What are the characteristics of the welding process of ERW pipe?

What are the characteristics of the welding process of ERW pipe?

(1) The heating time is short and the temperature rises quickly
During high-frequency welding, the time required for the edge of the tube blank to be rapidly heated from room temperature to the welding temperature of 1250-1450 °C is related to the diameter and wall thickness of the welded tube. In terms of pipe diameter alone, the larger the pipe diameter, the larger the diameter of the extrusion roller, the farther the heating induction coil is from the center line of the extrusion roller, the longer the V-shaped loop of the high-frequency current, and the longer the rapid heating range, reaching the required welding temperature. time is long
In addition, the contacts of the contact welding method can be closer to the welding meeting point, and the heating transmission distance is shorter, so that the heating time is shortened by 1/3~1/2 compared with the induction welding, and the heating temperature rise speed is increased by 30%~50%. And the larger the unit, the more obvious this advantage. Therefore, large and medium-sized welded pipe units can choose contact welding, so that the speed of the unit can be increased by about 30%, or even more.

(2) The heating area is narrowly distributed and uneven.
The width of the heating area of the high-frequency longitudinal welded pipe is closely related to the frequency of the heating power supply. Since the frequency of the high frequency current is usually between 250~450kHz, in this way, according to the skin effect principle of the high frequency current, the width b of the heated area at the edge of the tube blank is between 0.8~1mm. Therefore, during the welding process, the amount of metal directly heated at the edge of the tube wall accounts for about the percentage of the total tube blank width B:

Ji=2b/Bx100%

In the formula, Ji is the fusion ratio, which is the ratio of the metal that reaches the welding heating temperature to the base metal. This formula shows that since b does not change greatly due to changes in welded pipe specifications, the unit energy consumption and electricity consumption of welded large pipes is lower than that of welded small pipes.Moreover, the temperature distribution of the same cross-section is extremely uneven, and the inner and outer surfaces are higher than the central layer. There is also a transition zone between the direct heating zone and the base metal. The temperature gradient in this zone is large, and the temperature is lower than the welding temperature. The welding temperature is determined and affected by the welding temperature, so it is called "heat affected zone". The metal structure of the heat-affected zone is different from the weld and the base metal.

(3) Self-welding without welding consumables
High-frequency welding does not require any welding consumables, flux and shielding gas, and there is no matching problem between the welding consumables and the base metal, but self-welding in the air. This feature requires that, on the premise of ensuring the quality of the weld, the welding speed should be increased as much as possible to obtain a high-strength weld.Because the welding speed is fast, the welding surface is oxidized and the decarburization layer formed is thin, and the weld strength is high. The low-magnification morphology of the high-frequency longitudinal welded pipe weld seam confirms that there is a decarburization layer with a width of 0.05-0.15mm in the middle of the weld seam, and the content of elements that strengthen the weld seam strength such as carbon and manganese is higher than that of the base metal. It is much lower and some oxides are increased at the same time, hence the name of the decarburized layer.

The decarburization layer affects the strength of the weld, which can be confirmed from the case analysis of weld positive pressure and lateral pressure cracking. Most of the fractures start from the position of the decarburization layer. Under the existing process conditions, people cannot completely eliminate the decarburization layer, but the decarburization layer can be thinned by improving the welding process.

(4) High-speed dynamic welding
So far, the documented carbon steel welding speed of high-frequency welding is 200m/min, and the welded pipe is welded under high-speed moving state. , Extrusion force stability, billet running stability, roll precision, etc. all put forward higher requirements; any slight change in any factor will cause serious quality problems. Because at a certain welding speed, the welding current is a constant value during this period, and it is considered that the welding heat at this time matches the welding speed exactly, and neither overheating nor cold welding will occur. Once the speed fluctuates, the weld is either overheated or cold welded.

(5) Heating and extrusion welding are completed simultaneously
High-frequency welding is carried out simultaneously under high temperature of 1250~1450℃ and high pressure of 20~40MPa, both of which are indispensable. Not only that, but also requires the welding temperature to match the welding extrusion force.

(6) Forced cooling
Except for high-strength oil pipes and structural pipes, most of the pipes have a carbon equivalent of about 0.2. Rapid cooling after welding generally does not produce obvious hardening tendencies, and has little effect on weld performance. When the weld temperature drops from 1250~1450℃ to 30~50℃, the maximum cooling intensity of 32~50 units reaches 110~120℃/s, and the cooling intensity of 76~114 units also reaches 60~90℃/s. It is required that the operator must pay attention to the cooling of the welded pipe, so as not to affect the welding efficiency.

Another feature of welded pipe cooling is that the weld seam cooling intensity is extremely uneven, and the section with the highest cooling intensity is in the forced water cooling section. Strictly speaking, this unbalanced cooling will have a more or less negative impact on the weld and its heat-affected zone.