca):

AI: up to 3.0%,
Cr: up to 4.0%,
Cu: up to 1.0%,
Mo: up to 1.0%,
Ni: up to 2.0%,
Nb: up to 0.5%,
Ti: up to 0.5%,
V:  up to 0.5%,
B:  up to 0.01%,
Sn: up to 0.1%,
Ca: up to 0.1%.

The Mn-containing steel, including medium-manganese steel with Mn contents between 4% and 14% by weight or high-manganese steel with Mn contents between >14% and 25% by weight, as supplied, has an austenitic microstructure. After heat treatment in the course of production of the pressure vessel, there may indeed also be constituents of martensite, annealed martensite and/or ferrite in the microstructure, and a residue of residual austenite and unavoidable impurities.

In one configuration of the method of the invention, the second blank is provided simultaneously with the first blank, the creating of the wall section from the two blanks is performed by flow forming to give a pressure vessel preform, and then the neck section is created from the pressure vessel preform by swivel forming to give a pressure vessel. The providing of the two blanks has the benefit that it is possible to manufacture a pressure vessel with two layers in one process, although it has to be assured that the two blanks are arranged in such a way that, in the finished state, the austenitic steel forms the inner layer and the carbon steel the outer layer of the pressure vessel.

Alternatively, the second blank can be provided separately, in which case the second blank is used to create a wall section by flow forming to give a pressure vessel preform, wherein the external diameter of the pressure vessel preform made from the second blank is equal to or less than the internal diameter of the pressure vessel preform produced by flow forming from the first blank, wherein the pressure vessel preform made from the second blank is then introduced into the pressure vessel preform made from the first blank before the neck section is created from the pressure vessel preforms by swivel forming to give a pressure vessel. Other alternatives to flow forming for production of a pressure vessel preform from the second blank could also be deep drawing or an active media-based forming operation.

As well as the hardening, there may also be heat treatment of the at least partly and preferably fully hardened carbon steel of the pressure vessel in the course of annealing. The heat treatment is effected at temperatures between 200 and 500° C. for a period between 5 s and 30 min, which is associated with a reduction in tensile strength but with a rise in ductility. The heat-treated carbon steel of the pressure vessel, in the martensitic microstructure, includes at least one third and especially at least one half annealed martensite.

In a further teaching of the invention, the pressure vessel produced by the method of the invention is used to store pressurized fluids in mobile applications. Pressurized fluids are considered to be gases or liquids having a pressure of more than 200 bar that serve as energy source to drive a vehicle and have to be safely accommodated and stored accordingly in the vehicle. For example, the gas is hydrogen for hydrogen-driven vehicles or liquefied gas (LPG) as alternative fuel for internal combustion engines.

BRIEF DESCRIPTION OF DRAWINGS

There follows a detailed elucidation of the invention with reference to drawings. Identical parts are given the same reference numerals. The individual figures show:

FIG. 1 a schematic perspective diagram showing provision of a blank,

FIG. 2 a schematic perspective diagram showing provision of a blank with a base,

FIG. 3 a schematic perspective diagram showing heating of the blank prior to the creation of the pressure vessel preform,

FIG. 4 a schematic perspective diagram showing creation of the pressure vessel preform at different times,

FIG. 5 a schematic perspective partial diagram showing creation of the pressure vessel preform at different times from two blanks,

FIG. 6 a schematic perspective diagram showing combination of two separately produced pressure vessel preforms and

FIG. 7 a schematic side view of a finished pressure vessel.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a schematic perspective diagram showing provision of a first blank (1). The thickness of the blank (1) may, for example, be between 6 and 16 mm. Depending on the size of the pressure vessel (10) to be finished, the diameter of the blank may vary between 150 and 800 mm. The blank (1) consists of a carbon steel which is hardenable and/or heat-treatable. Examples include steels of C22 or C45 quality, but also manganese-boron steels, for example 22MnB5, 37MnB4.

FIG. 2 shows a schematic perspective diagram showing provision of a first blank (1) with a base (2). The step in FIG. 2 is optional if the finished pressure vessel (10) is not to have a planar base (2). For instance, prior to the creation of the pressure vessel preform (cf. FIG. 4), a base (2) may be formed into the blank (1) in a deep drawing step, said base (2) pointing outward in the finished pressure vessel (10) (cf. FIG. 7), or, alternatively and not shown here, if the construction space does not permit this, pointing inward in the finished pressure vessel. The deep drawing step for optional forming of the base (2) can be effected in the cold state of the blank (1), or else alternatively in the hot state, at least in the hot state in the region of the base (2) to be created, of the blank (1).

After the optional deep drawing step for creation of the base (2), FIG. 3 shows a schematic perspective diagram showing heating of the first blank (1) prior to the creation of the pressure vessel preform. The active heating can be effected here at least in some regions, such that at least the regions that still have to be shaped are heated. FIG. 3 shows the example of an inductor that heats only the region of the wall section (3) to be completed. In an alternative which is not shown here, the blank (1) may also be heated in full, in the furnace, by means of inductors or by means of burners.

FIG. 4 shows a schematic perspective diagram showing creation of the pressure vessel preform at different times. The optional deep drawing has the advantage that the correspondingly manufactured base (2) that has especially been created in the middle of the blank (1) can serve as fixing on the mandrel. The heating, or heating of subregions, need not necessarily be effected outside the apparatus for flow forming, but can also be effected within the apparatus before and/or during the creation of the pressure vessel preform. The heating is effected at a temperature of at least 300° C., where the blank (1) is heated at least in subregions, preferably to a temperature between 400 and 800° C. A pressure disk/roll, as shown schematically in FIG. 4, acts on the blank (1) fixed on the mandrel, and the flow forming creates a pressure vessel preform open to one side. After the flow forming, the base (2) and at least the main part of the wall section (3) have been completed.

If a second layer, especially an inner layer, is advisable, for example in the case of use of the pressure vessel (10) with hydrogen, a second blank (1.1) of an austenitic steel, especially a medium-Mn or high-Mn steel or preferably a CrNi steel, may be provided separately, in which case the second blank (1.1) is used to create a wall section (3.1), preferably by flow forming, to give a pressure vessel preform. The steps may be conducted analogously to the production of the pressure vessel preform from the first blank (1), in accordance with the steps as shown in FIGS. 1 to 4. It is optionally possible to shape a base (2.1) into the second blank (1.1) (cf. FIG. 2), and the creation of the pressure vessel preform may also be preceded by heating of the second blank (1.1) (cf. FIG. 3). In the flow forming of the individual pressure vessel preforms, it should be ensured that the external diameter (Da) of the pressure vessel preform made from the second blank (1.1) is equal to or less than the internal diameter (di) of the pressure vessel preform created by flow forming from the first blank (1), such that the pressure vessel preform made from the second blank (1.1) can be introduced into the pressure vessel preform made from the first blank (1) (see FIG. 6) before the neck section (4) is created from the pressure vessel preforms by swivel forming to give a pressure vessel (10).

Alternatively, the second blank (1.1) may be provided simultaneously with the first blank (1), and the creating of the wall section (3) from the two blanks (1, 1.1) can be conducted by flow forming to give a pressure vessel preform. FIG. 5 shows a schematic perspective part-diagram showing creation of the pressure vessel preform at different times from the two blanks (1, 1.1). The two blanks (1, 1.1) are arranged such that, in the finished state, the austenitic steel forms the inner layer and the carbon steel the outer layer of the pressure vessel (10).

In a step which is not shown, the neck section (4) is formed from the pressure vessel preform by swivel forming to give a pressure vessel (10). For example, this section can be performed in a swivel forming apparatus. Preferably, before and/or during the swivel forming, at least the neck section (4) to be manufactured is heated, preferably to a temperature between 700 and 1100° C., with introduction of an opening (5) in the course of or subsequent to the swivel forming (cf. FIG. 7).

After the swivel forming, the pressure vessel (10) is at least partly heated to a temperature of Ac1, at which the microstructure of the carbon steel is at least partly converted to austenite, and then is cooled down at least in sections by active cooling in such a way that the microstructure is converted at least partly to martensite and/or bainite, and hence, at least in sections, a tensile strength R_m of at least 1000 MPa is established in the carbon steel of the pressure vessel (10). The pressure vessel (10) is preferably heated completely at least to a temperature of Ac3 and completely cooled actively such that the homogeneous microstructure composed of essentially martensite having a tensile strength of at least 100 MPa, especially of at least 1100 MPa, preferably of at least 1200 MPa, more preferably of at least 1300 MPa, especially preferably of at least 1400 MPa, further preferably of at least 1900 MPa, is established throughout the carbon steel of the pressure vessel (10).

A final heat treatment may be conducted to increase ductility in the carbon steel of the pressure vessel (10).

The pressure vessel (10) may thus consist of a single-layer carbon steel or, if hydrogen is to be used as gas, of two individual layers composed of an outer layer of carbon steel and an inner layer of austenitic steel, preferably CrNi steel.

Claims

1. A method of manufacturing a pressure vessel having a base at one end of the pressure vessel, a wall section, and a neck section which is at the opposite end of the pressure vessel from the base and has an opening, wherein the method comprises the following steps: wherein the pressure vessel, after the swivel forming, is heated at least partly to a temperature Ac1 at which the microstructure of the carbon steel is transformed at least partly to austenite and then cooled down at least in sections by active cooling such that the microstructure is at least partly transformed to martensite and/or bainite, and hence a tensile strength R m of at least 1000 MPa is established at least in sections of the carbon steel of the pressure vessel.

providing at least a first blank, where the first blank consists of a carbon steel;

creating the wall section from the at least first blank by flow forming to give a pressure vessel preform;

creating the neck section from the pressure vessel preform by swivel forming to give a pressure vessel;

2. The method as claimed in claim 1, wherein the creation of the pressure vessel preform is preceded by forming of a base in the at least first blank in a deep drawing step.

3. The method as claimed in claim 2, wherein active heating is conducted at least in some regions at least one of before and during the creation of the pressure vessel preform and/or of the neck section.

4. The method as claimed in claim 3, wherein the active heating is conducted at a temperature of at least 300° C.

5. The method as claimed in claim 3, wherein the carbon steel, as well as Fe and unavoidable impurities from the production, contains the following chemical elements in % by weight: C: 0.01 to 0.7%, Si: 0.01 to 3.0%, Mn: 0.01 to 3.0%, N: up to 0.1%, P: up to 0.1%, S: up to 0.1%;

6. The method as claimed in claim 5, wherein a second blank consisting of an austenitic steel is provided.

7. The method as claimed in claim 6, wherein the austenitic steel, as well as Fe and unavoidable impurities from the production, contains the following chemical elements in % by weight: Cr: 11.0 to 22.0%, Ni: 5.0 to 15.0%, C: up to 0.2%, Si: up to 1.5%, Mn: up to 3.0%, N: up to 0.2%, P: up to 0.1%, S: up to 0.1%.

8. The method as claimed in claim 6, wherein the austenitic steel, as well as Fe and unavoidable impurities from the production, contains the following chemical elements in % by weight: C: up to 0.6%, Si: up to 1.5%, Mn: 4.0 to 25.0%, N: up to 0.2%, P: up to 0.1%, S: up to 0.1%

9. The method as claimed in claim 8, wherein the second blank is provided simultaneously with the first blank and the creating of the wall section from the two blanks is performed by flow forming to give a pressure vessel preform, and then the neck section is created from the pressure vessel preform by swivel forming to give a pressure vessel.

10. The method as claimed in claim 8, wherein the second blank is provided separately, wherein the second blank is used to create a wall section by flow forming to give a pressure vessel preform, wherein the external diameter (Da) of the pressure vessel preform made from the second blank is equal to or less than the internal diameter (di) of the pressure vessel preform produced by flow forming from the first blank, wherein the pressure vessel preform made from the second blank is then introduced into the pressure vessel preform made from the first blank before the neck section is created from the pressure vessel preforms by swivel forming to give a pressure vessel.

11. The use of a pressure vessel produced as claimed in claim 10 for storage of pressurized fluids in mobile applications.

12. The method of claim 5 wherein the carbon steel, as well as Fe and unavoidable impurities from the production, contains the following chemical elements in % by weight: Al: up to 1.0%, Cr: up to 1.0%, Cu: up to 1.0%, Mo: up to 1.0%, Ni: up to 1.0%, Nb: up to 0.2%, Ti: up to 0.2%, V: up to 0.2%, B: up to 0.01%, Sn: up to 0.1%, Ca: up to 0.1%, REM: up to 0.2%.

at least one or more than one element from the group of (Al, Cr, Cu, Mo, Ni, Nb, Ti, V, B, Sn, Ca, REM):

13. The method as claimed in claim 8, wherein the austenitic steel, as well as Fe and unavoidable impurities from the production, contains the following chemical elements in % by weight: Al: up to 3.0%, Cr: up to 4.0%, Cu: up to 1.0%, Mo: up to 1.0%, Ni: up to 2.0%, Nb: up to 0.5%, Ti: up to 0.5%, V: up to 0.5%, B: up to 0.01%, Sn: up to 0.1%, Ca: up to 0.1%.

at least one or more than one element from the group of (Al, Cr, Cu, Mo, Ni, Nb, Ti, V, B, Sn, Ca):