Control of Welding Deformation and Welding Stress in Thick Plates
# Control of Welding Deformation and Welding Stress in Thick Plates During welding, the main deformation after butt welding of thick plates is **angular distortion**. In actual production, to control distortion, we usually weld part of the passes on the front side first, turn over the workpiece, back-gouge the root by carbon arc gouging, then weld the passes on the reverse side, and turn over the workpiece repeatedly. Generally, the workpiece is turned over after welding **3 to 5 passes** each time until all passes on the front side are completed. Meanwhile, the angular distortion shall be monitored at any time during welding, and the workpiece shall be turned over promptly to minimize welding deformation and residual stress in the weld. In addition, **fixtures and jigs** can be set to restrain the components and control deformation. This method is generally applied to **special-shaped thick plate structures**. Due to their unique profiles and various cross-sectional shapes and sizes, it is difficult to guarantee dimensional accuracy under free-state conditions. Therefore, jigs and fixtures shall be manufactured according to the profile of the components, so that assembly, tack welding and welding can be carried out with the components fixed, thus controlling welding deformation. **Selecting and controlling a reasonable welding sequence** is an effective measure to prevent welding stress and also one of the most effective methods to prevent welding deformation. Different welding sequences shall be formulated for different welding processes: - Submerged Arc Welding (SAW): generally uses the **backstep method**; - CO₂ gas shielded welding and manual welding: adopt the **symmetric method and uniform dispersion method**. The principle for formulating a reasonable welding sequence is: **dispersion, symmetry, uniformity, and reducing restraint degree**. It is an objective law that **instantaneous stress** occurs during welding, and **residual stress** together with **residual deformation** appears after welding. In the fabrication process, much attention is usually paid to deformation control, and measures are often taken to increase the rigidity of welded components to reduce deformation, while the increased instantaneous stress and welding residual stress are ignored. For components with high rigidity and thick plates, although the residual deformation is relatively small, huge tensile stress will be generated, which may even lead to cracks. If no cracks occur, the residual stress will often cause instability, deformation or even failure of the components during the homogenization of internal force when the structure is under load. Therefore, the control and elimination of welding stress are very important during component fabrication. The goal of stress control is to **reduce the peak stress and make it distribute uniformly**. Under the assembly condition with many welds, the following principles shall be followed according to the component profile and weld layout: - Weld welds with **greater shrinkage** first, then welds with smaller shrinkage; - Weld welds with **higher restraint and unable to contract freely** first, then welds with lower restraint and able to contract freely. To reduce the rapid loss of welding heat input and avoid cracks during weld crystallization, **preheating** shall be carried out before welding when the plate thickness reaches a certain value. A certain range around the weld shall be heated, and the heating temperature depends on the plate thickness and the **Carbon Equivalent (CE)** of the base metal. When a weld on a component is welded after preheating, the temperature in the weld zone is very high. With the welding process, the phenomenon of thermal expansion and contraction will inevitably occur in this zone. However, this zone only accounts for a small part of the component section, while the rest of the base metal is in a cold (normal temperature) state, which imposes huge rigid restraint on the welding zone, resulting in high stress and even cracks. If the **symmetric position** of the weld zone is heated to a temperature slightly higher than the preheating temperature throughout the whole welding process, the above stress condition will be greatly reduced and the component deformation will be significantly improved. Although certain measures can be taken to control welding stress, considerable stress still remains in most thick plate components after welding. It is necessary to **bake the back or both sides of the welds** after the components are completed to relieve residual stress.