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Making joint design, equipment choices for successful orbital GTAW

Tube, joint, and weld head must work together

Autogenous gas tungsten arc welding (GTAW), by definition and in practice, produces the highest-purity weld when performed by an orbital closed-chamber weld head. Only the parent material, or base alloy, is involved. In this method of joining, the base alloy is melted in an inert environment consisting of argon, helium, or mixture of both. Certain applications call for a small amount of hydrogen, usually 5 percent, to provide a reducing environment for additional protection of the base alloy. Orbital welding of tubing, using a closed-chamber weld head rather than an open-arc weld head, provides a 360-degree protective atmosphere during the entire weld process—heating, fusion, and cooling.

The challenges associated with welding along a curved path, such as the circumference of a tube, make mechanization an attractive option when compared with manual welding. A mechanized system maintains a consistent tip-to-work distance and travel speed, hour after hour, without fatigue.

This doesn’t mean that orbital welding is advantageous for every project that involves joining tubes or pipes, but when it is determined to be the appropriate way to go, the project’s success hinges on several other related issues. These include:

  1. Compatible material type, tube size, and joint design.
  2. Welding equipment designed and optimized for welding tube.
  3. Personnel trained and certified in orbital welding.

This article focuses on the 300 series stainless steel alloys, which are the most common materials used in the semiconductor, aerospace, petrochemical, pharmaceutical, medical instrumentation, and sanitary industries. We assume that the tube’s wall thickness and diameter are consistent with the capabilities of the equipment.

Joint Design

Three types of joint design are used commonly in autogenous tube welding.

Straight Butt-to-Butt. A butt joint is the simplest and least expensive preparation of all weld joints. Consistent machining is necessary for successful, repeatable welds. Square ends and smooth faces are required so that the tube ends butt together without a measurable gap (see Figure 1). Burrs and other defects cause gaps between the tube ends, which probably will cause inconsistent penetration and concavity.

Socket Joint. A socket joint uses a sort of male-to-female coupler fashioned from one of the tubes to be welded (see Figure 2). The machined socket has a wall thickness approximately 20 percent of the tube’s original wall thickness and depth approximately half of the tube wall thickness. A properly machined socket is helpful for two reasons: It provides additional material reinforcement in the weld joint, and it aids in aligning the centerlines of the two tubes.

This type of joint commonly is used for welding valve bodies, manifolds, fittings, and similar devices that are challenging to fixture with respect to the tube centerline.

Weld Ring Insert. A weld ring joint uses a separate machined ring placed between the two tubes (see Figure 3). This is essentially a variation of the butt joint. The machined ring can be made from an alloy different from the tube material to change the weld joint’s metallurgy, or it can be used to introduce a catalyst, coated on the OD, to mitigate porosity or increase penetration.

Regardless of the joint-end preparation, the parts must be free of surface contaminants such as hydrocarbons, surface oxides, moisture, and particulate matter. Solvent cleaning is usually a necessary final step, preceded if necessary, and if permitted, by light abrasion using ceramic particle-embedded plastic pads. These come in a variety of coarseness levels and are readily available commercially.

Figure 1
For a simple butt joint preparation, the tube ends typically are machined on a lathe. If fabrication is done at a worksite, the end preparation can be done by means of a portable end-facing device.

For Class 1000 clean-room applications and other high-purity uses such as laminar flow tables, parts typically come clean, bagged, and ready to weld. In such cases, cleaning is not permitted.

Orbital Equipment Optimized for Tube

A well-designed weld head performs the following functions.

1. Guide the arc to keep it centered over the joint. The weld head is a fixture that aligns the arc with the weld joint and provides an orbital motion at a controlled speed. It also keeps the arc concentric and axially aligned with the weld joint and maintains a specified tungsten-to-surface distance (standoff distance).

The process must maintain a weld puddle that completely consumes the weld joint. Any lack of fusion of the weld joint can result in structural failure; leakage; or continuous fluid contamination caused by embedded hydrocarbons, bacterial growth, or effluence from the unwelded portion of the joint. A 360-degree weld is important for every application, but it is especially critical in the semiconductor and medical instrument industries. To provide the necessary alignment, the weld head’s clamping apparatus must be a precision mechanism that provides sufficient clamping pressure without marring the finish.

For heavy, cumbersome parts, several tack welds might be necessary to keep the parts aligned during the orbital welding process. They also help to prevent gaps from developing between the butted ends caused by plastic deformation from excessive thermal expansion. Tacking often is done by placing the tube ends butt-to-butt, supporting the tube lengths if necessary, and executing a programmed tacking procedure.

2. Provide an inert gas environment for the weld process. In the clamped configuration, the weld head must provide a fully closed chamber to produce a vectored gas flow. The gas input and output must be at opposite ends of the chamber, and any leak paths must be few and small. Such a setup is necessary to provide a constant, nonturbulent flow of inert gas to protect the weld from oxidation and to provide an undisturbed ionic path for the arc to follow. A key component is an internal gas diffuser that maintains a slow-moving gas cloud that surrounds the workpiece without creating excessive impinging gas jets.

A gas chamber setup like this one provides three effects. First, it minimizes the gas velocity, thereby reducing the likelihood of disturbing the arc, which helps to keep it centered on the weld seam. Second, keeping the gas velocity low minimizes the aspiration of air from gaps near the clamping surfaces and edges, thus improving weld quality by keeping the gas as pure as possible. Third, it reduces gas consumption, creating operational savings.

3. Provide adequate insulation and cooling to lengthen the weld head’s service life. Welding produces a lot of heat, and excess heat is detrimental to equipment operation. The weld head design therefore must have adequate self-protection against heat generated by the arc. Lowering the operating temperature of the weld head puts less stress on internal components. It also helps to keep the tungsten electrode properly aligned because it reduces the dimensional variation caused by thermal expansion and contraction. In extreme cases of thick-walled welding, a substantial amount of heat propagates through the tube to the collets and to the outside surface of the weld head body. Use of water-cooled collets eliminates this effect; when using noncooled collets, it’s necessary to reduce the duty cycle, allowing the weld head to recover thermally before starting the next weld.

4. Provide adequate shielding for personnel safety and protection of the surrounding environment. The three main hazards associated with arc welding are heat, ultraviolet (UV) light, and voltage. The weld head’s design can minimize these hazards.

First, because the weld head encloses the weld process, it must have a means of absorbing and dissipating heat. For thin-walled tubing and occasional welds, noncooled weld heads can perform adequately, but for thick-walled tubing and nearly continuous use, water-cooled weld heads make a big difference. The equipment is cooler to the touch after welding, making it easier to handle, and running cooler means a higher duty cycle. Minimizing the heat output also reduces the likelihood of damaging any nearby heat-sensitive components.

Figure 2
A socket joint increases the volume of weld material in the joint and has the same effect as wire feeding without the complexity of wire manipulation.

Second, the high spectrum of UV light that emanates from a welding arc has enough energy to cause sunburn and retina damage. Skin is vulnerable to UV burns after just a few minutes of exposure; retina damage occurs much more quickly. For these two reasons, the clamping mechanism and weld head body must be designed to minimize any gaps along their common interface. It is important to note that in some cases, the top covers of the clamps are made from a clear or translucent material, thereby requiring the operator to use eye protection.

The final hazard is the potential for high-voltage leakage during arc initiation, which can reach thousands of volts. Proper insulation and grounding inside the weld head are necessary to protect personnel, prevent weld head damage, and filter or block stray high voltage from coming back into the power supply circuit.

5. Provide ergonomic features that result in operational ease and reduced operator fatigue. One overlooked factor in weld head design is the elimination of hard corners and sharp edges. Operator comfort in handling equipment is key to maintaining smooth operation without distracting inconveniences (see Figure 4).

Furthermore, it’s not uncommon that many orbital welding equipment operators need a third hand for using the pendant controller, especially on scaffolding or in a precarious position. A small control panel built into the weld head’s handle eliminates this need (see Figure 5).

In confined areas such as gas boxes or panels, a means of fixturing and clamping must be included that can provide the operator with ease and access in presenting the weld head to the work. Side lever clamps or over-the-top clamps are two options.

Welding Power Supply Functions

The welding power supply must provide at least five critical functions. They are:

  1. Ample, precise, and synchronized welding current to provide code-compliant welds (per AWS D17.2).
  2. Monitoring and control of specific weld criteria such as motor speed and gas purge.
  3. Programming capabilities for developing and using automatic weld procedures stored in the power supply’s memory.
  4. Automatic real-time reporting and logging of weld status and performance with conclusive summaries per weld and per group of welds, as, for example, during a work shift.
  5. Ease of transporting and setting up the orbital welding power supply under different conditions through capabilities such as auto-ranging compatible power inputs from 110 to 240 volts.

For today’s ever-increasing requirements in manufacturing quality and reporting, it is important to have the equipment that can help to support procedural requirements such as ISO 9000.

In the early days of orbital welding,-during the Apollo missions and the early days of the Space Shuttle project, welding parameters were captured by a system that used pen recorders to generate graphs on long sheets of paper. Now the parameters are recorded digitally for long-term storage, and specialized software summarizes and reports in addition to recording. In this way, quality issues can be identified before parts are shipped or integrated into larger assemblies or systems.

Personnel Training and Certification

While modern orbital welding systems have many automated capabilities, success still hinges on the equipment operator. Training is a key factor, but personal attitude is just as important and, in some cases, overrides training. Therefore, personnel selection is one of the most important components to success.

Operator training typically lasts one to two days for welders skilled in GTAW. For personnel not previously involved in welding, training typically takes four to five days. The training outline usually includes classroom time to discuss equipment functions, causes and effects, and theory of operation; hands-on training in a shoplike environment, typically two trainees per welding system; a written exam; and certificates of successful completion.

Figure 3
Orbital GTAW is often autogenous, but not always. A machined ring placed between the tubes gives welding engineers a way to change the metallurgy of the joint.

Reaping the Rewards

Using a mechanized system means consistent results, but doesn’t necessarily lead to successful results. Success is a matter of careful research so that the equipment matches the application, the operators have proper training, and that they carry out appropriate procedures. Advanced equipment and demanding applications require a thorough understanding of the process, and orbital welding is no exception. The proof comes out in large construction projects. For example, a recently constructed solar power station had more than 77,000 orbital welds with a reject rate of less than 1 percent.

When using orbital welding equipment is feasible, the simplicity of the automated process has the potential for a hat trick: high productivity, low cost per joint, and a nominal number of discontinuities and defects.

About the Authors

Timothy Gittens

Business Development Manager

2599 Charlotte Hwy

Mooresville, NC 28117

V. John Jusionis

Sales and Project Engineer

2599 Charlotte Hwy.

Mooresville, NC 28117