Heat Flow during the Autogenous GTA Welding of Pipes
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I.
INTRODUCTION
HEATflow during welding is of great interest to welding engineers and metallurgists. It not only controls the size of the fusion and heat-affected zones, but also strongly affects the microstructure and properties of the resultant weld. Heat flow during the autogenous (i.e., without filler metal) GTA (i.e., gas tungsten arc) welding of plates has been studied extensively in recent years. However, far less work has been conducted on heat flow during the autogenous GTA welding of pipes. No analytical solutions have been made available. In fact, the only study known to the authors is the recent one by Grill, 1in which heat flow during girth welding was calculated using the finite difference method. A "temperature source" was assigned to each grid point in the workpiece, and the solution was obtained by using the alternating direction implicit scheme. No experiments were carried out to verify the calculated results. In the present study, both steady state, 3-dimensional heat flow during seam welding and unsteady state, 3dimensional heat flow during girth welding were theoretically calculated and experimentally verified.
SEAM WELDING -~ u Velocity,
Heat Source ,- rl Weld Fusion U/Pool / z o n e
(o) (a)
GIRTH WELDING
Heat Source
0/
Angular LYe I~ Q'
Weld Pool
I/Tt~" -'- z
(0)
(b) Fig. 1 --Schematic sketches of pipe welding: (a) seam welding; (b) girth welding.
II.
MATHEMATICAL MODEL
Shown in Figures l(a) and (b) are schematic sketches for seam and girth welding, respectively. In the former, the pipe is stationary while the heat source travels in the axial direction of the pipe at a constant speed U. In the latter, the pipe rotates about its axis at a constant angular velocity lq, while the heat source remains stationary. As a result of the heat input, a weld pool is created under the heat source. The weld pool can be either fully or partially penetrating, depending on the welding parameter used. Behind the weld pool is the solidified structure of the fusion zone, i.e., the weld bead. The cylindrical coordinate system shown in Figure l(a) travels with the heat source at the same velocity, while that in Figure l(b) remains stationary with the heat source. In SINDO KOU and Y. LE are, respectively, Associate Professor and Graduate Student at the Department of Metallurgical and Mineral Engineering, University of Wisconsin, Madison, WI 53706. Manuscript submitted October 24, 1983. METALLURGICALTRANSACTIONS A
both cases, the origin of the coordinate system is located at the intersection between the axis of the pipe and that of the tungsten electrode of the GTA torch. Due to the combined effects of the electromagnetic force, the plasma jet force, and the surface tension of the liquid metal, the convection of the liquid metal in the weld pool appears to be rather complex in arc welding. No attempts were made to simulate the weld pool convection. Rather, the effective thermal conductivity 1'2 was used to account for the effect of convection on heat flow during welding. The following integral energy
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