Electric heating – Metal heating – By arc
Reexamination Certificate
2000-02-25
2003-02-11
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S061000
Reexamination Certificate
active
06518545
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
An arc welding process for butt welding of parts, especially designed for orbital butt welding of tubes for piping applications.
2. Brief Description of the Prior Art
1. Preliminary
Welding has been used for many years. As it is well know in the art, when butt welding parts, the items to be welded must be held in position relative to one another. In the case of the parts being tubes welded together, a clamp is used for this purpose, generally placed inside the tubes and positioned at the junction of the two tubes. This clamp is equipped with rows of pistons, each row holding a tube.
The ends of the parts to be welded may first be chamfered
50
(depending on the thickness of the parts, the arc welding process used, the procedure—automatic or manual—and the like), to form ring
13
. See
FIGS. 1 and 2
for illustrations of the prior art components that are welded together.
2. Welding Principle
The process used for butt welding parts of the present invention is the MIG/MAG or GMAW metal arc welding with gas shield process. A metal wire
25
is connected at the weld torch
11
to one of the polarities of a power source and is fed to the welding location. An electric arc is created between this wire and the parts to be assembled, which parts are at the polarity different from that of the wire polarity (See
FIG. 5
for an illustration of the prior art equipment and process discussed in more detail below).
The electric arc causes the wire (sometimes referred to as the “filler metal”) to melt, as well as partial melt of the parts to be assembled. The wire feed rate (called V
f
) is made equal to its melt rate so as to provide for a stable arc. The first weld bead, which produces a perfect joint at the inner skin (with admissible imperfections according to implemented standard) between two chamfered parts is know as “the penetration pass.”
The current wave delivered by the generator is either continuous or sequential. Arc welding is a periodic phenomenon whose period is a few hundredths of a second. During the welding process, the electrical parameters therefore also vary in a periodic fashion.
The current and voltage waveforms of the welding arc change according to the type of transfer of the weld metal from the filler metal (See FIGS.
3
and
4
).
3. Types of Generators Used
MIG/MAG arc welding generators capable of generating sequential type arc modes (including the pulse and/or controlled short-circuit modes) illustrate the ability to have controlled waveforms as shown in
FIGS. 3 and 4
.
Parameters adjustable by the operator on such generators as shown in
FIGS. 3 and 4
are:
Ipeak
1
, Ibase
2
, Upeak
3
, Ubase
4
, peak time
5
, up ramp
6
, down ramp
7
(depending on source manufacturers), V
f
, frequency (inverse of cycle
51
) (adjustable or self-regulated), cool time
8
(adjustable or self-regulated).
In automatic welding, overall parameters managed by the program logic controller (“PLC”) are:
Welding unit speed (equivalent to welding speed), oscillation amplitude and frequency of the welding torch.
For Sequential mode: pure pulse mode (FIG.
3
), the pulse phase corresponds to the Ipeak dwell time
5
.
For Sequential mode: controlled short-circuit (FIG.
4
), Upeak: peak voltage. Voltage value
3
at which the current stops increasing, i.e. voltage at the inception of Ipeak
1
. Umax
9
: maximum achieved voltage. This is a phenomenon due to system inertia.
The pulse phase starts when the voltage becomes equal to Upeak and ends at the end of Ipeak.
In such generators, an intelligent system (microprocessor+programmable logic device, for example) allows certain parameters (Ipeak, frequency, cool time, and the like) to be adapted in real time according to the weld pool conditions (thermal emissivity of the weld pool, for example).
4. For Tube Welding (Penetration Pass), Several Techniques are Available
None of the techniques presented below makes use of real-time, self-assessment of the penetrating power of the electric welding arc.
a. Welding from the Inside:
An inner welding clamp is used. The clamp is usually equipped with two rows of pistons to hold the tubes in position. Further, mobile welding torches are mounted on the clamp for the purpose of completing the penetration from the inside. Parameters can be changed to fit predefined angular positions. The problem with this are that implementing this device is a complex process (positioning in the mating plane is difficult, centering of the welding torches, no direct check possible during welding, poor cost-effectiveness, complex machine with limited diameter range, and the like).
b. Welding from the Outside:
(1) For high wire feed rates, e.g. in the case of mechanized or automatic welding where the welding head movements are controlled by a carriage-type electromechanical assembly, the weld pool must be maintained using backing strips (copper, ceramics, and the like) to prevent the weld pool from collapsing. The backing strips are slaved to the piston rows and applied flat against the back of the joint to be welded when the pistons extend. During welding, parameters can be changed to fit predefined angular positions. The problem is that regardless of the type of medium, the backing strips gradually deteriorate as welding passes accumulate with current MIG-MAG type arc welding processes.
(2) In manual welding, or in automatic welding at low wire feed rates, the outside can be welded without backing strips. The welders are generally assisted by an operator who is requested to adjust the mean welding current according to the weld pool behavior. The problem is that low output occurs, with a welder-dependent process.
In the prior art, whether welding from the outside or from the inside, the welding parameters can be regulated according to the weld pool conditions (temperature, luminosity . . . etc.) or the bevel shape (width, gap, high-low . . . etc.) to control the penetration. To do so, some sensors (thermographic vision, laser, camera, pictures/vision analysis and the like) are available and can be used to measure the thermal evolution of the weld pool, the bevel shape evolution, and the like in real time while welding.
But these devices which need to be handled by the welding system are more cumbersome, fragile, expensive, and less time responsive to adapt the parameters.
5. Example of Configuration of Automatic Outside Welding
As shown in
FIGS. 5 and 13
, a motor-driven welding unit
10
is mounted on fixed tube
12
by means well known in the art. It is connected to a welding generator
14
and a wire reel
15
. It is equipped with one or several welding torches
11
and moves along a ring
13
integral with the fixed tube
12
, the welding unit
10
welds, for example, a half-circumference of ring
13
.
6. General Problems Experienced When Completing the Penetration Pass
a. A Requirement Exists to Control Weld Pool Fluidity in all Positions (Orbital Welding).
Fast cooling is controlled by varying the welding arc heat input. With no support, or with a support made of a low thermal conduction material, cooling of the weld pool during the penetration pass is slower than on a metal support (such a copper). The weld pool must therefore otherwise cool off fast enough to limit the effect of gravity on the fluid pool, thus preventing it from collapsing and avoiding gaps, overpenetration and concavity.
b. The Electric Arc Penetrating Power Varies as a Function of Angular Position.
Even under perfect pipe fit-up conditions (no gaps or high-low areas), the penetrating power of an electric arc is not constant. According to the local angular tilt of the interface of the parts to be welded, the position of the weld pool relative to the arc varies. Thus, for example, for tube welding, at the 12 and 6 o'clock positions, the weld pool is located vertically juxtaposed to the arc, as it is pushed away by the arc pressure: in this case, the arc penetrating power into the metal is high.
Conversely, at the 3 and 9 o'clock positions, the weld pool tends to run under
Richard Claude
Richard Gilles
Serimer Dasa
Shaw Clifford C.
Sheridan & Ross P.C.
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