(1) Welding deformation

Workpiece welding will generally produce deformation, if the deformation exceeds the allowable value, it will affect the use.

A few examples of welding deformation are shown below.

The main cause is uneven local heating and cooling of the weldment. Because welding, welding parts are only heated to high temperature in the local area, the closer to the weld, the higher the temperature, the larger the expansion.

However, the metal in the heated area cannot expand freely because it is prevented by the surrounding metal with lower temperature. When cooled, it could not contract freely because of the surrounding metal.

As a result, there is tensile stress in this part of the heated metal, and compressive stress in balance in the other parts of the metal.

When these stresses exceed the yield limit of the metal, welding deformation will occur. Cracks occur when the strength limit of the metal is exceeded.

(2) External defects of welds

1. The weld reinforcement is too high

This phenomenon occurs when the welding groove Angle is too small or the welding current is too small.

The dangerous plane of welds has changed from M-M plane to N-N plane of fusion zone. Due to the stress concentration, it is easy to be destroyed. Therefore, in order to improve the fatigue life of pressure vessels, the reinforcement of welds is required to be leveled.

2, weld too concave

The strength of the joint is reduced due to the reduction of the working section of the weld.

3. Weld edge biting

The indentation formed along the edge of the weld on the workpiece is called bite edge, as shown in Figure 2-22.

It not only reduces the working section of the joint, but also causes serious stress concentration at the biting edge.

4. The flash

Molten metal flows to the unmelted workpiece at the edge of the pool and accumulates to form welding nodules, which are not fused with the workpiece.

Weld nodules have no effect on static load strength, but can cause stress concentration and reduce dynamic load strength.

5. Burn through

Burn through refers to the leakage of part of the molten metal from the opposite side of the weld, or even burn through the hole, which makes the joint strength drop.

(3) Internal defects of welds

1. Lack of penetration

Lack of penetration refers to a defect between the workpiece and the weld metal or weld layer. Poor penetration weakens the working section of the weld, causes serious stress concentration and greatly reduces the strength of the joint, which often becomes the source of the weld cracking.

2. The slag

Non-metallic slag is sandwiched in the weld, which is called slag sandwiching. Slag inclusion reduces the working section of the weld and causes stress concentration, which will reduce the strength and impact toughness of the weld.

3. The porosity

When the weld metal is at high temperature, it absorbs too much gas (such as H2) or gas (such as CO) generated by metallurgical reaction in the molten pool. When the molten pool is cooled and solidified, it is too late to be discharged, and holes are formed in the interior or surface of the weld, that is, pores.

The existence of porosity reduces the effective working section of weld and the mechanical strength of joint. If there is penetration or continuous porosity, it will seriously affect the sealing of welding parts.

Crack in 4.

During the welding process or after welding, the local crack of the metal in the welding joint area is called crack. Cracks may occur in the weld or in the heat-affected zone on both sides of the weld. Sometimes it occurs on the surface of the metal, sometimes it occurs inside the metal.

Usually according to the different mechanism of crack generation, it can be divided into hot crack and cold crack.

4.1 hot crack

Hot cracks are produced in the crystallization process from liquid to solid in the weld metal, mostly in the weld metal.

The main reason is the existence of low melting point materials (such as FeS, melting point 1193℃) in the weld, which weakens the connection between grains. When subjected to a large welding stress, it is easy to cause the rupture between grains.

Welding parts and electrodes containing S, Cu and other impurities, it is easy to produce thermal cracks.

Hot cracks are distributed along grain boundaries. When the crack penetrates the surface and communicates with the outside world, it has obvious tendency of hydrogenation.

4.2 cold crack

Cold cracks occur in the cooling process after welding, and most of them occur in the fusion line at the junction between the base metal and the weld.

The main reason is that the quenched structure is formed in the heat-affected zone or the weld seam. Under the action of high stress, it causes the internal fracture of the grain. When welding the easily quenched steel with high carbon content or more alloy elements, it is easy to produce cold cracks.

Too much hydrogen in the weld can also cause cold cracks.

Crack is a very dangerous defect, it will not only reduce the bearing section, but also produce serious stress concentration, crack will gradually expand in use, may lead to the destruction of members. Therefore, such defects are generally not allowed in the welding structure. Once found, it is necessary to shovel and re-weld.

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