Gas cylinder fabrication

Abstract: A method of producing a cylindrical shell (4), the method comprising providing a billet (1) of circular, transverse cross-section; the billet being formed of first and second sections (10,12), the first section (10) formed of steel and having an end portion (14) and a recess (16) defined within the end portion and the second section (12) formed of a liner insert (24) material shaped to nest within the recess of the end portion of the first section; and billet piercing the billet into the cylindrical shell so that the first section produces an outer cylindrical form and the second section produces a liner insert for the cylindrical form (22).

Description: This invention relates to a method of producing a cylindrical shell used for fabricating a gas cylinder to contain a gas. More particularly, the invention relates to such a method in which a billet of circular, transverse cross-section is used to form the cylindrical shell by billet piercing. Even more particularly, the invention relates to such a method in which the billet is formed of a first section of steel and a second section of liner material so that the cylindrical shell has an outer cylindrical form made of steel and an inner liner insert formed of the liner insert material. Gas cylinders are widely used in various industries for storing gases. The storage of ultra-high purity gases used in the semiconductor industry is particularly problematical due their corrosive nature. Such corrosion can produce particulate contamination that in turn can produce unacceptable manufacturing defects. For instance, corrosive etching gases such as hydrogen chloride can corrode steel cylinders to produce particulate contaminants. If the resultant particulate material is drawn into a stage of the semiconductor manufacturing process, the product of such a stage might be ruined. Thus, gas cylinders have been specifically designed to maintain the purity of the gas by being fabricated of nickel. As may be appreciated, nickel gas cylinders are prohibitively expensive. Additionally, pure nickel cylinders generally cannot be used where the intended service pressure exceeds 35.15 kg/cm 2 . As a result, gas cylinders for high purity gas storage applications are formed with an outer layer of steel for structural integrity and an inner nickel plating for corrosion resistance. As has been indicated in our US Patent Specification 5,330,091, the electroplating of a cylindrical shell of steel with nickel is not a recommended technique for fabricating gas cylinders intended for high purity storage applications because the plating can contain voids or cracks which can trap corrosion products of steel. Therefore, this earlier specification disclosed that circular nickel and steel layers were bonded together by roll bonding or explosive cladding. The resultant two layer circular form is then used as a blank for a cold drawing process to produce the cylindrical shell used in forming the gas cylinder. In a cold drawing process, the blank is formed into a cup-like form with a mandrel and the cup-like form is then extruded by the mandrel, at room temperature, through a series of dies. The drawback of this earlier process is that it has not been found to be easily amenable toward the production of large gas cylinders. The invention is concerned with the provision of a method of forming a seamless, steel cylindrical shell having a corrosion resistant lining that can be used to produce larger gas cylinder sizes than are obtainable by cold drawing production techniques. WO 96 11757 teaches a method of forming a closed-end vessel by backward extrusion. The vessel having a thin (<0.1mm) lining material to enable a superior surface finish to be achieved. US3 648 351 teaches a method for impact extruding multi-metal cans. In accordance with the invention, there is provided a method of producing a corrosion resistant, steel, cylindrical shell for use as a gas cylinder, the method comprising providing a billet of circular, transverse cross-section; the billet being formed of first and second sections, the first section formed of steel and having an end portion and a recess defined within the end portion and the second section formed of a corrosion resistant liner insert material shaped to nest within the recess of the end portion of the first section; and billet piercing the billet into the cylindrical shell so that the first section produces an outer cylindrical form and the second section produces a liner insert for the cylindrical form. The recess may have a conical side wall and the second section therefore can be a frustum of a cone. In any method in accordance with the invention, the liner insert material may be nickel. The liner insert may also be Hastalloy (Trade Mark) C-22, tantalum, titanium, gold or platinum. Billet piercing, as used herein and in the claims, refers to a known method used in forming extruded cylindrical shells. In billet piercing, a billet, such as a billet in accordance with the invention, is heated to a temperature of between about 1093°C and about 1204°C. In a subsequent cupping operation, the heated billet is then pierced with a mandrel to form a cup. While still hot, the cup is further extruded through a series of dies by pressure of the mandrel The end result of the multiple extrusions is the cylindrical shell. The cylindrical shell is finished to form a gas cylinder by spinning the end of the shell into shoulder and neck regions. The cylinder is then thermally treated and then quenched and tempered. The billet piercing operation can be contrasted with older cold drawing methods in which disk-shaped plates containing layers of steel and nickel are drawn through dies at room temperature. Again, the problem with drawing is that the finished gas cylinder size is limited to about 21 litres. Larger, 43 litre gas cylinders cannot be cold drawn economically. In attempting simply to form a billet in two sections, steel and nickel, akin to the circular blank used in a cold deep drawing process resulting in a cylindrical shell that could be spun into a gas cylinder, it has been found that the problem with forming a cylindrical shell in such a manner is based on the thickness of nickel in the cylinder wall dramatically increasing towards the top of the cylindrical shell while the thickness of steel decreases. The reason for this is that the nickel or other liner insert materials during the piercing operation will flow faster than the steel. It is the steel, however, that adds sufficient structural integrity to the finished gas cylinder to allow for pressurisation. It has been found that nesting the nickel within the steel billet in accordance with the invention provides a greater uniformity of steel and nickel thickness so as to allow the cylindrical shell to be used for its intended purpose. For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a billet used in carrying out a method of the invention; Figure 2 is a cross-sectional view of the billet shown in Figure 1 after completion a cupping operation; Figure 3 is a cross-sectional view of a cylindrical shell extruded from the billet shown in Figure 1; and Figure 4 is a graph of nickel and steel thickness versus cylindrical shell length of the cylindrical shell shown in Figure 3. With reference to Figure 1, a billet 1 for carrying out a method of the invention is illustrated. The billet 1 has a circular, transfer-cross-section and is formed of first and second sections 10 and 20. The section 10 is fabricated from type 4130 steel and has an end portion 14 provided with a recess 16 defined within the end portion 14. A second section 12 is formed of a liner insert material which is shaped to nest within the recess 16 of the end portion 14. In gas cylinder used to retain speciality gases, the liner insert material is a corrosive resistant nickel or nickel alloy. Liner insert materials of Hastalloy (Trade Mark) C-22, tantalum, titanium, gold, or platinum are possible. As illustrated, the recess 16 has a conical side wall and thus the second section 12 is a frustum of a cone to nest within the recess 16. Other shapes are possible, such as hemispherical shapes. A series of billet dimensions were modelled using finite element techniques. Figures 2 to 4 represent the results of modelling the billet 1 with a height of about 22.86 cm and a diameter of about 20.32 cm. The second layer 12 was modelled as nickel with a thickness of about 5.08 cm, a top surface diameter of about 17.78 cm and a bottom surface diameter of about 15.24 cm. With specific reference to Figure 2, the billet 1 has been pierced by a mandrel to produce a cup-like form 3. The cup-like form 3 has an inner layer of nickel 18 derived from the liner insert material 12 and an outer portion 20 that is derived from the first section 10 of steel. With reference to Figures 3 and 4, a cylindrical shell 4 has been formed from a cup-like form 3 with an outer cylindrical form 22 that has been derived from an outer portion 20 of the cup-like form 3 and a liner insert 24 derived from the inner layer of nickel 18 thereof. As illustrated in Figure 4, although the nickel thickness increases toward the top of cylindrical shell 4, the steel retains a minimum transverse thickness that is greater than the minimum allowable wall thickness for a 141.7 kg/cm 2 cylinder under applicable Department of Transportation regulations of the United States. In Figure 4, the minimum transverse allowable wall thickness is shown by the dashed line and the length of the cylindrical shell 4 is measured from the closed to the open end or from bottom to top as viewed in Figure 4. Various billet shapes were modelled. For instance, billets having about a 17.78 cm diameter top surface and about a 10.16 cm diameter bottom surface and billets having about a 15.24 cm diameter top surface and about a 10.16 cm bottom surface. In all cases, the diameter of the steel remained at about 20.32 cm. The modelling indicated that decreasing the diameter of the bottom surface, for instance, from about 15.24 cm to about 10.16 cm, without changing the top surface diameter had only a modest effect on layer uniformity. Reducing the diameter on the bottom surface produced slightly more uniform nickel and steel layers. Reducing the diameter on the top surface of the nickel from about 17.78 cm to about 15.24 cm had a much greater effect on layer uniformity.

Claims: A method of producing a corrosion resistant, steel, cylindrical shell (4) for use as a gas container, the method comprising: providing a billet (1) of circular, transverse cross-section; the billet (1) being formed of first (10) and second (12) sections, the first section (10) formed of steel and having an end portion (14) and a recess (16) defined within the end portion (14) and the second section (12) formed of a corrosion resistant liner insert material shaped to nest within the recess (16) of the end portion (14) of the first section (10); and billet piercing the billet (1) into the cylindrical shell (4) so that the first section (10) produces an outer cylindrical form (22) and the second section (12) produces a liner insert (24) for the cylindrical form. A method according to Claim 1, in which the recess (16) has a conical sidewall and the second section (12) is a frustum of a cone. A method according to Claim 1 or Claim 2, in which the liner insert material is nickel or a nickel alloy. A method according to any preceding claim, in which the liner insert material is Hastalloy C-22, tantalum, titanium, gold or platinum.

Inventors: COLLIER JOHN PATRICK, COLLIER, JOHN PATRICK
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 2000
Main classification: B21C23
IPC codes: B21J5, B21K21, B21D22, F17C1, B21C23, B21D51