Coiled insert material or solid rings are intended for use in making sound, smooth, uniform, crack-free root welds where accessibility is limited to one side. Insert alloy composition and the composition of subsequently deposited weld metal should be compatible with base material.

Class 4 inserts tend to twist when rim is coiled. This is a result of an essentially triangular configuration, narrower at inner rim contact point than at outer surface. However, special coiling conditions may be specified by contract or order (see 6.2).