The welding process of used high frequency welded pipe equipment has 6 notable features: ① The heating time is short and the temperature rises quickly. ②The heating area is narrow and unevenly distributed. ③ Self-welding without welding material. ④High-speed dynamic welding. ⑤ Heating and extrusion welding are completed simultaneously. ⑥ Forced cooling.

(1) The heating time is short and the temperature rises quickly

When welding used high frequency welded pipe equipment, 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 of the extrusion roller, the longer the high-frequency current V-shaped loop, the longer the rapid heating interval, and the higher the welding temperature. time is long, as shown in Figure 1. When the high-frequency power is large enough, when the welding speed is 15~100m/min, the heating time used by the commonly used units is shown in Table 1.

From the heating time reflected in the table, it is in the millisecond level, and the time is extremely short; that is, heating from room temperature to welding temperature. The longest is no more than 1s, and the shortest is only 78ms. The smaller the unit, the shorter the time. Correspondingly, the larger the unit, the slower the heating rate; the smaller the unit, the faster the heating rate. For example, the 32 unit produces welded pipes with a diameter of 25mm X 0.7mm, and the maximum heating rate can reach about 20°C/ms. See Table 2 for details.

The characteristics of heating time and heating speed in Table 1 and Table 2 explain the reason why the welding speed is slow when the unit is large.

In addition, the contacts of the contact welding method can be closer to the welding confluence 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 rate is increased by 30%~50%. And the larger the unit, the more obvious this advantage is. Therefore, contact welding can be selected for large and medium-sized welded pipe units, so that the speed of the unit can be increased by about 30%, or even more.

(2) The narrow distribution of the heating area is uneven.

When making pipes with used high frequency welded pipe equipment, the width of the heating area of the high-frequency straight seam 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 and 450 kHz, 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 and 1 mm. Therefore, during the welding process, the amount of metal that is 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, that is, the ratio of the metal to the base metal that reaches the welding heating temperature. This formula shows that since b does not change greatly due to the change of welded pipe specifications, the unit energy consumption of welded large pipes is lower than that of welded small pipes, and the electricity consumption is less.

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 interval between the direct heating zone and the base metal, where the temperature gradient is large, and the temperature is lower than the welding temperature, and is determined by The welding temperature is determined and affected by the welding temperature, so it is called the "heat affected zone". The metal structure of the heat-affected zone is different from both the weld and the base metal.

(3) Self-welding without welding material

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 is exposed 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 decarburized layer formed is thin, and the weld strength is high. The low-magnification microstructure of the weld of the high-frequency straight seam welded pipe confirms that there is a decarburized layer with a width of 0. 05~0. 15mm in the middle of the weld, and the content of elements such as carbon and manganese to strengthen the weld strength is higher than that of the metal base metal. Much lower, and at the same time increased some oxides, hence the name of the decarburized layer.

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

(4) High-speed dynamic welding

So far, the documented welding speed of high-frequency welding for carbon steel is 200m/min, and the welded pipe is welded at high speed. , extrusion force stability, tube billet running stability, roll precision, etc. all put forward higher requirements; a small change in any one 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 is exactly matched with the welding speed, neither overburning nor cold welding will occur. Once the speed fluctuates, the weld is either over-burned or cold-welded.

(5) Heating and extrusion welding are completed simultaneously

The high-frequency welding of used high frequency welded pipe equipment 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 the welding temperature is required to match the welding extrusion force.

(6) Forced cooling

Except for high-strength oil pipes and structural pipes, the carbon equivalent of most pipes is about 0.2, and the rapid cooling after welding generally does not produce obvious hardening tendency, and has little effect on the weld performance. When the weld temperature drops from 1250~1450℃ to 30~50℃, the maximum cooling strength of 32~50 units is as high as 110~120℃/s, and the cooling strength of 76~114 units also reaches 60~90℃/s. It requires the operator to 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 cooling strength of the weld is extremely uneven, and the section with the highest cooling strength is in the forced water cooling section. Strictly speaking, this uneven cooling has a more or less negative effect on the weld and its heat-affected zone.

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