Cooling rates in a fillet weld are greater than in a similar thickness butt joint. There are three paths by which heat will be lost from the weld. This fact means that lack of fusion/cold start defects are more likely, particularly in high thermal conductivity metals such as aluminium and the risks of cold cracking are increased in carbon and low alloy steels. What may be acceptable in terms of heat input and/or preheat temperature for a butt weld may therefore not be acceptable with a fillet weld configuration. This point has sometimes been overlooked, particularly when welding on temporary attachments such as strongbacks, where quality control may be somewhat lax. This has led to major cracking problems for some fabricators.
With the conventional arc welding processes it is difficult to deposit a fillet weld with a throat less than some 2mm. This is in addition to the possibility of the lack of fusion/cold cracking mentioned above due to the rapid cooling rates experienced by small fillet welds. The maximum size of fillet weld is generally that of the thickness of the thinner of the two items being joined but very large fillet welds may cause unacceptable distortion and/or extremely high residual stresses. In addition, above a certain size it may be more economical to make a T-butt, rather than a fillet weld.
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