Effective measures to prevent crack formation at the ends of longitudinal welds in submerged arc welding

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In pressure vessel manufacturing, when submerged arc welding is used to weld the longitudinal seams of the cylinder, cracks often occur at the ends of these longitudinal seams or near them (hereinafter referred to as end cracks). Many studies have been conducted on this topic; it is believed that the main cause of end cracks is that when the welding arc approaches the end of the longitudinal weld, the weld undergoes axial expansion and deformation, along with lateral expansion and deformation perpendicular to the axial direction ； Moreover, cold working hardening stresses and assembly stresses also exist in the cylinder during rolling, manufacturing, and assembly processes ； During welding, due to the restraint exerted by the end positioning weld and the arc starting plate, large tensile stresses are generated at the ends of the weld ； When the arc moves to the end of the weld and the arc-starting plate, the heating and thermal expansion at these areas cause the transverse tensile stress at the end of the weld to be reduced, and the restraining force decreases. As a result, the weld metal that has just solidified at the end of the weld is subjected to high tensile stress, leading to the formation of end cracks. Based on the analysis of the above reasons, two solutions were proposed: one is to increase the width of the arc-starting plate to enhance its restraining force ； Second, a slotted elastic restraint arc-starting plate is used. However, in practice, even after implementing the above countermeasures, the problem was not effectively resolved: for instance, although flexible restraint arc-start plates were used, end cracks still occurred in the longitudinal welds, and end cracks also frequently appeared when welding cylinders with a small thickness and low rigidity that were assembled under forced conditions ； However, when a product test plate is present at the extended portion of the longitudinal weld of the cylinder, although conditions such as tack welding remain the same as when no product test plate is present, terminal cracks in the longitudinal weld rarely occur. Through repeated tests and analyses, it was found that the occurrence of cracks at the ends of longitudinal seams is indeed related to the relatively high tensile stresses inevitably present at those weld ends, but it is also associated with several other very important factors. I. Analysis of the causes of cracks at the end of the weld 1. Changes in the temperature field at the weld end During SAW welding, when the welding heat source is near the end of the longitudinal weld, the normal temperature field at the weld end changes, and these changes become more significant as one moves closer to the end. Since the size of the arc-starting plate is much smaller than that of the cylinder, its heat capacity is also much lower. Moreover, the arc-starting plate and the cylinder are connected only by tack welding, so it can be considered largely discontinuous. Therefore, the heat transfer conditions at the terminal weld area are very poor, which leads to an increase in the local temperature there. The shape of the molten pool changes, and its depth increases as well. Additionally, the molten pool remains at high temperatures for a longer period of time, resulting in a slower cooling rate. This phenomenon is particularly evident when the size of the arc-starting plate is too small, or when the positioning weld between the arc-starting plate and the cylinder is too short and too thin. 2. Influence of welding heat input: Since the welding heat input used in submerged arc welding is often much higher than that of other welding methods, the penetration depth is greater and the amount of metal deposited is larger. Additionally, the presence of a flux layer results in a larger molten pool; as a consequence, both the solidification rate of the molten pool and the cooling rate of the weld are slower compared to other welding methods. This leads to larger grain sizes and more severe segregation, all of which create highly favorable conditions for the formation of thermal cracks. Furthermore, the lateral contraction of the weld is much smaller than the expansion of the gap, resulting in a greater lateral tensile force at the terminal portion compared to other welding methods. This is particularly evident in thick plates with grooves and thinner plates without grooves. 3. Other situations: Forced assembly, assembly quality that does not meet requirements, high levels of impurities such as S and P in the base material, as well as segregation, can all lead to the formation of cracks. II. Nature of terminal cracks Terminal cracks are, by their nature, thermal cracks; and thermal cracks can be further divided into crystallization cracks and sub-solid phase cracks depending on the stage at which they form. Indeed, the location where terminal cracks form is sometimes at the terminal itself, sometimes within a 150 mm range around the terminal, sometimes as surface cracks, and sometimes as internal cracks; in most cases, they occur as internal cracks around the terminal. It can be seen that the nature of cracks at the weld ends is essentially that of sub-solid-state cracks; that is, while the weld ends are still in a liquid state, the molten pool surrounding them has already solidified but remains in a high-temperature state just below the solidus line, exhibiting zero strength. Cracks arise under the action of the complex welding stresses at the ends (mainly tensile stresses). The surface layer of the weld, being easier to dissipate heat, has a relatively lower temperature, possesses a certain degree of strength, and excellent plasticity; therefore, cracks at the weld ends tend to occur within the weld itself and cannot be detected with the naked eye. III. Measures to prevent cracks at the ends of welds As can be seen from the analysis of the causes of cracks at the ends of welds in submerged arc welding, the most important measure to overcome such cracks is: 1. Appropriately increase the size of the arc-starting plate. People often do not realize the importance of the arc-starting plate; they think its function is merely to guide the arc crater formed at the end of welding outside the workpiece. Sometimes, they simply use any piece of steel plate and weld it onto the cylinder. Some also make the arc-starting plate very small in order to save steel, turning it into a veritable \"arc-starting plate\"; such practices are highly incorrect. The arc starting plate has four main functions: (1) to guide the weld fracture at the start of arc formation and the arc crater at the end of arc formation outside the workpiece. (2) Increase the restraint at the ends of the longitudinal seam to withstand the high tensile stresses generated at those ends. (3) Improving the temperature field at the terminal end facilitates heat conduction and prevents the temperature at that location from becoming too high. (4) Improve the magnetic field distribution at the terminal area to reduce the degree of magnetic deflection. To achieve the above four purposes, the arc starting plate must be of sufficient size; its thickness should be the same as that of the workpiece. Its dimensions should be determined based on the size of the workpiece and the thickness of the steel plate. For ordinary pressure vessels, it is recommended that their length and width be at least 140 mm. 2. Pay attention to the assembly of the arc-starting plate and the tack welding. The tack weld between the arc-starting plate and the cylinder shell must have sufficient length and thickness; generally, the length and thickness of these tack welds should be at least 80% of those of the arc-starting plate. These welds need to be continuous, rather than being done as simple \"spot\" welds. On both sides of the longitudinal seam, sufficient weld thickness must be ensured for medium-thick plates, and grooves may need to be created if necessary. 3. Pay attention to the tack welding at the ends of the cylinder. When performing tack welding after rolling the cylinder into a cylindrical shape, in order to further increase the restraint at the ends of the longitudinal seams, the length of the tack welds at these ends should be no less than 100 mm. Additionally, there must be sufficient weld thickness, and no defects such as cracks or lack of fusion should be present. 4. Strictly control the heat input during welding. During the welding of pressure vessels, it is essential to strictly control the heat input; this is not only necessary to ensure the mechanical properties of the welded joints but also plays a crucial role in preventing crack formation. The magnitude of the submerged arc welding current has a significant impact on the sensitivity to end cracks, as it is directly related to the temperature field and the amount of heat input during welding. 5. Strictly control the shape of the molten pool and the weld formation factor. The shape of the molten pool and its formation factor in submerged arc welding are closely related to the susceptibility to welding cracks; therefore, it is also necessary to strictly control the size and shape of the molten pool as well as the weld formation factor. IV. Conclusion: Cracking at the ends of longitudinal seams is a very common phenomenon when welding longitudinal seams of cylinders using submerged arc welding; this problem has remained unresolved for many years. Through experiments and analysis, it is concluded that the main cause of cracks at the termination of submerged arc welding longitudinal seams is the combined effect of relatively high tensile stress and a special temperature field in this area. Practice has shown that measures such as appropriately increasing the size of the arc-starting plate, improving the quality control of tack welding, and strictly controlling the heat input during welding as well as the shape of the weld seam, can effectively prevent the occurrence of end cracks in submerged arc welding.