Cause of penetration

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

1.cold crack

Characteristics of cold cracks

They mostly appear in the heat-affected zone near the fusion line of the weld bead and the base metal, and are mostly transgranular cracks.

Cold crack has no oxidation color.

Cold cracking occurs in or at high carbon and alloy contents.

Cold cracks have delayed properties and are mainly delayed cracks.

Causes of cold cracks

The quenching tendency (and heat-affected zone and fusion zone) is serious, resulting in quenching structure, resulting in embrittlement of joint performance.

The hydrogen content is high, and it gathers together to form a large number of hydrogen molecules, causing very large local pressure and embrittlement of the joint; too high phosphorus content also causes 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 cracking.

Measures to prevent cold cracking

Choose alkaline or alkaline to reduce the hydrogen content in the metal and improve the plasticity of the metal.

It is necessary to dry, remove oil, water and rust from the welding seam and the nearby base material to reduce the source of hydrogen.

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

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

Dehydrogenation (post-heating) treatment is performed immediately after welding, heated to 250°C, and kept for 2 to 6 hours to allow the scattered hydrogen in the weld metal to escape from the metal surface.

2.Thermal cracks (also known as crystal cracks)

Characteristics of hot cracks

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

The microscopic characteristic of hot cracks is cracking along grain boundaries, so it is also called intergranular cracks. Because thermal cracks are formed at high temperatures, they have an oxidized color. Visible immediately after welding.

Causes of thermal cracks.

There are low-melting-point eutectics (containing impurities such as sulfur, phosphorus, copper, etc.) on the grain boundaries of the weld metal. Tensile stresses are present in the joint.

Precautions

Choose appropriate ones and strictly control the content of harmful impurities carbon, sulfur and phosphorus. Fe and FeS easily form a low melting point eutectic 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 heights and flat transitions.

Reduce 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 linear energy, preheating before welding, reasonable welding seam layout, etc.

Cold shrinkage holes:

【1】Cause:

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

(2) Because the welding current is too large and the arc burns during breakdown welding, and the heating time is too long, the temperature of the molten pool increases, the volume of the molten pool increases, and the liquid metal falls due to its own gravity to form a nodule. Most of the welding nodules exist on the flat surface. Welding, vertical welding speed is too slow, etc.

[2] Preventive measures: In order to prevent the occurrence of cold shrinkage cavities, measures should be taken mainly from the operating process. Before replacing the welding rod to extinguish the arc, the arc should be fired two or three times on the original molten pool or on the back of the pool to fill the molten pool, and then The arc is pulled back toward the side of the groove surface, gradually attenuating and extinguishing the arc. This can slightly increase the temperature of the molten pool and surroundings, slow down the cooling rate, and prevent the occurrence of cold shrinkage holes.

The concept of concave is also called root shrinkage. The concept in GB6417-86 "Classification and Description of Weld Defects in Metal Melting Welding" is "shallow grooves caused by root shrinkage of butt welds". A depression formed at the root of the weld below the surface of the base metal. Therefore, in the power industry, continuous concavities are usually called sag. Causes of concave: The main causes of concave are: excessive molten pool during welding, high temperature, reduced surface tension, falling molten iron; slow welding speed and inappropriate electrode angle. Many recesses are caused by the welding rod not staying for a short time when the arc is stopped.

1.The concept of undercutting:

Such defects are external defects. Excessive melting near the parent metal and the fusion line will also cause a depression in the transition zone between the deposited metal and the parent metal, which is an undercut.

According to the upper and lower sides of the undercut, it can be divided into outer undercut (on the side with the larger opening) and inner undercut (on the bottom side). Undercut can also be said to be a groove-like defect along the edge of the weld that is lower than the surface of the base metal.

2.Causes:

(1) The main reasons are that the welding current is too large, too long, and the angle of the far strip is inappropriate.

(2) When transporting the bar, the pause time on both sides of the weld is short and the molten pool cannot be filled. During horizontal welding, the pause time is too long, and incorrect transport and operation can also cause undercuts.

(3) The flame energy rate is too large during gas welding, the inclination angle of the welding tip is inappropriate, the welding torch and swing are inappropriate, etc.

3.Preventive measures

(1) Select the appropriate welding power source, rod transport angle, and perform short arc operation.

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

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

The main reasons for welding pores:

1. Caused by the gas generated during arc welding containing excessive hydrogen and carbon monoxide;

2. The base material steel contains too much sulfur;

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

4. The cooling speed of the welding part is too fast;

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

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

7. Arc blow occurs.

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