The welding design of steel structure should consider factors such as the weldability of the base metal, the matching of the welding material and the base metal, the welding efficiency, the welding method, the size of the weld bead, the welding deformation and the buckling of the joint plate. Take fillet weld bead as an example to illustrate. According to regulations, fillet weld bead is divided into three categories, as shown in Figure 1, including ideal weld bead, acceptable weld bead and unacceptable weld bead. The incorrect weld bead size includes insufficient throat depth of fillet weld bead, excessively long convex surface, weld erosion, overlap, insufficient foot length and poor fusion. The minimum foot length of fillet welding is determined by the thicker plate of the joint, but it should not be greater than the thickness of the thinner plate. If it exceeds, there should be sufficient preheating to ensure the quality of welding. The welding size can be greater than the thickness of the thin plate of the joint if the stress calculation is required.
Figure 1- fillet weld bead profile
How to reduce latent cracks
What is the reason for the longitudinal cracks in the flat fillet bead of submerged arc welding? How to prevent and avoid?
a.Causes of longitudinal cracks
When using the submerged arc welding technology to weld a flat fillet weld, if a longitudinal strip crack occurs on the metal surface of the weld bead, as shown in Figure 2-1. Usually, this kind of long strip cracks on the surface of the weld bead belong to the crystalline cracks in the thermal cracks. The reason for this welding crack can be illustrated by Figure 2-2. When the left weld bead gradually cools, the weld bead and the base metal will retain part of the welding thermal stress, so that the A plate will be affected by the shrinkage of the weld bead and slightly deviate to the left of center line. But when the right weld bead is completed, the solidification shrinkage thermal stress of the weld bead, coupled with the welding heat of the right weld bead, will cause the volume expansion of the left weld bead. This thermal stress will cause the plate A to pull back to the right and increase the probability of thermal cracking in the left weld bead. When the left and right sides of the weld bead start to cool down, the volume shrinkage effect of the weld bead will also gradually worsen the thermal cracking situation.
Generally, crystalline cracks are more likely to occur in the weld bead with carbon steel or low-alloy steel with a lot of impurities as the base material. Generally, crystal cracks only occur on the surface of the weld bead, most of which are longitudinally distributed in the centerline of the weld. Some of these cracks are arc-shaped on both sides of the centerline of the weld, and these arc-shaped cracks are perpendicular to the weld bead ripples, as shown in Figure 3. Normally, longitudinal cracks are longer and deeper, while arc cracks are shorter and shallower. In addition, crater cracks are also crystalline cracks, which are distributed in a star shape at the end of the weld bead.
Figure 3- Schematic diagram of the types and locations of crystal cracks
When the weld bead crystallizes, the first crystalline part will be purer, and the later crystalline part will contain more impurities and alloy components. As the columnar crystals continue to grow, impurities and alloy components are constantly being pushed out to the center of the weld bead. This phenomenon of component segregation will enrich these junctions with more impurities or alloying elements. When the weld bead solidifies and shrinks, it causes shrinkage tensile stress. Normally the eutectic between the grain boundaries cannot withstand this shrinkage stress. As a result, crystal cracks are formed at the junction.
b.Welding and pulling measures to avoid crystal cracks
To avoid the occurrence of crystal cracks, the first thing that must be taken into consideration is how to reduce the impurity content in the weld bead, and how to reduce the influence of deformation on the weld bead with appropriate construction methods.
1.Reduce the impurity content of the weld bead
The method to reduce the impurity content in the weld bead can be taken from two aspects, the base metal and the welding material. There are quite a few examples of crystal cracks caused by the unqualified base material in the engineering, such as the surface of the welded bead of a steel plate in a steel structure factory. Crystal cracks appeared in many places, and the chemical analysis showed that the carbon, sulfur and phosphorus components in the base metal exceeded the standard value a lot, which caused the impurity content in the weld bead to increase relatively, and finally caused the occurrence of crystal cracks. By choosing welding consumables with relatively low impurities, uniform composition and stable quality, then the probability of crystal cracks can be greatly reduced.
2.Appropriate construction method
Generally, insufficient throat depth or insufficient penetration of the weld bead will increase the probability of longitudinal cracks. The weld bead usually plays a key role in bearing the tensile and compressive stresses of the steel plates at both ends. If the throat depth of the weld bead or the degree of penetration is insufficient, then it means that the cross-sectional area (blue area in below figure) that can withstand the stress is reduced. As shown in Figure 4, it is quite prone to fracture as a thin wire cannot suspend heavy objects.
Figure 4- Schematic diagram of welding deformation and weld bead cracking
Appropriate construction methods must consider plate thickness, slotting of thick plates, welding conditions and welding methods. Take a thick plate as an example. Generally, if the thickness of the steel plate exceeds 17.5mm, it must be grooved, as shown in Figure 5. The welding method should adopt more than two welds to construct. The first one should increase the welding current or reduce the moving speed to achieve sufficient penetration. In the second welding, the appropriate welding conditions should be used to meet the requirement of throat depth and angle length. When the flat fillet weld bead of submerged arc welding has sufficient penetration and throat depth, the weld bead will has enough capacity to withstand the influence of welding deformation.
Figure 5- Schematic diagram of welding groove and bead
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