The main reasons for incomplete welding are: small groove angle, narrow root gap or thick blunt edge, improper selection of welding specifications, too small current, low wire energy, too fast welding speed, etc.

1. Cold cracks

Characteristics of cold cracks

Mostly appear in the heat-affected zone near the fusion line between the weld and the base material, mostly transgranular cracks.

Cold cracks have no oxidation color.

Cold cracks occur in or, with high carbon content and alloy content.

Cold cracks have delayed properties, mainly delayed cracks.

Causes of cold cracks

(and heat-affected zone and fusion zone) have a serious quenching tendency, resulting in quenched structure, leading to embrittlement of joint performance.

The hydrogen content is high, and a large number of hydrogen molecules are gathered at the joint, causing very large local pressure, making the joint embrittled; excessive phosphorus content also produces cold cracks.

There is a large tensile stress. Because the diffusion of hydrogen takes time, cold cracks need to be delayed for a period of time after welding. Because it is induced by hydrogen, it is also called hydrogen-induced cracks. Measures to prevent cold cracks

Use alkaline or, reduce the hydrogen content in the metal and improve the plasticity of the metal.

To dry, the weld and the nearby base material should be degreased, dehydrated, and derusted to reduce the source of hydrogen.

Preheating the workpiece before welding and slowly cooling it after welding (the temperature of most materials can be checked in the table) can reduce the cooling rate after welding, avoid the formation of hardened structure, and reduce welding.

Take process measures to reduce welding stress, such as symmetrical welding, multi-layer and multi-pass welding with small line energy, etc., and remove stress after welding.

Immediately after welding, dehydrogenation (post-heating) treatment is carried out, heating to 250℃, keeping warm for 2 to 6 hours, so that the scattered hydrogen in the weld metal escapes from the metal surface.

2. Thermal cracks (also known as crystallization cracks)

Characteristics of thermal cracks

Thermal cracks can occur in the weld area or heat-affected zone, and are distributed along the length of the weld.

The microscopic characteristics of thermal cracks are cracking along the grain boundary, so they are also called intergranular cracks. Because thermal cracks are formed at high temperatures, they have an oxidized color. They are visible immediately after welding.

Causes of thermal cracks.

There are low-melting eutectics (containing impurities such as sulfur, phosphorus, and copper) on the grain boundaries of the weld metal. There is tensile stress in the joint.

Preventive measures

Select appropriate and strictly control the content of harmful impurities such as carbon, sulfur, and phosphorus. Fe and FeS are easy to form low-melting eutectics with a melting point of 988°C, which can easily cause thermal cracks.

Strictly control the cross-sectional shape of the weld to avoid sudden height and flat transition.

Narrow the crystallization temperature range, improve the weld structure, refine the weld grains, and increase plasticity reduction.

Determine reasonable parameters and slow down the cooling rate of the weld to reduce welding stress. Such as using small wire energy, preheating before welding, and reasonable weld arrangement.

Shrinkage holes:

[1] Causes:

(1) Due to the thin blunt edge and large gap, the size of the breakdown hole is large.

(2) Due to the excessive welding current, the arc burns during breakdown welding, and the heating time is too long, causing the molten pool temperature to increase and the molten pool volume to increase. The liquid metal falls due to its own gravity and forms candle nodules. Most of the weld nodules exist in flat welding and vertical welding with slow speed.

[2] Prevention measures:

To prevent the formation of shrinkage holes, measures should be taken mainly from the operating process. Before replacing the welding rod to extinguish the arc, the arc should be continuously struck on the original molten pool or on the back of the pool for two or three times to fill the molten pool. Then the arc should be pulled back to the side of the groove surface to gradually attenuate and extinguish the arc. This can slightly increase the temperature of the molten pool and the surrounding area, slow down the cooling rate, and thus prevent the formation of shrinkage holes.

Concept of concave

Concave is also called root shrinkage. In GB6417-86 "Classification and Description of Weld Defects in Metal Fusion Welding", the concept is "shallow groove caused by the shrinkage of the root of the butt weld". The low-lying part below the surface of the parent material formed at the root of the weld. Therefore, the power industry usually calls continuous concave "waist collapse".

Causes of concave:

The main causes of concave are: too large molten pool during welding, high temperature, reduced surface tension, and molten iron falling; slow welding speed and inappropriate electrode angle. Many concave are caused by the electrode not staying for a short time when the arc is extinguished.

1. Concept of undercut:

This type of defect is an external defect. Because of excessive melting near the parent metal and the fusion line, the transition zone between the deposited metal and the parent metal will also form a depression, which is the undercut. According to the upper and lower surfaces of the undercut, it can be divided into external undercut (on the side with a larger opening) and internal undercut (on the bottom side). The undercut can also be said to be a groove-shaped defect along the edge of the weld that is lower than the surface of the parent material.

2. Causes:

(1) The main causes are excessive welding current, too long welding current, inappropriate rod angle, etc.

(2) When moving the rod, the pause time on both sides of the weld is short, the molten pool cannot be filled, the pause time on the top is too long during horizontal welding, and incorrect rod movement and operation will also cause undercut.

(3) When gas welding, the flame energy is too high, the welding nozzle is tilted at an improper angle, and the welding torch and swing are inappropriate.

3. Preventive measures

(1) Select appropriate welding power supply, rod movement angle, and perform short arc operation.

(2) Swing to the edge of the groove and pause for a while to stabilize the arc. The operation should be skillful and smooth.

(3) The gas welding flame energy should be appropriate, and the angle and swing of the welding torch should be appropriate.

The main reasons for welding porosity:

1. The gas produced during arc welding contains excessive hydrogen and carbon monoxide;

2. The base steel contains too much sulfur;

3. The properties of the flux and the baking temperature are not high enough;

4. The welding part cools down too fast;

5. The welding area is caused by oil, paint, rust, water or galvanized layer;

6. There is too much moisture and wind in the air;

7. The arc is blown.

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