Weld metal is the material that has melted and re-solidified as a result of the welding operation. In cases where no filler material is added (resistance, electron beam, laser and some autogenous arc welding), the weld metal has the same composition as the parent material. Where filler materials are added to the weld pool, the composition of the weld metal usually differs from that of the parent material.
For stainless steels, the differences between the parent material and weld compositions, along with the effect of dilution from the parent materials into the weld pool combined with the filler materials can be seen through the use of the Schaeffler Diagram.
Weld metal can be described as ' matching', 'undermatching' or 'overmatching' with respect to the parent material, which usually describes the strength of the weld metal, but it may also describe the chemical composition. For example, when welding a joint between 2.25Cr1Mo material and 9Cr1Mo material, a filler which matches the chemical composition of the lower alloyed material will be used, so 'undermatching' the higher alloyed 9Cr1Mo material with respect to the composition and strength.
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