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Quality, Repeatability And Documentation Drive Use Of Orbital Tube Welding
 Although orbital tube welding has long been employed in aerospace, semiconductor and other high-purity applications, general industrial markets now are beginning to view it as a viable and economical option for connecting stainless steel tubing.
This trend is due in part to the fact that orbital welding provides an entrapment-free, permanent connection that is highly resistant to vibration, often making it the best choice for joints that are not intended to be disassembled at some future time.
But the appeal of orbital welding also can be tied to another significant factor – the introduction of microprocessor technology and other advancements that are putting operators in a better position to meet stringent demands for quality and comply with industry standards and weld specifications. In short, today’s orbital welding systems are user-friendly and enhance the ability to make controlled, repeatable, high-quality and well-documented welds.
These improvements are making orbital welding a practical tool for the power, chemical, oil and gas and pulp and paper industries, for use in construction, OEM’s (original equipment manufacturers) and plant maintenance. And as more welders begin to utilize orbital welding, they should keep in mind that employing some basic pre- and post-weld techniques will further position them to achieve high-quality welds on a consistent basis.
Material matters
Quality welds invariably start with the material. Even the best orbital welding system cannot compensate for poor material used to manufacture tubing, fittings or other components. Selecting the appropriate material is a critical first step. Price should not be the only determinant, as there are inexpensive tubing and fittings made from sub-grade material hitting the domestic marketplace.
Reputable suppliers with appropriate material certification and documentation provide the greatest control over quality. It is absolutely critical to the welding process that certain elements in the material, such as sulfur, be controlled. The sulfur differential between two tubes is immensely important. Attempting to weld tubes together that have a significant differential in sulfur content will likely produce a bead shift toward the tube with the lower sulfur content, potentially causing the weld bead to partially miss the joint.
Variations in tube wall thickness, outside diameter and cleanliness will also affect the quality of the weld. Be sure to inspect all incoming tubing and require suppliers to provide material certifications. A mistake in material selection or control will almost certainly serve as a source for problems later in the welding process.
After selecting the proper material, it is essential to properly store and handle the tube. Do not store it outside, unprotected and uncovered where weather conditions can affect the cleanliness of the tube. The more dust, moisture and dirt inside the tube, the harder it will be to clean and the higher the likelihood of welding problems and contamination.
Cutting edge
Fabricating systems using tubing will involve cutting and facing the tubes in such a manner that they butt together properly in the weld fixture. Properly faced tubes are square or perpendicular to the axis of the tube so they butt together with little or no gap between them. The faced tube end should also have no hanging burrs, and chamfers should be kept to a minimum, typically less than 10 percent of the wall thickness, or 0.005 in., whichever is less. Excessive gaps or chamfering will have a significant affect on the weld bead profile, which may cause the weld to be rejected.
Another consideration when prepping a tube end for orbital welding is to prevent the metal shavings and chips generated from cutting and facing the tube ends from going inside the tube and becoming trapped there after welding. If this happens, these shavings or chips will most likely be moved downstream when the system is pressurized, potentially lodging in another component, such as a regulator or valve.
Sound tube preparing practices and proper equipment selection can minimize the likelihood of these kinds of problems. Orbital welding systems are not complete unless they include tube facing tools. Higher-quality tube facing tools feature chip shields that work in conjunction with the cutting insert to prevent the chip from curling back into the tube. Tube facing tools without chip shields increase the potential for the introduction of chips into the tube. I.D. plugs, or pigs as they are commonly called, are also used to clean the inside of the tube after preparation.
Good facing tools firmly secure the cutting insert so that there is no vibration at the cutting edge, eliminating the possibility of chatter or other irregularities on the tube face. Some tools position the cutting insert in a slot held in place by setscrews, while others feature a cutting insert holder that completely ensures proper placement of the insert each and every time.
Finally, a facing tool system with a properly sized and speed-regulated motor allows the user to select the proper cutting speed for the given material and size tubing that is being cut. This, coupled with proper training on cutting feeds, which the user does manually, will improve the overall success of tube end facing.
It is important to look at the entire design of the facing tool. The goal is to get a consistent square, burr-free and flat finish on the tube ends to produce the best joint fit-up possible and, subsequently, the best weld. Orbital tube welding typically is performed without the use of filler material. Therefore, it is extremely important to have the tube ends properly fit-up so that the wall thickness is maintained and the weld bead profile is acceptable.
Proper placement
Now that the tube has been properly prepped, it is time to position the two tube ends in the fixture. It is important to fixture the tubes securely to allow no movement during the welding process. Even the slightest movement could be the difference between a complete weld and an incomplete or misaligned weld that may be rejected and have to be cut out and re-welded.
 Integral to standard fixtures are the collets, which hold the tube in place during welding. Since small diameter tubing typically has an outside diameter tolerance range of 0.005 of an inch, collets need to be capable of securely holding tubing within this variation. Some collets utilize spring-loaded designs that can compensate for outside diameter variations while others use a solid design. The spring-loaded designs are more prone to allowing movement during welding.
Another fit-up challenge is the proper positioning of the weld joint into the fixture. For common welds, the joint needs to be centered in the fixture so the electrode will be precisely lined up with the weld joint. Some orbital welding systems feature a centering gage that exactly positions the tube joint every time. In other cases, centering is accomplished by visually positioning the joint. Obviously, positioning the joint with a gage will bring more consistent results.
Now that the weld joint has been positioned relative to the electrode position in the weld head, the gap between the electrode and the weld head also needs to be set. This involves positioning the electrode a predetermined distance away from the joint. This can be done in a number of ways, including visual adjustment or with pre-cut electrodes, calipers or a gage. Some equipment suppliers provide a gage, which is probably the most reliable method. The gages are adjustable and can be set to accommodate any diameter size. Setting and maintaining the proper arc gap is very important. Weld programs call out arc gap values, and changing the value or setting it incorrectly can have significant effects on the weld.
Get your program
After properly positioning the weld joint into the fixture, the operator must ensure that the appropriate program is entered into the power supply, either by loading an existing program or creating a new one. If a new one is required, the operator will need to make some calculations on a worksheet for heat input, build the new program and enter it into the power supply.
Some orbital welding system power supplies are microprocessor-based and have auto-programming options that create and adjust programs with minimal operator input. Once programs are developed, they can be stored in on-board memory in the power supply or on a PC data card. Programs can also be transferred between power supplies using these external memory devices.
Individual welds can be rejected for a number of reasons, but the success of orbital welding jobs can be greatly affected by using proper purging techniques. Proper selection of purge gas, typically Argon, is the first step. Argon is available in varying levels of purity, and selecting the proper level for the desired result must be considered. Defining and setting the correct flow and pressure through the tubing and across the weld joint is probably one of the most important procedural steps one can do to ensure successful welding. Conversely, it is probably one of the most likely areas of problems if not properly handled. The internal pressure keeps the weld bead flush to the tube wall inside the tube, while the proper flow will help keep the heat-affected zone clean.
Create documentation
Once the welding process is completed, many specifications call for comprehensive documentation, the amount and level of which often depends upon the industry or the specifications of the project.
Microprocessor-based orbital welding systems can provide a high level of documentation and eliminate the need for operators to track data by hand. They have sophisticated data-logging capabilities that capture an extensive amount of information for each weld in real time electronically. These systems readily interface with computers, enabling this data to be downloaded into spreadsheets or databases. This efficient means of data management can reduce the cost of quality control documentation while improving its accuracy by reducing the chances for human error.
Another benefit to capturing all weld data electronically is the ability to track and analyze specific data to help eliminate errors and increase productivity. Ultimately, data tracking and analysis can be used to ascertain a cost per weld, putting an operator into a position to reduce the cost per weld and improve efficiencies. It actually becomes a tool for contractors to separate themselves from their competition. Contract welding is a very competitive industry with increasingly smaller margins, and data analysis and statistical process control can lead to lower costs, helping contractors become more competitive bidding on contracts.
Still a skill
Orbital welding systems aid the operator in making a quality weld. A misconception exists that the automation associated with these systems eliminates the need for skilled welders. On the contrary, pressure is on operators to execute perfect welds and quickly troubleshoot problem welds. By saving time in both pre- and post-weld preparations and with documentation, microprocessor-based systems allow welders to sharpen their problem-solving skills.
The skill is no longer in making the connection and performing the weld, rather, the real skill is being able to develop solutions and make adjustments with as little scrap, cutouts, and downtime as possible.
Although the technology has been around for more than 25 years, advancements in orbital welding will result in finding even more applications in a wider range of industries.
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