When the metal deforms under the action of external force, it also produces an internal force that competes with the external force. The internal force induced per unit area of an object is called stress. Tensile stress is caused when stretched, and compressive stress is caused by compression. The strength of a metal is measured by its maximum stress (strength limit) before failure occurs. As long as the stress inside the metal reaches or exceeds its strength limit, the metal will crack.
According to the different causes of internal force, stress can be divided into two categories: one is working stress, which is the stress caused by external force acting on the object; the other is internal stress, which is caused by the chemical composition, metallographic structure of the object The stress caused by the uneven deformation inside the object is caused by changes in factors such as temperature and temperature. In other words, internal stress is the stress that balances inside an object without the action of external force.
The distinctive feature of internal stress is: in the interior of the object, the internal stress is naturally balanced, forming an internal balance force system. Internal stress exists in many engineering structures, such as riveted structures, cast structures, welded structures, etc. Welding stress is an internal stress.
Welding stress is the internal stress that exists in the weldment during and after the welding process. Welding stress is also generated from the beginning of welding and continuously changes its distribution in the component as the welding progresses. According to the time of stress action, welding stress can be divided into welding instantaneous stress and welding residual stress.
Welding instantaneous stress refers to the welding stress at a certain moment in the welding process, which changes with time. After the weldment is cooled, the residual stress in the weldment is called welding residual stress, and this stress has an impact on the performance of the structure. In addition, the transformation of the local tissue during the heating or cooling process of the metal will cause tissue stress, also known as phase transformation stress. The above stresses are all internal stresses generated without external force. During welding, if the weldment is subject to external constraints in the process of deformation, such as welding with a clamp clamped, a restraint stress will be generated in the weldment, and this stress will disappear when the limitation is removed. Excessive restraint stress can cause weld cracks.
The welding stress distribution on the weldment can be represented by a stress diagram. Figure Welding stress diagram representation method shows a stress diagram drawn directly on the section of the weldment to be studied. Figure Welding stress diagram representation method-a shows the distribution of longitudinal welding residual stress on section 1-1 (it is customary to specify that the stress in the direction parallel to the axis of the weld is called longitudinal stress, and the stress in the direction perpendicular to the axis of the weld is called transverse stress). The internal stress should be balanced on the entire section perpendicular to the stress, with tension and compression, and the resultant force is equal to zero.In the figure, the “⊕” symbol is tensile stress, and the “㊀” symbol is compressive stress.
In the figure, the tensile stress at point a is the largest, which is equal to the yield strength of the material σ8, the stress at point b is zero, and point c is the place where the compressive stress is the largest. The whole curve represents the magnitude and distribution of stress at each point on the section. Figure Welding stress diagram representation method-b is the transverse welding residual internal stress distribution on the 2-2 section, the middle part is compressive stress, and the two ends are tensile stress.