In addition, higher strength steels may require special techniques or treatments like tempering after welding. Spot weldability of HSLA (high-strength low-alloy) steels is directly related to composition and type of microalloying elements. It is advisable to check with the supplier before specifying spot welding here. Stainless steels are spot weldable, some grades more readily than others. Austenitic grades of the 300 series are the most commonly welded types, followed by ferritic. Martensitic stainlesses are the least common because welded joints are always much more brittle. All stainless steels require careful adjustment of welding parameters and/or special methods to obtain optimum quality welds.

One alternative to plug welding is “MIG spot welding”. It is similar to plug welding, although a hole is not drilled in the front sheet of metal. Instead the power of the MIG is relied upon to fully melt the top sheet and penetrate into the back sheet. This technique would require less preparation work than plug welding, but the two sheets need to be in tight contact and high amps used to complete the weld or else the weld could be very weak. Plug welding is a much more suitable technique for all but the most experienced welders.

Materials Appropriate for Spot Welding: Due to its lower thermal conductivity and higher electrical resistance, steel is comparatively easy to spot weld, with low carbon steel being most suited to spot welding. However, high carbon content steels (Carbon equivalence > 0.4wt%) are prone to poor fracture toughness or cracking in the welds as they tend to form hard and brittle microstructures. Galvanised steel (zinc coated) requires slightly higher welding currents to weld than uncoated steels. Also, with zinc alloys, the copper electrodes rapidly degrade the surface and lead to a loss of weld quality. When spot welding zinc coated steels, electrodes must either be frequently exchanged or the electrode tip surface should be ‘dressed’, where a cutter removes contaminated material to expose a clean copper surface and reshapes the electrode. Read extra details on Spot Welder.

As is often the case with machine tools, there are two types: portable (for ease of use but with limited performance); and stationary (better suited to intensive work and thicker metal sheet).