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Unit six Reading Material Edge _Preparation and Penetration Control for Arc Welding Reasons for Edge Preparation One of the most important welding variables is edge preparation .The reasons why edges are prepared are similar whatever fusion welding process is used ，and the choice of design is usually fundamental to the production of a successful joint . Why are edges prepared and masses of metal hewn (削掉) out of thick joints to replaced piecemeal (逐渐) by individual runs of weld metal ? To understand he reasons it is necessary to consider what happens as the thickness to welded is increased 。 When thin metal is welded it is possible to use the simplest of all joint designs –the square butt joint. The metal is penetrated completely and the penetration bead is held in position by surface tension. As the metal thickness is increased the welding current must de raised to ensure penetration. Two things then occur; either the weld bead width increases in keeping with the penetration causing the whole pool to become large or there is an increase in are force so that the weld metal cannot be contained. The forces acting downward on a molten weld pool derive from the mass of the liquid metal, the kinetic energy of transferring metal particles and the arc force as a result of plasma streaming. If these forces exceed the surface tension acting on the underside of the weld pool, the weld pool drops through the joint. The mass of liquid metal depends on the size of the weld pool and therefore on current and welding speed. Are force is controlled chiefly by current. Surface tension can be modified to some extent by the presence of fluxes and slag. Backing Techniques The difficulty of supporting the weld bead can be overcome by using a backing technique. Several such techniques are available. Permanent or fusible backing, in which a strip of the parent metal is inserted underneath the joint and is fused to become part of the joint. Temporary backing bars, in which the weld be