The welding time, the rate of advance is not as rapid as that assumed when filling with molten metal, so that, an area expanding and an area contracting as showing in picture A and picture B in the parent metal. In the picture A showing the condition when welding speed relatively slow speed. In this case, the expanding zone (which is marked green color) is small in compression to the width of the contraction zone (which is marked gray color). Therefore the contraction area is the predominating factor, because the plates to close-
The picture B showing the condition when welding is relatively high-
In the above cases, after welding has been completed and the plate has cooled to room temperature, the filler weld metal and parent metal in the yellow zone (which is shown in picture) which is subjected to plastic flow and its trying to contraction. But it is restricted by the base metal outside the plastic flow zone. The residual stress is happened during the arc welding. This residual stress can be measure by the strain gauge.
The weld filler metal and the base metal within the plastic flow zone (which is shown with yellow color) is in tension longitudinally, and the just outside of the base metal the plastic flow area is in compression stress longitudinally.