Welding quality directly affects the performance and safety of the boiler structure.
If the welding quality of the boiler is not good, leaks and even explosion accidents may occur, causing great damage to the lives of people and property.
Therefore, it is of great importance to discuss the causes of boiler welding defects and improve the boiler welding quality to ensure the safe operation of the boiler.
Incomplete penetration
Cause analysis
The main causes of incomplete penetration defects are as follows:
① Water wall membrane pipes are installed at the construction site.
Due to the narrow opening and obstacle operation, it is very easy to have incomplete penetration.
② Mounting clearance is inconsistent.
When assembling parts on site, it is necessary to assemble multiple welded joints at the same time.
However, it is very difficult to match the mounting clearance.
Furthermore, multiple welded joints cannot be welded at the same time.
When some welded joints are welded, the gap between the remaining welded joints will be reduced or even zero.
These welded joints are prone to incomplete penetration during welding.
③ Incomplete penetration is caused by the obstruction of the pipes on both sides and the strong assembly step joint during welding.
Preventative countermeasures
Measures to prevent incomplete penetration:
The size of the joint groove must be controlled.
For example, for the joint welded on one side and formed on both sides, the mounting gap should be the diameter of the electrode, and the blunt edge should be about half the diameter of the electrode;
The root must be removed carefully;
Suitable welding speed and welding current should be selected during welding; Use short arc welding, etc.
Start the arc at the most difficult part and seal the arc in the place with the least obstacle so as not to affect the welder's operation and vision during welding and reduce the quality of the welded joints;
To avoid very low temperatures in the welded joints, it is better to adopt butt welding by two people;
It is difficult to maintain the specified included angle between the nozzle, welding wire and welding at the obstacle position, which can be adjusted according to the actual situation.
After the difficult position, normal angle welding can be resumed.
For example, for membrane water wall:
① Before assembly, carefully check the flatness of the welded joint.
For small deformations, flame correction or mechanical method can be used for correction before assembly;
If the deformation is very large and the overall correction is difficult, the connection weld between the fins can be cut first.
The cutting length is determined according to the degree of deformation and the size of the stress, generally not more than 1500mm, and then the single tube is fixed.
After the assembly and welding of the entire water-cooled wall is completed, the cut fin weld must be welded again using the sectional back welding method.
② Strictly control the minimum and maximum gaps of various pairs of welded joints, so that the gap of the smallest pair can meet the welding quality requirements;
The maximum mounting clearance must not exceed 5 mm.
During welding, the welded joint with small gap should be welded first, and then the welded joint with large gap should be welded.
This can not only prevent incomplete penetration, but also help reduce welding stress and deformation and reduce wastage of the welded joint.
③ When welding on the fin, due to the limitation of tube spacing, it is better to use tungsten electrode with sharp edge grinding for welding to increase the accessibility of the welding gun and appropriately reduce the welding speed, increase the arc residence time here, and proceed when the size of the molten pool is equal to the other parts and the molten metal is in the “infiltration” state.
④ If the gap between the welded joints of each part is too small to ensure welding quality, tungsten electrodes with sharp tips can be used for welding to center the arc and facilitate penetration of the weld root.
Not fused
Cause analysis
When welding, the current is too small or the welding speed is too fast and the heat is not enough, so that the base metal groove or the weld metal that was welded first is not fully melted;
The selected current is too high, which turns the second half of the electrode red and causes very rapid melting.
The molten metal of the electrode has been covered before the edge of the base metal reaches the melt;
The welding heat dissipation rate is too fast or the temperature at the starting point is low, so that the starting end of the base metal does not melt, resulting in incomplete fusion;
The flame of the welding rod, welding wire or welding torch leans to one side of the groove, or the arc leans to one side due to the eccentricity of the welding rod, so that the base metal or the base metal previous weld being covered by the filler metal before being fully melted;
When there is rust or dirt in the base metal groove or the weld metal surface of the previous layer, the filler metal cannot be melted due to insufficient temperature during welding, and the edge and intermediate layer are not fused.
During the installation of the heating surface tubes of the power plants' boilers, the arc stopped during the welding of the same weld, which resulted in more “cold joints”, especially “cold joints” of large diameter tubes, in addition to causing no fusion between layers.
Preventative countermeasures
The angle of the electrode and welding torch must be reasonable, the electrode oscillation must be appropriate, and the fusion on both sides of the groove must be observed;
A slightly higher welding current and flame energy rate should be selected, and the welding speed should not be too fast to increase enough energy to melt the base metal or the weld metal of the previous layer;
If the electrode is eccentric during welding, the angle must be adjusted in time to make the arc in the correct direction;
Carefully clean the groove and weld from dirt;
After inspecting the root layer weld with hydrogen arc welding primer, secondary layer welding should be carried out in time;
To avoid “cold joint”, the length of the welding wire must be planned and the welding wire must not be replaced during welding.
To avoid wire shaking, the distance between the wire claw and the end of the wire should not be too long, and it is recommended to adopt the “hot joint” method without stopping the arc.
This method consists of first contacting the end of the welding wire with the weld pool when a joint occurs due to the need to change the position of the wire clamp.
At the same time, move the bow slightly back or take it to the side of the groove.
When the weld pool solidifies and the ends of the welding wire stick together, quickly change the position of the wire clamp.
Upon completion of this action, immediately restore the arc to its original position and continue welding.
The “hot joint” method can not only guarantee the welding quality but also improve work efficiency.
Stomach
Cause analysis
There are many factors that cause stomata.
When the pre-welding preparations at the construction site are sufficiently qualified, the following requirements have been strictly met:
① Strictly observe the welding parameters and keep them stable;
② Polarity and type of current have little influence;
③ The welding groove must meet the requirements and be clean. The low hydrogen type welding rod used must be strictly baked and placed in the isolation cylinder, and must be removed when used;
④ Strong quality awareness of welders;
⑤ Have strict protection measures against wind and rain;
⑥ When X-ray inspection results show that porosity is not caused by hydrogen arc welding, porosity is often caused by improper arc starting and stopping.
Preventative countermeasures
Control measures mainly include:
① Carefully wipe off the oil, rust, paint and other dirt within the range of 15~20mm on both sides of the groove surface and the surface of the welding wire until all the metal luster is exposed. During argon arc welding, clean it with acetone;
② Welding rods must be dried in strict accordance with regulations and placed in the special insulation cylinder for welding rods after heat preservation, and must be taken out when used;
③ Welding is not suitable when ventilation is large;
④ The heat input from the base metal increases and the cooling rate of the molten pool decreases when the arc starting current is increased appropriately;
⑤ When starting the arc, strike the arc at a distance along the joint, and then draw it back to the joint, increasing the gas escape time;
⑥ During argon arc welding, when honeycomb pores appear for some reason, stop welding immediately and grind with a grinder.
It is prohibited to eliminate air holes by arc reflow.
Honeycomb pores are usually caused by argon shielding failing for some reason, such as temporary failure of the air supply system).
At this time, the surface of the pores is an oxide film. Arc reflow not only makes the weld brittle, but also fails to eliminate pores;
⑦ When welding argon arc pipes with large specifications, pay attention to whether the water-cooled welding torque used is reliable, because the water-cooled welding torque is prone to air holes due to failure;
⑧ High purity argon (greater than 99.99%) must be used, the flow must be moderate and the inner wall of the nozzle must be clean.
To snap
Cause analysis
Welding cracks can occur during welding, or after welding or even after a certain period of time.
The main causes of welding cracks are the following:
① When the carbon or sulfur and phosphorus content of the base metal is too high, its weldability becomes poor and cracking is easy to occur.
② The higher the content of alloying elements and sulfur and phosphorus in welding materials such as electrodes and fluxes, the greater the tendency to crack.
③ Welding at low temperature or in wind will cause cracking if the cooling speed of the weld is too fast.
④ The welded thick plate is also easy to break due to its strong structural rigidity.
Prevention and control measures
Cracks are the most dangerous and unacceptable defects in welded structures.
Its occurrence can be controlled based on the following aspects:
① The adhesive solder must be thick and long enough;
② No strong gatherings are allowed;
③ Do not hit the weld with force;
④ The welding process of different steels should be determined according to the physical properties, chemical properties and weldability of the two materials.
For example, when welding hot steel and stainless steel, due to their different coefficients of thermal expansion, the arc must remain longer on the side with rapid heat dissipation;
⑤ When welding alloy steel, try to use equipment with an arc device for welding, on the one hand, reduce hardening, on the other hand, reduce the possibility of tungsten inclusion.
