Pulse MIG welder for welding thin-walled copper-nickel pipe

A welding station for welding thin-walled copper-nickel pipe. An electronic welding station (10) has an output circuit (13) which provides arc welding power having the arc characteristics specified by a weld parameter selection circuit (12). A welding torch and feeder assembly (11) has a controller (20) which is responsive to a predetermined event, such as the passage of time since the start of the arc, or the average temperature of the pipe (25) as indicated by two temperature-measuring devices (30, 32). The controller (20) adjusts the wire feed speed, the pulse frequency, the pulse width, the welding voltage, the welding current and/or other parameters so as to cause the arc to have the parameters most desired for welding copper-nickel pipe. The present invention provides for a hotter arc for starting the welding operation, thereby obtaining good penetration and bonding, and a cooler arc for continuing the welding operation, thereby preventing burn-through of the pipe (25).

TECHNICAL FIELD 
The present invention relates to arc welders and, more particularly, to a 
pulse MIG welder for welding thin-walled copper-nickel pipe. 
BACKGROUND OF THE INVENTION 
Copper-nickel pipe is commonly used in ships because of its resistance to 
the corrosive effects of the salt air outside of the pipe and the 
corrosive effects of any salt water inside the pip. This copper-nickel 
pipe is generally thin walled and is subject to burn through if the 
welding temperature is too high. However, if the welding temperature is 
too low, the weld penetration and bonding will be inadequate, thereby 
necessitating that the weld be ground out and that portion of the joint be 
rewelded. 
Tungsten inert gas (TIG) welding provides an arc which is concentrated on 
one spot. This allows the welding operator to cut the heat back and still 
maintain a good puddle and obtain good penetration and bonding. Metallic 
inert gas (MIG) welding provides an arc which is not as concentrated and 
therefore a high overall temperature is required in order to achieve a 
puddle which penetrates and bonds well to the pipe. If the welding power 
applied is too high, then the weld will penetrate through the joint. It is 
possible to cut the welding power down to the point where it is unlikely 
that the weld will penetrate through the pipe. However, in this case, the 
pipe will heat up very slowly, and the penetration and bonding may be 
inadequate for the first part of the weld. This requires grinding out the 
weld and rewelding the pipe. 
Furthermore, if the welding operator should start off with a hotter weld, 
break the arc long enough to readjust the welding power, and then continue 
on the weld, the penetration on the first part of the restarted weld will 
generally be inadequate. This occurs because the heat conductivity of the 
pipe draws the heat away from the weld area very quickly. Therefore, when 
the welder continues the welding operation with the lower power, the 
degree of penetration and bonding will be inadequate and it will take a 
short amount of time before the pipe temperature heats up to the point 
where the degree of penetration and bonding is proper. Again, this causes 
a poor weld and that part of the weld must be ground out and that 
particular portion of the pipe rewelded. Because of the difficulties of 
MIG welding of copper-nickel pipe, TIG welding is preferred for this type 
of pipe, especially where out-of-position welding is being performed. 
If the weld power could be controlled, so as to achieve adequate 
penetration at all times, while avoiding burning through of the pipe, MIG 
welding would be preferred over TIG welding because MIG welding operations 
proceed at five to six times the rate of TIG welding operations. 
Therefore, there is a need for a MIG welder which provides a high starting 
power so as to achieve good penetration and bonding and then provides a 
lower, continuous welding power so as to prevent burning through the pipe. 
Furthermore, there is a need for a MIG welder which monitors the 
temperature of the pipe and adjusts the welding power so as to achieve 
good penetration and bonding without burn-through. 
SUMMARY OF THE INVENTION 
The present invention is a welder which provides a high welding power when 
the weld is first started so as to quickly achieve good penetration and 
bonding and, after a predetermined amount of time, automatically cuts the 
welding power back to a point which continues to provide good penetration 
and bonding while preventing burn-through of the pipe. The present 
invention also is a welder which monitors the temperature of the pipe and 
automatically adjust the welding parameters in response to changes in the 
pipe temperature so as to provide good penetration and bonding while 
preventing burn-through. 
In one embodiment of the present invention, the welding operator adjust the 
controls on a welding parameter selection circuit to provide the welding 
parameters desired for the particular welding operation to be performed. 
When the welding operator strikes the arc, a timer sends a signal to the 
welding parameter selection circuit which causes this circuit to provide 
welding parameters which correspond to a higher welding power. This 
provides for good penetration and bonding on arc starting. After a 
predetermined amount of time, the timer causes the welding parameter 
selection circuit to revert to the welding parameters selected by the 
welding operator so that welding is continued using welding parameters 
which provide for good penetration and boding while avoiding burn-through. 
The present invention also provides for the welding parameters to be 
chanted at a predetermined rate of change from the welding parameters 
desired for the initial welding operation to the parameters desired for 
the continued welding operation. Each welding parameter may changed at a 
different rate from any other welding parameter and, furthermore, any 
particular rate of change may be a linear or a non-linear function of 
time. 
The present invention also provides a welder which adjusts the welding 
parameters so as to maintain the pipe temperature within a temperature 
range which provides good penetration and bonding and avoids burn-through. 
The present invention uses temperature-sensitive devices to monitor the 
temperature of the pipe, preferably on both sides of the weld area. If the 
pipe temperature begins to rise the present invention cuts back on 
selected welding parameters so as to cause the pipe temperature to fall 
back within the desired temperature range. This prevents burn-through of 
the pipe. If the pipe temperature tends to decrease then the present 
invention changes selected welding parameters so as to increase the pipe 
temperature, thereby assuring good penetration of the weld and good 
bonding of the weld to the pipe. The selected parameters may vary linearly 
or non-linearly, as required, with respect to the pipe temperature. 
Furthermore, any parameter may vary in a manner which is different than 
any other parameter. 
Therefore, it is an object of the present invention to provide a welder 
which provides a first set of welding parameters selected to cause rapid 
heating of the pipe for starting the welding operation and, in response to 
a predetermined event, such as the passage of a predetermined amount of 
time, provides a second set of welding parameters for continuing the 
welding operation. 
It is another object of the present invention to provide a welder which 
provides a first set of welding parameters selected to cause rapid heating 
of the pipe for starting the welding operation and, in response to a 
predetermined event, such as the temperature of the pipe exceeding a 
predetermined threshold, provides a second set of welding parameters for 
continuing the welding operation. 
It is another object of the present invention to provide a welder which 
monitors a selected condition, such as the pipe temperature, and adjusts 
the welding parameters so as to maintain the pipe temperature within a 
predetermined temperature range. 
It is a further object of the present invention to provide a welder which 
maintains the temperature at the weld area within a predetermined range by 
adjusting the pulse frequency, the pulse width, the welding voltage, 
and/or the welding current.

DETAILED DESCRIPTION 
Turn now to the drawing in which like numerals represent like components 
throughout the several figures. The present invention represents an 
improvement to electronic welding stations in general and, in particular, 
to the electronic welding stations described in U.S. Pat. No. 4,716,274, 
issued Dec. 29, 1987, entitled "Distributed Station Welding System", U.S. 
patent application Ser. No. 062,543, filed Jun. 12, 1987, entitled 
"Improved Arc Welding System", U.S. patent application Ser. No. 181,985, 
filed Apr. 15, 1988, entitled "Arc Welder With Improved Arc Striking 
Capability", and U.S. patent application No. 374,465, filed Jun. 30, 1989, 
entitled "Electronic Welding Station With AC and Reversible Polarity DC 
Outputs", the inventor of all of which is Malcolm T. Gilliland, and all of 
which are hereby incorporated herein by reference as if fully set forth 
herein. 
FIG. 1 is a block diagram of the preferred embodiment of the present 
invention. The preferred embodiment comprises an electronic welding 
station 10, a welding torch and feeder assembly 11, and a power supply 
(not shown) for providing operating power to electronic welding station 
10. Electronic welding station 10 comprises a weld parameter selection 
circuit 12, an output circuit 13, and a welding power relay circuit 14 for 
selectively providing operating power to output circuit 13. Weld parameter 
selection circuit 12 allows the welding operator to adjust the welding 
parameters so as to provide the arc characteristics most desirable for the 
particular welding operation to be performed. The welding parameters 
selected by the operator may include the pulse frequency, the pulse width, 
arc voltage, arc current, constant current or constant voltage modes, 
chopped or unchopped output, MIG or TIG, etc. Weld parameter selection 
circuit 12 provides drive signals to output circuit 13. Output circuit 13 
steps up the drive signals provided by weld parameter selection circuit 12 
so as to provide the output voltage and output current levels required for 
arc welding. 
Welding torch and feeder assembly 11 comprises a controller 20, a trigger 
21, a wire feed speed control circuit 22, and a motor and a wire feed 
mechanism 23. Trigger 21 is a conventional trigger on the welding torch 
and feeder assembly 11 which allows the welding operator to start or stop 
the welding operation. Wire feed speed control circuit 22 allows the 
welding operator to select the wire feed speed desired for the particular 
welding operation to be performed. Motor and wire feed mechanism 23 is 
responsive to speed control signals from wire feed speed control circuit 
22 for controlling the speed at which wire 24 is fed from assembly 11. 
Electronic welding station 10 and welding torch and feeder assembly 11 are 
used to weld together a first pipe 25A and a second pipe 25B. In the 
preferred embodiment, pipes 25A and 25B are copper-nickel pipe which may 
have bevelled ends (as shown) or straight ends (not shown). The arc 26 
melts the wire 24 and causes a bead 27 to be deposited at the junction of 
pipes 25A and 25B. As stated above, if the temperature of pipes 25A and 
25B is too low the penetration and the bonding of the weld will be 
inadequate and the bead 27 will have to be ground out and the welding 
operation performed again. Also, if the welding temperature is too high 
then the pipe will burn through and the pipe ends must be refinished 
before proceeding with the welding operation. It will be appreciated that, 
initially, the pipe will be an ambient temperature so a hotter arc should 
be used so as to quickly bring the pipe temperature up to the desired 
welding temperature. Copper-nickel pipe has a substantial heat 
conductivity so the heat applied to the immediate welding area quickly 
spreads to the surrounding pipe. Therefore, by the time the welding 
operation has travelled to a different point on the junction of the pipes, 
the temperature of the pipes will be substantially above the ambient 
temperature. If the hotter arc is allowed to continue the pipes will be 
burned through. Therefore, the arc power must be cut back so that 
burn-through is avoided. 
Controller 20 controls the arc power and therefore the pipe temperature. 
When the welding operator first squeezes trigger 21 controller 20 causes 
weld parameter selection circuit 12 to increase the pulse frequency and 
the pulse width above that selected by the welding operator so as to 
provide a hotter arc. The hotter arc will cause wire 24 to be liquified 
and deposited more quickly so controlled 20 also causes wire feed speed 
control circuit 22 to increase the feed rate of wire 24. Once the pipes 25 
have heated to the proper temperature then controller 20 causes weld 
parameter selection circuit 12 and wire feed speed control circuit 22 to 
revert to the parameters initially selected by the welding operator. This 
provides a cooler arc which is sufficient to obtain good penetration and 
good bonding while avoiding burn-through of the pipes 25. In one 
embodiment of the present invention, each time trigger 21 is squeezed, 
controller 20 causes the hotter arc to be provided for a predetermined 
amount of time and then automatically switches to the arc selected by the 
welding operator. 
In another embodiment of the present invention controller 20 monitors the 
temperature of pipes 25A and 25B by a pair of temperature-measuring 
devices 30 and 32, such as thermistors. This allows controller 20 to more 
accurately determine the temperature of the pipes 25 so that switching 
from the initial, hotter arc to the cooler arc will occur at a point which 
is advantageous for producing a quality weld. In one embodiment of the 
present invention controller 20, via thermistors 30 and 32, determines an 
average temperature for pipes 25A and 25B. Controller 20 initially causes 
the hotter arc to be provided for the welding operation. Once the average 
temperature reaches a predetermined temperature then controller 20 causes 
the cooler arc to be provided. In addition, any time that the average 
temperature drops below another predetermined temperature, then controller 
20 causes the hotter arc to be provided again. The first predetermined 
temperature may be high than the second predetermined temperature so that 
a hysteresis is provided which prevents controller 20 from rapidly 
alternating between the hotter arc and the cooler arc. The term "average" 
is used herein in a rough sense and includes, but is not limited to, the 
precise mathematical average of the two temperatures. The present 
invention contemplates the generation of an "average" temperature wherein 
one temperature is weighted more heavily than the other temperature so 
that the "average" temperature lies between the two temperatures. 
The use of two thermistors is preferred since one of the pipes, for example 
pipe 25B, may be clamped, laying against a bulkhead, or in contact with 
some other mechanism which dissipates heat. If just a single thermistor is 
used, such as thermistor 30, then switching from the hotter arc to the 
cooler arc might occur before the temperature of the junction of pipes 25 
had reached the optimum temperature, so that poor penetration and bonding 
could result. Similarly, if only thermistor 32 is used, then the 
temperature of the junction of pipes 25 may exceed the desired temperature 
to the point where burn-through occurs. However, even though two 
thermistors are preferred, it should be understood that the present 
invention encompasses the concept of using just one thermistor as well as 
the concept of using more than two thermistors. Thermistors 30 and 32 are 
held to pipes 25 by straps 31 and 33. Straps 31 and 33 are disposable 
straps, such as Teflon cable ties. Metal straps may be used if desired or 
required because of proximity to the weld. 
In another embodiment of the present invention controller 20 monitors the 
average temperature of the pipes 25 via thermistors 30 and 32 so as to 
provide a hotter arc upon starting and then, as the temperature of the 
pipes 25 increases, changes the welding parameters in a manner so as to 
maintain the average temperature at a predetermined temperature. In this 
manner, controller 20 again causes the temperature of the junction of 
pipes 25 to be maintained at the temperature optimum for providing good 
penetration and good bonding while avoiding burn-through. In this and the 
preceding embodiment, a closed loop feedback configuration is provided. 
In the first two embodiments, the transition from the hotter arc to the 
cooler arc may be an abrupt transition, that is, a step transition, may be 
a linear transition, or may be a non-linear transition. Furthermore, one 
welding parameter may make the transition in a different manner than 
another welding parameter. All of the embodiments have the same purpose: 
to provide a hotter arc for heating up the material to be welded so as to 
provide good penetration and good bonding and, thereafter, providing a 
cooler arc which still provides good penetration and good bonding while 
avoiding burn-through problems. 
FIG. 2 is a schematic diagram of a first embodiment of the present 
invention. Weld parameter selection circuit 12 includes a pulse width 
modulator circuit 50. Pulse width modulator circuit 50 has two control 
inputs: a pulse width control, and a pulse frequency control. If switch 51 
is in the LOCAL position, then potentiometer 52 controls the pulse width. 
If switch 51 is in the REMOTE position, then potentiometers 53 and/or 54 
control the pulse width. Relay contacts 55 are normally closed contacts. 
Therefore, when switch 51 is in the REMOTE position, potentiometer 54 is 
bypassed by contacts 55 so that only potentiometer 53 controls the pulse 
width. Therefore, potentiometer 53 controls the pulse width for the hotter 
arc. When contacts 55 are opened, then potentiometers 53 and 54 control 
the pulse width for the cooler arc. In this embodiment, less resistance 
across the pulse width control input of pulse width modulator 50 causes 
modulator 50 to produce a larger pulse width. 
Jack 61 is a shorting jack. If nothing is plugged into jack 61 
potentiometer 60 controls the pulse frequency provided by pulse width 
modulator 50. However, if plug 62 is inserted into jack 61 the pulse 
frequency is also determined by potentiometer 63 and contacts 64. If 
contacts 64 are closed, then potentiometer 63 is bypassed and the 
frequency is controlled only by potentiometer 60. When contacts 64 are 
opened the pulse frequency for the cooler arc is determined by 
potentiometers 60 and 63. In this embodiment, less resistance across the 
frequency control input of pulse width modulator 50 causes modulator 50 to 
produce a higher pulse frequency. 
The wire feed speed is determined by the resistance connected across the 
speed control input terminals of wire feed speed control 22. Initially, 
contacts 71 short out potentiometer 72 so that only potentiometer 70 
controls the wire feed speed during the hotter arc. However, once contacts 
71 open, then the wire feed speed for the cooler arc is determined by 
potentiometers 70 and 72. In this embodiment, a lower resistance across 
the speed control terminals causes speed control 22 to provide a higher 
wire feed speed. 
Contacts 55, 64 and 71 are single pole, single throw, normally closed 
contacts. These contacts are part of a time delay relay (TDR) 75. The 
operation of relay 75 is controlled by switch 76 and by gas control 
circuit 19. Switch 76 is a two pole, double throw, center off switch. In 
the HIGH (center off) position switch 76 prevents relay 75 from being 
activated. Therefore, contacts 55, 64 and 71 bypass potentiometers 54, 63 
and 72, respectively, so that the pulse width, pulse frequency, and wire 
feed speed, respectively, are controlled by potentiometers 53, 60 and 70, 
respectively. Potentiometers 53, 60 and 70 are therefore used to adjust 
the desired characteristics for the hotter arc. When switch 76 is in the 
LOW position, relay position 75 is energized so that, after the delay, 
contacts 55, 64 and 71 are opened. This allows potentiometers 54, 63 and 
72 to affect the pulse width, pulse frequency, and wire feed speed, 
respectively. Therefore, potentiometers 54, 63 and 72 are sued to adjust 
the characteristics for the cooler arc. The HIGH and LOW positions are 
preferably used to adjust the arc characteristics and the STANDARD 
position is the operating position. When switch 76 is in the STANDARD 
position, relay 75 is connected to an output of the gas control circuit 
19. When trigger 21 is squeezed and activates gas control circuit 19, then 
gas control circuit 19 applied 80 volts to relay 75. Initially, contacts 
55, 64 and 71 remain closed so that a hotter arc is provided. After the 
delay time has passed contacts 55, 64 and 71 are opened so that the cooler 
arc is provided. 
FIG. 3 is a schematic diagram of a second embodiment of the present 
invention. In this embodiment, relay 75' is a standard relay, and not a 
time delay relay. Relay 75' controls contacts 55, 64 and 71 of FIG. 2. 
Switch 76, as in FIG. 2, provides LOW, HIGH, and STANDARD positions. These 
three positions perform the same functions as described in FIG. 2. 
However, instead of merely monitoring the passage of time, control 20 is 
responsive to the average temperature of pipes 25A and 25B. Thermistors 30 
and 32 are connected to +12 volts through pull-up resistors 105 and 106 
and to the non-inverting input of comparator 100 through resistors 101 and 
102. The output of comparator 100 is connected by a hysteresis setting 
potentiometer 103 to the non-inverting input of comparator 100. The 
inverting input of comparator 100 is connected to a reference temperature 
setting potentiometer 104. The output of comparator 100 is connected, when 
switch 76 is in the STANDARD position, to relay 75'. Potentiometer 104 is 
used to set the reference temperature at which controller 20 will cause a 
transition from the hotter arc to the cooler arc. Potentiometer 103 is 
used to provide a hysteresis so that minor changes in the average 
temperature of pipes 25 do not cause controller 20 to switch back and 
forth between the hotter arc and the cooler arc. 
When pipes 25 are an ambient temperature thermistors 30 and 32 will have a 
high resistance and the voltage at the non-inverting input of comparator 
100 will be less than the voltage provided by the reference temperature 
setting potentiometer 104. Therefore, the output of comparator 100 will be 
low and relay 75' will not be energized. Contacts 55, 64 and 71 of FIG. 2 
will therefore cause the higher pulse frequency, high pulse width, and 
higher wire feed speed to be provided, as required for the hotter arc. 
Once the welding operation has caused the temperature of pipes 25A and 25B 
to heat up, the resistance of thermistors 30 and 32 will decrease, thereby 
increasing the voltage at the non-inverting input of comparator 100 above 
the reference voltage. This causes the output of comparator 100 to go high 
and energize relay 75'. Relay 75' then opens contacts 55, 64 and 71, 
thereby causing the pulse frequency, pulse width, and wire feed speed to 
drop to a lower value appropriate for the cooler arc. However, if the 
cooler arc causes the average temperature to drop below the reference 
setting, after considering any hysteresis provided by potentiometer 103, 
then comparator 104 will de-energize relay 75', thereby causing the hotter 
arc to be provided again. 
Turn now to FIG. 4 which is a schematic diagram of the third embodiment of 
the present invention. In this embodiment, the outputs of thermistors 30 
and 32 are summed in an adder 120 and then provided to an 
analog-to-digital (A/D) converter 121. Converter 121 converts the signals 
from adder 120 into a form suitable for use by a microprocessor 122. 
Microprocessor 122 determines the average temperature of pipes 25 by 
reading output of converter 121. Microprocessor 122 then uses this average 
temperature to determine the proper pulse frequency, pulse width, wire 
feed speed, or other necessary parameter, so as to obtain the arc 
characteristics and wire feed speed necessary to obtain a proper weld. 
Although the use of a lookup table is preferred, it will be appreciated 
that ROM 123 may contain an algorithm which allows microprocessor 122 to 
compute the proper pulse frequency, etc. Microprocessor 122 then, through 
digital-to-analog (D/A) converters 124, 125 and 126, changes the voltage 
on the gates of field effect transistors (FETs) 130, 131 and 132, 
respectively, so as to provide the pulse frequency, pulse width, and wire 
feed speed, respectively, necessary to provide a good weld. As the average 
temperature of pipes 25 increases controller 20 will cause the pulse 
frequency, pulse width, and wire feed speed to decrease, and, conversely, 
as the average temperature of pipes 25 decreases controller 20 will cause 
the pulse frequency, pulse width and wire feed speed to increase. 
Controller 20 therefore adjust the arc characteristics so as to maintain 
pipes 25 at the temperature optimum for welding and also adjusts the wire 
feed speed to match the rate of deposit of the welding bead. 
Although the use of a microprocessor and lookup table are preferred, it 
will be understood that a microprocessor which executes an algorithm which 
defines the pulse frequency, pulse width, and wire feed speed as a 
function of the average pipe temperature can also be implemented. 
Furthermore, it will also be appreciated that, at least to some degree, a 
linear or a nonlinear analog circuit, as appropriate, can also be used to 
define the parameters as a function of the average temperature of the 
pipes 25. 
It will be appreciated that changing the voltage on the gates of the FETs 
130, 131 and 132 serve to change the drain-source resistance of the FETs 
and therefore causes an appropriate change in the desired welding 
parameter. 
It will be appreciated that different welding operations may require 
slightly different welding parameters. Therefore, a replaceable ROM 123 
could be used, whereby each ROM would have the characteristics desirable 
for a specified welding operation. Alternatively, the ROM 123 could 
contain tables for several different types of welding operations and the 
welding operator, through a switch, could instruct microprocessor 122 
which table was to be used for the particular welding operation. However, 
the preferred implementation of this embodiment is to use a SETUP switch 
133 in conjunction with three rotary switches 134, 135 and 136. When 
switch 133 is in the SETUP position, switches 134, 135 and 136 are used to 
set the desired pulse frequency, pulse width, and wire feed speed, 
respectively. Microprocessor 122 stores the settings of these switches. 
When switch 133 is in the OPERATE mode, microprocessor 122 uses the stored 
values to offset the parameter value specified by the lookup table in ROM 
123. Alternatively, switches 133-136 would be used to specify certain 
physical characteristics, such as the pipe diameter, the pipe thickness, 
the pipe material, etc., which would cause microprocessor 122 to select a 
certain lookup table or to offset the values obtained by a lookup table 
when determining the welding parameters to be used. Therefore, the present 
invention accommodates different types of welding operations. 
The values for the parameters in the lookup table in the ROM, and the 
particulars of any algorithm used to compute the parameters, are a 
function of the output voltage rating, output current rating, frequency 
and pulse width range, output inductor/resistor values, etc., of the 
particular type of electronic welder being used. Therefore, these values 
and particulars are determined by experimentation. 
It will be appreciated from the above that the present invention describes 
a method and an apparatus for obtaining an optimum weld by maintaining the 
average temperature of the pipes to be welded together at a temperature 
which achieves good penetration and boding while avoiding burn-through, 
The present invention provides a hotter arc for starting the welding 
operation so that good penetration and bonding will be obtained and, once 
the pipe temperature has reached a predetermined value, provides a cooler 
arc so that good penetration and bonding will continue to be obtained 
while avoiding the problems of burn-through. 
Although the preferred embodiment of the present invention is used with a 
pulse MIG welder the present invention is not so limited. The present 
invention may also be used with other types of welders, especially 
non-pulsed (DC) MIG welders. Generally, out-of-position welding is more 
difficult when a non-pulsed MIG welder is used. If the present invention 
is implemented in a non-pulsed MIG welder then the pipe should be rotated 
about its longitudinal axis at a rate which maintains a fixed relationship 
between the welding torch and the pipe. Alternatively, welding may be 
performed at a number of selected positions around the pipe junction so 
that, for each position, a near-fixed relationship is obtained. 
It will be appreciated that the welding arc provided by a non-pulsed MIG 
welder is very similar to the welding arc provided by a pulse MIG welder 
if the pulse MIG welder has a pulse frequency and an output inductance 
which are high enough to smooth the output of the welder. If the smoothing 
effect is large enough then the output of the welder will closely 
approximate a direct current. 
It will also be appreciated that, when the present invention is used with a 
non-pulsed welder, weld parameter selection circuit 12 would adjust the 
welding voltage and welding current, rather than the pulse frequency and 
pulse width. In this case, controller 20 would cause weld parameter 
selection circuit 12 to vary the welding voltage and/or welding current so 
as to obtain the preferred arc characteristics and pipe temperature. 
Furthermore, different embodiments of the present invention have been 
described which are responsive to the amount of time since the welding 
operation was started, the temperature of the pipe, and other 
characteristics of the welding operation. Furthermore, different 
embodiments of the present invention have been described which provide a 
step transition between the hotter arc and the cooler arc, or provide a 
tapered transition, which can be linear or non-linear, between the hotter 
arc and the cooler arc. Furthermore, some of the embodiments described 
above provide for increasing the temperature of the arc when the 
temperature of the pipe decreases below the desired temperature. From the 
above, other embodiments of and variations to the present invention will 
suggest themselves to those skilled in the art. Therefore, the present 
invention is to be limited only by the claims below.