Abstract: The reasons for the occurrence of cracks at the end of submerged arc welding longitudinal welds were analyzed from the aspects of temperature changes, stress conditions, and welding heat input energy. Measures to prevent the occurrence of terminal cracks were formulated.

I. Overview

In the manufacturing of pressure vessels, when using submerged arc welding to weld simplified longitudinal welds, cracks (hereinafter referred to as terminal cracks) often occur at or near the end of the longitudinal weld. Many people have conducted research on this issue and believe that the main cause of terminal cracks is that when the welding arc approaches the longitudinal weld seam terminal, the weld seam expands and deforms along the axial direction, accompanied by transverse opening deformation in the vertical axial direction; During the rolling and assembly process of the cylinder, there are also cold work hardening stresses and assembly stresses; During the welding process, significant tensile stress is generated at the end of the weld due to the constraint effect of the terminal positioning weld and the arc initiation plate; When the arc moves to the terminal positioning weld and the arc initiation plate, due to the thermal expansion and deformation of this part, the transverse tensile stress of the weld terminal is relaxed, the restraining force is reduced, and the newly solidified weld metal at the weld terminal is subjected to greater tensile stress, forming terminal cracks.

Based on the above reasons, two solutions have been proposed: one is to increase the width of the arc initiation plate to increase its restraining force; The second is to use slotted elastic restraint arc plates. However, after adopting the above measures in practice, the problem has not been effectively solved: for example, although elastic restraint arc plates are used, terminal cracks in longitudinal welds still occur, and terminal cracks often occur when the welding thickness is small, the rigidity is small, and the simplified structure is forcibly assembled; However, when a product test plate is attached to the extension of the simplified longitudinal weld seam, although the situation of positioning welding is the same as that without the product test plate, terminal cracks rarely occur in the longitudinal seam. After repeated experimentation and analysis, we believe that the occurrence of longitudinal seam terminal cracks is not only related to the inevitable presence of significant tensile stress at the terminal weld seam, but also to several other extremely important reasons.

2、 Analysis of the causes of terminal cracks

1. Changes in temperature field at the terminal weld seam location

During submerged 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. Because the size of the arc initiation plate is much smaller than that of the cylinder, its heat capacity is also much smaller, and the arc initiation plate and the cylinder are only connected by positioning welding, so it can be considered as mostly discontinuous. So the heat transfer conditions at the terminal weld seam are very poor, resulting in a local temperature increase, a change in the shape of the molten pool, and an increase in the depth of fusion. At the same time, the molten pool stays at high temperatures for a longer time, and the solidification rate of the molten pool slows down, especially when the size of the arc initiation plate is too small and the positioning weld seam between the arc initiation plate and the simplified body is too short or thin.

2. The influence of welding heat input

Due to the fact that the welding heat input used in submerged arc welding is often much larger than other welding methods, the melting depth is deep, the amount of deposited metal is large, and there is a flux layer covering it, resulting in a larger molten pool. The solidification rate of the molten pool and the cooling rate of the weld seam are slower than other welding methods, leading to coarser particles and more severe segregation, which creates extremely favorable conditions for the occurrence of hot cracks. In addition, the transverse shrinkage of the weld seam is much smaller than the opening of the gap, resulting in a greater transverse tensile force at the terminal area compared to other welding methods. This is particularly significant for medium thick plates with beveled edges and thinner plates without beveled edges.

3. Other situations

If there is forced assembly, the assembly quality does not meet the requirements, and the content and segregation of impurities such as S and P in the base material are high, it can also lead to the occurrence of cracks.

3、 The nature of terminal cracks

Terminal cracks belong to thermal cracks according to their properties, and thermal cracks can be divided into crystalline cracks and sub solid phase cracks according to their formation stages. Although the location where terminal cracks form is sometimes at the terminal and sometimes at a distance of 150mm from the terminal, within this range, there are sometimes surface cracks, sometimes internal cracks, and most of the time internal cracks occur near the terminal. From this, it can be seen that the nature of terminal cracks is basically a sub solid phase crack, that is, when the weld seam terminal is still in the liquid state, although the molten pool near the terminal has solidified, it is still in a high-temperature zero strength state slightly below the solidus line, and cracks are generated under the complex welding stress (mainly tensile stress) at the terminal. The surface layer of the weld seam near the surface is easy to dissipate heat, has a relatively low temperature, and has a certain strength and excellent plasticity, so terminal cracks often exist inside the weld seam and cannot be detected by the naked eye.

4、 Preventive measures

From the analysis of the causes of terminal cracks mentioned above, it can be seen that the most important measure to overcome longitudinal seam terminal cracks in submerged arc welding is:
1. Appropriately increase the size of the arc starting plate

People often have insufficient understanding of the importance of the arc starting plate, believing that its function is only to lead the arc pit outside the welded part during arc extinguishing. Sometimes, they can simply spot weld a section of steel plate onto the cylinder body. Some people also make the arc starting plate very small to save steel, becoming a true "arc starting plate", but these practices are very wrong. The arc starting plate has four main functions:

(1) Lead the weld seam fracture during arc initiation and the arc crater during arc termination to the outside of the weldment.

(2) Strengthen the restraint at the end of the longitudinal seam to withstand the significant tensile stress generated at the end.

(3) Improving the temperature field of the terminal area is beneficial for heat conduction and prevents excessive temperature in the terminal area.

(4) Improve the magnetic field distribution at the terminal area and reduce the degree of magnetic bias blowing.

To achieve the above four objectives, the arc plate must have sufficient size and thickness, which should be the same as the welded piece. The size should depend on the size of the welded piece and the thickness of the steel plate. For general pressure vessels, it is recommended that their length and width be no less than 140mm.

2. Pay attention to the assembly and positioning welding of the arc starting plate

The positioning welding between the arc plate and the simplified body must have sufficient length and thickness. Generally speaking, the length and thickness of the positioning weld should not be less than 80% of the width and thickness of the arc plate, and it is required to be continuous welding. It cannot be simply "spot" welding. On both sides of the longitudinal seam, for medium thick plates, sufficient weld thickness should be ensured, and a certain groove should be opened if necessary.

3. Pay attention to the positioning welding of simplified terminal parts

In order to further increase the restraint at the end of the longitudinal seam during the positioning welding after the simplified rolling, the length of the positioning weld at the end of the longitudinal seam should not be less than 100mm, and there should be sufficient weld thickness without defects such as cracks or lack of fusion.

4. Strictly control the welding heat input

During the welding process of pressure vessels, it is necessary to strictly control the welding heat input, which is not only necessary to ensure the mechanical properties of the welded joint, but also plays a very important role in preventing the occurrence of cracks. The magnitude of the submerged arc welding current has a significant impact on the sensitivity of terminal cracks, as the magnitude of the welding current is directly related to the temperature field and welding heat input.

5. Strictly control the shape of the molten pool and the formation coefficient of the weld seam

The shape and forming coefficient of the weld pool in submerged arc welding are closely related to the sensitivity of welding cracks. Therefore, the size, shape, and forming coefficient of the weld pool should also be strictly controlled.

V. Conclusion

It is extremely common for longitudinal seam terminal cracks to occur during submerged arc welding of simplified pipes, and they have not been well resolved for many years. Through experiments and analysis, the main reason for the occurrence of longitudinal seam terminal cracks in submerged arc welding is due to the presence of significant tensile stress and a special temperature field in that area, which are the result of their combined action.

Practice has proven that measures such as appropriately increasing the size of the arc initiation plate, strengthening the quality control of positioning welding, strictly controlling the welding heat input and the shape of the weld seam can effectively prevent the occurrence of terminal cracks in submerged arc welding.