Luders Deformation in Weld Joints

The elastoplastic transition in welded low-carbon steel samples is considered. Two methods of manual arc welding of floating-electrode type are employed: a traditional steady arc; and pulsed welding with controlled heat consumption. In terms of the structural characteristics and mechanical properties of the welded metal, the methods are identical. In both cases, extended elastoplastic transition by the nucleation and propagation of Luders bands is observed. However, the underlying process is different. In the traditional steady arc, the Luders bands are formed in the applied metal initially as diffuse regions where the deformation is localized. These regions fill the seam and convert it to the plastically deformed state. The moving fronts of the bands are finally shaped in the thermal-influence zones and pass to the basic metal. The velocity and morphology of the fronts match those of fronts in uniform objects of the same steel. When using a pulsed arc, Luders bands appear some distance from the weld seam, at the clamps of the loading device. Up to the thermal- influence zones, the velocity and morphology of the fronts correspond to those for the basic metal. At the fusion boundary, the front stops and forms the nucleus of a new band, which expands in the seam metal. This new band first transforms the applied metal to the deformed state and then creates a moving front in the opposite thermal-influence zone. The velocity of the front differs by an order of magnitude in the applied metal and the basic metal. The weld seam determines the nucleation of Luders bands. An explanation is offered for the different origins of the elastoplastic transition in the two welding methods. In traditional welding with a steady arc, the local long-range stress is considerably higher in the thermal-influence zones than in the basic metal. Therefore, the nucleation of Luders bands is a relaxational process in this case. In pulsed arc welding, the local long-range stress is higher in the basic metal, where the Luders bands appear. The results may be used in selecting the test conditions for power-system equipment.