CONSIDERING ORBITAL GMAW/FCAW PIPE WELDING?
A Primer for the Prospective First-time User

Orbital GMAW/FCAW welding is increasingly being used in a wide variety of industries. It is being used to replace manual SMAW, semiautomatic GMAW/FCAW, and even orbital GTAW. The following is a brief introduction to this mechanized process and a discussion of many of the questions typically raised by the manufacturer or fabricator considering the first-time use of mechanized GMAW/FCAW pipe welding. Some of the common pitfalls to avoid are identified.

There is a misconception that orbital GMAW/FCAW is a exotic new process, and today few companies can afford to be guinea pigs for product development. Orbital pipe welding systems using the GMAW process were first used for pipeline applications in 1969 and since that time in excess of 25,000 miles of pipeline have been welding using the process. Orbital FCAW was introduced 10 years ago and has been used successfully in industries ranging from ship building, power plant construction and maintenance, chemical plant/refinery construction and maintenance, as well as offshore gas production, to name just a few. The Electric Power Research Institute performs research and development for 800 of the nations Electric Utilities. Mr. David Gandy (Program Manager of Materials and Fossil Applications) states that Ait has been conclusively demonstrated to Utility members that current technology makes orbital GMAW/FCAW a valid option for maintenance applications in both nuclear and fossil power generation facilities.
Equipment Configuration

All of the equipment on the market essentially consists of:
- a weld power supply
- a weld Head which mounts the torch and manipulates it, similar to the motions a manual welder would use
- a band or "guide ring" which clamps on the pipe and provides mounting for the Head and is involved in propulsion of the Head around the pipe
- a filler wire feeder which may be mounted on the floor or on the rotating Head
- a weld programmer/controller which may be integrated with the power supply or the weld Head. A remote control Pendant is usually provided.

Weld Head models are available in Wire Feeder on Floor (WFOF) and Wire Feeder on Head (WFOH) configurations, allowing use for a wide variety of applications. Each version has advantages and disadvantages for specific applications.

Wire Feeder on Floor systems use a conventional wire feeder mounting standard 15 kg (25 lb) spools, resulting in lower electrode costs. This version has advantages for fabrication shop use with smaller workpieces and when the wire feeder can be conveniently located near the weld Head. Torch cable lengths are limited to 4.6m (15') between weld Head and wire feeder however, which is a limitation for many applications. Future development of push-pull systems will possibly minimize these shortcomings. The lower weight of the weld Head makes it easier to move from joint to joint.

Wire Feed on Head systems use a smaller 5 kg (10 lb) spool of wire mounted on the rotating Head with the feed mechanism. This configuration is frequently necessary for use on large workpieces or for field erection where the weld Head must be operated at a distance (50'/15m or more) from the power source and a floor-mounted feeder would be inconvenient.

One advantage of orbital GMAW is that the process allows use of conventional bevel geometries that are normally used for manual welding. A standard 30 or 37.5 bevel is used for pipe with wall thicknesses up to 19mm (3/4") and a compound bevel for heavier wall thicknesses to minimize the amount of wire that must be deposited.

Although it is possible to obtain significant productivity improvements with solid wire, there are even greater benefits using flux core wire when welding pipe out of position. The deposition rate is higher and the welding process is more forgiving, and thus less dependant on operator skill level.

Flux core wires were first introduced in 1957. The evolution of filler wire technology is one of the principle ingredients making orbital FCAW a viable process today. Flux core electrodes are available today for virtually any grade of carbon steel, low alloy steel, and stainless pipe material. The proper handling of FCAW electrode, however, remains one of the most poorly understood subjects and is the most frequent cause of start up problems for the first-time user.

Flux core electrode consists of a metal sheath which is filled with a core of various powder materials. These may include fluxing elements, deoxidizers, denitrating compounds, and other alloying materials to variously increase hardness, strength, or improve corrosion resistance and arc stability. Virtually all of the flux core wire on the market today does not have a continuous 360 sheath, but has either a butt or overlap joint. This interruption in the metal sheath can permit moisture into the flux.
Flux core electrode is essentially stick electrode turned inside out. The seam along the entire length of the electrode can allow moisture to invade the flux. It is common practice to keep stick electrodes in a sealed container until ready for use and if they are not used that day, they are put in a "rod oven" to drive out the moisture. Yet customers often will improperly store or allow partially used spools of electrode to be exposed to moisture without realizing that they will experience the same problems. A frequent early complaint is "worm hole porosity" which is usually attributable to moisture in the flux.

Flux-core electrode is frequently made in two formulations: a standard formulation and a "all position" formulation. The flux in the "all position" wire causes the puddle to freeze much faster which is required when welding "out of position" relative to gravity. "All position" versions of wire must be used for orbital pipe welding.

An unfortunate fact is that FCAW electrodes from different manufacturers, which may have the identical specification, do not all perform the same. An electrode from one manufacturer may have very different welding characteristics than that from another despite the fact that they have the same specification. In addition, there has been so much consolidation in the industry that one manufacturer may operate many facilities, with electrodes marketed under their original labels. The wire from one factory may be a little different in formulation than another. Ask for advise from the orbital equipment manufacturer for a list of electrode brands with which their customers have had good results. Or evaluate electrodes from several manufacturers and form your own opinion before making a large purchase.

When ordering electrode, be sure to specify:
- Level Wound, regardless of spool size
- Wire to be supplied in Sealed Bags with desiccant. (This is often available as an option.)

With the equipment on the market today, the root pass is generally done manually using either manual GTAW, SMAW, or semiautomatic GMAW. The pipe ends will be gapped apart using pre-established procedures. Generally, two passes will be required - a root pass and subsequent hot pass - before mechanized GMAW/FCAW welding can be used to complete the weld. A single pass is generally insufficient to avoid repenetration by the higher voltage/amperage FCAW parameters used for the fill passes. There are semiautomatic GMAW power sources on the market today, however, which will deposit a root pass sufficiently thick to eliminate the need for a subsequent hot pass.

Can the root pass be done using a mechanized GMAW system? The answer is a qualified Yes, but with limitations which make it impractical for most users. Flux core wire cannot be used for doing the root pass because of the potential of slag entrapment. Therefore, solid wire must be used. If the fill pass procedures require flux core, which offers substantially higher deposition rates, the user will either have to exchange the filler wire spool, or have two systems, with one dedicated strictly to root pass welding. While the root pass can be done with orbital GMAW, it does require precise fit up including a very uniform gap around the 360 bevel as well as tight control over high/low mismatch. In addition to tight fit up tolerances, a greater amount of operator skill is required when doing the root pass. One company has developed an orbital pipe welding system specifically for making the root pass for pipeline applications. This system, however, requires an internal spacer clamp which precisely aligns the pipe ends and simultaneously creates a precise gap. While this system is practical for pipeline welding, the need for a very large internal clamp designed for a specific pipe size makes it impractical for most fabrication or field erection applications. Presently, it may be better to plan to do the root and hot pass manually or at least until welders have gained sufficient experienced with equipment.

While the term orbital has become closely associated with mechanized GTAW welding, it is not totally appropriate when applied to mechanized FCAW welding. With the pipe in the horizontal (5G position), welding is done using a double up technique - 180 clockwise followed by 180 counterclockwise. An orbital technique is used only with the pipe vertical (2G position).

A double-down welding technique can be used with GMAW. This technique is generally only used for cross-country or marine pipeline laying operations. Joints-per-day productivity of these operations mandate multiple weld stations with two or more weld Heads per station used on a joint simultaneously. Each station is usually dedicated to one pass - for example, a Root Pass station followed by Hot Pass stations, two Fill Pass stations, and the final Cap Pass station. As each pass is completed, all the stations move forward to begin the same pass on the next joint in sequence. Because all stations must move-up in lockstep, the weld cycle time is dependant on the speed at the initial Root Pass station. While much higher rotational travel speeds can be used, less metal is deposited per pass. Great care must be taken by the operator to avoid sidewall fusion defects.

When welding in the 5G position, a weave technique is generally used. Most of the systems available allow a programmable dwell or delay period at either end of the oscillation stroke which guarantees proper sidewall fusion. Oscillation dwell is programmed in tenths of a second. A useful feature is the ability to index the rotation of the torch around the pipe with the weave or oscillation motion of the torch. As the torch moves across the joint, forward motion of the torch (weld Head) is interrupted. When the torch reaches the end of its stroke and begins its programmable dwell period on the sidewall, the weld Head tractor moves the torch forward at the programmed speed. This approximates a technique used by many manual welders.

All of the equipment on the market today using either solid and flux core electrodes require an appropriate shielding gas. The gasless type wires, while excellent for many semi-automatic (manual) applications, still produce excessive spatter which would ultimately degrade the orbital equipment where the torch is mounted inches from the weld Head mechanisms.

This is an important consideration that must be planned for if the equipment is to be used for field construction purposes. A contractor accustomed to welding with stick will have to consider erecting wind breaks, tarps, or tents over the equipment to prevent the shielding gas from being disrupted by wind, as well as to protect the equipment from the elements. He must consider the added cost of gas and the logistics of moving gas bottles around the job site and safely restraining them.

Particular attention should be paid to the manufacturer's recommendation for shielding gas composition for a particular filler wire type. Shielding gas not only affects the arc stability and general "weldability," but also the metallurgy of the deposited metal. Frequently, a manufacturer will recommend more than one gas, for example 100% CO2 or 75% Argon/25%CO2. The user should consider the trade offs when making the selection. For example, CO2 gas is least expensive, but argon/CO2 mixtures will produce a more stable arc with less spatter. Other factors will affect filler wire performance such as the characteristics of the power supply used and the metal transfer mode (short circuit, spray, pulse spray).

Torch oscillation may be accomplished by a straight line linear motion or a pendular motion. Each technique has its proponents. Either technique will produce satisfactory results with FCAW. Several systems on the market provide the capability of both modes, which allows the customer to select the mode for a specific application. Fillet or socket welds are often best done using pendular oscillation.

Typical parameters for 5G fill passes with FCAW electrode:
- Electrode Diameter: 0.045" (1.2mm)
- Volts: 24 - 26
- Amps: 180 - 260
- Electrode Speed: 180 -300 ipm (4.6 - 7.6 mpm)
- Head (Torch) Rotation Speed (Average): 6 ipm (15cm/min)
- Deposition Rate: 4.6 - 6.8 lbs/hr (2.1 - 3.1 kg/hr)
- Gas Usage: 40 - 50 cfh (15 - 20 liters/min)

The potential user must consider whether his application will allow the productivity gains to justify the purchase of equipment and cost of qualifying new procedures. For example, a contractor with a job consisting primarily of smaller diameter thinner wall (Schedule 40) pipe may not be a logical candidate for mechanized FCAW. An application consisting of welding heavy wall CrMo piping with a preheat which must be maintained during the welding process is likely a very suitable candidate for mechanized FCAW.

Such applications which can take advantage of the high duty cycle of a machine are more obvious. Less obvious may be a limited welding requirement in a fabricated product, where the duration of the welding operation, subsequent inspection and repair proves to be a critical path item affecting delivery schedule. A major manufacturer of medical imaging devices uses orbital GMAW for a single-pass 84" diameter fillet weld which must undergo helium leakage testing.

Many manufacturers have a rental program with purchase option which allows the customer to evaluate the equipment for his specific application using his personnel. This Atry before you buy@ approach may be a viable technique to evaluating the technology for those customers that are unsure about whether productivity improvements can be obtained for their specific application.

All manufacturers of pipe welding equipment will offer training at the customer's facility. It is extremely important that management realize that training is required and there is a learning curve associate with any new equipment. Equipment marketed today is "mechanized," not "automatic." Too often there is an attempt to train welders "on the job." The manufacturer of the equipment will provide a structured training course, but time must still be allowed for the welder to practice, make the inevitable mistakes, and become confident before the equipment should be used in production.

Orbital FCAW is capable of making welds meeting the same quality standards as SMAW, Submerged Arc Welding and in most cases GTAW. Many companies are already using semiautomatic GMAW/FCAW and the transition to mechanized orbital equipment for out of position pipe welding is a simple economic decision based upon pipe size, wall thickness and quantity of welds made. Other companies are hesitant through lack of experience with the process. However, numerous businesses and organizations have evaluated equipment, tested welds and approved the process for even some of the most demanding applications such as welding high-pressure piping in a power plant or Duplex stainless steel piping assemblies for offshore gas production and transportation.