The properties of a welded joint depend partly on the correct preparation of the edges being welded. All mill scale, rust oxides, and other impurities must be removed from the joint edges or surfaces to prevent their inclusion in the weld metal. You should prepare the edges to permit fusion without excessive melting, and you should take care to keep to a minimum the heat loss due to radiation into the base metal from the weld. A properly prepared joint will give a minimum of expansion on heating and a minimum of contraction on cooling.

The preparation of the metal for welding is governed by the form, thickness, kind of metal, the load that the weld will be required to support, and the available means for preparing the edges to be joined. The five basic types of welded joints arc the butt, tee joints, lap, edge, and corner. (See figure 15-35.)