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.

Product function: it is mainly used to switch, cut off or connect the medium in the pipeline, and can also be used for fluid regulation and control. It can not only easily control the pipeline switch and flow, but also has a beautiful effect.

Product features: the valve has compact structure, reliable sealing between sealing surface and spherical surface, simple structure, convenient maintenance, closed sealing state, not easy to be eroded by medium, easy operation and maintenance

1. It is suitable for frequent operation, rapid opening and closing and light weight.

2. Low liquid resistance.

3. The utility model has the advantages of simple structure, small relative volume, light weight and convenient maintenance.

4. Good sealing performance.

5. Not limited by the installation direction, the flow direction of media can be arbitrary.

6. No vibration, low noise.

7. Maximum allowable pressure:

Normal temperature water, oil and gas 5.49mpa

Saturated steam 0.5MPa

Sealing inspection pressure: 0.59mpa

Operating temperature range - 20 ℃ ~ 180 ℃

8. Product size: 1 / 8 1 / 4 3 / 8 3 / 4 1

Introduction to ball valve:

Ball valve is mainly used to cut off, distribute and change the flow direction of medium in the pipeline. Ball valve is a new type of valve widely used. Its working principle is to make the valve unblocked or blocked by rotating the valve core. The ball valve has the advantages of light switch and small volume. It can be made into a large diameter, reliable sealing, simple structure and convenient maintenance. The sealing surface and spherical surface are often closed, which is not easy to be eroded by the medium. It has been widely used in various industries.

Features of ball valve:

Stainless steel ball valve can be closed tightly only by rotating 90 degrees and small rotating torque. The completely equal inner cavity of the valve body provides a very small resistance and straight flow channel for the medium. The main feature of the ball valve is its compact structure and easy operation and maintenance. The stainless steel ball valve can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil products, liquid metal and radioactive media. The ball valve body can be integral or combined. This type of valve shall be installed horizontally in the pipeline. Classification of stainless steel ball valve: stainless steel pneumatic ball valve, stainless steel electric ball valve and stainless steel manual ball valve. Stainless steel ball valves are made of 304316321 stainless steel ball valves.

Precautions for ball valve operation:

1. Stainless steel valves shall avoid contact with other metals. Note that there shall be no garbage and other foreign matters inside, and shall be stored in a well ventilated environment.

2. In order to protect the inner gasket of the ball valve, the ball valve shall be in the fully open state, and the gate valve, stop valve and check valve shall be kept in the fully closed state.

3. Before piping construction, confirm the state of the interface, whether there is oil stain, etc., and remove the attached foreign matters if any.

4. There is a specified flow direction on the valve body. During piping construction, the flow direction shown on the valve body shall be confirmed. During installation, the fluid direction shall be consistent with the arrow on the valve body,

5. When the pipe is connected with the valve, fix it with a wrench at the valve end near the side of the pipe, and then tighten the steel pipe with a pipe wrench. If the thread is screwed too far, the end of the steel pipe will damage the inside of the ball valve.

6. When tightening the bolts, do not tighten only one side, but tighten the bolts on the corresponding diagonal.

7. For the valve with sealing ring, due to the characteristics of the sealing ring, the pressure of the sealing parts will drop during storage, resulting in leakage. Be sure to tighten the loose screws before use. Loose screws should also be checked regularly during normal use.

Stainless steel is certainly familiar to everyone. It can be seen everywhere in life, such as stainless steel thermos cups, stainless steel bowls, etc. in fact, stainless steel pipe fittings are widely used in food and medical treatment, metal products, mechanical equipment and other industries. According to different specific uses, people have different requirements for the hardness of stainless steel pipe fittings. If you loose Xiaobian, please give us a popular science: what parameters are used to express the hardness and softness of stainless steel pipe fittings? In fact, Brinell, Rockwell and Vickers indexes are mainly used to measure the hardness of stainless steel pipe fittings. Different methods have the following differences:

1. Vickers hardness (HV)

Vickers hardness test of stainless steel pipe fittings is an indentation test method, which can be used to measure the hardness of very thin metal materials and surface layers. It has the main advantages of Brinell and Rockwell test methods, but it is not as simple as Rockwell method, and Vickers method is rarely used in the standard of stainless steel pipe fittings. However, Vickers hardness tester has a wide measurement range and can measure almost all metal materials used in industry.

2. Brinell hardness (HB)

In the standard of stainless steel pipe fittings, Brinell hardness is the most widely used. Indentation diameter is often used to express the hardness of materials, which is intuitive and convenient. However, it is not suitable for harder or thinner steel and steel pipes.

3. Rockwell hardness (HR)

Rockwell hardness, like Brinell hardness test, uses indentation test method. But it measures the depth of the indentation. Rockwell hardness test is a widely used method at present, but there are three standards for Rockwell hardness grade: A, B and C, which are usually recorded as HRA, HRB and HRC. The representation method is hardness data and hardness symbol. Among them, HRC is second only to Brinell hardness Hb in steel pipe standards. Rockwell hardness can be used to measure metal materials from very soft to very hard, and the hardness value can be read directly from the dial of the hardness tester. However, the hardness value is not as accurate as Brinell method. When the test sample is too small or the Brinell hardness (HB) is greater than 450, Rockwell hardness measurement shall be used.

Ruosung stainless steel pipe fittings manufacturer strictly adheres to the requirements of ISO9001 quality management system to realize the trinity of post self inspection, process mutual inspection and inspector special inspection, so as to prevent unqualified products from flowing into customers.