Weldability of Steels
11.3 Solidification cracking
The technically correct name for cracks that form during weld metal solidification is solidification cracks but other names are sometimes used when referring to this type of cracking.
Hot cracking - they occur at high temperatures – while the weld is hot.
Centreline cracking - cracks may appear down the centreline of the weld bead.
Crater cracking - small cracks in weld craters are solidification cracks.
WIS10-30816
Weldability of Steels 11-5 Copyright © TWI Ltd
Because a weld metal may be particularly susceptible to solidification cracking it may be said to show hot shortness because it is short of ductility when hot and so tends to crack.
Figure 11.4 shows a transverse section of a weld with a typical centreline solidification crack.
11.3.1 Factors influencing susceptibility to solidification cracking
Solidification cracking occurs when three conditions exist at the same time:
Weld metal has a susceptible chemical composition.
Welding conditions used give an unfavourable bead shape.
High level of restraint or tensile stresses present in the weld area.
11.3.2 Cracking mechanism
All weld metals solidify over a temperature range and since solidification starts at the fusion line towards the centreline of the weld pool, during the last stages of weld bead solidification there may be enough liquid present to form a weak zone in the centre of the bead. This liquid film is the result of low melting point constituents being pushed ahead of the solidification front.
During solidification, tensile stresses start to build-up due to contraction of the solid parts of the weld bead, and it is these stresses that can cause the weld bead to rupture. These circumstances result in a weld bead showing a centreline crack that is present as soon as the bead has been deposited.
Centreline solidification cracks tend to be surface breaking at some point in their length and can be easily seen during visual inspection because they tend to be relatively wide cracks.
11.3.3 Avoiding solidification cracking
Avoiding solidification cracking requires the influence of one of the factors responsible, to be reduced to an inactive level.
Weld metal composition
Most C and C-Mn steel weld metals made by modern steelmaking methods do not have chemical compositions that are particularly sensitive to solidification cracking.
However, these weld metals can become sensitive to this type of cracking if they are contaminated with elements, or compounds, that produce relatively low melting point films in weld metal.
Sulphur and copper are elements that can make steel weld metal sensitive to solidification cracking if they are present in the weld at relatively high levels.
Sulphur contamination may lead to the formation of iron sulphides that remain liquid when the bead has cooled down as low as ~980°C, whereas bead solidification starts at above 1400°C.
The source of sulphur may be contamination by oil or grease or it could be picked up from the less refined parent steel being welded by dilution into the weld.
Copper contamination in weld metal can be similarly harmful because it has low solubility in steel and can form films that are still molten at ~1100°C.
WIS10-30816
Weldability of Steels 11-6 Copyright © TWI Ltd
Avoiding solidification cracking (of an otherwise non-sensitive weld metal) requires the avoidance of contamination with potentially harmful materials by ensuring:
Weld joints are thoroughly cleaned immediately before welding.
Any copper containing welding accessories are suitable/in suitable condition - such as backing-bars and contact tips used for GMAW, FCAW and SAW.
Unfavourable welding conditions
Unfavourable welding conditions are those that encourage weld beads to solidify so that low melting point films become trapped at the centre of a solidifying weld bead and become the weak zones for easy crack formation.
Figure 11.5 shows a weld bead that has solidified using unfavourable welding conditions associated with centreline solidification cracking.
The weld bead has a cross-section that is quite deep and narrow – a width-to-depth ratio <~2 and the solidifying dendrites have pushed the lower melting point liquid to the centre of the bead where it has become trapped. Since the surrounding material is shrinking as a result of cooling, this film would be subjected to tensile stress, which leads to cracking.
In contrast, Figure 11.6 shows a bead that has a width-to-depth ratio that is
>>2. This bead shape shows lower melting point liquid pushed ahead of the solidifying dendrites but it does not become trapped at the bead centre. Thus, even under tensile stresses resulting from cooling, this film is self-healing and cracking is avoided.
SAW and spray-transfer GMAW are more likely to give weld beads with an unfavourable width-to-depth ratio than the other arc welding processes. Also, electron beam and laser welding processes are extremely sensitive to this kind of cracking as a result of the deep, narrow beads produced.
Avoiding unfavourable welding conditions that lead to centreline solidification cracking (of weld metals with sensitive compositions) may require significant changes to welding parameters, such as reducing the:
Welding current (to give a shallower bead).
Welding speed (to give a wider weld bead).
Avoiding unfavourable welding conditions that lead to crater cracking of a sensitive weld metal requires changes to the technique used at the end of a weld when the arc is extinguished, such as:
For TIG welding, use a current slope-out device so that the current, and weld pool depth gradually reduce before the arc is extinguished (gives more favourable weld bead width-to-depth ratio). It is also a common practice to backtrack the bead slightly before breaking the arc or lengthen the arc gradually to avoid crater cracks.
For TIG welding, modify weld pool solidification mode by feeding the filler wire into the pool until solidification is almost complete and avoiding a concave crater.
For MMA, modify the weld pool solidification mode by reversing the direction of travel at the end of the weld run so that crater is filled.
WIS10-30816
Weldability of Steels 11-7 Copyright © TWI Ltd