Weldments are available only in the assembly environment. I would recommend bringing in the sheet metal part into an assembly, convert it to an weldment and then apply the appropriate weld (fillet or groove weld). Welds work on faces of multiple/same part. I have heard that some users derive the part and then create modeling features (sweep) to represent it as a weld in the IPT. IMHO, by this, one would surely miss the rich tools in the weldment environment in assemblies. Thanks.