Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~l
A METHOD FOR DIFFUSION WELDII~G.
The present invention relate3 to a method or joining parts
of metal by means of diffusion welding, comprising the
following steps: establishing a joint between opposed
bounding surfaces on the respective parts to be joined,
heating of the parts to a predetermined temperature in at
least those areas bordering on the joint, joining the pa~ts
by pressing the parts together, and cooling the parts.
Advantages and disadvantages of diffusion welding are known
i.a. from Welding Handbook, 7. edition, volume III, page~
312 et sec.
Among the advantages can be mentioned that joints can be
formed having properties and microstructures which are very
similar to those of the base material and joining is possible
where the form of the work p:ieces makes it difficult to use
other methods. Furthermore, join-ts can be made having mini~al
2Q deformations and without con,ecutive machining or forming.
~mong the disadvantages can be mentioned that the cost of
the equipment is usually verv high, a fact -that limit the
size of the parts that may be diffusion welded in an economic
manner. Furthermore, the necessity of supplying heat and a
high compressive force in an environment such as vacuum or
~rotective atmosphere will g ve rise to grave problems as
regards the necessary equipment and t:herefore constitute a
substantial limitation in the applicability of the method.
In addition, one has assumed that the method requires very
careful and thorough surface preparation and, besides, that
it requires more time than conventional me~hods.
Nevertheless, it is an object of the present invention to
adapt a method of the above mentioned type for making a pipe-
line for offshore transportat:ion of gas and oil. Making of
such pipelines today is done by welding together pipe sections
manually or by CO2 automatic equipment on board a laying
vessel. Usually the welding of the pipes occurs at several
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stations concurrently in order to incxease the laying speed.
Due to practical reasons the stations are placed on a hori-
zontal line ana several welders can weld on each station
concurrently. The finished pipe is let ou-t behind the
laying vessel over a so-called "stinger" which prevents the
pipe from breaking directly behind the laying vessel and
runs in a S-shaped curve down towards the sea floor. A
certain tension is maintained in the laid pipeline, and for
this purpose the laying vessel must have anchors by means of
which it is pulling itself forward during the laying of the
pipeline. These anchors must from time to time be moved
forward in front of the vessel and for this purpose it is
necessary to use auxiliary vessels~
For various reasons it would be desirable to be able to run
the pipeline generally vertically downwards from the laying
vessel, so that the pipeline would run in a single hend down
towards the sea floor. This would i.a. entail better control
of the stresses in the laid pipeline, and one could avoid
anchors and auxiliary vessels and instead make do with
dynamic positioning of the laying vessel. With the pipeline
vertically oriented on board the laying vessel, however, it
would be very difficult to use more than one welding station.
With the present slow welding procedures this would therefore
not give sufficient laying speed to be of practical interest.
The present invention enables two pipe sections to be joined
so quickly that sufficient laying speed can be obtained in
order to use vertical orientation of the pipeline on board a
layi~g vessel, thus enjoying the advantages this will entail.
Thus, the invention provides a method for joining elongate
metal parts of generally constant cross-section by diffusion
welding, comprising the steps ofo
(a) establishing a joint between opposed bounding surfaces
on the respective parts to be joined, which parts in the
joint area initially are reduced in cross section, so that
the joint initially is formed with a cavity with generally
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increasing height from the periphery to~ards the middle and
is provided with at least one connecting conduit,
(b) heating the parts to a predetermined temperature at
least in those areas bordering on said joint, the cavity of
the joint during the heating being supplied with reducing
flushing gas until the joint is tightly closed due to
diffusion welding along the periphery,
(c) joining the parts by pressing them together with a pre-
determined speed for the further diffusion welding and for
restoring said reduced cross-section to substantially said
generally constant cross-section of the parts, and
(d) thereafter coolinq the parts.
By means of the stated form of the joint one obtains high
surface pressure along the periphery of the joint at the
beginning of the method. Therefore, diffusion can start
along the periphery at such low temperatures that oxidation
of the surfaces are avoided. Flushing with inert gas pre-
vents access of oxygen so that oxidation is prevented. If a
reducing gas is used, one can remove oxides which previously
may have formed on the surfaces of the joint~ After closing
of the joint along the periphery in this way, which will
manifest itself in that flushing gas no longer escapes, one
may connect the connecting conduit to a vacuum source so
that the joint is put under vacuum. This will remove resi-
dual gases and make oxides and other impurities dissociate
to a certain degree upon increasing temperature. By adapt-
ing temperature and pressure the joint is concurrently closed
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quickly, whereupon complete diffusion welding can take place in
a short time. Quick compression is advantageous because it
produces higher pressure in the joint area, which is due -to
the creep stress of the material increasing wi-th the speed
5 of deformation.
According to an advantageous feature of the invention, at
least one of the bounding surfaces of the joint is provided
with serrations along the periphery. This will ensure
10 sufficient escape of flushing gas during the early phases of
the joining.
Furthermore, it is very advantageous to provide at least one
of the bounding sur~aces of the joint with a concave form.
15 Thereby an advantageous stress distribution is obtained during
the final compression of -the parts, with resulting assurance
of complete fusion along the entire joint.
I
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A further advantageous form~of the joint is obtained if ajt
least one, preferably both bounding surfaces are given conical
~orm. The conical form is easier to machine in a sufficiently
exact manner, and if both surfaces are conical, one convex
and the other concave, one will obtain a self-allgning effec~
between the parts when they are pressed together. Such a
form will also provide good assurance of complete fusion.
In order to increase the con act pressure in the joint area
during compression, it is suqgested according to the invention
to reduce the cross section of the parts in the joint area.
Thereby a pressure in the joint area is obtained which is
several times higher than the uniaxial creep stress of the
material at the prevailing temperature, the result being
quicker and better diffusion.
Several different inert flushing gases may be used, but
helium is preferred because ~1uring its manufacture it is
cooled down to such a temperature that it is virtually
guaranteed to be oxygen free. Besides commercially
available and reliable helium detectors can be used to co~trol
that the periphery of the joint is absolutely closed before
the vacuum is applied.
If one instead uses a reducing gas for flushing, hydrogen
will be very advantageous. I~lot only will it remove any
oxide layers but it will also make it possible to observe
visually if the joint is clo!;ed because any leaking hydrogen
will burn on the surface. Hydrogen will not have any harmful
effect on the relatively soft structural steels to be joined
by the method.
According to the invention it: is also proposed to provide the
joint with an activating alloy, i.a. 60/40 Palladium/Nickel.
This alloy may be introduced as a thin strip or possibly be
applied on one or both bounding surfaces by means of electro-
plating. This will i.a. result in reduced diffusion time~
The connecting conduit for supplying flushing gas or removing
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gas for pulling a vacuum, may advantageously be arranged in
one of the parts ~o be joined, preferably near its periphery.
Thereby the conduit may be plugged immediately after the
diffusion welding by means of a deep welding electrode ox
Plasma-TIG while the temperature oE the parts still is
sufficiently highl for instance 400 ~ 600C.
The invention shall be furthPr explained wi~h reference to
the exemplifying embodiments shown in the accompanying
drawings.
Figures l and 2 show a first embodiment of two parts at
beginning and completed diEfusion welding, respectively.
Figures 3 and 4 show a second embodiment of the two parts
at beginning and completed difusion welding, respectlvely.
Figures S and 6 show a further exemplifying embodiment of
two parts at beginning and completed diffusion welding,
respeotively.
Figure 7 shows the parts of Figure 6 upon further compression.
Corresponding parts are given the same reference numerals
in all the figures.
Figure l shows a section through a por-tion of two parts l and
2, which may be bolts of structural steel such as ST 52-3.
Figure l may also be visualized as showing an axial section
through a thick-walled pipe, the axis of the pipe being at the
left of the figure. Between the parts l and 2 a joint 3 is
formed, which is bounded by a concavely curved bounding
surface 4 on the part l and a plane bounding surface 5 on the
part 2. A connec~ing conduit 6 is arranged in the part l,
said conduit opening into the cavity 3 of the joint and with
its other end being connectably alternately with a source of
flushing gas and a vacuum source ( not shown)O The arrows
F indicate a variable compressing force, while the bell-
shaped curve to the left of the parts l and ~ indicates the
axial temperature distribution in the parts.
When the parts are to be joined, they are first brought in
the position shown in Figure l and subjected to a suitable com-
pressing force F. This force may be created by means o~ asimple jack system (not shown), which for instance may
comprise a clamping ring on each of the parts l, 2, the
clamping rings being interconnected by means of hydraulic
cylinders. O-ther forms may of course be envisioned, depending
upon the form and cross section of the parts. Thereupon
the heating is started, for instance by means of an induction
coil (not shown). Concurrently,flushing gas is supplied
through the connecting conduit 6. The flushing gas will first
leak out along the periphery 7 of the joint due to minor
`15 irregularities or serrating on the bounding surfaces of the
joint. The purpose of the flushing is to keep oxygen away
from the joint surfa~es in order to prevent oxidation of these
during the heating, possibly also to remove oxides that may
already be present. Regardless of how well the bounding
surfaces 4, 5 have been cleaned before the welding, even a
short exposure to the oxygen of the air will cause an o~ide
layer of a thickness in the ~rder o~ 350 - lO00 ~, depending
on the air temperature and humidity.
When the temperature of the material near the periphery 7 of
the joint has reached 600 - 800C, for instance with a
temperature distribution as shown schematicly to the lef~ in
Figure l, diffusion bet~een the parts l, 2 will occur under
a suitable compressing force F so that the joint 3 is closed
along the periphPry 7. This may be registered in several
ways, or instance in that g~s no longer leaks out of the
joint, or in that a pressure imposed on the cavit~ 3 o~ the
joint no longer decreases.
When the joint thus is closed along its periphery 7, the
connecting conduit 6 is connQcted to a vacuum source, which
thereupon reduces the pressure in the cavity 3 to about
torr. Concurrently,the temperature of the parts l, 2
is increased as suggested schematicly by the curve to the
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left in Figure 2, to a maximum temperature of about 1350C.
With a suitable compressing iorce F the joint 3 will be
closed in a ma-tter of seconds. This result is schematicly
shown in Figure 2. Colnplete diffusion welding will take
place in the course of 15 - 30 minu-tes. ~However, one has
obtained complete welding in diffusion times as short as
8 minutes at about 1350C.) Thereafter the parts are cooled
in calm air down to about 60~)C, which will take about ~
minutes for a material thickness of ~0 mm. ~t this temperature
the connecting conduit may be plugged,i.a. by means of a
deep weldiny electrode or Plasma-TI~ welding.
Figure 3 shows two parts l and 2 with generally the same
outer form as in Figure l. ]lowever, the welding joint 3 here
has a different form, its bounding surfaces 4 and 5 both
being conical, one concave and the other convex. The top
angles are different so that the joint will have increasing
thickness towards the middle. The distance between the apices
of the joint surfaces can amount to about 10% of the thickness
f the parts 1, 2~ The conical form of the bounding surfaces
4, 5 makes the parts l, 2 self-aligning when they are pressed
together. The conical form also helps to avoid insufficient
diffusion in the middle of the joint, as easily could happen
if the lower bounding surface S was plane and the upper
bounding surface ~ conicc~lly concave.
Otherwise, joining of the parts l, 2 takes place in the same
manner as indicated with respect to Figures `l and 2. The
final result is schematicly shown in Figure 4.
The pressure during the diffusion welding is a very important
parameter which is difficult to control. According to the
Welding Handbook previously mentioned it is supposed that
the pressure cannot be brouyht higher than the uniaxial
creep stress of the material at the temperature prevailing
at any time without using forms to prevent floatingO The use
of such forms is cumbersome and entails increased cost,
and in some applications such forms cannot be used due to
lack of access. Since the diffusion speed generally increases
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with the square of the pressure, by increasing the pressure
to the double or triple of the creep stress, one could bring
the diffusion time down to about one four-th or one ninth,
respectively, of the corresponding diffusion -time for uniaxial
pressure. Alternatively one could permit considerably more
of oxides on the joint surfaces and thereby possibly delete
flushing of the joint with a reducing gas.
In order to obtain such high pressures, the invention aims a-t
establishing a triaxial stress condition in the joint area.
It will be known that in tensile testing of thicker test
rods a contraction will OCCUL- by shear deformation, a cleavage
fracture finally taking place in the middle ~one. This
cleavage fracture is caused ~y the axial tension in the middle
being very much higher than 1he yield stress of the material,
the material on both sides o the contraction holding back
by means of radial stresses. For this reason the axial stress
increases, the yieldin~ subs:idin~ when the difference between
the axial stress and the rad:ial stress becomes less than the
yield stress (Trescas principle).
According to the invention one aims at utilizing the triaxial
phenomenon by providing the parts l, 2 with a substantial
constriction in the joint area as compaxed to the adjacent
material. An example is shown in Figure 5 In this example
hydrogen is used as flushing gas, as is evidenced by the
flames indicated along the periphexy of the ~oint.
When the flames disappear one knows that the joint is closed,
i.e. one does not have to use any form of detection equipment
in this connection. When the joint has ~een reduced and
closed along the edge, application of the full compressive
force will make the joint close under a very complicated
stress picture of dynamic character. The triaxial stress
condition exactly at the closing moment ~ill, due to the
constriction and the relatively cold surrounding material,
give stresses transversely of the joint which are 5-6 times
the creep stress for equally thick parts. This situation
is illustrated in Figure 6. Next, the parts are pressed
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together wlth a relatively mc,dest force until they have
obtained generally the same cross section over the entire
length, as is suggested in Figure 7.
Thus, one has obtained the correct shape in the welded parts
without the use of an external form, a very high contact
pressure in the deciding phase, and a distribution of
oxides tnat might have been present to double the area or
more, with a corresponding thinning of the oxide layer a~d
increasing globular conversion, including accelerated
recrystallization.
When -the desired thickness has been reached, the compressive
force may be removed and the diffusion time and temperature
be held until the residual pores have been closed by re-
crystallization. If a much quicker joining is desired, this
can be obtained by electroplating the joint surfaces or
introducing in the joint a st:rip of activating alloy, for
instance of the type Pd/Ni 60/qO.
Due to the very high stresses occuring in the joint area
when it is cons-tricted as shown in Figure 5l it is supposecl
that the original form of the joint and the pre-finishing of
the join-t surfaces are less critical.
Tests performed ~ith the method according to the invention
by using bolts of structural steel ST 52-3 having 40 ~n
diameter have given excellent: results with respect to for
instance ductility and tensile strength. More than 60%
elongation has been measured in tha heat effected zone for
bolts which have been bent to hair needle shape. The
rupture surface showed essentially shear fracture.
It will be understood that the invention may also be
advantageously utilized for other purposes than fabrication
of large pipelines. For inst:ance,it may be used in welding
joints in trusses for bridges and offshore structures.
It is mentioned above that serration may be used on one or
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both of the bounding surface, of the joint along their
periphery in order to ascertain discharge of flushing gas
during the preliminary phases of the welding. In certain
cases it may be advantageous to use a slight serration on -the
entire bounding surface. Such serration will give an echo
during later ultrasonic testing if the joint is not completely
fusioned.
