Under bead Cracking
This form of cracking occurs within the base metal at a very short distance away from the fusion line. It occurs in low-alloy and high-alloy steels. The factors responsible for this form of cracking are not, as yet, completely understood. It is known that dissolved hydrogen gas must be present; otherwise underbead cracking does not occur. It is believed to be caused by stresses within the metal that are the result of:
1. The unequal contraction of the base metal and the weld metal
2. The restraint of the cooler base metal
3. Stresses caused by the expansion of the metal when martensite is formed
4. Stresses set up by the precipitation of hydrogen out of the metal to form molecular hydrogen.
The presence of hydrogen seems to act as a trigger to start the
formation of these cracks.
The cracks occur below the surface of the metal and when a pipe is subjected to a load, they spread very rapidly, causing the joint to fail.
Since the presence of hydrogen is the primary cause of underbead cracking, everything possible should be done to prevent hydrogen from entering the molten metal. It can enter this molten metal from the atmosphere, from ingredients in electrode coatings, and from moisture.
To prevent hydrogen from entering from the atmosphere, the molten metal should always be blanketed with the gaseous shield formed by the electrode coating or, in the case of GTAW welding, by the inert gas. Low-hydrogen electrodes are so designed that the coating contains only the smallest trace of hydrogen. While these electrodes eliminate underbead cracking, they are difficult to use in some pipe-welding applications and, in the case of downhill welding, they cannot be used at all.
A primary source of hydrogen in welding is moisture. Some welders like to soak their electrodes in water in order to obtain a better arc characteristic. This practice should be discouraged. The electrode coating should be protected from any form of dampness. Before attempting to weld, the weld joint should be dry and welding should never be attempted in rain or snow.
If the weld metal cools more slowly, some of the hydrogen gas that has been dissolved will have an opportunity to escape from the weld by precipitating out, For this reason, preheating the metal before welding helps to prevent underbead cracking. Preheating will also prevent the formation of martensite, as explained before, thereby eliminating one of the factors contributing to underbead cracking. While postheating will help to relieve locked-up stresses in the metal, it is seldom effective in preventing underbead cracking.