INTRODUCTION TO TIG WELDING

There are many methods of welding Austenitic grades of stainless steel, including High Frequency, Plasma, TIG (GTAW), Low Frequency Square Wave, and Laser. The most common method of welding tubular products is the TIG (GTAW) method. Simply put, the TIG method uses a DC power supply to provide a power source. The electrode is non-consumable and is composed of a doped tungsten material. The doping agent used in the past has been 2% Thorium; however, in Europe the use of Thorium as a doping agent has been dramatically curtailed. New compounds of Cesium and Lanthanum are the most common replacements. It is expected that the United States will also curtail the use of Thorium over the next several years. The other component of TIG welding is the shield gas. Many gases and combinations of gases are used; the most common are Helium, Argon, Helium-Argon mix, and Hydrogen-Argon mix. Nitrogen is often used to create an inert atmosphere on the ID of the tube in certain applications. The cost of the gases used can be a very important consideration, especially if 100% Helium is used.

LIMITATION OF SINGLE TIG

A single TIG torch is an effective way to weld tubular products; however, when the producer wants to increase productivity, certain issues must be considered. To do this, it is important to understand what is actually happening to the weld arc during the welding process. With a properly sharpened electrode, the arc is projected as a circle on the weld seam. The size of this circle is directly related to the amperage applied to the arc, hence, the more amps, the larger the circle. (fig. 1) This means that, in an attempt to increase line speeds by applying more amperage and thus heat to fuse the seam, a condition results wherein more amperage does little or no good. By augmenting the amperage, the diameter of the arc circle increases and the extra heat is applied farther away from the weld seam where it is needed.

MULTICATHODE

The use of multicathode welding is the next step in improving performance. By multicathode, we mean the use of more than one electrode in a uni-body torch. Early experimentation determined that the electrodes must be adjacent enough to each other to maintain the liquid puddle from torch to torch, but not so close that the individual arcs affect one another. Multicathode torch configuration is usually determined by the thickness of the material to be welded and the weld box design. Below is a basic guide to choosing a multicathode system.

Wall Thickness	# of Electrodes	Magnetic Deviation	Magnetic Oscillation
.5 mm	1	yes	no
.8 to 1.0 mm	2	yes	yes
1.0 to 3.5 mm	3	yes	yes
2.0 to 4.0 mm	4	yes	yes
2.0 to 9.0 mm	plasma	yes	yes
3.5 to 9.0 mm	plasma-tig	no	yes
4.0 to 9.0 mm	tig-plasma-tig	no	yes

The use of magnetic deviation and arc oscillation (which will be explained later) in addition to a multicathode system can further improve productivity and weld quality. When the material thickness exceeds 4 mm, the use of plasma is almost always needed. It is possible to weld up to 9 mm without the use of filler metal and with the addition of a TIG torch with arc oscillation after the plasma. On large diameter tubes an ID torch can be added to improve the surface bead of the ID. The ID torch should be arc oscillated and the use of an automatic voltage control is recommended to maintain the arc voltage.

ARC DEVIATION

Increased heat can be focussed on the weld seam without increasing the width of the arc through the process of arc deviation. Arc deviation uses a magnetic field to elongate the arc, giving it a teardrop shape as opposed to the normal round shape.(fig. 2) This elongated arc allows an actual increase in heat applied to the seam for a given amperage, and thus, an increased line speed. There is a limit to the amount of amperage that arc deviation allows to be applied, however; even with the elongated shape, the width of the arc will grow to a point where adding more amps is futile.

ARC OSCILLATION

The use of arc oscillation on the last torch of a multicathode system is one of the ways that producers are increasing mill speeds. Like in deviation, a magnetic field is applied to the arc; but unlike deviation, the field applied continuously changes polarity during operation. This action makes the arc oscillate on the seam in the same manner as a sine wave, with the seam being the horizontal axis. The two main adjustments on an arc oscillation control are the frequency and the amplitude. The frequency is simply the speed of the oscillating action, or how many times the arc passes over the seam in a given time period. By increasing the frequency, the arc will cross the seam more times during this period, allowing for faster line speeds. The amplitude describes how far past the seam the arc will travel. The higher the amplitude, the farther away from the seam the arc will move. In effect, the amplitude is a direct function of the strength of the magnetic force applied to the arc. A key benefit of arc oscillation is a stirring of the weld puddle, which is favorable for three reasons. First, the stirring allows for impurities in the weld puddle to come out of solution and rise to the top, where they can be removed by a bead flashing unit. This stirring action also creates good puddle flow, which makes for a smoother surface of the weld. Lastly, the hottest metal at the surface is circulated to the bottom of the puddle, melting the solid metal it encounters and securing a quality weld near the ID, while at the same time controlling the amount of ID bead.

ID GAS

When welding with a multicathode system it is important to know that the length of the weld puddle is relative to the number of electrodes used. For example, a single TIG torch at 200 amps could form a puddle about 12 mm in length. When using a tricathode with magnetic deviation and 150 amps at each electrode, the puddle could be up to 40 mm long. This puddle is more difficult to control and must be supported by the ID purge gas pressure. The factors in maintaining this pressure are the gas dams used, flow meters, method of measuring gas deployment, and operator knowledge of what’s happening inside the tube. The supplier of a multicathode system should be able to help with establishing guidelines to be used for ID gas application.

FORMING PROBLEMS AND MILL COMPONENTS

When considering implementing a multicathode welding system on an existing mill, one cannot ignore the importance tube forming before and in the welding station. The size of the puddle makes it extremely important to create as close to a neutral pressure environment under the torches as possible. For this effect, the use of weld shoes is the most favorable method. The ideal situation is to have the proper shape of tube coming out of the last finpass allowing the shoes to only hold the tube, and consequently the seam, in the same condition while passing under the torches, neither applying any squeeze, nor allowing the tube to spring open. This condition is facilitated by the length of contact in a shoe as opposed to the minimal contact of rolls. When using rolls, there is always going to be some spring back, causing the seam to try to open up. This is something that must be considered when trying to maintain a long molten puddle. It is possible to position the torch in the weld rolls to compensate for this somewhat; however, the condition of the weld box must be excellent. This means properly contoured rolls, no bent shafts, and no worn bearings. Also, the alignment of all the rolls in the box must be perfect. Any imperfections in any of the above mentioned variables will create a condition wherein the finished product will not be the best it can be. Often, a decrease in speed will be the answer to compensate, which defeats the purpose of installing a multicathode system. Whether using rolls or shoes, the presentation of the seam to the torches is critical. This is controlled not so much in the weld box, but in the forming of the tube up to that point. The edges of the seam should touch from top to bottom, making sure that one side is not higher than the other. Poor forming can be caused by poor tooling condition or set-up, poor mill condition, or wrong slit width. Often the previous answer to these problems was to slow the mill down. A single TIG torch running at a slow speed can be very forgiving, but it is not a long term solution A final problem that has been encountered due to poor forming is the twisting of the weld seam during production. Some facilities have installed tracking units which move the torch as the seam moves; this solution does not fix the problem. Generally twisting is caused by mill misalignment, tooling problems, or sometimes a malfunction in multi-motor mill drive systems. It is suggested that the source of the twisting be found and corrected by the producer and the tooling or mill manufacturer. An anti-twist stand installed on the mill can be an inexpensive compensation for twisting problems. Before installing a multicathode system, one must make sure that these forming problems are resolved. Experience in the stainless steel industry has shown that most manufacturers pay little or no attention to the quality of weld box covers. It is foolish to assume that a weld box cover must only shield the operator from the arc. The cover is a very important part of the welding process. A properly designed cover can improve weld quality, reduce surface oxides, increase tooling life, and provide a safer workplace. A good cover houses all sides of the weldbox while still providing good vantage points for viewing the welding process. It should be airtight enough so that a small flow of inert gas will provide a positive pressure within the box. If possible, it should provide a means for the application of a trailing inert gas to the weld at the exit side of the uni-body. When installing the torch on the mill, alignment of the electrodes is crucial. Two, three, or four torches must be carefully aligned to the seam and the uni-body must be leveled with a precision device. Most torches come with an aligning tool or method that insures correct position over the seam.

LATEST ADVANCEMENTS IN MULTICATHODE WELDING

The latest advancements in multicathode welding include a tricathode torch that can have magnetic deviation on either the first or second torch giving the ability to weld a greater range of wall thickness using the same uni-body. Another advancement has been a quadracathode unit with electrode separation as small as 15 mm. One of the units includes an additional gas port for the application of a trailing inert gas after the weld. This allows for faster cooling of the weld puddle and reduces the formation of oxides on the surface of the heat-affected zone.

CHOOSING A SYSTEM

Of all the factors to be considered when choosing a multicathode system, the least important is cost. A more expensive system which provides the desired quality at the maximum speed will recoup the cost difference in a short period of time. Some other factors to consider are:

• What is the availability of consumable parts for the system?
• What is the availability of spare parts for the system?
• Does the system require special power supplies?
• Will the unit cover the required range of product on the mill?
• Can your supplier help you with operation of the unit, or do they just install it?
• Is there a production guarantee for the unit?

SUMMARY

The competitive world market for welded stainless steel tubular products requires continuous improvement in both productivity and quality. Multicathode welding is one of the solutions to these problems, but it is not an easy solution. Many producers will find that the addition of a multicathode system is the equivalent of replacing a four cylinder engine with a turbocharged V-8 in their car, in that many new problems become evident as a result of the higher speeds. A carefully formulated plan with as much attention to forming as welding will result in a reduction in the learning curve and a faster rise in productivity. With this in mind, it is a good idea to work with a consultant who has experience operating multicathode systems before purchasing a new welding system.

The Authors

Christopher "Kip" Cole is President of Kipco International. He is past President of the American Tube Association and past Chairman of the Board of the Fabricators and Manufacturers Association. He has Chaired many of the conferences on the production of Stainless Steel Tubular Products. His Company acts as a sales agent for a multicathode welding systems manufacturer, and as consultants to the Stainless Steel Industry worldwide.

Stacy Reese co-authored this paper. He is the Senior Sales Engineer for Kipco International. He holds a BSME from the US Merchant Marine Academy in Kings Point, NY.

For more information please contact:
Kipco international
P.O. Box 187
Auburndale, Florida 33823
Tel. 863-299-0040
Fax. 863-294-2896
Email.
www. http://www.kipcointl.com
Contact. Stacy Reese