In recent years, as manufacturers have become more concerned about environmental issues, automakers are under increasing pressure to improve fuel efficiency. More stringent and more restrictive regulations pose technical challenges for industrial production and material processing. Among these trends are reduced emissions, lighter bodies and longer component life.
Advances in material processing have created unique opportunities in the field of stainless Steel Tube production. Specifically, manufacturers are required to produce such parts that must have a lighter weight, but must still have corrosion protection characteristics and meet strength requirements. In addition, the space limitations of the body emphasize the importance of formability. Typical applications include exhaust pipes, fuel lines, fuel injectors, and other components.
In the production of stainless steel pipes, a flat Steel Strip is formed first, and then its shape is rounded. Once formed, the joints of the tubes must be welded together. This weld greatly affects the formability of the part. Therefore, to obtain a welding profile that meets the stringent testing requirements of the manufacturing industry, it is extremely important to choose the right welding technology. Undoubtedly, tungsten gas shielded arc welding (GTAW), high frequency (HF) welding, and laser welding have been used in the manufacture of stainless steel tubes.
High frequency induction welding
In high-frequency contact welding and high-frequency induction welding, the device that supplies the current and the device that supplies the pressing force are independent of each other. In addition, both methods can use a magnetic bar, which is a soft magnetic element that is placed inside the tube and helps to concentrate the weld flow at the edge of the strip.
In both cases, the strip is cut and cleaned, rolled up, and sent to the weld. In addition, a coolant is used for cooling the induction coil used in the heating process. Finally, some coolant will be used in the extrusion process. Here, a large force is applied to the squeezing pulley to avoid porosity in the welded area; however, the use of a larger pressing force will result in an increase in burrs (or beads). Therefore, specially designed tools are used to remove burrs inside and outside the tube.
One of the main advantages of the high-frequency welding process is its ability to machine steel tubes at high speeds. However, the typical case in most solid phase forgings is that high frequency welded joints are not easily tested reliably using conventional non-destructive techniques (NDT). Welding cracks may occur in flat areas of low-strength joints that cannot be detected using conventional methods and may lack reliability in some demanding automotive applications.
Tungsten gas shielded arc welding (GTAW)
Traditionally, Steel Pipe manufacturers have chosen to complete the welding process with tungsten gas shielded arc welding (GTAW). GTAW creates an electric arc between two non-consumable tungsten electrodes. At the same time, an inert shielding gas is introduced from the lance to shield the electrodes, create an ionized plasma stream, and protect the molten weld pool. This is an established process that has been understood and will repeat the high quality welding process.
The advantage of this process is repeatability, no spattering during the soldering process, and elimination of porosity. GTAW is considered to be a process of electrical conduction, so the process is relatively slow.
High frequency arc pulse
In recent years, GTAW welding power supplies, also known as high speed switches, have caused arc pulses to exceed 10,000 Hz. Customers of steel pipe processing plants benefited first from this new technology, and high-frequency arc pulses caused the downward pressure of the arc to be five times greater than that of conventional GTAW. Representative improvements include improved blasting strength, faster weld line speeds, and reduced scrap.
Customers of steel pipe production plants quickly discovered that the welding profile required for this welding process needs 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 the edges solidify when the edges of the steel tubes are pressed together using a clamping bracket. However, a unique property for laser welding is its high energy beam density. The laser beam not only melts the surface of the material, but also creates a keyhole so that the weld is very narrow.
If the power density is less than 1 MW/cm2, such as GTAW technology, sufficient energy density cannot be produced to produce a keyhole. Thus, the keyless hole process results in a wide and shallow weld profile. The high precision of laser welding leads to higher efficiency penetration, which in turn reduces grain growth and results in better metallographic quality; on the other hand, GTAW's higher thermal input and slower cooling process result Rough welded structure.
In general, 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 burst strength and higher formability. When compared with high-frequency welding, the laser processing material process does not oxidize, which results in lower scrap rate and higher formability.
Spot size effect
In the welding of stainless steel pipe plants, 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 weld width while achieving higher speeds. When choosing the most suitable laser, one cannot only consider the beam quality, but must also consider the accuracy of the tube mill. In addition, the tube mill must first consider the limitations encountered in reducing the spot size before the size error occurs.
There are many problems in the dimensions unique to steel pipe welding, however, the main factor affecting the welding is the seam on the welded box (more specifically, the welded roll). Once the steel strip is ready for welding, the weld features include: strip gap, severe/slight weld misalignment, and weld centerline variation. 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 inner diameter of the steel tube. On the other hand, severe or slight solder misalignment can result in poor soldering profile.
In addition, the steel pipe will be further trimmed after passing through the welded box. This includes adjustments in size and shape (shape). On the other hand, extra work can remove some serious/slight soldering defects, but may not be completely removed. Of course, we hope to achieve zero defects. In general, the rule of thumb is that 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 tubing. With the increasing emphasis on formability in automotive markets, it is directly related to the need for smaller heat affected zones (HAZ) and reduced weld profiles. This, in turn, facilitates the development of laser technology that increases beam quality to reduce spot size. As spot sizes continue to shrink, we need more attention to the accuracy of scanning the seam centerline. In general, steel pipe manufacturers will reduce this deviation as much as possible, but in practice, it is difficult to achieve a deviation of 0.2 mm (0.008 inch).
This brings the need to use a weld tracking system. The two most common tracking techniques are mechanical scanning and laser scanning. On the one hand, mechanical systems use probes to contact upstream of the joints of the weld pool, which are subject to ash, wear and vibration. The accuracy of these systems is 0.25 mm (0.01 inch), which is not accurate enough for high beam quality laser welding.
Laser weld seam tracking, on the other hand, can achieve the required accuracy. In general, laser light or laser spots are projected onto the surface of the weld and the resulting image is fed back to the CMOS camera, which determines the location of the weld, mis-engagement and gap by an algorithm.
While imaging speed is important, when providing the necessary closed loop control to move the laser focus head directly over the seam, the laser weld bead tracker must have a fast enough controller to accurately compile the weld position. Therefore, the accuracy of weld tracking is important, and response time is equally important.
In general, weld seam tracking technology has been fully developed to allow steel tube manufacturers to produce higher quality thermoplastic tubes using higher quality laser beams.
Therefore, laser welding has found its use, which 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 and further improved formability by reducing weld width. This development has led to the need for tighter dimensional control and laser weld seam tracking in steel pipe plants.
In this way, the success of the stainless steel pipe plant welding process depends on the integration of all individual technologies, so it must be treated as a complete system.
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