Intro4u2u

Generally, solidification cracking can occur when:
•the weld metal has a high carbon or impurity element content
•the depth-to-width ratio of the solidifying weld bead is large
•disruption of the heat flow condition occurs, e.g. stop/start condition
The cracks can be wide and open to the surface like shrinkage voids or sub-surface and possibly narrow.
Solidification cracking is most likely to occur in compositions which result in a wide freezing temperature range. In steels this is commonly created by a higher than normal content of carbon and impurity elements such as sulphur and phosphorus. These elements segregate during solidification, so that intergranular liquid films remain after the bulk of the weld has solidified. The thermal shrinkage of the cooling weld bead can cause these to rupture and form a crack.
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It is important that the welding fabricator does not weld on or near metal surfaces covered with scale or which have been contaminated with oil or grease. Scale can have a high sulphur content, and oils and greases can supply both carbon and sulphur. Contamination with lead, or other low melting point metals such as copper, tin or zinc should also be avoided.
A common form of solidification cracking is referred to as centre-line cracking. The cracks lie on the solidification centre-line where solidification fronts from opposite sides of the weld pool meet. The most unfavourable case is where weld beads are made with a large depth-to-width ratio, and end-to-end contact between opposing solidifying columnar grains results over a large proportion of the depth of the weld bead.
Generally, high depth-to-width ratio weld beads are produced when attempting to deposit weld metal at a high rate, using a process such as Submerged Arc Welding with a single electrode wire and a high welding current. If the same size of weld bead can be made using the process with two or more electrode wires operating in tandem, then a modified solidification pattern is usually formed and solidification cracking avoided.