The welding quality directly affects the performance and safety of the boiler structure. Poor welding quality of the boiler may lead to leakage or even explosion accidents, causing significant losses to people's lives and property. Therefore, exploring the causes of welding defects in boilers, mastering their preventive measures, and effectively improving the welding quality of boilers are of great significance for ensuring the safe operation of boilers.

1、 Not fully welded

1. Cause analysis

The main reasons for the occurrence of incomplete welding defects are as follows: ① During the installation of membrane water-cooled wall pipes at the construction site, due to narrow gaps and obstacles in operation, incomplete welding is easily caused. ② When assembling pieces on the construction site, it is necessary to simultaneously assemble multiple welding joints due to inconsistent gaps between pairs. However, it is very difficult to make the gaps between pairs the same. In addition, multiple welds cannot be welded at the same time. After welding a portion of the welds, the gaps between the remaining welds that have not been welded will shrink or even become zero. These welds are prone to incomplete penetration defects during welding. ③ During welding, there are obstacles from both sides of the pipe and misalignment caused by strong assembly, resulting in incomplete penetration.

2. Preventive measures

The measures to prevent incomplete welding are: controlling the size of the joint groove, such as single-sided welding and double-sided forming joints, the assembly gap should be the diameter of the welding rod, and the blunt edge should be about half of the diameter of the welding rod; Be careful to clean the roots; Suitable welding speed and welding current should be selected during welding; Use short arc welding, etc. Start the arc from the most difficult part and seal it at the place with the smallest obstacle to avoid affecting the operation and welder's line of sight during welding, and reducing the quality of the welded joint; To avoid welding joints with excessively low temperatures, it is best to use a two person butt welding method for welding; In areas with obstacles, it is difficult to maintain the specified angle between the nozzle, welding wire, and weldment. Adjustments can be made according to the actual situation, and after the difficult position is overcome, normal angle welding can be restored.
Welding of membrane water-cooled wall: ① Before assembly, the flatness of the weld joint should be carefully checked. For minor deformations, flame correction or mechanical methods can be used for correction before assembly; If the deformation is too large and the overall correction is difficult, the connecting welds between the fins can be cut first. The length of the cut depends on the degree of deformation and stress, generally not exceeding 1500mm, and then a single tube can be corrected. After the entire water-cooled wall assembly is welded, the cut fin welds are re welded using the segmented back welding method. ② Strictly control the minimum and maximum gaps between multiple weld pairs, so that the gaps between the smallest pairs can meet the welding quality requirements; The maximum gap between pairs should not exceed 5mm. When welding, the smaller gap should be welded first, followed by the larger gap. This can not only avoid incomplete welding, but also help reduce welding stress and deformation, while reducing weld waste. ③ When welding to the fin area, due to the limitation of tube spacing, it is best to use tungsten electrodes with sharp end grinding for welding to increase the accessibility of the welding gun. The welding speed should be appropriately reduced to increase the residence time of the arc at this point. When the size of the molten pool is equal to other parts and the molten metal is in a "penetrating" state, it should be moved forward. ④ When the gap between each final weld joint is too small to ensure welding quality, a tungsten electrode with sharp end grinding can be used for welding to concentrate the arc and facilitate root penetration of the weld seam.

2、 Not fused

1. Cause analysis

When welding, if the current is too low or the welding speed is too fast, due to insufficient heat, the base metal groove or the weld metal that was welded first may not be fully melted; The selected current is too high, causing the second half of the welding rod to turn red and melt too quickly. Before the edge of the base metal reaches melting, the molten metal of the welding rod has already covered it; The heat dissipation speed of the welded component is too fast, or the temperature at the welding point is low, causing the starting end of the base material to not melt, resulting in incomplete fusion; The flame of the welding rod, welding wire or welding torch is biased towards one side of the groove, or the arc is biased towards one side due to the eccentricity of the welding rod, causing the base metal or the previous weld metal to be covered by the filler metal before being fully melted; When there is rust or dirt on the surface of the base metal groove or the previous layer of weld metal, due to insufficient temperature during welding, it cannot be melted and covered with filler metal, which can also form edge and interlayer lack of fusion. In the installation of heating surface tubes in power plant boilers, the occurrence of arc stopping during the welding process of the same weld seam has resulted in more "cold joints", especially for large-diameter tubes, which is also the reason for interlayer lack of fusion.

2. Preventive measures

The angle between the welding rod and the welding torch should be reasonable, the swinging of the welding rod should be appropriate, and attention should be paid to observing the melting situation on both sides of the groove; Choose a slightly higher welding current and flame rate, and the welding speed should not be too fast, so that the energy increase is sufficient to melt the base metal or the previous layer of weld metal; If the welding rod is found to be eccentric during the welding process, the angle should be adjusted in a timely manner to ensure that the arc is in the correct direction; Carefully clean the dirt on the groove and weld seam; After inspecting the root layer weld seam using hydrogen arc welding as the base, the welding of the secondary layer weld seam should be carried out in a timely manner; In order to avoid the occurrence of "cold joints", the length of the welding wire should be planned carefully, and it is advisable not to replace the welding wire during the welding process. To avoid wire shaking, the distance between the wire grip and the end of the wire should not be too long. It is recommended to use the non-stop arc "hot joint" method. This method involves contacting the end of the welding wire with the molten pool when a joint occurs due to the need to change the wire grip position. At the same time, the arc is slightly moved back or directed towards one side of the groove. When the molten pool solidifies and sticks to the end of the welding wire, the wire grip position is quickly changed. After completing this action, the arc is immediately restored to its original position and welding continues. Adopting the "hot joint" method can ensure welding quality and improve work efficiency.

3、 Stomata

1. Cause analysis

The factors that cause porosity are multifaceted: ① Strictly adhere to welding parameters and maintain stability; ② The polarity and type of current have little effect; ③ The welding groove meets the requirements and is cleaned thoroughly. The low hydrogen welding rod used is strictly baked and placed in an insulation cylinder for immediate use and retrieval; ④ Strong quality awareness among welders; ⑤ Having strict wind and rain prevention measures; ⑥ Pores caused by arc deviation of DC welding machine.

2. Preventive measures

The main control measures are: ① Carefully clean the oil, rust, paint and other dirt within the range of 15-20mm on both sides of the groove surface and the welding wire surface, until all the metallic luster is exposed. During argon arc welding, wipe it with acetone; ② Welding rods should be dried strictly according to regulations, and after insulation, they should be placed in a dedicated insulation cylinder for welding rods. They can be taken as needed. Any deviation or blowing during the welding process should be controlled by adjusting the welding rod angle and welding specifications; ③ When there is a strong draft, welding is not suitable; ④ Appropriately increase the arc current to increase the heat input of the base material and slow down the cooling rate of the molten pool; ⑤ When starting the arc, start the arc slightly away from the joint and then pull it back to the joint to prolong the gas escape time; ⑥ During argon arc welding, if honeycomb like pores appear for some reason, the welding should be stopped immediately and polished with a grinding machine. It is prohibited to use arc remelting to eliminate porosity. Due to the appearance of honeycomb like pores, it is often caused by the failure of argon gas protection. At this time, the surface of the pores is covered with oxide film. Using arc remelting not only makes the weld brittle, but also cannot eliminate the pores; ⑦ When welding pipes with larger specifications using argon arc welding, attention should be paid to whether the water-cooled welding torque used is reliable, as water-cooled welding torque failures can easily lead to porosity; ⑧ Using high-purity (greater than 99.99%) argon gas, the flow rate should be moderate, and the inner wall of the nozzle must be clean.

4、 Crack

1. Cause analysis

Welding cracks may occur during the welding process, as well as after welding or even after a certain period of time. The main reasons for the formation of welding cracks are as follows: ① When the carbon content or sulfur and phosphorus content of the base metal is too high, its weldability deteriorates and cracks are easily generated. ② The higher the alloy elements and sulfur phosphorus content in welding materials such as welding rods and fluxes, the greater the tendency for cracks to occur. ③ Welding at low temperatures or in windy conditions can also lead to cracking if the cooling rate of the weld seam is too fast. ④ Welding thick plates is prone to cracking due to their high structural rigidity.

2. Preventive measures

Cracks are the most dangerous and unacceptable defects in welded structures, and their occurrence can be controlled from the following aspects: ① The spot weld seam should be thick and long enough; ② Do not forcefully assemble; ③ Do not forcefully strike the weld seam; ④ When welding dissimilar steels, the welding process should be determined based on the physical properties, chemical properties, and weldability of the two materials. For example, when welding high-strength steel and stainless steel, due to their different coefficients of thermal expansion, the arc should stay on the side with faster heat dissipation for a longer time; ⑤ When welding alloy steel, try to use equipment with arc ignition devices for welding, which can reduce quenching hardening and minimize the possibility of tungsten inclusions.