Analysis of three common welding techniques for stainless steel pipe

In recent years, as people pay more and more attention to environmental issues, automobile manufacturers have been under increasing pressure to improve fuel efficiency. Stricter and more restrictive regulations have brought technical challenges to industrial production and material processing. Among these trends are lower exhaust emissions, lighter bodies and longer service life of parts.

The progress in material processing has brought unique opportunities to the field of stainless steel pipe production. Specifically, manufacturers are required to produce such parts. They must have lighter weight, but they must still have anti-corrosion characteristics and meet the strength requirements. In addition, the space limitation of the body emphasizes the importance of formability. Typical applications include exhaust pipes, fuel pipes, fuel injection nozzles and other components.

In the production of stainless steel pipe, the flat steel strip is formed first, and then its shape is made into a circular tube. Once formed, the joints of the tubes must be welded together. This weld greatly affects the formability of the part. Therefore, in order to obtain the welding shape that can meet the strict test requirements in the manufacturing industry, it is very important to select the appropriate welding technology. There is no doubt that gas tungsten arc welding (GTAW), high frequency (HF) welding and laser welding have been respectively applied in the manufacture of stainless steel pipes.

1、 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 magnetic rod, which is a soft magnetic element, which is placed inside the pipe body, which helps to converge the welding flow at the edge of the steel strip.

In both cases, the steel strip is cut and cleaned, rolled up and sent to the welding point. In addition, coolant is used to cool the induction coil used in the heating process. Finally, some coolant will be used in the extrusion process. Here, a great force is applied on the extrusion pulley to avoid porosity in the welding area; However, the use of a larger extrusion force will result in an increase in burrs (or weld beads). Therefore, specially designed tools are used to remove burrs inside and outside the pipe.

One of the main advantages of high frequency welding process is that it can process steel pipes at high speed. In the case of non-traditional welding, however, it is not easy to use high-frequency NDT. Welding cracks may appear in the flat and thin area of low-strength joints, which cannot be detected by traditional methods, so it may lack reliability in some high-demand automotive applications.

Gas tungsten arc welding (GTAW)

Traditionally, steel pipe manufacturers choose gas tungsten arc welding (GTAW) to complete the welding process. GTAW produces a welding arc between two non expendable tungsten electrodes. At the same time, inert shielding gas is introduced from the spray gun to shield the electrode, generate ionized plasma flow and protect the molten weld pool. This is an established and understood process, which will repeat the high-quality welding process.

The advantage of this process is repeatability, no spatter in the welding process, and the elimination of porosity. GTAW is considered to be an electrical conduction process, so the process is relatively slow.

2、 High frequency arc pulse

In recent years, GTAW welding power supply, also known as high-speed switch, makes the arc pulse exceed 10000hz. Customers of steel pipe processing plants are the first to benefit from this new technology. The downward pressure of arc caused by high-frequency arc pulse is five times higher than that of traditional GTAW. The representative improvement characteristics also include: the blasting strength is improved, the welding line speed is faster, and the waste products are reduced.

Customers of steel pipe manufacturers soon found that the welding shape obtained by this welding process needs to be reduced. In addition, the welding speed is relatively slow.

3、 Laser welding

In all steel pipe welding applications, the edge of the steel strip is melted, and the edge solidifies when the steel pipe edges are squeezed together with the clamping support. However, for laser welding, its unique property is that it has high energy beam density. The laser beam not only melted the surface layer of the material, but also produced a keyhole, so that the shape of the weld was very narrow.

If the power density is lower than 1 MW / cm2, such as GTAW technology, it will not produce enough energy density to produce keyhole. In this way, the welding shape obtained by the keyless process is wide and shallow. The high precision of laser welding brings more efficient penetration, which reduces grain growth and brings better metallographic quality; On the other hand, higher heat input and slower cooling process of GTAW lead to rough welding structure.

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 characteristics, which brings higher blasting strength and higher formability. When compared with high-frequency welding, the laser processing material process does not oxidize, which makes the scrap rate lower and the formability higher.

4、 Influence of spot size

In the welding of stainless steel pipe factory, the welding depth is determined by the thickness of steel pipe. In this way, the production goal is to improve formability and achieve higher speed by reducing the welding width. When choosing the most suitable laser, people should not only consider the beam quality, but also the accuracy of the pipe mill. In addition, the limitation of reducing the light spot must be considered before the dimensional error of the pipe mill plays a role.

There are many unique dimensional problems in steel pipe welding. However, the main factor affecting welding is the joint on the welding box (more specifically, the welding coil). Once the steel strip is formed and ready for welding, the characteristics of the weld include: steel strip gap, serious / slight welding dislocation and change of weld centerline. The gap determines how much material is used to form the weld pool. Too much pressure will lead to excess material on the top or inner diameter of the steel pipe. On the other hand, serious or slight welding misalignment will lead to poor welding appearance.

In addition, after welding the box, the steel pipe will be further trimmed. This includes size adjustment and shape (shape) adjustment. On the other hand, additional work can remove some serious / minor welding defects, but it may not be able to remove them all. Of course, we want to achieve zero defects. Generally speaking, the rule of thumb is that welding defects should not exceed 5% of the material thickness. Exceeding this value will affect the strength of welded products.

Finally, the existence of welding center line is very important for the production of high-quality stainless steel pipe. With the increasing emphasis on formability in the automotive market, it is directly related to the need for smaller heat affected zone (HAZ) and reduced welding profile. In turn, this promotes the development of laser technology, that is, improve the beam quality to reduce the spot size. As the spot size becomes smaller and smaller, we need to pay more attention to the accuracy of scanning the centerline of the seam. Generally speaking, steel pipe manufacturers will reduce this deviation as much as possible, but in fact, it is difficult to achieve a deviation of 0.2mm (0.008 inch).

This brings the need to use weld tracking system. The two most common tracking technologies are mechanical scanning and laser scanning. On the one hand, the mechanical system uses probes to contact the upstream of the joint of the welding pool, which will be stained with ash, worn and vibrated. The accuracy of these systems is 0.25 mm (0.01 inch), which is not accurate enough for high beam quality laser welding.

On the other hand, laser seam tracking can achieve the required accuracy. Generally speaking, the laser light or laser spot is projected on the weld surface, and the obtained image is fed back to the CMOS camera, which determines the position of weld, wrong joint and gap through algorithm.

Although imaging speed is important, when providing the necessary closed-loop control to move the laser focusing head directly on the joint, the laser weld tracker must have a fast enough controller to accurately compile the position of the weld. Therefore, the accuracy of weld tracking is very important, and the response time is also important.

In general, weld tracking technology has been fully developed, which 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 a place to use. It is used to reduce the porosity of welding, reduce the welding shape, and maintain or improve the welding speed at the same time. Laser systems, such as diffusion cooled slab lasers, have improved beam quality and further improved formability by reducing welding width. This development has led to the need for stricter dimensional control and laser weld tracking in steel pipe plants.

In this way, the success of welding process in stainless steel pipe factory depends on the integration of all individual technologies, so it must be treated as a complete system.