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Common Defects and preventive measures in High Frequency Welding — Inclusions

High-frequency welded pipes may have various defects, and the defects are not completely uniform. In combination with the characteristics of our welded pipe production, there are several defects that often occur:

1 Inclusions;

2 Insufficient fusion;

3 Bonding;

4 Casting and welding;

5 Air holes;

6 Skip welding.

These defects are not all present, but are often found in high frequency welded tubes.

The following is a sketch of each defect rather than an actual picture. It is mainly used to illustrate the main features of the defect, and not to mistake it for a welding defect only occurring under a specific process condition. The defect shown in this paper is the fracture morphology of the weld cracked by flattening test.

Inclusions – Black Over-burned Inclusions

This kind of defect is that the metal oxide is not clamped on the melt surface with the extrusion of molten metal, and is formed on the molten metal surface of V-shaped month. If the approaching speed of the steel strip edge is less than the melting speed and the melting speed is higher than the discharging speed of the molten metal, a narrow sector containing molten metal and metal oxides is formed after the vertex of the V-shaped mouth. These molten metals and metal oxides can not be completely discharged after normal extrusion, thus forming an inclusion zone, as shown in Figure 1.

图片1 

Figure 1 The formation mechanism of black over-burned inclusions

After the weld is flattened, it is easy to see black over-burned inclusions at the weld fracture. Compared with the fibrous fracture of the weld, the black over-burned inclusion has a flat fracture and no metallic luster. Such defects may occur individually or as a chain. as shown in Figure 2.

 图片2

Fig. 2 Fracture profile of black over-burned inclusion defect

 

The inclusion probability increases when the angle of V-shaped month becomes narrower, for example, the angle is less than 4 degrees or the ratio of silicon to manganese in steel is less than 8:1. However, the ratio of silicon to manganese in steel is difficult to control compared with other factors, which mainly depends on the chemical composition of the base metal.

Preventive measures for black over-burned inclusions:

(1) V-shaped angle is controlled at 4°~6°;

(2) Reliable fixture installation guarantees stable V-shaped length;

(3) Better weld quality can be obtained at relatively low welding temperature.

(4) Avoid the ratio of silicon to manganese in the chemical composition of the base metal of steel strip being less than 8:1.

Inclusions – White Peroxide Inclusions

This kind of defect is called white peroxide inclusion,which is not exact. In fact, it is inadequate fusion caused by pre-arc, and there is no foreign body clamped on the fusion surface. Usually, burrs or rust fall in front of the V-shaped vertex to form a bridge, resulting in short circuit and current jump to produce pre-arc phenomenon. Short-circuit current instantaneously changes the direction of current and reduces the heat of V-month, as shown in Figure 3.

 图片3

Figure 3 The formation mechanism of white peroxide inclusions

 

The instantaneous current produces very small defects. Generally, the length of defects does not exceed the wall thickness. From the weld fracture, we can see that a small bright, flat plane is surrounded by a fibrous fracture, as shown in Figure 4.

 图片4

Fig. 4 Fracture profile of white peroxide inclusions defect

 

Preventive measures for white peroxide inclusions:

(1) The angle of V-shaped mouth is controlled from 4°~6°;

(2) Reduce trimming burr;

(3) Appropriate edge treatment or reduction of edge damage of steel strip;

(4) Keep the cooling water clean and do not flow to V-shaped port.


Post time: Mar-05-2019

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