In the manufacture of pressure vessels, when submerged arc welding is used to weld the longitudinal welds of the cylinder, cracks (hereinafter referred to as terminal cracks) often occur at or near the ends of the longitudinal welds.
Many people have studied this problem and thought that the main reason for terminal cracks is that when the welding arc is close to the end of the longitudinal weld, the weld expands and deforms along the axial direction, accompanied by the transverse opening deformation in the vertical axial direction.
The cold work hardening stress and assembly stress also exist in the process of coiling and fabrication and assembly of the cylinder;
In the welding process, due to the restraint of the end tack weld and the run on tab, a large tensile stress is generated at the end of the weld.
When the arc moves to the end tack weld and run on plate, the transverse tensile stress at the end of the weld is relaxed and the restraint force is reduced due to the thermal expansion deformation of this part, so that the weld metal just solidified at the end of the weld is subject to a large tensile stress and a terminal crack is formed.
According to the above reason analysis, two solutions are proposed:
First, increase the width of the run on plate to increase its restraint force;
The other is to use the slotted elastic restraint run on plate.
However, after taking the above measures in practice, the problem has not been effectively solved:
For example, although the elastic restraint run on plate is used, the terminal crack of the longitudinal weld will still occur, and the terminal crack often occurs when the welding thickness is small, the rigidity is small and the cylinder is forcibly assembled;
However, when there is a product test plate in the extended part of the longitudinal weld of the cylinder, although the situation of tack welding is the same as that without the product test plate, there are few end cracks in the longitudinal weld.
Through repeated tests and analysis, it is found that the crack at the end of the longitudinal seam is not only related to the inevitable large tensile stress at the end weld, but also related to several other extremely important reasons.
1. Cause analysis of terminal crack
1. Change of temperature field at the terminal weld
During arc welding, when the welding heat source is close to the end of the longitudinal weld, the normal temperature field at the end of the weld will change, and the closer to the end, the greater the change.
Since the size of the run on tab is much smaller than the cylinder, its heat capacity is much smaller, and the run on tab is connected to the cylinder only by tack welding, it can be regarded as most discontinuities.
Therefore, the heat transfer condition of the terminal weld is very poor, which causes the local temperature of this part to rise, the shape of the molten pool to change, and the penetration will also increase.
At the same time, the molten pool will stay longer under high temperature, and the solidification speed of the molten pool will become slower, especially when the size of the run on plate is too small, and the tack weld between the run on plate and the cylinder is too short and thin.
2. Effect of welding heat input
Because the welding heat input used by submerged arc welding is often much larger than other welding methods, so the penetration is large, the amount of deposited metal is large, and there is a flux layer covering, so the molten pool is large, the solidification speed of the molten pool and the cooling speed of the weld are slower than other welding methods, resulting in coarse grains and severe segregation, which create extremely favorable conditions for the generation of hot cracks.
In addition, the transverse shrinkage of the weld seam is much smaller than the opening of the gap, which makes the transverse tensile force at the end greater than that of other welding methods.
This is especially obvious for the medium thick plate with groove and the thin plate without groove.
3. Other information
If there is forced assembly, the assembly quality does not meet the requirements, and the content and segregation of S, P and other impurities in the base metal are high, they will also lead to cracks.
2. Nature of terminal crack
The terminal crack belongs to hot crack according to its nature, and the hot crack can be divided into crystalline crack and sub solid crack according to its formation stage.
Although the terminal crack is sometimes formed at the terminal, sometimes within 150 mm from the area around the terminal, sometimes at the surface, sometimes at the internal crack, and most of the time at the internal crack around the terminal.
It can be seen that the nature of the terminal crack is basically a sub solid crack, that is, when the weld end is still in the liquid state, although the molten pool around the terminal has solidified, it is still in a high temperature zero strength state slightly lower than the solidus, and cracks are generated under the action of the complex welding stress (mainly tensile stress) at the terminal, while the weld surface near the surface is relatively low due to its easy heat dissipation.
It has a certain strength and excellent plasticity, so terminal cracks often exist in the weld and cannot be found with the naked eyes.
3. Measures to prevent terminal cracks
It can be seen from the above analysis on the causes of terminal cracks that the most important measures to overcome the terminal cracks of submerged arc welding longitudinal joints are:
1. Increase the size of run on tab appropriately
People are often not familiar with the importance of the run on tab.
They think that the role of the run on tab is just to lead the arc crater out of the weldment. Sometimes they just find a section of steel plate to spot weld the barrel.
Others make the run on plate very small to save steel and become a “run on plate” worthy of the name, which is very wrong.
The run on tab has four functions:
(1) The weld break during arc striking and the arc crater during arc stopping shall be led out of the weldment.
(2) Strengthen the restraint at the end of the longitudinal joint, and bear the large tensile stress generated at the end.
(3) The temperature field at the terminal is improved, which is conducive to heat conduction and does not make the temperature at the terminal too high.
(4) Improve the magnetic field distribution at the terminal and reduce the degree of magnetic bias blow.
In order to achieve the above four purposes, the run on tab must have sufficient size, and the thickness should be the same as that of the weldment.
The size should depend on the size of the weldment and the thickness of the steel plate.
For general pressure vessels, it is recommended that their length and width should not be less than 140mm.
2. Pay attention to the assembly and tack welding of run on tab
The tack welding between the run on plate and the barrel must have sufficient length and thickness.
Generally speaking, the length and thickness of the tack weld should not be less than 80% of the run on plate width and thickness, and it is required to be continuous welding, instead of simply “spot” welding.
On both sides of the longitudinal seam, the thickness of the weld should be sufficient for the medium plate, and a certain groove should be made if necessary.
3. Pay attention to tack welding of the cylinder terminal
When the cylinder is rolled and tack welded, to further increase the restraint of the longitudinal seam end, the length of tack weld at the longitudinal seam end shall not be less than 100mm, and there shall be sufficient weld thickness, and there shall be no cracks, lack of fusion and other defects.
4. Strictly control the welding heat input
During the welding process of pressure vessels, the welding heat input must be strictly controlled, which is not only ensures the mechanical properties of welded joints, but also plays a very important role in preventing cracks.
The welding current of submerged arc welding has a great influence on the sensitivity of terminal crack, because the welding current is directly related to the temperature field and welding heat input.
5. Strictly control the weld pool shape and weld formation coefficient
The weld pool shape and formation factor of submerged arc welding are closely related to the sensitivity of welding cracks. Therefore, the size, shape and formation factor of weld pool should be strictly controlled.
It is very common to have cracks at the end of the longitudinal seam when submerged arc welding the longitudinal seam of the cylinder, which has not been solved well for many years.
Through the test and analysis, the main reason for the cracks at the end of the longitudinal seam of submerged arc welding is that there are large tensile stress and special temperature field in this part, which are the result of the joint action of the two.
The practice has proved that measures such as appropriately increasing the size of the run on tab, strengthening the quality control of tack welding, and strictly controlling the welding heat input and weld appearance can effectively prevent the occurrence of cracks at the end of submerged arc welding.