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Analysis of welding defects

Analysis of welding defects

  • Categories:Industry trends
  • Author:
  • Origin:
  • Time of issue:2016-06-13
  • Views:44

(Summary description)The harm of welding defects to product components, especially the hidden dangers of boiler pressure vessels and pressure pipes in the use process cannot be underestimated. 1. Due to the existence of welding defects in the weld, the consequence is to reduce the load-bearing sectional area of the weld and weaken the static tensile strength. 2. If the defect forms a notch, stress concentration and embrittlement will occur at the tip of the notch, which is easy to produce cracks and expand. 3. The defects may penetrate the weld and leak, affecting the tightness of equipment or pipeline. No matter what kind of welding defect is, it will do harm to the structural parts to a certain extent. The lesser one will reduce the mechanical properties of the product and shorten the production

Analysis of welding defects

(Summary description)The harm of welding defects to product components, especially the hidden dangers of boiler pressure vessels and pressure pipes in the use process cannot be underestimated. 1. Due to the existence of welding defects in the weld, the consequence is to reduce the load-bearing sectional area of the weld and weaken the static tensile strength. 2. If the defect forms a notch, stress concentration and embrittlement will occur at the tip of the notch, which is easy to produce cracks and expand. 3. The defects may penetrate the weld and leak, affecting the tightness of equipment or pipeline. No matter what kind of welding defect is, it will do harm to the structural parts to a certain extent. The lesser one will reduce the mechanical properties of the product and shorten the production

  • Categories:Industry trends
  • Author:
  • Origin:
  • Time of issue:2016-06-13
  • Views:44
Information

The harm of welding defects to product components, especially the hidden dangers of boiler pressure vessels and pressure pipes in the use process cannot be underestimated.

1. Due to the existence of welding defects in the weld, the consequence is to reduce the load-bearing sectional area of the weld and weaken the static tensile strength.

2. If the defect forms a notch, stress concentration and embrittlement will occur at the tip of the notch, which is easy to produce cracks and expand.

3. The defects may penetrate the weld and leak, affecting the tightness of equipment or pipeline.

No matter what kind of welding defect is, it will do harm to the structural parts to a certain extent, and the light ones will reduce the mechanical properties of the products and shorten the service life of the products; In severe cases, brittle fracture may occur, and in severe cases, catastrophic accidents may occur, bringing huge losses to the national economy.

It can be divided into surface defects and internal defects.

Common surface defects: porosity, slag inclusion, incomplete penetration, incomplete fusion, cracks, etc.

Porosity: When welding, the cavity formed by the bubbles in the molten pool that fail to overflow in time during solidification and remain is called porosity. Porosity is one of the serious process defects in welding. Porosity weakens the effective working area of welded joints, but also brings stress concentration, reducing the strength, plasticity and fatigue strength of joints. It can be divided into surface pores and internal pores according to different positions of pores. According to the different distribution of pores, they can be divided into single pores, open pores, dense pores and continuous pores. According to the shape of pores, they can be divided into strip pores, needle pores, round pores, elliptical pores, etc. Image description of stomatal defect on the negative film: the image on the negative film is black dot, but also black strip or other irregular shapes. The contour of the stomatal is generally smooth, and the density of the stomatal center is large, and it is slightly reduced to the edge.

Cause of air hole:

There are two types:

1. The influence of metallurgical factors on porosity.

When welding, the weld metal will produce porosity when metallurgical reaction occurs. The chemical reaction between the coating and flux of the arc welding electrode when they play a protective role will also generate pores. The oil stain, rust spot and dirt decomposition on the surface of the base metal, and the chemical composition content of the flux may cause porosity.

2. Influence of process factors on porosity.

Process factors include: welding procedures, current types, arc height, operation technology, etc. When DC is connected reversely, the tendency of air hole is the minimum, which means that the workpiece is negative.

The influence of metallurgical factors and technological factors on porosity is explained in detail as follows:

1. Welding rods or fluxes are not dried as required before use, or the drying temperature does not meet the requirements, and the use of the above welding materials and fluxes is very easy to produce air holes in the welding process. The drying temperature of alkaline electrode is 350-450 ℃, and that of acid electrode is not more than 200 ℃, generally 70-150 ℃. The welding rod insulation barrel shall be used for receiving the welding rod and shall be used within 4 hours. The welding rod that has not been used for more than 4 hours must be returned to the warehouse for drying again. During submerged arc automatic welding, if the flux is affected with moisture, there will be continuous porosity in the weld bead.

2. The coating of welding rod is invalid, peeled off or the drying temperature is too high, causing some components in the coating to deteriorate.

3. The metal rust and oil stain of base metal are not removed before welding. Or mixed with different gas making substances in the flux.

4. The cooling speed of molten metal is too fast, the heat energy of arc is small or the welding speed is too fast. As a result, the gas can not float up and overflow in the molten pool in time.

5. Excessive current is used, because the electrode is red and the coating fails, or the arc is too long during welding with alkaline low hydrogen electrode.

6. During manual argon tungsten arc welding, the argon purity is low and the protection is poor. Argon with purity of 99.99% shall be used for welding; During welding, air supply shall be carried out in advance and air stop shall be carried out later; Correctly connect the air pipe and water pipe without confusion; Clean up before welding; Select the protective air flow, nozzle size, electrode extension length, etc.

7. The electric arc is too long or blown laterally, the protection effect of the molten pool is poor, and the air invades the molten pool. Short arc welding shall be adopted for welding with low hydrogen type electrode, and the swing is conducive to gas escape.

8. Improper rod handling method, too fast arc extinguishing action, easy to produce shrinkage cavity, incorrect arc striking action of joint, easy to produce dense pores.

9. The welding current is too small or the welding speed is too fast, the existence time of the molten pool is too short, and the gas cannot escape from the metal in the molten pool.

10. The carbon content of base metal and electrode steel core is high, and the deoxidization ability of electrode coating is poor.

Measures to prevent air holes:

1. Before welding, remove the oil, rust and moisture within 20-30mm on both sides of the groove.

2. Dry the welding rod in strict accordance with the temperature and time specified in the welding rod manual.

3. Select welding process parameters correctly, operate correctly and preheat.

4. Short arc welding shall be adopted as far as possible, and wind proof facilities shall be provided for field welding.

5. It is not allowed to use invalid welding rods, such as welding core corrosion, coating cracking, peeling, excessive eccentricity, etc.

Causes of inclusions:

Type of inclusions in the weld:

1. Oxides. When welding steel materials, oxide inclusions are common. In manual arc welding and automatic submerged arc welding of low carbon steel, the composition of oxide inclusions is mainly SiO2; These oxide inclusions are mainly produced in the process of molten pool reaction.

2. Nitride.

3. Metal inclusion

4. Slag inclusion. The inclusions mentioned above are all precipitated or generated by reaction and belong to microscopic inclusions. There is also a kind of slag inclusion that is directly mixed by slag due to improper process. For example, in manual arc welding, slag inclusion may occur at the edge of the groove due to improper electrode handling, and it often occurs in horizontal welding.

5. Tungsten slag is generated when the tungsten electrode touches the molten pool or the welding wire peels off and melts into the weld due to the burning of the tungsten electrode in tungsten argon arc welding.

Measures to prevent slag inclusion in welds:

The most important measure to prevent slag inclusion in the weld is to correctly select the type of welding rod and flux. The second is to pay attention to the operation of the process.

1. Select appropriate welding specifications to ensure the existence time of the molten pool is not too short to prevent the solidification of the molten pool metal too fast.

2. During multi-layer welding, attention shall be paid to eliminating the slag of the previous weld.

3. The welding rod shall swing properly, and the electrode angle and electrode moving method shall be adjusted at any time to separate the molten iron from the slag, so as to facilitate the slag floating out.

4. Pay attention to protecting the molten pool during operation to prevent air from invading the molten pool.

5. The welding speed is not easy to be too fast to ensure that the slag in the molten pool has sufficient time to float.

6. The welding current shall not be too small.

Image description of slag inclusion defect on negative film:

The image of slag inclusion on the negative film is black spot, black strip or black block, with irregular shape, irregular change of blackness, uneven contour, and some with edges and corners. Tungsten inclusions are white spots on the negative.

Concept and hazard of non fusion

Lack of fusion refers to the defect that the weld metal and base metal, or the weld metal are not fused together. Incomplete fusion can be divided into groove incomplete fusion, root incomplete fusion and interlayer incomplete fusion. Lack of fusion is an area type defect. The lack of fusion at groove and root reduces the bearing area obviously, and the stress concentration is serious, causing great harm.

Reasons for non fusion:

1. The welding current is too small and the welding speed is too fast.

2. The angle of welding rod is incorrect.

3. Arc deflection blowing occurs.

4. The welding is in the downhill welding position, and the base metal has been covered by molten iron before melting.

5. Dirt or oxide on the surface of the base metal affects the fusion and bonding between the deposited metal and the base metal.

Measures to prevent non fusion:

1. Increase welding line energy.

2. The groove edge shall be fully penetrated.

3. The welding specification is correct, the operation is proper, the welding speed is uniform, and the welding rod swings in place.

4. Clean the dirt on the groove surface or the edge of the groove bottom.

Image description not fused on the negative:

The typical image of incomplete fusion of groove is continuous or intermittent black line with different width and uneven blackness. One side has a relatively uniform contour with large blackness, while the other side has an irregular contour with small blackness. The position on the negative film is generally 1/2 of the weld center to the edge, extending longitudinally along the weld.

The typical image of interlayer non fusion is block shadow with small blackness and irregular shape. If there is slag inclusion, the blackness of the slag inclusion part is large.

Concept and hazard of incomplete penetration:

Incomplete penetration refers to the defect caused by the fact that the base metal is not melted and the weld metal does not enter the root of the joint. Incomplete penetration includes incomplete penetration on both sides and incomplete penetration on one side. Incomplete penetration defect is a relatively dangerous defect, and its harmfulness depends on the shape, depth and length of the defect.

Causes of incomplete penetration:

1. The welding current is too small and the welding speed is too fast.

2. The groove angle is too small.

3. The root blunt edge is too thick.

4. The groove clearance is too small.

5. The angle of welding rod is improper.

6. The arc is too long.

Image description of incomplete penetration on negative film:

The typical image of incomplete penetration is a thin straight black line, with neat contours on both sides, which is a blunt edge trace of the groove, and the width is exactly the width of the blunt edge gap. Sometimes the blunt edge of the groove is partially melted, the image contour becomes irregular, and the current width and blackness change locally. However, as long as it can be judged that it is a linear defect at the root of the weld, it should be judged as incomplete penetration. Incomplete penetration is located on the negative film at the projection position of the weld root, generally in the middle of the weld. It may also be biased to one side due to radiographic deviation, welding deviation and other reasons. Incomplete penetration is distributed intermittently or continuously, and sometimes it can penetrate the entire negative film.

Measures to prevent incomplete penetration:

1. The groove type shall be selected reasonably. 2. The assembly clearance shall be appropriate. 3. Use correct welding process.

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Time of issue:2019-11-26 00:00:00
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