Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF JOINING CLAD METALS AND VESSEL
PRODUCED THEREBY
FIELD OF THE INVENTION
The present invention relates to clad vessels. Embodiments of the present
invention
have particular application to methods to fabricate apparatus for carrying
corrosive
and highly pressurized fluids. Such apparatus include vessels and conduits,
pipes,
tanks, pressure vessels, autoclaves and heat exchangers.
BACKGROUND TO THE INVENTION
There are many applications in industry where there is a need to have pressure
vessels of various types made from rare metals such as titanium, zirconium and
tantalum to contain various chemicals. Since rare metals such as titanium (Ti)
are
very expensive and only the internal contact surfaces of the vessels need to
be of
such a material, a preferred approach is to use explosive-clad welded (EXW)
plate or
roll-clad plate. EXW plate & roll-clad plate comprises a titanium plate forced
onto a
base material e.g. carbon steel, resulting in a sufficiently strong bond to
enable the
clad plate to be shaped in the manner of a single plate.
The use of EXW or roll-clad plate is especially indicated where, if a titanium-
only construction was employed, the titanium wall thickness would have to be
greater
than say 10mm. In such a situation it is usually a far more economical
solution to
use clad plate consisting of a thin, e.g. 2mm to 16mm thick titanium plate as
the
cladding material.
For the purposes of explanation the present invention will be described with
reference to clad plate comprising a layer of titanium bonded to a thicker
steel layer.
It will be realised that the invention encompasses methods for joining clad
plate, and
apparatus formed by such methods, wherein the clad plate is comprised of other
combinations of metals than titanium and carbon steel.
In order to fabricate a vessel of desired shape and size from clad plate a
number of pre-shaped pieces of clad plate must be joined. Thus there are
various
seams which need to be welded in order to join all the individual plates
together.
Whilst welding the base material, e.g. carbon steel, is usually
straightforward, in order
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to do so it is necessary to firstly remove a margin of the titanium covering
along the
opposing edges of each of the clad metal pieces that are to be joined. A
problem
then arises in that subsequent to welding the base metal the titanium layer
must then
be repaired in order to cover the welded base metal. This problem is
compounded
because, due to metallurgical reasons, titanium, for example cannot be
successfully
welded to the base metal but rather must be welded to other titanium.
A prior art approach to joining clad metal pieces will now be described with
reference to Figure 1.
Figure 1 is a cross section through a typical prior art join 26 between a
first
piece 2 and a second piece 4 of clad plate in the form of titanium clad steel.
The join
makes use of a batten strap technique. Initially titanium cladding is removed
from the
area around all edges where steel welds 6 and 8 are to be made, typically 12mm
inward from the steel weld preparation edge. The steel base metal pieces 10
and 12
are then prepared and welds 6 and 8 applied using conventional steel
fabrication
procedures. The join is then cleaned-up and prepared for titanium welding. In
the
conventional batten strap technique, a filler-metal strip 7 is inserted into
the space
where the titanium has been removed. The choice of filler is dependent upon
fabrication preferences; commonly used materials include copper, steel,
aluminium
and titanium. A wider strip of titanium 16, comprising the batten strap, is
then placed
over the weld area. The batten strap is welded along its edges with fillet
welds 18
and 20 to adjacent titanium cladding portions 11 and 13 respectively.
Figure 2 depicts a reaction crucible formed by joining a number of pieces of
clad plate together using the batten strap technique described in relation to
Figure 1.
It will be noted that the batten strap joins 26A-26D protrude into the
interior of the
crucible. Often agitators are used to swirl the contents of vessels made from
clad
plate in order to enable mixing of the contents of the vessel and to encourage
reaction. The contents of the vessels are often erosive to metals. A problem
has
been found to occur in that the batten strap joins 26A-26D have been found to
be
susceptible to erosion. The batten strips therefore become the first point of
failure in
the titanium cladding which then exposes the base material to corrosion and
pressure vessel failure if undetected.
Titanium has a substantially lower coefficient of thermal expansion than the
base material, which is usually of steel. As a result, since the contents of
the vessel
are often at high temperatures, differential expansion and contraction of the
titanium
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and base material can occur which lowers the fatigue life of a titanium
envelope of
the vessel.
In light of the above, it is an object of the present invention to provide a
method for joining clad metals which can be used to fabricate vessels and
which
addresses one or more of the above-described problems.
It is a further object of the present invention to provide a clad plate vessel
that
is not as susceptible to the above-described problems as has hitherto been the
case.
The clad plate vessel may include a titanium envelope.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a
method for
joining clad metal plates having a protective layer and a substrate layer, the
method
including:
removing margins of protective layer along edges of the clad metal plates to
be joined;
welding the substrate layers together to form an exposed substrate weld;
locating covering material of the same type as the protective layer along the
exposed substrate weld to a level substantially flush with an outer surface of
the
protective layer; and
welding the covering material to the protective layer so that the protective
layer, covering material and welds located there-between from an outer layer
of
substantially uniform thickness.
The covering material may include one or more backing strips.
In one embodiment edges of the one or more backing strips are located
between respective interfaces of the protective layer and the substrate layer
adjacent
the edges of the clad metal plates to be joined.
Preferably slots are formed between said respective interfaces to receive the
edges of the one or more backing strips.
The covering material will generally include a batten strip.
In some embodiments the method includes locating the batten strip over the
one or more backing strips.
The batten strip may be formed with sloping sides that open out towards the
surface of the protective layer. In that case the step of welding the covering
material
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includes making a number of welding runs to form a butt weld joining the
batten strip
to the protective layers.
Alternatively the batten strip may be formed with sides that are substantially
at
right angles to the one or more backing strips. In that case the step of
welding the
covering material to the protective layer includes high current narrow gap
welding the
batten strip to the protective layers.
In a further embodiment the method includes forming steps along opposing
edges of the protective layers.
In that case the covering material will typically comprise a batten strip
having a
base portion dimensioned to be located between said steps and a body portion
having lips overhanging the base portion to rest upon said steps.
The method may include the step of preheating the substrate layer to a
predetermined temperature subsequent to locating the covering material and
before
welding the covering material to the protective layer to achieve pre-stressing
of the
covering material.
The method may also include the step if preheating the covering material and
the protective layer to said predetermined temperature.
According to a second aspect of the invention, there is provided a method for
joining clad metal plates having a protective layer and a substrate layer, the
method
including:
removing margins of protective layer along edges of the clad metal plates to
be joined;
welding the substrate layers together to form an exposed substrate weld;
locating covering material of the same type as the protective layer along the
exposed substrate weld to a level substantially flush with an outer surface of
the
protective layer;
preheating the substrate layer to a predetermined temperature; and
welding the covering material to the protective layer to achieve pre-stressing
of the
covering material upon subsequent cooling.
This aspect may include the step of preheating the covering material and the
protective layer to said predetermined temperature.
According to a third aspect of the present invention, there is provided a
method for joining clad metal plates having a protective layer and a substrate
layer,
the method including:
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removing margins of the protective layer along edges of the clad metal plates
to be joined;
welding the substrate layers together to form an exposed substrate weld;
locating filler material along the exposed substrate weld to a level
substantially
5 flush with an outer surface of the protective layer;
preheating the substrate layer to a predetermined temperature;
locating covering material of the same type as the protective layer to cover
the
filler material; and
welding the covering material to the protective layer to achieve pre-stressing
of the covering material upon subsequent cooling.
Preferred features, embodiments and variations of the invention may be
discerned from the following Detailed Description which provides sufficient
information for those skilled in the art to perform the invention. The
Detailed
Description will make reference to a number of drawings as follows.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross section through a prior art clad metal plate join.
Figure 2 is a cross section of a prior art vessel formed with joins of the
type
depicted in Figure 1.
Figure 3A depicts opposed clad metal plates to be joined according to an
embodiment of the present invention.
Figure 3B depicts a step in a method according to an embodiment of the
present invention.
Figure 3C depicts a further step in a method according to an embodiment of
the present invention.
Figure 3D depicts a further step in a method according to an embodiment of
the present invention.
Figure 3E depicts a further step in a method according to an embodiment of
the present invention.
Figure 3F depicts a further step in a method according to an embodiment of
the present invention.
Figure 3G depicts a further step in a method according to an embodiment of
the present invention.
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Figure 3H depicts a further step in a method according to an embodiment of
the present invention.
Figure 4A depicts a join produced by a method according to second
embodiment of the present invention.
Figure 4B depicts a step in the production of the join of Figure 4A.
Figure 5A depicts a join produced by a method according to a third
embodiment of the present invention.
Figure 5B depicts a step in the production of the join of Figure 5A.
Figure 5C depicts covering material in the form of a batten strip used to form
the join in Figure 5A.
Figure 6 depicts a join produced by a method according to a fourth
embodiment of the present invention.
Figure 7 depicts a vessel according to an embodiment of the present
invention.
Figure 8 depicts a flowchart showing the steps for joining clad metal plates
together according to an embodiment of the invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
A method for joining clad metal according to a first embodiment of the present
invention will now be described with reference to Figures 3A-3F.
Figure 3A depicts cross sections of two opposing pieces of clad plate 25A and
25B to be joined. Each of the pieces consists of a substrate or base metal
layer 28A,
28B, in the present example carbon steel, and a protective cladding layer 27A,
27B,
in the present example titanium.
In order to allow welding access to the base metal layers 28A and 28B, a
margin 29 of titanium is stripped off along eachJ of the opposing edges as
shown in
Figure 3B.
Metal is removed from base metal layers 28A and 28B as shown in Figure 3C
to form surfaces 50A and 50B which are typically formed at a depth of 2mm to
3mm
below the interface between cladding portions 27A, 27B and base portions 28A,
28B
respectively. At the same time base material weld preparations are cut or
machined
into base metal layers 28A and 28B to form opposing angled surfaces 51A, 51 B
and
53A, 53B.
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A machining operation is then performed to produce grooves 55A, 55B below
the lower titanium cladding edges 30A and 30B as shown in Figure 3D. Formation
of
the grooves may alternatively be performed subsequent to joining the base
material
portions 28A and 28B as will be described shortly.
Figure 3E depicts the base metal layers 28A and 28B subsequently joined
using conventional steel welding techniques. This involves preparing the
opposing
edges of the base metal and usually firstly making an inner weld 31. The root
of the
inner weld is then back-gouged from the outer side i.e. the underside as it
appears in
Figure 3E, and a second weld 34 is formed to complete the join between base
metal
portion 28A and base metal portion 28B. The inner weld surface may be machined
or dressed flush to ensure a flat inside surface 52.
At Figure 3F covering material in the form of a titanium backing strip 36A is
placed over and to one side of weld 31 and the edge 50A of the base material.
Backing strip 36A is then pushed under the cladding edge 30A by means of light
mallets to bring it to a final position as shown in Figure 3F.
Backing strip 36B is then placed on weld 31 and base material edge 50B.
Backing strip 36B is then pushed under the cladding edge 27B by means of light
mallets to bring it to a final position as shown in Figure 3G.
Further covering material in the form of a titanium batten strip 38 is then
located over backing strips 36A and 36B as shown in Figure 3H. The edges of
batten strip 38, and opposing edges 30A, 30B of the titanium cladding layers
27A,
27B have previously been prepared for titanium welding. Multiple run titanium
welds
40A and 40B are then formed using conventional methods. It will be noted that
welds 40A and 40B are each three-way welds in that they join the batten strip
both to
the adjacent cladding layer and also to the underlying backing strips 36A and
36B. It
will be further noted that because batten strip 38 is the same thickness as
the
adjacent cladding layers, and because the upper surface of the backing strip
is
substantially flush with the lower surface of the cladding layers 27A, 27B,
the batten
strip does not protrude substantially above the adjacent cladding layers.
Consequently, a vessel formed according to the previously described embodiment
does not have protruding joins which are susceptible to erosion. A further
advantage
is that the welds 40A and 40B are strong due to the fact that they run the
full
thickness of the cladding layer and are a butt-type weld in contrast to the
fillet welds
18 and 20 of the prior art embodiment of Figure 1.
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Referring now to Figure 4A, a join between two pieces of clad metal is
depicted that is formed by a method according to a further embodiment of the
present invention. The join shown in Figure 4A is formed using the steps that
were
previously described in relation to Figures 3A-3G. Subsequent to the step
described
with reference to Figure 3G covering material in the form of a precisely
dimensioned
batten strip 42 is located over backing strips 36A and 36B as shown in Figure
4A.
This is so that the gaps 43A, 43B between the batten strip and adjacent
cladding
layers 27A and 27B are about 1 mm in width. High current narrow gap welds 44A,
44B, or as they are sometimes called, "keyhole welds" are then formed to join
the
batten strip to the adjacent cladding layers 27A, 27B and to the underlying
backing
strips as shown in Figure 4A. An advantage of the embodiment used to form the
weld depicted in Figure 4A is that it reduces residual stresses in the batten
strip joint
and it is not necessary to prepare the edges of batten strip 42 and adjacent
titanium
cladding layers 27A and 27B prior to making each of the keyhole welds.
Furthermore, multiple welding runs are not necessary.
The join of Figure 4A is relatively smooth so that a vessel formed according
to
this embodiment does not have protruding joins that are susceptible to
erosion.
Referring now to Figure 5A, there is depicted a clad metal join formed by a
method according to a further embodiment of the present invention. The clad
metal
join of Figure 5A is formed by initially performing the steps described in
relation to
Figures 3A to 3C, though without machining surfaces 50A and 50B in Figure 3C.
The opposing edges of clad layers 27A and 27B are then machined to produce
opposing steps 46A and 46B as shown in Figure 5B. A batten strip 47 of the
same
material as the cladding 27A, 27B, e.g. titanium is machined so that it is
stepped with
a base portion 49 that locates between steps 46A and 46B and overhanging lips
57A
and 57B that rest on steps 46A and 46B respectively. The batten strip 47 is
formed
with sloping sides 59A and 59B that provide space for titanium welding runs
61A and
61 B to be formed between the batten strip and the opposing edges 30A and 30B
of
cladding 27A, 27B.
Figure 6 depicts a variation to the join depicted in Figure 5A wherein batten
strip 63 has vertical sides and is precisely shaped to fit snugly between the
opposing
edges of cladding 27A and 27B with a gap on either side of about 1 mm. High
current,
narrow gap welds 65A and 65B are then performed to join batten 63 to cladding
27A
and 27B.
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In order to address problems associated with differential thermal expansion of
the base metal portions 28A, 28B and batten strip 38, 42, 47 or 63, the
following
steps (shown in Figure 8) are added to any of the methods described in
relation to
Figures 3A to 6.
After the base metal portions 28A, 28B have been welded together at step 80
and the batten strip 38, 42, 47 or 63 has been positioned between the cladding
portions 27A, 27B at step 82, but not yet welded, an outer side of the base
metal
portions 28A, 28B is heated at step 84. The heating is carried out until a
suitable
titanium pre-weld temperature is reached on the inside surface of the
titanium. The
temperature is selected to suit the particular application and is over 50 C.
The batten
strip 38, 42, 47 or 63 is then welded at step 86 as described above.
As a result of this process, when the crucible cools, the batten strip 38, 42,
47
or 63 is placed in compression, since carbon steel has a higher coefficient of
thermal
expansion than titanium. This improves the fatigue life of the crucible.
An analogous process can also be applied to join clad metal plates as shown
in Figure 1. Initially, the method involves removing margins of the protective
layer 11,
13 along edges of the clad metal plates to be joined and welding the substrate
layers
10, 12 together to form an exposed substrate weld 6. Filler material 7 is then
located
along the exposed substrate weld 6 to a level substantially flush with an
outer surface
of the protective layer 11, 13. The method further involves preheating the
substrate
layer 10, 12 to a predetermined temperature and locating covering material 16,
of the
same type as the protective layer 11, 13, to cover the filler material 7.
Finally, the
covering material 16 is welded to the protective layer to achieve pre-
stressing of the
covering material 16 upon subsequent cooling.
Figure 7 depicts an apparatus in the form of a crucible formed from.clad plate
using one or more of the joining methods described herein. It will be noted
that
seams 65A-65D do not protrude into the crucible so that they are considerably
less
susceptible to erosion than was the case for the prior art crucible of Figure
1.
It is to be understood that the invention is not limited to specific features
shown or described since the means herein described comprises preferred forms
of
putting the invention into effect. The invention is, therefore, claimed in any
of its
forms or modifications within the proper scope of the appended claims
appropriately
interpreted by those skilled in the art.
