During the welding heat is not transmitted in the uniform manner. The certain part of the metal part is heated to a very high temperature, while the metal temperature on the sections adjacent to the weld zone remains lower. This leads to the fact that in areas located at different distances from the near-weld zone, the expansion occurs at different scales. That part of the metal part that is exposed to the straight will expand, while at other sites the metal that has the lower temperature will resist expansion.
Figure 1 shows the changes in the dimensions of the direct sample (see Fig. 1A), which is heated by the welding arc.
In Fig. 1 B, the sample begins to heat up under the influence of the lighted arc. The heated area expands (see Fig. 1 C), and due to the fact that it is partially held back by that part of the sample that does not undergo heating, under the influence of the arc, the sample is bent at the ends remote from the heat source. Since the hot zone is less durable (part of this zone, turned into a liquid, has an extremely low strength), it cannot contribute too much to the bending of the bar. As the minimum amount of restraining forces act on the hot part in the lateral direction, it expands from the side with which heat is supplied.
After the arc is extinguished (Fig. 1 D), the hot molten part begins to cool and contract. The heat is always transferred from the hot to the cold section, so when cooling the heat spreads to the area that was previously cold and heats it. Further, as the hot expanded section begins to cool, it contracts, changing the direction of the deforming forces, which ultimately leads to the decrease in the length of the sample, and also to the fact that the ends of the sample are bent upward. In this connection, upon cooling, the sample acquires the concave shape, Fig. 1 E. Thus, when we bring heat to the part in an uneven manner (which happens when welding), as a result of the effect of the resulting thermal stress, the change in size takes place, in which the part is bent or deformed during cooling. In Fig. 1 F, the sample is shown after returning to the solid state, in which there is a residual voltage, conventionally indicated in the figure in the spring form.