What is the impact toughness of ERW steel pipe?

Posted by zora li on November 28th, 2022

How to improve the impact toughness of erw pipe weld is a major problem in pipe making technology. In order to further improve the impact toughness of welds and meet the toughness requirements of long-distance pipelines for ERW steel pipes, it is necessary to control the quality of raw material steel coils and strengthen the forming quality control combined with the manufacturing process. , welding and post-weld in-line heat treatment.

Characteristics of impact toughness of ERW steel pipe welds:

The welding process of ERW steel pipe is to use the principle of skin effect generated by high-frequency current to reheat the edge of the hot-rolled coil to a molten state, and then weld by mechanical extrusion. The steel pipe produced by this process will have a white fusion line in the center of the weld, and the heat-affected zone on both sides of the fusion line will produce metal flow lines extending from the middle to the inner and outer surfaces.

Since the ERW steel pipe uses the base metal as the welding material, its weld performance has a great relationship with the base metal performance. In common materials, the ductile-brittle transition temperature curve of the weld and the base metal. In ordinary materials, the difference in energy absorbed by the weld and the base metal in the impact test is very small; on the contrary, the difference in the high toughness material is large, and the impact toughness of the weld is obviously inferior to that of the base metal. It can be seen that the impact toughness of the ERW steel pipe weld center is generally lower than that of the base metal. Even if ductile materials are used, although the toughness of the weld is improved, the impact toughness at the center of the weld cannot be completely improved.

Factors affecting the impact toughness of welds:

1. Physical and chemical properties of raw materials

ERW steel pipe uses the base metal as the welding material, so the physical and chemical properties of the raw material directly determine the physical and chemical properties of the weld.

In order to study the impact toughness of base metal and welds with different chemical compositions, the impact toughness of base metal and welds is affected by the change of chemical composition. In order to exclude the influence of the pipe making process on the test results, all samples were standardized.

2. Crystallinity of raw materials and non-metallic inclusions

During the welding process of ERW steel pipe, due to mechanical pressure, the heat affected zone on both sides of the weld will produce metal streamlines extending from the middle of the steel plate to the inner and outer surfaces. The direction of the metal flow line near the center of the weld is basically the same, and the fuse line and the fuse line are parallel and coincident. Metal streamlines are band-like structures and inclusions extending along the rolling direction during hot rolling of the steel sheet. The thickness of the metal streamline is related to the grain size of the raw material and the amount of non-metallic inclusions.

The larger the grain size of the raw material or the more non-metallic inclusions, the thicker and more obvious the metal streamline. When there are many non-metallic inclusions and the distribution is uneven, the base metal segregation is serious. During high-frequency welding, under the pressing force of the squeeze roller, a large number of non-metallic inclusions in the segregation zone will enter the welding fusion zone along the metal streamline, and the segregation line can be clearly seen from above. The low heat acid sample meets the fusion line at an angle.

In this case, the impact toughness of the weld will be severely reduced, and the impact absorption energy will also be greatly reduced. Therefore, measures should be taken to reduce inclusions and refine grains in the smelting and rolling stages of raw steel coils, and the rising angle of the metal streamline should also be reasonably controlled during the tube-making process.

3. Weld heat treatment conditions

Metal flow lines in welds and heat-affected zones can be eliminated by heat treatment. In order to study the difference in impact toughness between heat-treated and non-heat-treated weld samples under the same conditions, two groups of samples were taken respectively. Heat treated, with obvious metal fluidity.

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