In the manufacture of pressure vessels, when submerged arc welding is used to weld the longitudinal weld of the cylinder, cracks are often generated at or near the end of the longitudinal weld (hereinafter referred to as terminal cracks).
Many people have studied this, and they think that the main reason for the terminal crack is that when the welding arc is close to the end of the longitudinal weld, the weld expands and deforms in the axial direction, and is accompanied by transverse tension in the vertical axial direction. open deformation;
In the process of rolling, manufacturing and assembling, the cylinder also has cold work hardening stress and assembly stress; during the welding process, due to the restraint of the terminal positioning weld and the arc strike plate, a large stretch is generated at the end of the weld. stress;
When the arc moves to the terminal positioning weld and the arc strike plate, due to the thermal expansion and deformation of this part, the transverse tensile stress at the end of the weld is relaxed, and the restraint force is reduced, so that the weld metal that has just solidified at the end of the weld is reduced. Terminal cracks are formed due to large tensile stress.
Based on the above analysis of the reasons, two solutions are proposed:
One is to increase the width of the arc strike plate to increase its binding force;
The second is to use a slotted elastic restraint arc-starting plate.
However, after taking the above countermeasures in practice, the problem has not been effectively solved:
For example, although the elastic restraint arc ignition plate is used, the terminal crack of the longitudinal weld will still be generated, and the terminal crack often occurs when the welding thickness is small, the rigidity is small and the cylinder body 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, the longitudinal seam is rarely generated to produce terminal cracks.
After repeated tests and analysis, it is found that the generation of longitudinal seam terminal cracks is not only related to the inevitable large tensile stress at the terminal weld, but also related to several other extremely important reasons.
One. Analysis of the causes of terminal cracks
1. Changes in the temperature field of 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 it is to the end, the greater the change.
Since the size of the arc pilot plate is much smaller than that of the cylinder, its heat capacity is also much smaller, and the arc pilot plate and the cylinder are only connected by tack welding, so most of them can be regarded as discontinuous.
Therefore, the heat transfer condition of the terminal weld is very poor, resulting in an increase in the local temperature of the part, the shape of the molten pool will change, the penetration depth will also increase, and the time of the molten pool staying at high temperature will also become longer. The speed of molten pool solidification slows down, especially when the size of the arc ignition plate is too small, and the tack weld between the arc ignition plate and the cylinder is too short or too thin.
2. Influence of welding heat input
Since the welding heat input used in submerged arc welding is often much larger than other welding methods, the penetration depth is large, the amount of deposited metal is large, and it is covered by a flux layer, so the molten pool is large and the speed of the molten pool solidification. And the cooling rate of the weld is slower than other welding methods, resulting in coarser grains and more serious segregation, which have created extremely favorable conditions for the generation of hot cracks.
In addition, the lateral shrinkage of the welding seam is much smaller than the opening of the gap, so that the lateral tensile force of the terminal part is larger than that of other welding methods. This is especially true for beveled medium plates and unbevelled thinner plates.
3. Other situations
If there is forced assembly, the assembly quality does not meet the requirements, and the content of impurities such as S and P in the base metal is too high and segregation, which will also lead to cracks.
Two: The nature of terminal cracks
Terminal cracks belong to hot cracks according to their nature, and hot cracks can be divided into crystalline cracks and sub-solid cracks according to their formation stages. Although the part where the terminal crack is formed is sometimes the terminal, sometimes within 150mm from the area around the terminal, sometimes it is a surface crack, sometimes it is an internal crack, and most of the cases are internal cracks 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 terminal is still in a liquid state, although the molten pool near the terminal has solidified, it is still at a high temperature slightly below the solidus. In the zero-strength state, cracks are generated under the action of complex welding stress (mainly tensile stress) at the terminal,
The surface layer of the weld near the surface is easy to dissipate heat, the temperature is relatively low, and it has a certain strength and excellent plasticity, so the terminal crack often exists inside the weld and cannot be detected with the naked eye.
Three. Measures to prevent terminal cracks
From the above analysis of the causes of terminal cracks, it can be seen that the most important measures to overcome the terminal cracks of submerged arc welding longitudinal seams are:
1. Properly increase the size of the arc pilot plate
People are often not familiar enough with the importance of the arc strike plate, thinking that the function of the arc strike plate is only to lead the arc crater during arc closing to the outside of the weldment. In order to save steel, some make the arc pilot plate very small and become a veritable "arc pilot plate". These practices are very wrong. The arc ignition plate has four functions:
(1) Lead the broken part of the weld during arc striking and the arc crater during arc closing to the outside of the weldment.
(2) Strengthen the restraint of the terminal part of the longitudinal seam, and bear the large tensile stress generated by the terminal part.
(3) Improve the temperature field of the terminal part, which is conducive to heat conduction and does not make the temperature of the terminal part too high.
(4) Improve the magnetic field distribution at the terminal and reduce the degree of magnetic deflection.
In order to achieve the above four purposes, the arc strike plate must have sufficient size, and the thickness should be the same as that of the weldment. The size should be determined by the size of the weldment and the thickness of the steel plate. For general pressure vessels, it is recommended that the length and width should not be less than 140mm.
2. Pay attention to the assembly and tack welding of the arc ignition plate
The tack welding between the arc pilot plate and the cylinder must have sufficient length and thickness. Generally speaking, the length and thickness of the tack weld should not be less than 80% of the width and thickness of the arc pilot plate, and continuous welding is required. It is not possible to simply "spot" welding. On both sides of the longitudinal seam, sufficient weld thickness should be ensured for the medium and heavy plate, and a certain groove should be opened if necessary.
3. Pay attention to the tack welding of the terminal part of the cylinder
In order to further increase the restraint at the end of the longitudinal seam during tack welding after the cylinder is rounded, the length of the tack weld at the end of the longitudinal seam should not be less than 100mm, and the weld thickness should be sufficient, and there should be no cracks, Defects such as lack of fusion.
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 to ensure the mechanical properties of the welded joints, but also plays a very important role in preventing the occurrence of cracks. The magnitude of the welding current of submerged arc welding has a great influence on the sensitivity of the terminal crack, because the magnitude of the welding current is directly related to the temperature field and the amount of welding heat input.
5. Strictly control the shape of the molten pool and the welding seam forming coefficient
The weld pool shape and forming coefficient of submerged arc welding are closely related to the susceptibility of welding cracks. Therefore, the size, shape and forming coefficient of the weld pool should also be strictly controlled.
Four. Conclusion
It is extremely common to produce longitudinal seam terminal cracks when submerged arc welding is used to weld the longitudinal seam of the cylinder, and it has not been well solved for many years. Through the test and analysis, the main reason for the terminal crack of the submerged arc welding longitudinal seam is due to the large tensile stress and special temperature field in this part, which are the result of the combined effect of the two.
Practice has proved that measures such as appropriately increasing the size of the arc ignition plate, strengthening the quality control of tack welding, and strictly controlling the amount of welding heat input and the shape of the weld can effectively prevent the occurrence of terminal cracks in submerged arc welding.