Frequently Asked Questions
When welds in sheet metal are tested to check quality, it is normal to expect the weld to tear out of one sheet to give a plug or button fracture, the plug size being specified for the sheet thickness being welded. The target weld diameter is typically 5 √t (where t is the sheet thickness in mm). The minimum acceptable size is 70 to 80% of this target value depending on the application standard.
Fracture of a weld at the interface between the sheets, without tearing the parent material, can indicate possible weld quality problems.
Interface fracture can be caused by the following reasons:
- Small weld diameter.
- Lack of fusion of the nugget.
- Low ductility weld nugget.
- Shear or torsion loading of an acceptable weld.
In the first two cases, the weld area usually appears dull and flat grey in the case of uncoated steels with perhaps a small central crystalline area. The factors affecting weld size need to be checked and, if appropriate, electrode condition and weld settings may need to be adjusted.
In the case of the low ductility weld nugget, the fused zone is correctly formed but the weld interface fails in peel or tension, because of excessive hardness or grain growth. The fracture appearance is then usually crystalline although it is sometimes fine, and difficult to distinguish. Low ductility due to hardening of slightly high carbon or high strength steels can sometimes be overcome by introducing more heat in the weld and by using longer weld times or pulsed weld current. In more difficult cases, a long cool time (which allows the weld to quench) can be followed by a long, low current pulse to temper the weld nugget. If the temper pulse is too hot, then the weld will quench afterwards to form a hard weld again. The result depends heavily on the material being welded and the sheet thickness. Suitable settings need to be selected by experiment and confirmed by destructive tests. Certain ferritic stainless steels suffer low ductility due to the formation coarse grains in the weld nugget and this condition cannot be corrected by heat treatment.
Interface failures can be caused by excessive cracking or porosity in the nugget, particularly with thick steel, if insufficient force or excessive splash occurs. Longer time/pulsed conditions and sufficient hold time may help to minimise such imperfections.
If the loading on the weld is in shear or torsion, a good weld may not fracture as a plug but the fracture surface usually shows smearing of the surface to indicate ductile fractures. Confirm the quality by peel or chisel testing.
See also Resistance Welding of Sheet Metals Best Practice Guide