Orbital GTAW Boosts Production of Leak‑Free Tubing
Orbital GTAW Boosts Production of Leak‑Free Tubing
Automating the orbital GTAW process allowed a fabricator to meet both the high productivity and leak free requirements its application demanded
Leaks have long been considered to he inherent to industrial hydraulic systems, but recent safety and environmental concerns have made it imperative to explore alternative joining technologies that could produce leak‑free assemblies. In 2001, Iracroft Ltd., a specialist in the manufacture of rigid tube assemblies for high-pressure pneumatic and hydraulic applications, received a request from a major customer for a regular supply of tube assemblies. The customer wanted to replace tubing that was rolled to produce a pipe thread on one end, and which required an "0" ring for the connection, with a leak free welded connection. To make the tube assemblies, which are used for hydraulic lines for tractors, forklifts, and similar industrial equipment, fittings were to be welded onto the ends of low‑carbon steel tubes. The specification for the new tube assembly called for an improved fitting design with a deeper cone to provide a better sealing surface. It also required the welds to be guaranteed leak‑free.
Investment in Orbital Gas Tungsten Arc Welding
Because the customer had a consistent high demand for the product over a number of years, Iracroft decided to invest in joining technology capable of producing the high‑integrity welds in the specified quantities. The company is well‑experienced in gas metal arc welding (GMAW), which could provide the needed quantities, but the GMAW process does not lend itself to leak‑free welds. The gas tungsten arc welding (GTAW) process can produce high‑quality, high‑integrity welds, but is typically not a high‑production process. In order to meet both the high‑productivity and the leak‑free requirements, Iracroft consulted with Arc Machines, UK Ltd., in January 2002 to see if orbital GTA welding technology could be adapted to its application.
Developing an Orbital Welding Workstation
Fig. 1 - The Model 2O7 powersupply and water cooling unit with controls used for the Irucioft workstation.
Fig. 2 Diagram of fitting-to-tube weld joint. It is a square butt joint with no root opening between the fitting and tube.
Arc Machines worked with the company to develop a workstation in which the welding operations would be carried out with minimal operator intervention. The prototype workstation incorporated a Model 207A orbital welding power supply and a Model 9‑1500 orbital fusion weld head Fig. 1. With orbital welding, the part being welded remains in place while an electrode inside the weld head moves the arc circumferentially around the joint to complete the weld. In this case, the weld was done autogenously without the addition of filler metal to the weld.
The power supply was standard equipment, but the weld head was modified to accept the fitting from the loading mechanism and to hold it securely in place during welding. A PLC logic controller was used to manage loading and clamping of the fittings and to interface with the power supply.
The operator handled tube loading. A tungsten electrode holder was designed to simplify electrode changes, which are accomplished using the power supply remote operator pendant. The arc length, which is the distance between the end of the electrode and the work, is set with a special gauge made for the purpose. Maintaining a consistent arc length is critical for maintaining consistency of the welds. An inert gas purge was provided to prevent excessive oxidation inside the tube during welding. Guarding and safety interlocks were installed to protect the welding operators.
Fast‑Track to Production Welding
The company began with a single workstation to weld the fittings to 'X‑in.‑diameter tubing. Weld programs for each size of tubing are stored in the memory of the microprocessor‑controlled power supply. A two‑day orbital welding training class was presented to the lead workstation operators; others received in‑house training.
There was a fairly steep learning curve, but Iracroft gradually improved the process and reduced the number of welds that required rework. Other orbital welding workstations were added to reach the present level of 13 power supplies to weld 6 different tube sizes to the carbon steel end fitting ‑ Fig. 2. Model 9‑1500 weld heads are used for tube sizes from 12 to
20 mm, while Model 9‑2500 weld heads are used to weld 1/4‑in. OD with 3.2‑mm wall and 11/4‑in. OD with 4.06‑mm wall thickness. The larger sizes were the most difficult because high welding currents were required to achieve complete joint penetration. If not corrected, excessive heat buildup in the weld head can cause the weld head to malfunction. The equipment manufacturer worked closely with the fabricator to design special supplementary water cooling for the fitting clamp housing. This was in addition to the built‑in water cooling system in the standard weld head. Water cooling of the weld head and fitting clamp was essential for maintaining productivity goals.
Quality Control/Weld Inspection
Pig. 3 ‑ Borescopic inspection of all orbital welds to verify' complete joint penetration is done routinely at the company's manufacturing facility in Dorset, UK.
Fig. 4 - A completed orbital tube-to-fitting weld. The company is experiencing a reject rate of only 3 ppm.
Quality control is built into the production process. Borescopes (endoscopes) are positioned adjacent to the workstations in order to inspect the inside diameter of the weld soon after completion ‑ Fig. 3. The image of the weld is displayed on a video monitor to verify that the weld has completely penetrated to the inside of the joint and that the weld was properly purged with inert gas during welding. Rework of the weld is permitted in the case of incomplete penetration, but the fabricator has a very low rework rate.
Iracroft bends its assemblies with tube bending equipment to the specific configuration required for the job after the welding and inspection steps have been completed.
The finished assemblies (Fig. 4) are subjected to high working pressures and vibration stresses in the field. It is a seri ous safety issue it' it fitting fails in the field. To guard against this, the welded assemblies were subjected to fatigue testing by the customer, pressure testing, and underwater leak testing. The reject rate is now so low that it is expressed in parts per million (ppm) rather than in percentage. The 3‑ppm reject rate is considered to be wonderful."
At present, the GTA workstations produce about 3500 to 4000 orbital welds per day or about 75,000 per month. This number has been higher at times and sometimes the stations work a double shift, but not continuously. Not all workstations are in operation all the time; instead, the work schedule is demand driven. In the 3V years since the fabricator began its orbital welding operation, it has welded more than 1.7 million parts.
Iracroft has known about orbital welding for years and had considered using it previously, hut didn't attempt it until it had the requirement for leak‑free welds and a commitment for delivery of parts in sufficient quantities over a sufficient period of time to make it practical. The company felt that it would have been very difficult to tool up for just one size. It had to subsidize the first workstation hut realized the potential, and after that first effort, it continued to increase its capability for orbital welding by adding more stations. The company does a minimum of advertising but is well aware of its customer base. They "know who the big guys are." Automation of the orbital GTAW process is fairly unusual, but this will grow as the demand for quality as well as quantity in manufacturing increases.
The author acknowledges the kind assistance of Iracroft Ltd. and John Morris of Arc Machines UK Ltd. in the preparation of this article.
Reprinted from Welding Journal, June, 2006