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Fundamentals of Orbital Welding

Modern orbital-welding systems offer computer controls that store welding schedules in memory. The skills of a certified welder are thus built into the system, enabling the production of enormous numbers of identical welds and leaving little room for error or defects. This article serves as a guide to the technical and financial considerations of orbital welding.

Bernard Mannion

Orbital welding first found use in the 1960s when the aerospace industry recognized the need for a superior joining technique for aircraft hydraulic lines. The solution: a mechanism to rotate a welding arc from a tungsten electrode around a tube-weld joint. Regulating weld current with a control system automated the entire process. The result was a more precise, reliable method than manual welding.

Orbital welding became practical for many industries in the early 1980s with the development of portable combination power supply/control systems that operated from 110-V AC. Modern orbital-welding systems offer computer controls that store welding schedules in memory. The skills of a certified welder are thus built into the system, enabling the production of enormous numbers of identical welds and leaving little room for error or defects.

Orbital welding uses the gas-tungsten-arc-welding (GTAW) process as the source of the electric arc that melts the base material and forms the weld. During GTAW an electric arc forms between a tungsten electrode and the workpiece. To initiate the arc, an RF or high-voltage signal will ionize the shielding gas to generate a path for the weld current. A capacitor dumps current into the arc to reduce arc voltage to a point where the power supply can regulate. The power supply responds to the demand and provides current to maintain the arc.

A typical orbital tube weld. Note the overlapping, pulsed arc finish to the weld surface.

Material weldability

In general, the commonly used 300-series stainless steels offer a high degree of weldability using orbital equipment, except for types 303/303SE, which contain additives for ease of machining. The 400-series stainless steels, while generally weldable, may require post-weld heat treatment.

Fabricators should be prepared to adjust the orbital-welding setup to accommodate for potential differences among material heats.

Weld joint fitup

Fitup is dependent on the weld specification requirements on tube straightness, weld concavity, reinforcement, and drop through. If no specification exists, the laws of physics require that the molten material flow and compensate for tube mismatch and any gap in the weld joint.

The use of tube and pipe end-prep facing equipment helps to ensure end squareness and flatness. The i.d. and o.d. should be burr-free with no chamfer.

Sidebar - Industries and Applications for Orbital Welding

Shielding gas basics

During welding, an inert gas directed to the tube o.d. and i.d. prevents the molten material from combining with oxygen in the ambient atmosphere. With sufficient shielding gas coverage, welds can have practically zero tint at the weld zone i.d.

Argon is the most commonly used shielding gas for the o.d. and as the i.d. purge gas. Argon/hydrogen gas mixtures or helium gas may be used as the shielding gas for benefits to specific applications.

Tungsten electrode choice: The right tool for the job

The tungsten electrode, the source of the welding arc, is singularly the most important element of the welding system most often ignored by orbital welders. While no one would refute the importance of the ignition device on an automobile airbag, the rip cord for a parachute, or quality tires for our automobiles, the importance of the tungsten electrode is often overlooked. Whether in manual or automatic welding, this is one area where users can improve the consistency of their welding output with minor effort.

The welding procedure

Many welding equipment suppliers offer a series of pre-engineered weld schedules for a variety of tube diameters, wall thicknesses, and base materials. Welders should always follow these suggested procedures first. However, there will always exist a trade-off in maximum possible weld speed and weld quality and repeatability. Where weld parameter specifications do not exist or the engineer would like to change those settings, follow these guidelines:

Arc Length: Arc-gap setting depends on the weld current, arc stability, and tube concentricity or ovality. The welding engineer must keep the electrode at a constant distance from the tube surface with sufficient gap to avoid stubbing-out.

As a rule of thumb, set a base arc gap of 0.010 in. and add to this half the tube-wall thickness (or required penetration) expressed in thousandths of an inch. Thus, if the tube wall is 0.030 in. thick, then a good starting arc gap would be 0.010 + 0.015 = .025 in.

Weld Speed: Weld speed depends on flow rate of material to be welded and wall thickness. The objective: to weld as fast as possible while still producing a high-quality weld.

As a starting point, set welding speed at 4 to 10 in./min, running faster on thinner-wall materials and lower on heavy-wall tube.

To produce consistent, high-quality welds, the tungsten electrode must be of high-quality material and tip dimensions must be held to close tolerances.

Welding Current: Welding current depends on the base material, wall thickness, weld speed, and shielding gas. The objective: to achieve full penetration defect-free welds.

As a starting point, for welding of stainless steel use 1 A of weld current for every 0.001 in. of wall thickness. Thus, for 0.030 in.-wall tubing, set average weld current to 30A.

Weld Current Levels: Orbital welding typically calls for multiple levels of weld current to compensate for heat buildup in the tube during welding. If the current used to initially penetrate the tubing was held at the same level for the complete weld, penetration would increase as the weld progressed around the tube, resulting in excessive penetration.

Typically, orbital-welding procedures employ a minimum of four levels of weld time, with amperage decreasing from level 1 to level 4.

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