Restraint cracking is usually associated with small welds made on thick metal sections. This form of cracking occurs in the weld bead and it is caused in part by the solidification and cooling pattern of the weld. Because the parent, or base, metal is much cooler than the weld metal, solidification progresses from the fusion line inward. For this reason the center of the weld is at a higher temperature than the remainder of the weld while it cools.

The mechanism of restraint cracking can be illustrated by again resorting to an imaginary situation, as seen in Fig. 13-2. Cross sections through an imaginary weld are indicated. For simplicity, a square butt joint is shown. Above each joint a small graph indicates the distribution of the temperature across the joint.

Shown on the graph are the solidification temperature and the strengthening temperature of the weld metal. Actually, there is no fixed strengthening temperature, although each metal does have a temperature range below which it attains a reasonable amount of strength and above which it is very weak when slowly cooled.

in Fig. 13-2A, the weld metal has just solidified and, as can be seen in the graph above the weld, the temperature drops off very rapidly at the fusion line because of the chilling effect of the heavy plate. In Fig, 13-2B, C, D and E, the temperature is assumed to have

dropped an equal amount; i. e., they represent the same instant during the cooling cycle.

For a moment, let it be assumed again that the weld metal could be separated from the sides of the weld as it cools from the solidifica­tion temperature shown in Fig. 13-2A to the temperature shown in the other diagrams. In this case the weld metal would separate completely from the sides because it would have to shrink as it cools, as shown in Fig, 13-2B.

In order to attach the weld metal to the sides while at this temperature, it would be necessary to pull on the weld in order to stretch it as shown in Fig. 13-2C. When this is done, the weld metal is in tension and tensile stresses exist within it.

Most of the weld metal is strong enough at this temperature to resist the tensile stresses; however, as shown in Fig. 13-2D, the metal in the center of the weld, which is at the highest temperature, is still very weak. The weak metal in the center will crack when subjected to internal stresses. Of course, the weld metal does not actually separate from the wall, but in shrinking as it cools, stresses are set up within the metal that can cause the weaker metal in the center of the weld to crack.

As the weld shrinks upon cooling it must deform if it remains attached to the sides of the weld. The amount of deformation that can take place before cracking occurs depends upon the amount of metal that is available. A thick weld, having a larger volume of metal, will accommodate more deformation without cracking than will a thin weld. Therefore, as shown in Fig. 13-2E, a heavy weld is less likely to crack than a small weld, when they are on thick metal sections.