Narrow prep MIG welding

MIG welding apparatus includes a torch body; a machine barrel extending forwardly of the torch body; a contact tip extending forwardly from the machine barrel; and a pair of gas supply tubes extending along and substantially parallel to the contact tip, enabling reduction in the weld prep angle to about three degrees.

TECHNICAL FIELD 
This invention relates to automated MIG welding apparatus with particular 
application to the welding of turbine components. 
BACKGROUND 
By way of brief background, Metal Inert Gas (MIG) welding employs a 
continuously fed wire electrode (consumable) and a shielding gas. 
Generally, the wire consumable is fed through a torch to the workpiece. 
The latter is connected by an earth lead which completes the circuit. The 
wire electrode is held at a potential above ground using a power source 
capable of supplying several hundred amps of current to produce an arc. 
When the wire consumable touches the workpiece, an arc is formed which 
melts localized metal of the workpiece and the wire, forming a molten pool 
which forms the weld. The shielding gas, usually an argon/CO.sub.2 
mixture, is also supplied through the torch and protects the weld from 
oxidation and provides the desired arc characteristics. 
Up until 1989, MIG welding in steam turbine diaphragms required 22.degree. 
weld prep angles in order for the traditional weld cone to access the 
bottom of the weld and to provide adequate gas shielding. In other words, 
the access groove required a 22.degree. angle to enable the welding torch 
to reach the weld area. Although a good quality weld resulted, this led to 
excessive weld material being deposited into the diaphragm, increasing the 
product's distortion, cycle and cost. 
In 1989, an adjustable copper sheath was developed which extended from 1" 
to 8" beyond the original end of the cone. This configuration was 
developed in order to reduce the weld prep angle from 22.degree. to 
11.degree.. This significantly reduced the weld volume and direct costs 
for each diaphragm, but still left substantial room for improvement. 
A concept of narrow prep welding known as "Fine Line Welding" was recently 
introduced in connection with Tungsten Inert Gas (TIG) welding. Although 
this technology would certainly reduce the distortion found in standard 
diaphragms, it has two qualities which make it inappropriate for diaphragm 
production use, namely, a) the machinery currently being used for this 
application is very delicate, still in the development stage and not 
rugged enough for the manufacturing environment and b) the material 
deposition rate is too low to be satisfactory for diaphragm structural 
welding. The development of a weld process that permitted even smaller 
weld prep angles consequently focused on modification of existing MIG 
welding techniques. 
Weld distortion associated with traditional MIG welding apparatus has 
typically resulted in unfavorable: 
1) Steam path dimensional deviations between before and after welding 
conditions; 
2) Circumferential shrinkage of the diaphragm halves leading to poor joint 
blade matching in subsequent finishing operations; 
3) Excessive manual deburring of steam path configuration which had been 
distorted due to welding; 
4) Weld splatter as a result of excess weld and weld buildups required for 
access to traditional sized preps; and 
5) Distortion of radial location of steam path sidewalls and setbacks to 
the trailing edges of the blades. 
Labor costs associated with MIG welding in the turbine environment are also 
high. Examples of operations which drive these costs in a steam turbine 
diaphragm welding application are, 
1) Welding and cleaning of the structural and cover welds of the diaphragm, 
2) Welding of diaphragm appendage rings and outer ring buildups required to 
allow access to the traditional sized weld preps, 
3) Use of excess shielding gas and filler material in welding larger weld 
preps, 
4) Machining time for weld preps and appendage rings, 
5) Material costs for appendage rings. 
SUMMARY OF THE INVENTION 
The objective that led to the development of this invention was to reduce 
the distortion caused by large weld volume and heat input, and direct 
costs associated with traditional automated MIG welding apparatus. The 
tooling modifications of this invention contribute to achieving these 
objectives through a reduction in weld volume and heat input into the 
diaphragm. This is accomplished by enabling a smaller angle prep opening 
or angle (on the order of 3.degree.) and through redesigned hardware which 
allows welding to be performed in greater than normal depths. The hardware 
allows for adjustments in depth of weld based on the given product 
requirements, resulting in a maximum of 8" groove depth. 
This invention also provides shielding gas coverage both upstream and 
downstream of the weld puddle to yield good quality welds. In addition, 
since weld buildups and appendage rings are no longer needed, weld 
distortion and applied labor are further reduced. 
Although hardware has been modified in accordance with this invention, no 
changes have been made to the standard MIG welding parameters. In fact, 
with the exception of the trailing and leading extended gas cup 
configuration and related hardware, and the ceramic coated contact tip, 
the invention utilizes all of the hardware traditionally used for standard 
MIG welding. 
In the exemplary embodiment, rather than having the torch cone or extending 
copper sheath protect the contact tip from arcing out with the sidewalls 
of the prep, a ceramic coated contact tip is used. The resulting 
configuration allows for greater depths to be achieved while reducing the 
required weld prep sidewall angle for clearance. The tip is a copper tube 
with a 1/4" outer diameter, with lengths ranging from 4" to 12". At the 
same time, in order to provide adequate gas shielding for the leading and 
trailing portions of the weld puddle, small diameter extended gas tubes 
are employed, and are located externally of the torch. Again, the gas flow 
rates used in connection with this invention are comparable to those used 
in traditional MIG welding. 
The shielding gas hardware configuration consists of an adjustable head 
which is mounted to the existing MIG weld torch. The two pronged gas 
shielding dispenser is connected directly into a gas line. This 
configuration deviates from the traditional MIG welding practice of 
dispensing the gas through the cones surrounding the contact tip with the 
tubing line directly connected with the gas supply. This separation of the 
gas supply tubes from the main torch head allows for easy removal and 
replacement, and also to a smaller front-to-back profile. 
Improvements to the control of the aforementioned parameters will lead to 
improved turbine efficiency, reduced rework and greater control of 
critical dimensional characteristics of the gas turbine diaphragms. 
In addition, the reduced weld volume leads to a significant reduction in 
direct labor costs. 
In its broader aspects, therefore, the present invention relates to welding 
apparatus comprising a torch body; a machine barrel extending forwardly of 
the torch body; a contact tip extending forwardly from the machine barrel; 
and a pair of supply gas tubes extending externally along and 
substantially parallel to the contact tip. 
Other objects and advantages of the invention will become apparent from the 
detailed description which follows.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIGS. 1, 1A and 1B, a conventional MIG welding tool is 
illustrated. The tool 10 in FIG. 1 includes a torch body 12 and a narrower 
(smaller diameter) barrel portion 14 connected to the torch body at a 
shoulder 16. The barrel typically has a diameter of about 1 inch. At the 
forward end of the barrel 14, there is located a core cup or cone 18 with 
a weld wire feed tube 20 projecting out of the core cup in concentric 
relationship therewith. The core cup 18 has a diameter at its smaller, 
forwardmost end of about 5/8 inch. Feed tube 20 serves to supply the 
consumable weld wire 22 to the weld site. Shielding gas is supplied to the 
weld area through the torch 12, in the annular space between the cone 18 
and the tube 20. 
For the manner in which the tool 10 is employed in the context of welding a 
steam turbine diaphragm, reference is made to FIG. 1B where the tool 10 is 
located in a groove or slot between a steam turbine diaphragm band 24 and 
ring or web 26. The weld prep angle A required with this tooling setup is 
approximately 22.degree.. 
Turning to FIGS. 2, 2A and 2B, a modified tooling arrangement is 
illustrated and, for convenience, reference numerals used to designate 
corresponding components are the same as those used in connection with 
FIGS. 1-1B but with the prefix "1" added. Thus, the MIG welding tool 110 
includes a torch body 112 connected to a barrel portion 114 by means of a 
tapered shoulder 116. The core cup 118 is located at the forward end of 
the barrel. But in this embodiment, an adjustable gas cup 19 extends 
forwardly of the core cup 118. The wire supply tube or contact tip 120 
extends beyond the adjustable gas cup and supplies the consumable wire 122 
to the weld site. The shielding gas is supplied to the weld site in the 
oval space between the cup 19 and the contact tip 120. As best seen in 
FIG. 2A, the cross-sectional shape of the adjustable gas cup is elliptical 
or oval and has a minor axis dimension of about 7/16 to 5/8 inch. In use, 
the tool is moved in a direction parallel to the major axis, so that it is 
the minor axis which is critical to the weld prep angle. The minor axis 
dimension leads to a narrower weld prep angle as shown in FIG. 2B. 
Specifically, the weld prep angle B has been reduced from 22.degree. to 
11.degree. in this prior embodiment. 
With reference now to FIGS. 3, 3A and 3B, an improved MIG welding tool in 
accordance with this invention is illustrated. In this embodiment, the 
tool 210 includes a torch body 212 connected to a barrel portion 214 by 
means of a tapered shoulder 216. A standard core cup 218 is affixed to the 
forwardmost end of the barrel. In this embodiment, an extended contact tip 
or wire supply tube 220 (preferably copper) projects forwardly of the core 
cup 218, and supplies the consumable wire 222 to the weld site. The tube 
220 has a diameter of about 1/4" and a length ranging from 4 to 12". Tube 
220 is coated with a ceramic material to insulate the tube, so that there 
is no need for the core cup to extend axially substantially to the free 
end of the contact tip. 
The shielding gas is supplied in two discrete small diameter (3/8 inch 
diameter) copper tubes 230 and 232 which are located on diametrically 
opposite sides of the contact tube 220. The gas supply tubes 230 and 232 
are connected to a gas supply conduit 234 at a gas inlet fitting 236 where 
the pair of supply tubes 230 and 232 are merged with the supply tube 234. 
The gas supply tubes 230 and 232 are secured to the machine barrel 214 by 
means of an adjustable clamp 238. In this manner, the circumferential 
location of the gas supply tubes 230 and 232 relative to the contact tip 
220 may be adjusted as desired. Tubes 230 and 232 may also be adjusted 
axially to a limited extent. In this regard, the tubes 230, 232 need not 
extend to the same linear or axial degree as the contact tip 220. 
It is readily apparent from a comparison of FIGS. 3A with FIGS. 1A and 2A, 
that the cross-sectional profile of the tooling in accordance with this 
invention has been significantly reduced. In fact, the width dimension at 
the tip of the tool is only 3/8", as determined by the diameters of the 
tubes 230, 232. As a result, and with reference specifically to FIG. 3B, 
it will be appreciated that the tool 210 requires a weld prep angle of 
only 3.degree.. In this regard, the apparatus shown in FIG. 3 is moved in 
use parallel to the plane containing the tubes 230, 232 and contact tip 
220, although the tubes 230, 232 need not be perfectly linearly aligned. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiment, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims.