Just as inverter-power-supply technology sweeps through the ranks of manual welding-power supplies, it gains popularity by the tube-and pipe-welding equipment manufacturers. Fabricators can now select among orbital-welding equipment that is smaller and lighter than ever, for portability at the job site and to fit into cramped working conditions. One worker can transport a miniaturized power source integrated with a water-cooler package. A decade ago, a power supply suitable for pipe welding tipped the scale at 600 pounds for a welding output of 300 amps. Today, the equivalent power supply weighs in a svelte 100 pounds.

Modern power supplies carry microprocessor-based controls that offer numerous benefits compared to older analog-based controls. For tube-welding, equipment with analog programming typically carried four preset levels. The equipment operator would record weld parameters on paper and carefully input them into the controller for each size of tube to be welded. By comparison, modern microprocessor-based controls can store up to 100 weld programs. The operator merely calls up the appropriate weld schedule and pushes the START-WELD switch. Rather than four levels of parameters per program, these programs frequently offer 100 levels, and allow changes in welding parameters as a function of rotational position.

Has this increased programming capability actually led to better welds? Probably not for welders of tubing; analog units deposit welds equal in quality to the welds of microprocessor-based units. However, the microprocessor has led to many conveniences not previously available in analog tube-welding equipment. For example, automatic procedure generation allows the operator to enter basic parameters such as tube diameter, wall thickness, and travel speed. The program then automatically creates a near-perfect welding procedure, simplifying the weld-development process, with only one or two welds required to fine-tune the program. This capability eliminates the tedious requirement of depositing numerous test welds, and requires a minimal amount of welding expertise by the operator.

Microprocessor-based controls also carry software that automatically selects the appropriate weld head based on the application. Using an analog system, the operator needs to determine desires rotation speeds, convert them to RPM using a formula specific to each model of weld head, and enter an arbitrary speed setting based on a calibration curve provided with the equipment. Conversely, most modern systems immediately display actual RPM for the model of weld head selected. The operator selects a desired tungsten speed and the software calculates the RPM setting for the selected weld head.

Moving up to pipe welding

Unlike the pro-and-con argument that can be made for the fusion-only tube-welding industry, for multipass pipe welding microprocessor-based programming provides distinct advantages. Old analog machines required the operator to carefully enter the parameters for each weld pass using an array of switches and dials. Changing parameters during welding required the operator to turn the dials on a remote pendant control at precisely the right moment, demanding a high level of skill and attentiveness. Typically, the operator would terminate the welding process after each pass and reset the dials for the subsequent pass working from a setup sheet. Each dial-spinning episode became a possible source of error. Although several companies offered analog programmers that allowed programming an entire weld on small sizes of pipe, these controllers consisted of a large number of conventional electrical components, including potentiometers, switches, and relays. This complexity ultimately effected reliability, since the relays were often required to operate in harsh environments for thousands of cycles. The introduction of the microprocessor allows programming a multipass weld from root pass through cap pass with an uninterrupted weld, eliminating mistakes in setting parameters.

Microprocessors bring other advantages to pipe welders. For example, parameter-override lockouts set limits on each welding parameter; the operator can set variable override percentages from zero to 100 percent. A welding supervisor can limit the amount of changes an operator can make during a program, preventing him from accessing the override screen through the use of a key switch or password. Thus, the supervisor might set override limits at zero for a particularly critical on duplex stainless-steel tubing for example. Conversely, retubing a furnace at a chemical plant might require a greater degree of discretion or override capability by the welder, to compensate for imperfect fitup between the new tubes and existing tubes.

In addition to the ability to adjust parameters between weld passes, operators of microprocessor-based pipe-welding machines can also program changes to weld parameters during each pass or orbit around the pipe, which offers distinct advantages for certain applications. An example is welding on copper-nickel or high-copper-content tube and piping. The high thermal conductivity of these materials requires high current at arc start that tapers quickly to maintain stable puddle size and uniform penetration around the joint. Another example is the welding of high-nickel alloys, extremely sensitive to crater cracking at welding start and stop points. Microprocessor-based controls allow the operator to set initial and final current levels with precise control over the rate of upslope and downslope to minimize thermal shock. A software option allows the feeding of filler wire during part of the downslope period to further minimize thermal shock and the likelihood of cracking.

As an added benefit, many microprocessor-based power sources can store programmed welding procedures on solid-state data cards. This allows the operator to easily transfer programs developed on one machine to other machines being used either in the same facility doing the same type of work. Data cards store 30 to 100 programs, created on a standard personal computer using software provided by the manufacturer.

Microprocessor-based machines can immediately alert the operator if weld parameters fall outside preselected limits, and can print-out records of preset and actual parameters. Parameters can be downloaded to PC data cares or directly to a quality-control computer program for storage or statistical analysis. This can eliminate time-consuming and expensive radiography in many applications.

Audible alarms alert the operator to out-of-tolerance conditions, enabling him to perform timely functions and prevent the deposit of faulty welds. For example, an out-of-limit voltage signal might indicate that the tungsten electrode tip has deteriorated and requires replacement. To ensure equipment repeatability of older systems, users could only periodically calibrate meters, and call on strip-chart recorders to monitor critical parameters for each weld, such as amperage, voltage, and rotation speed. The charts required skilled interpretation. If a user did not want to monitor parameters for every weld, he had to use radiography and destructive testing to determine if the equipment was operating correctly.

Selecting the optimum weld head

A variety of weld heads have been developed to meet specific application requirements and constraints. Weld heads fall into one of four categories: enclosed, or cassette, limited-function; full function-in-place; or orbital or guide-ring full-function.

Applications for enclosed heads abound in the aerospace, pharmaceutical, nuclear, semiconductor, food processing and dairy, and medical and biotechnology industries.

Enclosed heads offer superior gas shielding, as they enclose the entire joint. Also, the enclosed arc eliminates the need for weld-shield and eye-protection devices. They are the simplest type of head to operate. However, their high-temperature plastic bodies limit duty cycles on heavy-wall stainless-and carbon-steel tubes. Tube and pipe must be round to ensure a tight seal. The heads require periodic cleanings to avoid internal arcing. Each head model only covers a finite range of tube sizes, and radial clearance is dependent on tube o.d.

In-place limited-function heads, of a relatively simple design, adjust to clamp to pipe or tube of a specified o.d. range. An adjustable lever-actuated clamp holds the head on the workpiece and remains stationary. A torch and wire feeder mount on a rotating disc; a torch-cable assembly delivers weld current, cooling water, and shielding gas to the head. Position of the tungsten electrode adjusts laterally relative to the joint. A mechanical follower device, using the pipe surface as reference, corrects arc length changes due to any out-of-roundness of the pipe. This head type enables use of filler-wire feeders positioned either on the floor or mounted on the rotating portion of the weld head.

Applications include aerospace, pharmaceutical, semiconductor and high purity, food process and dairy, process pipe, heat exchangers, chemical, and power generation. Benefits include clamping on only one side of the joint; uses for either autogenous welding or with filler wire; can weld out-of-round pipe; all-metal construction tolerates heat well; and can be used with standard fittings purchased from the manufacturer. Limitations: each head covers a finite size range of diameters; shielding-gas coverage is inferior to that of enclosed heads; radial clearance depends on pipe o.d.; and multipass welding requires the use of the stringer-bead technique.

Full-function in-place heads operate similarly to the limited-function heads, but can incorporate electronic arc-voltage control and electronic torch oscillation. These heads find work depositing multipass welds on pressure pipe. Applications include fossil- and nuclear-power plant construction and maintenance, steam-generation equipment, process piping, chemical and refinery plants construction and maintenance, and ship construction and maintenance. They allow clamping on one side of the joint, and are quickly installed. However, each head can weld only a finite range of tube and pipe sizes, radial clearance depends on pipe o.d., and the heads are not water-cooled, limiting their use on alloys requiring preheat.

Full-function orbital heads incorporate torch rotation, filler-wire feed, electronic arc-voltage control, and electronic torch oscillation. Unlike the in-place head, however, the entire orbital weld head rotates around the workpiece. The head attaches to the pipe using a metal band or guide ring fabricated to match the size of the tube or pipe. Generally, the guide ring attaches to the pipe, then the head installs on the ring. Applications include fossil-and nuclear-power plant construction and maintenance, steam-generation equipment, process piping, chemical and refinery plant construction and maintenance, ship construction and maintenance. One model covers a broad size range, and fixed radial clearance remains constant on all sizes. These heads find use on the simplest fusion-welding jobs as well as on complex multipass welding of heavy-wall pipe. Head design permits water cooling of the head body for use on alloys requiring preheat.

Training operators in the use of orbital tube and pipe welding systems seems to follow the general trend in vocational training in the manufacturing sciences - inadequate, with too few courses in technical schools for welding in general, and few apprenticeship programs.

Unfortunately, too-few first-time users understand the amount of training required, and the length of the associated learning curve, before operators can become proficient at running orbital-welding equipment. Too often contractors will rent or purchase equipment on Friday and expect it to be productive on Monday morning. Even companies with long-term experience fail to properly train operators when new equipment comes on-board.

A few bright spots appear on the horizon concerning training. Many first-time operators come from a non-welding background and expect a more-realistic learning curve. Many union locals are purchasing equipment and conducting training programs for their members to support local companies. Many companies are finally realizing that there is a real payback in worker-training programs.