Generally speaking, limiting the discussion to ferrous metals, codes expect the welded joint to be at least as strong as the adjoining base metal. When the strength of the weld and the base metal are similar, i.e., within 10ksi, it is said to be "matching." When the filler metal is not as strong as the base metal, it is said to be undermatching. Again, 10 ksi is the figure often used to determine whether the filler metal is matching or undermatching.

Undermatching the filler metal tensile strength to the ultimate tensile strength of the base metal is preferable when improved ductility is desirable. This is often the case when the welds are loaded in compression. It is common for the weld metal to "gain" strength when subjected to loads that exceed the yield strength of the weld through a strain hardening mechanism. 10ksi can be the expected gain in the weld deposit properties. As such, undermatched filler metal often gains enough strength to match the properties of the base metal.

Overmatching the filler metal simply means the filler metal is stronger than the adjoining base metal. The concern is that the weld, in an over load condition, will force the failure into the adjacent base metal. As a preventive measure, the structural code limits the calculated stress on the adjacent base metal to 0.4 times the yield strength of the base metal.

Best regards - Al