Stainless steel pipe welding process method

Advances in material processing present unique opportunities in the field of stainless steel pipe production. Typical applications include exhaust pipes, fuel lines, fuel injectors, and other components. In the production of stainless steel tubes, a flat steel strip is first formed, which is then shaped into a round tube. Once formed, the pipe's seams must be welded together. This weld greatly affects the formability of the part. Therefore, choosing the right welding technique is extremely important to obtain a weld profile that can meet the stringent testing requirements in the manufacturing industry. Undoubtedly, 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 providing current and the equipment providing extrusion force are independent of each other. In addition, both methods can use a bar magnet, which is a soft magnetic element placed inside the tube body, which helps to focus the welding flow at the edge of the strip. In both cases, the strip is cut and cleaned before being rolled up and sent to the welding point. In addition, coolant is used to cool the induction coils used in the heating process. Finally, some coolant will be used in the extrusion process. Here, a lot of force is applied to the squeeze pulley to avoid creating porosity in the weld area; however, using more 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 pipes. However, as is typical in most solid phase forgings, high-frequency welded joints cannot be reliably tested 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, pipe manufacturers have chosen to complete the welding process with gas tungsten arc welding (GTAW). GTAW creates a welding 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 produce repeatable high-quality welds. 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, allow arc pulses over 10,000 Hz. Customers in steel pipe processing plants benefit from this new technology, where high-frequency arc pulses result in five times greater arc downward pressure compared to conventional GTAW. Typical improvements brought about include increased burst strength, faster weld line speeds, and reduced scrap. Customers of steel pipe producers quickly 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 steel pipe edges are pressed together using clamping brackets. However, the 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 bead profile. Power densities below 1 MW/cm2, such as GTAW technology, do not produce sufficient energy density to produce keyholes. Thus, the keyhole-less process results in a wide and shallow weld profile. The high precision of laser welding brings more efficient 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 lead to Rough welded construction. Generally speaking, it is considered that the laser welding process is faster than GTAW, they have the same reject rate, and the former leads to better metallographic properties, which leads to higher burst strength and higher formability. When compared to high-frequency welding, the laser processes materials without oxidation, 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. Thus, 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 mill. In addition, before the dimensional error of the pipe mill can play a role, the limitation of reducing the light spot must be considered first. There are many dimensional problems specific to steel pipe welding, however, the main factor affecting welding is the seam on the welding box (more specifically, the welding coil). Once the strip has been formed for welding, weld characteristics include strip gaps, severe/slight weld misalignment, and centerline variation of the weld. The gap determines how much material is used to form the weld pool. Too much pressure will result in excess material on the top or inside diameter of the pipe. On the other hand, severe or slight weld misalignment can result in a poor weld profile. In addition, after passing through the welding box, the steel pipe will be further trimmed. This includes size adjustments and shape (shape) adjustments. On the other hand, extra work can remove some major/minor weld 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. Directly related to the increasing focus on formability in the automotive market is the need for smaller heat-affected zones (HAZ) and reduced weld profiles. In turn, this promotes the development of laser technology, that is, improving beam quality to reduce spot size. As the spot size continues to decrease, 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 up 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 weld pool upstream of the seam, where they get dusty, abrasive, and vibrate. The accuracy of these systems is 0.25mm (0.01 inches), which is not precise enough for high-beam-quality laser welding.

Laser seam tracking, on the other hand, can achieve the required precision. 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, misjoins, and gaps.

While imaging speed is important, a laser seam tracker 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, but so is response time.

In general, seam tracking technology has developed sufficiently to also allow steel pipe manufacturers to utilize higher quality laser beams to produce more formable stainless steel pipe.

Therefore, laser welding has found a place where it is used to reduce the porosity of the weld and reduce the weld profile 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 has led to the need for tighter dimensional control and laser seam tracking in steel pipe mills.