Cracking

To say that 'a weld has cracked' is too general a statement to be really useful. Why, well because there are several different mechanisms, which may have formed the crack and one of the most useful pieces of information to know is when the crack occurred.

Cold cracking occurs after the weld has cooled and the cracks usually form in the heat affected zone. The most common form of cold cracking is Hydrogen Induced Cold Cracking (HICC) in ferritic steels. HICC is caused by a combination of three factors a susceptible microstructure, the presence of stress and the presence of hydrogen.

Solidification cracking occurs when the solidifying weld pool is exposed to transverse shrinkage. The last part of the weld pool to solidify is usually rich in low melting point impurity elements and liquid films are often present at the centreline of the welds; as a result the ductility of the weld metal is lowered.

Low melting point grain boundary films may also be present in the affected zone (HAZ), and these may melt and form micro-cracks, HAZ liquation cracks, under the influence of the thermal stresses induced by welding. Sulphur and phosphorus are known to increase the liquation cracking susceptibility of steel, and the problem is more common when high heat input processes are used.

In some materials, and particularly Cr/Mo/Mn alloy steels containing titanium and vanadium, the heat treatment or equivalent service conditions may cause cracking. Reheat cracking is related to the low ductility of the material at elevated temperatures and is particularly severe when thick sections and stress raisers are present.