Quality, repeatability, and documentation drive the technology
July 12, 2001
Although orbital tube welding has been used in aerospace, semiconductor, and other high-purity applications for a long time, general industrial markets just now are beginning to view it as a viable and economical option for joining stainless steel tubing.
This trend is due partly to orbital welding's capability to make an entrapment-free, permanent connection that is highly resistant to vibration. This makes it a good choice for joints that are not intended to be disassembled in the future.
However, the appeal of orbital welding also is due to another significant factor—the introduction of microprocessor technology and other advancements that put operators in a better position to meet stringent demands for quality and to comply with industry standards and weld specifications. In short, orbital welding systems are becoming more user-friendly and enhancing welders' ability to make controlled, repeatable, high-quality, and well-documented welds.
These improvements are making orbital welding a more practical tool for the power, chemical, oil and gas, and pulp and paper industries; for use in construction; for original equipment manufacturers (OEMs); and for plant maintenance.
Furthermore, employing some basic pre- and postweld techniques increases the consistency of weld quality.
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, because inexpensive tubing and fittings made from subgrade material are hitting the domestic marketplace.
Reputable suppliers with appropriate material certification and documentation provide the greatest control over quality. It is 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 sulfur content differential will likely produce a bead shift toward the tube with the lower sulfur content, potentially causing the weld bead to miss the joint partially.
Variations in tube wall thickness, outside diameter, and cleanliness also will affect the quality of the weld. All incoming tubing should be inspected, and suppliers should be required to provide material certifications. A mistake in material selection or control almost certainly will serve as a source for problems later in the welding process.
After the proper material is selected, it is essential to store and handle the tube properly. It should not be stored outside, unprotected, uncovered, or where weather conditions can affect its cleanliness. The more dust, moisture, and dirt that are inside the tube, the harder it is to clean, and the higher the likelihood of welding problems and contamination.
Processes for fabricating tubing involve cutting and facing the tubes in such a manner that they butt together properly (see Figure 1) 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 have no hanging burrs, and chamfers should be kept to a minimum—less than 10 percent of the wall thickness, or 0.005 inch, whichever is less. Excessive gaps or chamfering will have a significant effect on the weld bead profile, which may cause the weld to be rejected.
Another consideration when preparing a tube end for orbital welding is to prevent the metal shavings and chips that are generated from going inside the tube and becoming trapped there after welding. If this happens, these shavings or chips most likely will be moved downstream when the system is pressurized, potentially lodging in another component, such as a regulator or valve. Sound tube preparation practices and proper equipment selection can minimize the likelihood of these kinds of problems.
Some orbital welding systems include tube facing tools. The tools should secure the cutting insert firmly so that there is no vibration at the cutting edge, eliminating the possibility of introducing irregularities to the tube face. Furthermore, some facing tools feature chip shields that work in conjunction with the cutting insert to prevent the chip from curling back into the tube. ID plugs, or pigs as they are commonly called, also are used to clean the inside of the tube after preparation.
Finally, an orbital welding system with a properly sized and speed-regulated motor allows the user to select the proper cutting speed for the given material and size of 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.
With the tube properly prepared, it is necessary to position the two tube ends in the fixture. It is important to fixture the tubes securely to prevent movement during the welding process. Even the slightest movement could result in an incomplete or misaligned weld, which would require rework.
Integral to standard fixtures are the collets, which hold the tube in place during welding. Because small-diameter tubing typically has an outside diameter tolerance range of ±0.005 inch, collets need to be able to hold tubing securely to this tolerance. Some collets have 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 lined up precisely with the weld joint. Some orbital welding systems feature a centering gauge that positions the tube joint exactly every time. In other cases, centering is accomplished by positioning the joint visually. Obviously, positioning the joint with a gauge brings results that are more consistent.
After the weld joint has been positioned relative to the electrode position in the weld head, the gap between the electrode and the weld head must be set. This involves positioning the electrode a predetermined distance from the joint. This can be done in a number of ways, including visual adjustment or with precut electrodes, calipers, or a gauge. Gauges 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 affect the weld significantly.
With the weld joint properly positioned in the fixture, the operator must ensure that the appropriate program is entered into the power supply, by either loading an existing program or creating a new one. If a new one is required, the operator needs to make some calculations on a worksheet for heat input, build the new program, and enter it into the power supply.
Some power supplies are microprocessor-based and have autoprogramming options that create and adjust programs with minimal operator input. Once programs are developed, they can be stored in onboard memory in the power supply or on a PC data card. Programs also can be transferred between power supplies using these external memory devices.
The success of orbital welding jobs can be greatly affected by the purging techniques used. Selecting the proper purge gas, typically argon, is the first step. Argon is available in varying levels of purity, and choosing 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 probably are two of the most important procedural steps one can take to ensure successful welding. Conversely, they are likely to cause problems if not handled properly. The internal pressure keeps the weld bead flush to the tube wall inside the tube, while the proper flow helps to keep the heat-affected zone clean.
Once the welding process is completed, many specifications call for comprehensive documentation, the amount and level of which often depend on 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 electronically capture an extensive amount of information for each weld in real time. These systems readily interface with computers, enabling this data to be downloaded to 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.
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 postweld preparations and producing documentation, microprocessor-based systems allow welders to sharpen their problem-solving skills.
The skill required no longer is 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.