Complete collection of stainless steel pipe welding process methods

When producing stainless steel pipes, a flat steel strip is first formed and then shaped into a round tube shape. Once formed, the pipe's seams must be welded together. This weld greatly affects the formability of the part. Therefore, it is extremely important to select the appropriate welding technology 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 current and the equipment that provides extrusion force are independent of each other. Additionally, both methods make use of bar magnets, which are soft magnetic elements placed inside the tube body and help focus the welding flow at the edge of the steel strip. In both cases, the strip is cut and cleaned, rolled up, and delivered to the welding point. In addition, a coolant is used to cool the induction coil used during the heating process. Finally, some coolant will be used in the extrusion process. Here, a high force is applied to the extrusion pulley to avoid porosity in the weld area; however, using greater extrusion force will increase burrs (or beads). Therefore, specially designed knives are used to remove burrs from the inside and outside of the tube. The main advantage of the high-frequency welding process is its ability to process steel pipes at high speeds. However, as is typical in most solid-phase forged joints, high-frequency welded joints are not easy to reliably test using traditional 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 therefore lack reliability in some demanding automotive applications.

Gas tungsten arc welding (GTAW):
Traditionally, steel pipe manufacturers have chosen gas tungsten arc welding (GTAW) to complete the welding process. GTAW creates an electric welding arc between two non-consumable tungsten electrodes. At the same time, an inert shielding gas is introduced from the spray gun to shield the electrodes, generate an ionized plasma flow, and protect the molten weld pool. This is an established and understood process that will result in repeatable, high-quality welding. The advantages of this process are repeatability, spatter-free welding, and elimination of porosity. GTAW is considered 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, enable arc pulses exceeding 10,000Hz. Customers in steel pipe processing plants benefit from this new technology, with high-frequency arc pulses resulting in arc downward pressure that is five times greater compared to conventional GTAW. Representative improvements include improved burst strength, faster welding line speeds, and reduced scrap. Customers of steel pipe manufacturers soon discovered 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 edges of the pipe are squeezed together using clamping brackets. However, a unique property of laser welding is its high energy beam density. The laser beam not only melts the surface layer of the material but also creates a keyhole, resulting in a narrow weld profile. If the power density is lower than 1MW/cm2, such as GTAW technology, it will not produce enough energy density to create keyholes. In this way, the keyholeless process results in a weld profile that is wide and shallow. The high precision of laser welding brings higher efficiency penetration, which in turn reduces grain growth and brings better metallographic quality; on the other hand, the higher heat energy input and slower cooling process of GTAW result in Rough welded construction.

Generally speaking, it is believed that the laser welding process is faster than GTAW, they have the same scrap rate, and the former brings better metallographic properties, which leads to higher blast strength and higher formability. When compared with high-frequency welding, no oxidation occurs during laser processing of materials, which results in lower scrap rates and higher formability. Influence of spot size: In welding in stainless steel pipe factories, the welding depth is determined by the thickness of the steel pipe. In this way, the production goal is to improve formability by reducing weld width while achieving higher speeds. When choosing the most suitable laser, one cannot only consider the beam quality but also the accuracy of the pipe rolling machine. In addition, the limitations of reducing the light spot must be considered before the dimensional errors of the pipe rolling machine can play a role.

There are many dimensional issues specific to steel pipe welding, however, the main factor affecting welding is the seam in the welding box (more specifically, the welding coil). Once the strip has been formed and prepared for welding, characteristics of the weld include strip gaps, severe/minor weld misalignment, and changes in the centerline of the weld. 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 steel pipe. On the other hand, severe or slight welding misalignment can lead to poor welding appearance. In addition, after passing through the welding box, the steel pipe will be further trimmed. This includes size and shape adjustments. On the other hand, additional work can remove some major/minor weld defects, but may not remove them all. Of course, we want to achieve zero defects. Generally speaking, the rule of thumb is that welding 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 the welding centerline is important for the production of high-quality stainless steel pipes. As the automotive market places increasing emphasis on formability, it is directly related to the need for smaller heat-affected zones (HAZ) and reduced welding profiles. This, in turn, promotes the development of laser technology that improves 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 center line of the seam. Generally speaking, steel pipe manufacturers will try to reduce this deviation as much as possible, but in fact, it is very difficult to achieve a deviation of 0.2mm (0.008 inches). This brings with it the need to use a weld tracking system. The two most common tracking technologies are mechanical scanning and laser scanning. On the one hand, mechanical systems use probes to contact the joints upstream of the weld pool, which is subject to dirt, wear, and vibration. The accuracy of these systems is 0.25mm (0.01 inch), which is not accurate enough for high-beam-quality laser welding.

Laser seam tracking, on the other hand, can achieve the required accuracy. Generally speaking, laser light or laser spots are 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, faulty joints, and gaps. While imaging speed is important, laser seam trackers must have controllers 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 weld seam tracking is important, but so is the response time.

In general, weld seam tracking technology has been fully developed and can also allow steel pipe manufacturers to use higher quality laser beams to produce stainless steel pipes with better formability. Therefore, laser welding has found its place. It is used to reduce the porosity of the weld and reduce the weld shape while maintaining or increasing the welding speed. Laser systems, such as diffusion-cooled slab lasers, have improved beam quality, further improving formability by reducing weld width. This development resulted in the need for tighter dimensional control and laser weld tracking in steel pipe mills.