Advances in material processing have brought unique opportunities in the field of stainless steel tube production. Typical applications include exhaust pipes, fuel pipes, fuel injectors, and other components. In the production of stainless steel pipes, a flat steel strip is formed first, and then its shape is made into a round tube. Once formed, the seams of the tubes must be welded together. This weld greatly affects the formability of the part. Therefore, it is extremely important to select the appropriate welding technique to obtain a welding profile that can meet the stringent testing requirements in the manufacturing industry. There is no doubt that gas tungsten arc welding (GTAW), high frequency (HF) welding, and laser welding have each been applied in the manufacture of stainless steel pipes.
High-frequency induction welding
In high-frequency contact welding and high-frequency induction welding, the equipment that provides the current and the equipment that provides the pressing force are independent of each other. In addition, both methods can use bar magnets, which are soft magnetic elements placed inside the tube body, which help to concentrate the welding flow at the edge of the strip.
In both cases, the strip is cut and cleaned, rolled up, and sent to the welding point. In addition, a coolant is used to cool the induction coils used in the heating process. Finally, some coolant will be used for the extrusion process. Here, a lot of force is applied to the squeeze pulley to avoid creating porosity in the weld area; however, using a higher squeeze force will result in increased burrs (or weld beads). Therefore, specially designed knives are used to deburr the inside and outside of the tube.
The main advantage of the high-frequency welding process is that it enables high-speed machining of steel tubes. However, as is typical in most solid phase forging joints, high-frequency welded joints are not easily tested reliably using conventional non-destructive techniques (NDT). Weld cracks can occur in flat, thin areas of low-strength joints that cannot be detected using traditional methods and may lack reliability in some demanding automotive applications.
Gas tungsten arc welding (GTAW)
Traditionally, steel pipe manufacturers have opted for gas tungsten arc welding (GTAW) to complete the welding process. GTAW creates an electric arc between two non-consumable tungsten electrodes. At the same time, an inert shielding gas is introduced from the torch to shield the electrodes, generate an ionized plasma stream, and protect the molten weld pool. This is an established and understood process that will result in a repeatable high-quality welding process.
The advantages of this process are repeatability, spatter-free welding, and the elimination of porosity. GTAW is considered to be an electrical conduction process, so, relatively speaking, the process is relatively slow.
High-frequency arc pulse
In recent years, GTAW welding power sources, also known as high-speed switches, have enabled arc pulses over 10,000 Hz. The customers of the steel pipe processing plant benefit from this new technology, the high-frequency arc pulse causes the arc down pressure which is five times greater than that of conventional GTAW. Representative improvements include increased burst strength, faster weld line speeds, and reduced scrap.
The customer of the steel pipe manufacturer quickly found that the weld profile obtained by this welding process needed to be reduced. In addition, the welding speed is still relatively slow.
Laser welding
In all steel pipe welding applications, the edges of the steel strip are melted and solidified when the steel pipe edges are squeezed together using clamping brackets. However, a unique property of laser welding is its high energy beam density. The laser beam not only melted the surface layer of the material but also created a keyhole so that the weld profile is very narrow. Power densities below 1 MW/cm2, such as GTAW technology, do not produce enough energy density to produce keyholes. In this way, the keyhole-less process results in a wide and shallow weld profile. The high precision of laser welding leads to more efficient penetration, which in turn reduces grain growth and leads to better metallographic quality; on the other hand, the higher thermal energy input and slower cooling process of GTAW lead to Rough welded construction.
Generally speaking, the laser welding process is considered to be faster than GTAW, they have the same scrap rate, and the former brings better metallographic properties, which leads to higher burst strength and higher formability. When compared to high-frequency welding, no oxidation occurs during laser processing of the material, which results in lower scrap rates and higher formability. Influence of spot size: In the welding of stainless steel pipe factories, the welding depth is determined by the thickness of the steel pipe. In this way, the production goal is to increase formability by reducing the width of the weld, while achieving higher speeds. When choosing the most suitable laser, one must not only consider the beam quality, but also the accuracy of the mill. In addition, the limitations of reducing the spot must be considered before the dimensional error of the tube mill can come into play.
There are many dimensional problems specific to steel pipe welding, however, the main factor affecting the welding is the seam on the welded box (more specifically, the welded coil). Once the strip is formed and ready for welding, the characteristics of the weld include strip gap, severe/slight weld misalignment, and weld centerline changes. The gap determines how much material is used to form the weld pool. Too much pressure will result in excess material at the top or inside diameter of the pipe. On the other hand, severe or slight weld misalignment can result in poor weld profile. In addition, after passing through the welded box, the steel pipe will be further trimmed. This includes size adjustment and shape (shape) adjustment. On the other hand, extra work can remove some serious/minor solder defects, but probably not all of them. Of course, we want to achieve zero defects. As a general rule of thumb, weld defects should not exceed five percent of the material thickness. Exceeding this value will affect the strength of the welded product.
Finally, the presence of a weld centerline is important for the production of high-quality stainless steel pipes. With the increasing emphasis on formability in the automotive market, there is a direct correlation between the need for a smaller heat-affected zone (HAZ) and reduced weld profile. This, in turn, has led to advances in laser technology that improve beam quality to reduce spot size. As the spot size continues to get smaller, we need to pay more attention to the accuracy of scanning the seam centerline. Generally speaking, steel pipe manufacturers will try to reduce this deviation as much as possible, but in practice, it is very difficult to achieve a deviation of 0.2mm (0.008 inches). This brings the need to use a seam tracking system. The two most common tracking techniques are mechanical scanning and laser scanning. On the one hand, mechanical systems use probes to contact the seam upstream of the weld pool, which is subject to dust, wear, and vibration. The accuracy of these systems is 0.25mm (0.01 inches), which is not precise enough for high beam quality laser welding.
On the other hand, laser seam tracking can achieve the required accuracy. Typically, a laser beam or laser spot is projected on the surface of the weld, and the resulting image is fed back to a CMOS camera, which uses algorithms to determine the location of welds, mis-joints, and gaps. While imaging speed is important, laser seam trackers must have a controller fast enough to accurately compile the position of the weld while providing the necessary closed-loop control to move the laser focus head directly over the seam. Therefore, the accuracy of seam tracking is important, and so is the response time.
In general, seam tracking technology is sufficiently developed to also allow steel pipe manufacturers to utilize higher quality laser beams to produce better formable stainless steel pipes. As a result, laser welding has found a place to reduce weld porosity and reduce weld profile while maintaining or increasing welding speed. Laser systems, such as diffusion-cooled slab lasers, have improved beam quality, further improving formability by reducing weld width. This development has led to the need for tighter dimensional control and laser seam tracking in steel pipe mills.
Post time: Aug-29-2022