Do you know how welding residual stress is generated?

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Welding residual stress: The internal stress generated in welded components as a result of welding is called welding stress. Based on their duration of action, welding stresses can be divided into welding instantaneous stresses and welding residual stresses. During the welding process, the welding stress at a particular moment is known as the instantaneous welding stress, and it changes over time. The welding stress that remains in the welded part after welding is known as welding residual stress, and it is caused by the uneven temperature field resulting from welding heating. Classification of welding residual stresses: Welding stresses can be divided into the following three categories based on their source. (1) Direct stress: This results from uneven heating and cooling, and is a thermal stress that arises depending on the temperature gradients during heating and cooling. It is the main cause of welding residual stress. (2) Indirect stress: This is the stress resulting from the machining conditions prior to welding. After rolling and cold drawing, tensile stress is generated on the surface of the components; this stress combines with the stress resulting from welding, exerting an additional influence on the deformation of the welded components. In addition, the constraint stress generated in components due to external constraints also falls into this category of stresses. (3) Structural stress: This is the stress resulting from structural changes, that is, the stress arising from the change in specific volume caused by phase transitions. It is related to the carbon content and other components of the material. The formation process of welding residual stress: During welding, the workpiece is subjected to uneven heating by the arc heat, which generates welding stress. The residual stress that remains within the workpiece after it cools down is known as welding residual stress. The welding process is a non-uniform heating process, in which the metal of the weld and its adjacent areas are heated to very high temperatures before being cooled down rapidly. During the welding process, since the temperatures of different parts of the weldment vary, the cooling rates also differ. As a result, under the influence of thermal expansion and contraction as well as plastic deformation, welding stresses are inevitably generated in various parts of the weldment. The figure below shows the variation of temperature distribution over time during post-weld cooling. When the time is zero, the area near the weld is rapidly heated to a high temperature. Therefore, first, large thermal stresses are generated in the welds that reach this temperature, and plastic deformation occurs in their vicinity. It is clearly in a constrained state in the joining direction; therefore, significant compressive plastic deformation will occur ； In the direction perpendicular to the joint, plastic deformation also occurs under normal welding conditions. Since the length of the actual part of the weld seam becomes shorter at this point, if all areas are cooled from their current state to room temperature, the weld seam section with a smaller actual size will inevitably be under tensile stress, and this tensile stress will not exceed the yield strength of the material. Image: The figure below shows the temperature distribution during the cooling process and the corresponding thermal stress conditions. The graph above in Figure a shows the temperature distribution curve with a starting time of zero. It is distributed in a mountain-like pattern in the central melting zone, and the corresponding thermal stress distribution is shown in Figures c and d below. The bend that appears at the center of the thermal stress distribution curve is caused by the yield strength of the melted portion being almost zero. As time passes, its temperature distribution gradually becomes more gradual. Figure b below shows the temperature change from one moment to another during the cooling process. Figure c below shows the change in thermal stress corresponding to temperature changes. At this point, the thermal stress does not exceed the yield strength at that temperature. The actual thermal stress (see figure d below) is the final residual stress state obtained by adding the thermal stress change at that time to the thermal stress at the start of cooling. The generation of welding residual stress is determined by the thermal stresses during heating and cooling, as well as the plastic deformation resulting therefrom.