In the manufacturing process of wind power towers, welding is a very important process. The quality of welding directly affects the tower production quality. Therefore, it is necessary to understand the causes of weld defects and various control measures. In tower production, welding defects are divided into internal defects (such as cracks, incomplete penetration), external defects, and other defects.
First, appearance defects
Appearance defects (surface defects) are defects that can be found from the surface of the workpiece without the aid of an instrument. Common appearance defects include undercuts, soldering bumps, and welding distortions. Sometimes, there are surface pores and surface cracks, and the roots of single-side welding are not thoroughly welded.
Preventive measures: Correct postures, use reasonable specifications, use non-polarized electrodes, reduce assembly gaps, and add pads or pads on the back of the weld.
Second, pores and slag
Air hole: Air hole refers to the hole formed in the weld pool when the gas in the weld pool does not escape before the metal is solidified and remains in the weld. The gas may be absorbed from the outside by the bath, and may also be generated during the welding metallurgy process.
(1) The main reason for generating pores is that the surface of the base metal or filler metal has rust, oil, etc. The electrode and flux do not dry will increase the amount of pores, because the moisture in the rust, oil, electrode coating, and flux is decomposed at high temperatures. Gases increase the content of gases in high-temperature metals. Welding line energy is too small, the molten pool cooling speed, is not conducive to gas evolution. Deoxidation of the weld metal will also increase oxygen pores.
(2) The hazard of stomata: The stomata reduces the effective cross-sectional area of ​​the weld and loosens the weld, which reduces the strength of the joint, reduces the plasticity, and causes leakage. Stomata are also factors that cause stress concentration. Hydrogen pores may also contribute to cold cracking.
Prevention:
a. Remove oil, rust, moisture, and debris from the surface of the wire, working groove, and nearby surfaces.
b, using a basic electrode, flux, and thoroughly dried.
c. Adopt DC reverse connection and short arc welding.
d. Preheating before welding, slowing the cooling rate.
e. Welded with strong specifications.
Slagging: Slagging refers to the phenomenon that residual slag remains in the weld after welding.
(1) Reasons for slag inclusions: a. The size of the groove is irrational; b. There are dirt on the groove; c. When the multi-layer welding, the clearance between the layers is not complete; d. The welding line energy is small; e. Welds The heat dissipation is too fast, the liquid metal solidifies too fast; f, the coating material of the electrode, the chemical composition of the flux is unreasonable, and the melting point is too high; g. When the manual welding is performed, the welding rod swings poorly, which is not conducive to the floating of the slag. According to the above reasons, corresponding measures can be taken to prevent slag formation.
(2) Hazard of slag inclusions: The hazards of spotted slag inclusions are similar to those of pores. Slag inclusions with sharp corners will produce stress concentration at the tip, and the tip will also develop as a source of cracks, which will cause serious damage.
Third, crack
Classification of cracks: According to the size of cracks, they are divided into three categories: (1) Macroscopic cracks: cracks that are visible to the naked eye. (2) Micro cracks: Can be found under the microscope. (3) Ultramicrocracking: It can be found under a high magnification microscope, generally referring to intergranular cracking and intracrystalline cracking.
From the point of view of temperature, cracks fall into two categories:
(1) Hot cracks: cracks generated near the Ac3 line. The welding is usually completed, also known as crystal cracks. This kind of crack mainly occurs in the grain boundary. The crack surface has an oxidized color and loses its metallic luster.
(2) Cold crack: It refers to the crack produced after the welding is cold to the point of M3 of martensite transformation temperature. It usually occurs after welding for a period of time (a few hours, days or even longer), so it is also called delay crack.
According to the causes of cracks, cracks can be divided into: (1) reheat cracking; (2) layered tearing; (3) stress corrosion cracking.
Cracking hazards: Cracks, especially cold cracks, are catastrophic. The world's pressure vessel accidents, except for a few due to irrational design and improper material selection, are mostly caused by brittle fractures.
Measures to prevent crystal cracks:
a. Reduce the content of harmful elements such as sulfur and phosphorus, and weld them with materials with low carbon content.
b. Add certain alloying elements to reduce columnar crystals and segregation. Such as aluminum, iron, etc. can refine the grain.
c. Welds with shallow penetrations are used to improve the cooling conditions so that low-melting substances float on the surface of the weld but do not exist in the weld.
d, a reasonable choice of welding specifications, and the use of preheating and post-heat, reducing the cooling rate.
e. Use a reasonable assembly sequence to reduce welding stress.
Measures to prevent reheat cracks:
a. Pay attention to the strengthening of metallurgical elements and their effect on reheat cracking.
b. Reasonably preheat or use post-heat, control cooling rate.
c. Reduce residual stress to avoid stress concentration.
d. Try to avoid the sensitive temperature zone of reheat cracks or shorten the residence time in this temperature zone when tempering.
Measures to prevent cold cracks:
a, using low-hydrogen alkaline electrode, strictly drying, stored at 100 - 150 °C, with access to use.
b. Increase the preheating temperature, adopt post-heating measures, and ensure that the temperature between the layers is not less than the preheating temperature, choose a reasonable welding specification, and avoid brittle and hard tissues in the weld.
c. Choose a reasonable welding sequence to reduce welding distortion and welding stress.
d. Hydrogen removal heat treatment is performed in time after welding.
Fourth, incomplete penetration
Means that the base metal is not melted, and the weld metal does not enter the joint.
Causes of incomplete penetration:
(1) Small welding current and shallow penetration.
(2) The size of groove and gap is irrational, and the blunt edge is too large.
(3) Magnetic bias blow effect
(4) The eccentricity of the electrode is too great
(5) Poor cleaning between layers and weld roots.
Incomplete weld hazards: One is to reduce the effective cross-sectional area of ​​the weld and reduce the joint strength. Second, the harm caused by stress concentration caused by incomplete penetration is much greater than the damage caused by strength reduction. Incomplete penetration severely reduces the fatigue strength of the weld. Incomplete penetration may be a source of cracks and an important cause of weld failure.
Incomplete prevention measures: Using a large current for welding is the basic method to prevent incomplete penetration. In addition, when welding fillet welds, AC is used instead of DC to prevent magnetic bias blows, and the groove is properly designed and cleaned up. Short arc welding and other measures can also effectively prevent incomplete penetration.
V. Not fused
It refers to a defect in which the weld metal and the base metal, or the weld metal, are not melted together. According to its location, the unfused can be divided into three types: the groove is not fused, the layers are not fused, and the root is not fused.
Causes of unfused defects:
(1) The welding current is too small;
(2) The welding speed is too fast;
(3) The electrode angle is not correct;
(4) The arc bias blowing phenomenon has been generated;
(5) The welding is in the down-slope welding position, and the base metal is covered with hot metal when it is not melted;
(6) Dirt or oxide on the surface of the base metal affects the fusion bonding between the deposited metal and the base metal.
Unfused hazards: Unfused is an area-type defect. The unfused and unfused bases of the groove have a significant reduction in the bearing area. The stress concentration is also more serious and the damage is second only to the crack.
Unfused prevention and control measures: Properly increase the welding current and increase the energy of the welding line; the welding speed is appropriate, not too fast; skilled operation skills, the electrode (gun) angle is correct.
Sixth, other defects
The chemical composition or composition of the weld does not meet the requirements: improper matching of the welding material and the base metal, or burning of the elements during the welding process, etc., easily causing the chemical composition of the weld metal to change or causing the weld structure not to meet the requirements. This may lead to a decrease in the mechanical properties of the weld seam and also affect the corrosion resistance of the joint.
Overheating and overheating: If the welding specification is used improperly, the heat affected zone will stay at a high temperature for a long time and the grains will become coarse, ie, overheating occurs. If the temperature is further increased and the residence time is longer, the grain boundary may be oxidized or partially melted, resulting in an over-fired structure. Overheating can be eliminated by heat treatment, and overburning is an irreversible defect.
Seven, conclusion
By analyzing the causes of welding defects, formulating various prevention and control measures and training employees on welding skills and welding knowledge, they not only accelerated the production schedule, reduced the cost, but also greatly improved the welding quality.
Digital Signage for hotel Digital signage includes digital signage hardware, digital signage solutions, information cloud publishing systems, LCD screens, touch screens and other advertising display equipment. JMSX digital signage is widely used in shopping malls, restaurants, churches, enterprises and other places.
digital signage for hotel,Wall-mounted advertising player,digital signage
Jumei Video(Shenzhen)Co.,Ltd , https://www.jmsxdisplay.com