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This is also called centreline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulphur and phosphorus, are more likely to occur in higher carbon steels.
Solidification cracks are normally distinguishable from other types of cracks by the following features:
- they occur only in the weld metal – although the parent metal is almost always the source of the low melting point contaminants associated with the cracking
- they normally appear in straight lines along the centreline of the weld bead, but may occasionally appear as transverse cracking
- solidification cracks in the final crater may have a branching appearance
- as the cracks are ‘open’ they are visible to the naked eye
A schematic diagram of a centreline crack is shown below:
On breaking open the weld the crack surface may have a blue appearance, showing the cracks formed while the metal was still hot. The cracks form at the solidification boundaries and are characteristically inter dendritic. There may be evidence of segregation associated with the solidification boundary.
The main cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:
- insufficient weld bead size or inappropriate shape
- welding under excessive restraint
- material properties – such as a high impurity content or a relatively large shrinkage on solidification
Joint design can have an influence on the level of residual stresses. Large gaps between conponents will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Hence weld beads with a small depth to width ratio, such as is formed when bridging a large wide gap with a thin bead, will be more susceptible to solidification cracking.
In steels, cracking is associated with impurities, particularly sulphur and phosphorus and is promoted by carbon, whereas manganese and sulphur can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. However when welding a highly restrained joint using high strength steels, a combined level below 0.03% might be needed.
Weld metal composition is dominated by the filler and as this is usually cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Parent metal composition becomes more important with autogenous welding techniques, such as TIG with no filler.
Avoiding Solidification Cracking
Apart from choice of material and filler, the main techniques for avoiding solidification cracking are:
- control the joint fit up to reduce the gaps
- clean off all contaminants before welding
- ensure that the welding sequence will not lead to a buildup of thermally induced stresses
- choose welding parameters to produce a weld bead with adequate depth to width ratio or with sufficient throat thickness (fillet weld) to ensure the bead has sufficient resistance to solidificatiuon stresses. Recommended minimum depth to width ratio is 0.5:1
- avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains. As a rule, weld beads with a depth to width ratio exceeds 2:1 will be prone to solidification cracking
- avoid high welding speeds (at high current levels) which increase segregation and stress levels accross the weld bead
- at the run stop, ensure adequate filling of the crater to avoid an unfavourable concave shape