Defining new dimensionless parameters β, in, jn, k and functions Fon, F∞n, the temperature distribution of welded plates under unstationary states, which was obtained in the authors' preceding paper is more simply shown in equations (2) to (5).1)2) In equations (6) to (9) the cooling rate of welded plates is shown. The cooling rate passing through a constant temperature, which is very large at the start of the welding, rapidly decreases during welding, and increases again near the finishing point of the welding (crater). The rapid cooling at the both ends of weld bead is due to the unstationarity of the temperature distribution. The effect of the welding conditions on the unstationarity of the cooling rate is discussed. Figs 6 to 9.
As a first step of the fundamental study of thermal stresses induced by welding operation, some experiments were made on the thermal stresses induced when a circular hole made in the central part of a disc of small thickness was filled up by welding. The specimens were made of mild steel plate, and their surfaces were preliminary polished to a degree of 000 by means of sand paper. When the central hole was filled up by are welding process, Luders lines were observed on the polished surfaces of the specimens. These lines were found to belong to three different systems, and their times of appearance differed from each other. Namely, a radial line system appeared first within the welding interval, Then soon after the welding was finished, a logarithmic spiral system was observed. Finally after a considerably long time a concentric circle system appeared. For determining the direction of slip plane along which the flow phenomena took place, cross section was made of the specimen by cutting it perpendicularly to each system of Luders lines, and 'was etched with Fry's etchant. From this experiment was clarified that the radial line system slips along a plane inclined, at 45°to the surface, the logarithmic spiral aystem along a curved plane perpendicular to the surface, and finally the concentric circle system along a plane inclined at 45°to the surface. The state of appearance of each system of Luders lines considerably changed with the dimensions of specimens and degree of preheating. Temperature distribution of the dise during welding and the welding strains were also measured in connection with the appearance of Luders lines. A general consideration is given to the above-mentioned experimental data and the cause of Luders lines.