Both large metal containers and hundreds of kilometers of fluid transmission pipelines are connected by welding. Through the welding method, the welded joint with excellent mechanical properties can be obtained. However, in the process of welding, due to the influence of human factors and other natural factors, there are inevitably various defects in the process of weld formation. A large part of the leakage on the weld is caused by the welding defects left over in the welding process.

The most common welding methods are electric welding and gas welding. The common weld defects of both are described below.

1. Defects of electric welding seam

Electric welding is the process of obtaining the whole metal joint through the high-temperature arc generated by electric energy. Common defects of electric welding seam mainly include the following.

(1) The gap or edge of the weldment that is not fully welded is not melted, and the gap left is called incomplete welding, as shown in Figure 6-6. Due to incomplete penetration, the pressure medium will leak along the small gap between layers, and jet leakage will also occur in severe cases.

(2) The non-metallic substance of slag inclusion in the weld is called slag inclusion, as shown in Figure 6-7. Slag inclusion is mainly due to poor operation technology, so that the slag in the molten pool does not float out and exists in the weld. The slag inclusion may also come from the dirt of the base metal.

Some slag inclusions can be seen, which are called external defects; Some exist in the depth of the weld, which can not be seen by the naked eye, and can be seen through nondestructive testing, which is called internal defect. Both internal and external defects do great harm to the weld, and their existence reduces the mechanical properties of the weld. For some micro inclusions with acicular shape, the sharp angle of slag inclusion will cause stress concentration, which is almost equal to that of crack. Acicular nitrides and phosphates in the weld will make the metal brittle, and iron oxide and iron sulfide can also form cracks.

Weld leakage caused by slag inclusion is also common, especially on fluid transmission pipelines and vessels with low weld quality requirements. The weld section with slag inclusion will cause stress concentration in local area and expand the micro crack at the tip of slag inclusion. When this crack penetrates the pipe wall thickness, leakage will occur.

(3) During metal welding, due to some reasons, the gas in the molten pool does not have time to escape and stay in the molten pool. The fluid metal in the weld solidifies to form pores, which are called pores, as shown in Fig. 6-8. The shape, size and quantity of pores are related to the steel type of base metal, the nature of welding rod, welding position and the operation technical level of welders. Some of the gases forming pores are originally dissolved in the base metal or the steel core of the electrode; Some are gases produced when the drug skin melts; Some are produced by the decomposition of oil rust and scale on the base metal after heating; Others come from the atmosphere. The pores in low carbon steel welds are mainly hydrogen or carbon monoxide pores.

According to the different positions of pores, they can be divided into surface pores and internal pores; According to different distribution, it can be divided into evacuation pores, dense pores, continuous pores, etc. The causes of these stomata are various, and the shapes and sizes of stomata are also different, including spherical, oval, vortex and caterpillar.

Porosity has a great impact on the strength of the weld. It can reduce the effective working load of the weld, reduce the mechanical properties of the weld, especially the bending and impact toughness, and destroy the compactness of the weld. Continuous porosity can also lead to the destruction of welded structure.

A single small air hole generally does not cause leakage. However, under the action of temperature difference stress, installation stress or other natural forces, the tip of long pores will have the phenomenon of stress concentration, resulting in cracks at the tip of pores and continuous expansion, and finally lead to leakage; Several continuous honeycomb pores will cause point leakage. The simple and easy method introduced in the pressure bonding and sealing technology can be used to eliminate the leakage caused by such weld pores; When the leakage pressure and amount are large and it is difficult for personnel to get close to the leakage part, the agent type pressurized sealing technology can be used to eliminate it; The parts where hot work is allowed can also be eliminated by the method introduced in the pressurized welding sealing technology, which will have higher strength and longer service life.

(4) Cracks are the most dangerous defects in metals. It is also a problem often encountered in the welding process of various materials. The dangerous defects in this metal have the trend of continuous expansion and extension. From the perspective of sealing, the expansion of cracks will eventually lead to the leakage of the sealed fluid medium.

Cracks can be divided into longitudinal cracks, transverse cracks, weld center cracks, root cracks, crater cracks, heat affected zone cracks, etc.

Sometimes cracks appear on the surface of the weld and sometimes inside the weld. Sometimes macroscopic, sometimes microscopic, which can be observed only with a microscope. Common cracks are as follows.

1) Hot crack of welding metal. The characteristic of this kind of crack is that the fracture is blue black, that is, the color of the metal oxidized at high temperature. The crack always occurs in the center of the weld or perpendicular to the weld fish scale ripple. The thermal crack visible on the weld surface is in an inconspicuous zigzag shape. The pattern like or slightly zigzag linear crack at the arc pit also belongs to the thermal crack.

2) Cold crack of welding metal. Cold crack is different from hot crack. It is produced at a lower temperature after welding, generally at 200-300 ℃. Cold cracks can appear immediately during weld cooling, and some can also appear after several hours, days or even one or two months. Therefore, cold cracks are also called delayed cracks. Most of the delayed cracks occur on the base metal or on the fusion line at the junction between the base metal and the weld, most of them are longitudinal distribution, and in a few cases, they may also be transverse cracks. Its appearance features are: there is no obvious oxidation color on the cold crack section exposed on the welding metal surface, and the fracture is bright. The metallographic characteristics are that the cold crack may occur on the grain boundary or penetrate into the grain body

The above only introduces several common welding seam defects and their causes. Of course, some other factors will also cause weld defects. Generally speaking, no matter what kind of welding defect exists in the afternoon weld, it will affect the quality of the weld, weaken the strength of the weld, and also be an important reason for the leakage of hazardous chemical media in equipment and pipelines.

2. Weld defects of gas welding

Gas welding is a process in which the combustible gas emitted by the welding torch and oxygen are mixed and burned, the heat heats the joint of the two weldments to the molten state, and the weldments are connected with or without filler materials to obtain the overall welded joint. In the process of gas welding, like electric welding, some welding defects sometimes appear in the weld due to some reasons.

(1) Overheating and overburning overheating and overburning generally refer to the change of metal structure after the metal is heated to a certain extent during gas welding. The overheating of metal is characterized by blackening on the metal surface and the appearance of oxide scale. The grain size is coarse; During overburning, in addition to the coarse grain, the grain boundary is also strongly oxidized, and the macro feature of the weld is "slagging". Overheated metal will become brittle, and if it is burned too much, it will be more brittle. The main reasons for this defect are:

1) The flame energy rate is too high;

2) Welding speed is too slow;

3) The torch stays in one place too long.

In addition, it is also related to the objective factors such as the oxidation flame with excess oxygen, unqualified welding wire composition and welding at the place with excessive wind force. Obviously, the existence of such welding defects will inevitably affect the weld quality,

(2) Pores are bubbles left in the weld. The main causes of air holes in gas welding are:

1) The surface of workpiece and welding wire is not clean, with oil, rust, paint and oxide scale;

2) The chemical composition of welding wire and base metal does not meet the requirements;

3) Welding speed is too fast;

4) The heating and melting coordination between welding wire and base metal is inconsistent.

The existence of pores will reduce the effective cross-sectional area of the weld, destroy the compactness of the weld and reduce the mechanical properties of the welded joint.

(3) Slag inclusion may occur when there are dirt such as oil, paint and rust on the welded parts and welding wires, and the necessary means are not taken to clean them during assembly welding. This kind of slag inclusion causes the same harm as the slag inclusion generated during electric welding.

(4) Undercut undercut is a pit or groove formed at the junction of base metal and weld metal. When welding transverse welds, undercut is most likely to form on the upper part of the weld. The reason is that the inclination angle of welding nozzle is incorrect, the swing of welding nozzle and welding wire is improper, and the flame energy rate is too large.

After the undercut defect is formed in the weld, the effective cross-sectional area of the metal is reduced, and a stress concentration is formed at the undercut. This stress concentration will also cause the propagation of small cracks in the weld and leakage.

(5) The main causes of cracks during gas welding are:

1) Unqualified composition and structure of weldment and welding wire (such as excessive carbon content in metal, excessive sulfur and phosphorus impurities, uneven structure, etc.);

2) Excessive stress during welding, insufficient weld reinforcement height or poor weld fusion;

3) Improper welding sequence when welding long welds;

4) During spot welding, the weld is too short or the fusion is poor;

5) Low temperature in the workplace;

6) The welded junction is not filled at the end.

For metal, crack is the most dangerous welding defect. Its existence obviously reduces the bearing capacity of welded components, and stress concentration will inevitably appear at the tip of crack. Stress concentration will make the crack expand continuously. When the crack reaches a certain depth, it will destroy the sealing performance of pipeline and equipment, and the fluid medium will leak along these cracks.

Both the defects of electric welding and gas welding are the root causes of weld leakage. It is absolutely necessary to improve the welding quality from the perspective of permanent cure. However, the leakage on the welds of equipment and pipelines that have been put into operation must be eliminated by pressure sealing technology to ensure the safety of production.