For a full penetration butt weld on thick plates, why is a double-V or double-U groove preparation often preferred over a single-V groove, especially for minimizing distortion?

For a full penetration butt weld, which is a weld where the weld metal completely fills the joint thickness to fuse the plates together, particularly on thick plates, a double-V or double-U groove preparation is significantly preferred over a single-V groove for minimizing distortion. Distortion refers to the unwanted warpage or change in shape of the welded component, primarily caused by the localized heating and subsequent cooling and solidification shrinkage of the weld metal. When weld metal solidifies, it shrinks, creating internal stresses that pull the base material. The amount and distribution of this shrinkage dictate the degree and type of distortion. Angular distortion, a common type, is characterized by the plate edges rotating out of their original plane.

In a single-V groove, the weld material is deposited entirely from one side of the plate. This creates a highly asymmetrical heating and cooling cycle. As the weld metal cools and shrinks, the shrinkage forces are concentrated on one side of the joint, pulling the top surface of the plate edges together while the bottom surface remains relatively unaffected. This unbalanced shrinkage causes significant angular distortion, bowing the plates away from the side where the weld was deposited. Furthermore, to achieve full penetration on thick plates with a single-V groove, a large volume of weld metal is required, which necessitates a wide groove angle and deep penetration. This large volume of weld metal means higher total heat input, greater overall thermal expansion and contraction, and consequently, more severe shrinkage and distortion.

Conversely, a double-V or double-U groove involves preparing the plate edges with a V or U shape from both sides of the plate, effectively creating two smaller grooves that meet in the middle. When welding with these preparations, the weld metal is deposited alternately from one side and then the other. This symmetrical, or nearly symmetrical, deposition and cooling process is the key to minimizing distortion. As weld passes are made on one side, they cause localized shrinkage. However, subsequent passes made on the opposite side of the joint generate opposing shrinkage forces that effectively counteract the distortion introduced by the previous passes. The alternating weld passes balance the thermal stresses and shrinkage across the plate thickness, preventing the severe angular distortion seen with single-sided welding.

Additionally, double-V and double-U grooves often require less total weld metal volume compared to a single-V groove for thick plates because the groove angles on each side can be smaller. For example, two 30-degree bevels for a double-V might replace one 60-degree bevel for a single-V. Less weld metal volume translates to less heat input, which further reduces the overall thermal expansion and contraction experienced by the base material, thereby mitigating distortion. The double-U groove offers an even greater advantage in reducing weld metal volume due to its rounded profile, which minimizes the open area to be filled, further contributing to lower heat input and reduced distortion, while also providing excellent root access and strength.