The cause of generation of cracking in the bead end part in one side high speed automatic arc welding of heavy plate is studied. It is well known that the base metal tends to bend as shown in Fig. 3-b due to the heat delivered to the plate. The actual rotational deformation might be restricted by some restraining devices. When these devices are constituted only by tack weld beads along the weld line, cracking may occur because the welding arc will melt off these tack weld metals one by one as the arc proceeds, and at the terminal end when the last tack weld is melted off, the constraining force also vanishes and the rotational deformation takes place abruptly in large amount which results in a cracking of the bead at the part whose temperature is about 1200-1300°C. See Fig. 8.
Fig. 8-10 and Fig. 15 show the deflection of point N on the base metal shown in Fig. 6 under the welding condition shown in Table 1. The abrupt increase of deflection observable in Fig. 8 causes cracking shown in Fig. 2 and Fig. 13, and it is noticeable that in case of Fig. 10 no cracking is observed.
The point R in these figures corresponds to the time when the arc proceeds into the tab plate passing over the border line of the base metal. The increase in displacement of the measuring point N in RS part in Figs. 9, 10 is caused by the rotational moment of the tab plate side as illustrated in Fig. 12-b.
In Fig. 15, the tab is separated into two parts in order to eliminate the above mentioned tab actions.
Fig. 21 shows the deflection of points N
1-N
5 along the bead, Z being the tack weld. Figs. 21, 23 are the results of explanational experiment to illustrate the action of the tack weld at root.
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