In the high-frequency welding process of used roll forming equipment, some of them also need to perform on-line heat treatment of the weld, during which the welded pipe absorbs a lot of heat, and about 90% of the heat is concentrated in the weld area. That is to say, the distribution of heat on the cross-section of the welded pipe is extremely uneven, with the narrow weld area where most of the heat is concentrated, while only a very small amount of heat is distributed on the pipe body. If the pipe body is not forcedly cooled or cooled sufficiently, and the welded pipe is allowed to enter the sizing area, there will be three consequences: one is that it cannot be straightened, or even if it is straightened, it will bend after leaving the unit; The local wear of the profile is serious; the third is to have an adverse effect on the microstructure and properties of the welds of some high-strength and high-carbon equivalent pipes.

Welded pipe cooling is divided into two stages: high temperature cooling and low temperature cooling

High temperature cooling: The temperature of the weld part drops from 1450~1250°C to about 350°C.

Low temperature cooling: The temperature of the weld joint is cooled from 350°C to room temperature.

Influence of high temperature cooling intensity on welded pipe quality

When the used roll forming equipment is used for the cooling strength at the high temperature stage, for the welded pipes made of Q195, Q215, Q235, 08AI and SPCC tube blanks, because their carbon equivalents usually do not exceed 0.25, the cooling speed is different for the welding seam. The effect of its heat-affected zone performance is not obvious.

However, this is not the case for welded pipes with higher carbon equivalents. For some low-alloy steels such as X42~X100 pipeline steel, the fast or slow cooling rate will affect the structure and properties of the welding heat-affected zone. Taking X80 pipeline steel welded pipe as an example, the cooling rate is different, and the hardness and impact properties of the weld heat-affected zone are different. big different.

Cryogenic cooling of welded pipes

It is generally believed that after the temperature of the weld and the heat affected zone is lower than 350°C, the influence of the cooling rate on the welded pipe is mainly reflected in the physical shape of the pipe and the life of the sizing roll pass. Therefore, when the used roll forming equipment implements the function of low temperature cooling of the welded pipe, one is to cool the welded pipe to room temperature in the sizing section as much as possible, and the other is to fully cool the hole profile of the sizing roll as much as possible.

Criteria for evaluating the final cooling effect of welded pipes

(1) The surface temperature of the welded pipe after exiting the sizing machine is lower than 50°C, and the pipe can be touched for relevant inspection without gloves.

(2) There is a little coolant left on the pipe surface between the straightening head and the flying saw, indicating that the cooling is sufficient.

(3) There is no change in the straightness of the tubes on the header frame.

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