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/cm2.
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
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/cm2 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
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.