ERW steel pipe weld

Factors Affecting Impact Toughness of Weld Seam of ERW Pipe

Initial impact performance reflects the ability of a material to resist impact loads. The practical significance of the impact toughness index is to reveal the brittle tendency of the material, which is generally measured by impact tests. Combined with the ERW welded pipe production process and its actual production process, the main factors affecting the impact toughness of the ERW welded pipe weld are as follows:

1. Raw materials
Since the ERW steel pipe uses the base metal as the welding material, its weld performance is closely related to that of the base metal. The main factors affecting the impact toughness of raw materials are as follows:
①Ingredient
In addition to Fe, C and alloy elements that often appear in steel, there are also impurity elements such as S and P, among which C and S have the greatest impact on the initial stage. As the C content increases, the pearlite content in the steel also increases accordingly. Because pearlite contains a large amount of brittle layered cementite, the impact toughness is poor. S often exists in the form of compound FeS in steel, and the eutectic with low melting point on the grain boundary melts at a higher temperature, causing the steel to crack along the grain boundary. P is partially soluble in ferrite in steel, which increases the strength and hardness of ferrite and reduces plasticity and plasticity; most of Si dissolves in ferrite to strengthen it, thereby increasing the strength and hardness of steel and reducing the strength of steel and hardness. plasticity and toughness.
②Inclusions
The inclusions in the steel will reduce the impact toughness of the steel, and the gray spots at the weld contain a large amount of oxides, which mostly exist in the form of composite inclusions at high temperatures.
In the actual production of ERW welded pipes, it is found that the transverse impact fracture of the weld often appears gray, also known as "oxidation inclusions". It has a great influence on the toughness of the weld. Regardless of whether it has been heat treated or not, the weld still has this defect. Its existence is an important reason why the quality of ERW welded pipes is difficult to improve.
③Granularity
The finer the grain, the more uniformly the deformation caused by the external force is dispersed in the grain, reducing the phenomenon of excessive stress concentration. At the same time, the finer the grain, the more unfavorable it is to the expansion of cracks, so the finer the grain, the better the impact toughness of the material itself.
Therefore, in the process of pipe making, the raw materials should keep as few inclusions as possible, the grains should be refined, and the rising angle of the metal streamline should also be reasonably controlled.

2. Forming quality
During ERW welding, the shape of the pipe edge determines the stability of the weld. During the molding process, it is easy to produce ripples, wrinkles, bulges and work hardening, which affect the quality of toughness. Before entering the extruded spokes, the two sides of the steel belt should be as parallel as possible to ensure that the sides of the two plates have a high degree of parallelism, which is convenient for high-frequency welding, facilitates the discharge of materials and inclusions during welding, and reduces the formation of gray spots. The welding temperature of local bumps on the welding surface is high, which makes the edge of the plate melt unevenly, increases the degree of local oxidation, and is prone to defects.
If there are burrs on the edge of the steel strip, it will cause early contact and sparks during welding, resulting in local cold welding defects. Bends at the edges of the strip or reduced edge thickness will cause the edges to appear wavy, causing gray spots to form and affecting toughness.

3. Welding process
The welding process is closely related to the welding quality. The main parameters are: heat input, welding extrusion force, welding extrusion amount, welding speed, opening angle, welding temperature.
The welding process parameters should be selected according to the wall thickness of the pipe, and the opening angle should be set reasonably. In the case of ensuring sufficient welding power, gradually increase the welding speed and apply an appropriate extrusion force, so that when the extrusion amount reaches 2/3 times the wall thickness, the impact toughness of the weld can be appropriately improved.

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