Laser welding of stainless pressure tubes
Not all laser welded tubes are created equal. Know what to look for in the final product to ensure that you're buying quality and not just an imposter.
The vast majority of welded stainless steel pressure tubing is produced with the gas tungsten arc welding (GTAW) process. This process often is referred to as tungsten inert gas (TIG) welding. However, with the recent advent of reliable, high-power laser power sources, the laser beam welding (LBW) process has moved quickly into stainless welded tube manufacturing.
The first uses for this technology were ornamental and mechanical tubing, which is not manufactured for pressure applications, but rather for such uses as swimming pool ladders, truck mirror frames, and automotive exhaust pipes. These applications do not require additional operations beyond welding and possibly heat treating (annealing) to achieve an acceptable product. Recently laser welding has become more common in the production of pressure tubing to ASTM A249 or ASME SA249 specifications. As with GTAW tubing, the quality of laser-welded tubing can vary dramatically by producer.
Causes of Poor Quality
Variations in stainless pressure tubing quality usually are related to the cold work of the weld area on both the inside diameter (ID) and outside diameter (OD) of the tube. The weld area of a high-quality pressure tube has been forged out and annealed properly so that it effectively disappears and blends with the base metal of the tube. This is known as a full finished tube.
Most poor-quality tubes are the result of inadequate cold working of the weld area. The cold-work procedures that usually are effective for GTAW pressure tubes are not always effective at the higher speeds at which laser weld mills run. Consequently, many producers perform no cold work or use a bead roller inside the tube to blend the weld into the base metal, often with no attempt to cold-work the OD of the weld.
A poor-quality tube may have a weld on the OD that has been sanded or ground to remove weld buildup, and this may be hard to spot. However, the ID is the place to check for indications of poor quality. Tubing with welds that have not been cold-worked can be spotted readily-the weld appears as a rough surface or line that runs the length of the ID. Some producers attempt to roll the weld area on the ID, which leaves a weld area thicker than the base metal. Generally, the adjacent area on either side of the weld (the heat-affected zone) appears to be undercut. When looking down the inside of such a tube, you can clearly see the ID weld with the naked eye, and it is easy to catch a fingernail on the weld bead in this area.
Conventional fusion welding processes exhibit characteristics found on the left side of the graph, while high energy density processes such as LBW are characterized on the right side of the graph.
Tubing with improper cold work is inferior for pressure tubing applications in process systems because welds may trap particulate in the process stream, which can result in premature failure because of pitting or crevice corrosion. Lack of proper forge-down or annealing also may result in sensitization of the weld area for preferential attack from process fluids, especially acids or those containing chlorine. In addition, the weld and heat affected zone may not have the strength of a properly forged and annealed weld.
The left photo in Figure 1shows a laser weld with no cold work. Some tube manufacturers stop here with no or marginal cold work. The weld in the center photo has been cold-worked properly, and the weld in the photo on the right has been cold-worked and annealed properly to achieve full recrystallization.
LBW has some unique aspects, but it is still just a fusion welding process in which a molten puddle is generated between two edges of steel and allowed to solidify to form a weldment. Unlike the GTAW process, LBW is a high-energy density process, meaning that it uses a moderate amount of light energy that is focused to form a very small-diameter beam. This concentration of welding energy generates a narrow, deep-penetrating weld.
This energy density is so high that a minute surface layer of metal is vaporized to develop the deep penetration. The ejection (vaporization) of metal from the puddle forces the puddle surface down violently until the beam literally bores a small keyhole through the metal. The critical energy density required to form this type of deep-penetrating weld is approximately 1,000,000 watts/cm2. Processes that operate below this critical level, such as GTAW, produce welds that are relatively wide and shallow (see Figure 2). Narrow welds also develop less thermal distortion than typical GTAW.
The total heat energy required to penetrate a given thickness of steel is greater for GTAW than for LBW. A metallurgical result of this higher heat input and slower cooling rate is a coarser weld structure. Both welding processes exhibit similar weld segregation, but the distance between depleted and enriched zones can be dramatically different (see Figure 3). This distance has a major impact on the time required at annealing temperatures to effect diffusion and homogenization of the cast structures.
Einstein's diffusion equation (seeFigure 4) can demonstrate that a doubling of the average diffusion distance requires a fourfold increase in annealing time to achieve equivalent homogenization. Thus, welded tube producers that anneal as required by pressure tube specifications can benefit from the LBW process.
A high-quality laser weld that has been properly cold-worked and annealed can completely recrystallize of the weld. Results can approach seamless tubing or welded and cold-drawn tubing for microstructure and overall quality (see Figure 5).
Photo on left displays the weld and heat affected zone (HAZ) at the ID, or the root, of a GTAW weld. The photo on the lower right is the same weld area at higher magnification. The dark etching retained in the ferrite phase is slightly enriched in chromium and slightly depleted in the nickel and corresponds to inter-dendritic zones during solidification. The photo on upper right is laser welded for comparison to the photo directly under it, which is at the same magnification. Please note the LB weld structure is much finer and no ferrite is detectable.
Performance of a tube in corrosion tests also gives an indication of the tube quality. A tube with a well-forged and -annealed weld area will yield better corrosion test results. See Figure 6for corrosion test results for a high-quality tube. Figure 7shows corrosion test results for a poor-quality laser-welded tube.
An informed consumer recognizes that not all laser-welded tubing is equal. Significant differences may be evident in mechanical properties, appearance, roughness of inside surfaces, and corrosion resistance as a result of differences in the total manufacturing process. When properly designed and executed manufacturing processes are used, laser beam welding produces a viable welded stainless steel tube.
Transverse section of high quality laser welded tubing. Left photo is at lower magnification with the tube ID surface visible at the bottom. Right photo is the same area at higher magnification. Note complete weld recrystallization and that the ID weld is well-forged.
The transverse section of a laser-welded tube in the left photo, seen as a scanning electron microscope (SEM) secondary electron image, was decay-corrosion tested using hydrochloric acid per the ASTM A249 S7 supplement. The base metal thinning in excess of weld thinning is evident. In the photo on the right, an optical image of the tube's laser weld at higher magnification reveals complete weld recrystallization.