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Analysis of the Causes of Terminal Cracks in Longitudinal Welds in Submerged Arc Welding and Preventive Measures
Release time:
2018-05-03
I. Overview In the manufacturing of pressure vessels, when submerged arc welding is used to join the longitudinal weld seams of cylindrical shells, cracks—hereinafter referred to as end-terminal cracks—often occur at or near the ends of these longitudinal welds. Many researchers have already studied this issue and concluded that the primary cause of end-terminal cracks is as follows: As the welding arc approaches the end of the longitudinal weld seam, the weld undergoes axial expansion and deformation, while simultaneously experiencing lateral tensile deformation perpendicular to the axial direction. Moreover, during the rolling and fabrication processes, the cylindrical shell itself experiences cold-work hardening stresses and assembly stresses. During welding, due to the restraint effect of the end-positioning welds and arc-starting plates, significant stresses accumulate at the weld’s end terminal.
I. Overview
In the manufacturing of pressure vessels, when submerged arc welding is used to join the longitudinal weld seams of cylindrical shells, cracks—hereinafter referred to as "terminal cracks"—often occur at or near the ends of these longitudinal welds. Many researchers have already studied this issue and concluded that the primary cause of terminal cracks lies in the fact that, as the welding arc approaches the end of a longitudinal weld seam, the weld undergoes not only axial expansion and deformation but also lateral opening deformation perpendicular to the axial direction. Moreover, during the rolling and fabrication assembly processes, the cylindrical shell experiences both cold-work hardening stresses and assembly-induced stresses. During welding, the restraint imposed by the terminal tack welds and arc-starting plates leads to significant tensile stresses at the weld ends. When the arc moves onto the terminal tack welds and arc-starting plates, the localized heating causes these areas to expand and deform, thereby relieving the lateral tensile stress at the weld ends and reducing the restraining forces. As a result, the freshly solidified weld metal at the weld ends is subjected to substantial tensile stress, ultimately leading to the formation of terminal cracks.
Based on the analysis of the above-mentioned reasons, two countermeasures were proposed: first, increasing the width of the arc-starting plate to enhance its restraining effect; second, adopting an elastic restraining arc-starting plate with grooves. However, after implementing these countermeasures in practice, the problem still failed to be effectively resolved. For instance, although the elastic restraining arc-starting plate was used, longitudinal weld seam termination cracks still occurred. Moreover, even when welding thin-walled cylinders with low rigidity that had been forcibly assembled, termination cracks frequently appeared as well. Yet, when a product test plate was attached to the extended section of the cylinder’s longitudinal weld seam—despite identical conditions for tack welding and other procedures compared to cases without the test plate—termination cracks in the longitudinal weld seam rarely occurred. After repeated experiments and analyses, we concluded that the occurrence of longitudinal weld seam termination cracks, while inevitably linked to the significant tensile stresses present at the weld termination points, is also associated with several other critically important factors.
II. Analysis of the Causes of Crack Formation at the Terminal
1. Variation of the temperature field at the terminal weld area
During submerged arc welding, as the welding heat source approaches the terminal region of the longitudinal weld seam, the normal temperature distribution at the weld seam’s end will undergo significant changes—these changes become more pronounced the closer one gets to the terminal. Since the size of the arc-starting plate is much smaller than that of the cylindrical shell, its thermal capacity is also considerably lower. Moreover, the arc-starting plate is connected to the cylindrical shell only by spot welds, which can thus be regarded as largely discontinuous. Consequently, the heat transfer conditions at the terminal weld region are extremely poor, leading to a localized increase in temperature, a change in the shape of the molten pool, and a corresponding increase in penetration depth. At the same time, the molten pool remains at high temperatures for a longer duration, slowing down its solidification rate—this effect becomes particularly pronounced when the arc-starting plate is too small or when the spot welds connecting the arc-starting plate to the cylindrical shell are too short and thin.
2. The influence of welding heat input.
Because submerged arc welding typically employs a much higher heat input than other welding methods, it results in greater penetration, larger weld metal deposition, and coverage by a flux layer. Consequently, the weld pool is large, and both the solidification rate of the weld pool and the cooling rate of the weld seam are slower than those in other welding processes. This leads to coarser grain structures and more severe segregation, creating highly favorable conditions for the formation of hot cracks. Moreover, the transverse contraction of the weld is significantly less than the opening of the joint gap, resulting in greater transverse tensile stresses at the terminal regions compared to other welding methods. This effect is particularly pronounced in medium- and thick-walled plates with beveled edges as well as in thinner plates without bevels.
3. Other circumstances
If forced assembly is present, the assembly quality fails to meet requirements, or if the content of impurities such as S and P in the base material is excessively high or segregation occurs, these factors can also lead to crack formation.
III. Nature of Terminal Cracks
The terminal cracks are classified as thermal cracks in terms of their nature. Thermal cracks, in turn, can be further divided into solidification cracks and sub-solidus cracks depending on the stage at which they form. Although the location where terminal cracks initially appear varies—sometimes at the terminal itself, sometimes within a 150-mm radius near the terminal, and sometimes as surface cracks or internal cracks—the majority of such cracks occur as internal cracks located close to the terminal. Thus, it is evident that the nature of terminal cracks essentially corresponds to sub-solidus cracks. Specifically, when the weld terminal is still in the liquid state, although the molten pool near the terminal has already begun to solidify, it remains in a high-temperature, zero-strength state just below the solidus line. Under the influence of complex welding stresses—primarily tensile stresses—at the terminal, cracks begin to form. In contrast, the weld surface layer near the surface, being more readily dissipating heat, maintains a relatively lower temperature and has already developed a certain degree of strength and excellent plasticity. Consequently, terminal cracks tend to remain hidden within the weld and are typically invisible to the naked eye.
IV. Preventive Measures
As can be seen from the analysis of the causes of terminal cracks mentioned above, the most important measure to overcome longitudinal weld seam terminal cracks in submerged arc welding is:
1. Appropriately increase the size of the arc-starting plate.
People often fail to recognize the importance of arc-starting plates, thinking that their sole function is merely to direct the arc pit formed during arc termination away from the weldment. Sometimes, they simply find a random piece of steel plate and tack-weld it onto the cylinder shell—completely neglecting proper procedures. Others, in an effort to save on steel, make the arc-starting plates excessively small, turning them into mere “arc-starting plates” in name only. Such practices are profoundly mistaken. Arc-starting plates serve four key functions:
(1) Guide the weld seam break formed during arc initiation and the arc crater formed during arc termination to outside the workpiece.
(2) Strengthen the constraint at the termination of longitudinal seams to withstand the significant tensile stresses generated at the termination points.
(3) Improving the temperature distribution at the terminal section promotes heat conduction and prevents the temperature at the terminal section from becoming excessively high.
(4) Improve the magnetic field distribution at the terminal end to reduce the degree of magnetic deflection.
To achieve the four objectives mentioned above, the arc-starting plate must be of sufficient size; its thickness should ideally match that of the workpiece, and its dimensions should be determined based on the size of the workpiece and the thickness of the steel plate. For typical pressure vessels, it is recommended that the length and width of the arc-starting plate be no less than 140 mm.
2. Pay close attention to the assembly and tack welding of the arc-starting plate.
The tack welds between the arc-start plate and the main body must have sufficient length and thickness. Generally, the length and thickness of the tack welds should be no less than 80% of the width and thickness of the arc-start plate, respectively. Moreover, these tack welds must be continuous and cannot simply be “spot” welded. On both sides of the longitudinal seam, for medium- and thick-walled plates, it is essential to ensure adequate weld thickness; if necessary, a bevel should be prepared.
3. Pay close attention to the tack welding at the simplified terminal sections.
When performing tack welding after rounding the simplified Chinese sheet, to further enhance the constraint at the ends of the longitudinal weld seam, the length of the tack welds at the termination points of the longitudinal weld seam should be no less than 100 mm. The welds must also have sufficient thickness and shall be free from defects such as cracks and lack of fusion.
4. Strictly control the amount of heat input during welding.
During the welding of pressure vessels, it is essential to strictly control the heat input during welding. This is not only necessary to ensure the mechanical properties of the weld joints but also plays a crucial role in preventing crack formation. The magnitude of the welding current in submerged arc welding significantly affects the susceptibility to terminal cracks, as the welding current directly influences the temperature field and the heat input during welding.
5. Strictly control the shape of the molten pool and the weld bead formation coefficient.
The shape of the weld pool and the formation coefficient in submerged arc welding are closely related to the susceptibility to welding cracks. Therefore, it is also essential to strictly control the size and shape of the weld pool as well as the formation coefficient of the weld seam.
V. Conclusion
When using submerged arc welding to join the longitudinal seams of cylindrical shells, longitudinal seam termination cracks are extremely common and have remained poorly addressed for many years. Through experimentation and analysis, it has been determined that the primary cause of these longitudinal seam termination cracks is the combination of significant tensile stresses and a unique temperature field present at this location.
Practice has proven that adopting measures such as appropriately increasing the size of the arc-starting plate, strengthening quality control over tack welds, strictly controlling the heat input during welding, and carefully managing the shape of the weld bead can effectively prevent the occurrence of terminal cracks in submerged arc welding.
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