Welding and welding symbols

This chapter describes in brief the process of welding and presents the welding symbols used in engineering drawings. Welding may be described as a process of uniting two pieces of metal or alloy by raising the temperature of the surfaces to be joined so that they become plastic or molten. This may be done with or without the application of pressure and with or without the use of added metal. This definition excludes the more recently developed method of coldwelding, in which pressure alone is used and which is used principally for aluminum and its alloys. When complex joints involve multiple welds it is often easier to detail such constructions on separate drawing sheets. Each type of weld is characterized by a symbol, which is representative of the shape of the weld, or the edge preparation. It may be necessary to specify the shape of the weld surface on the drawing as flat, convex, or concave, and so a supplementary symbol is then added to the elementary symbol. A welding symbol is applied to a drawing by using a reference line and an arrow line. The reference line should be drawn parallel to the bottom edge of the drawing sheet and the arrow line should form an angle with the reference line. Welding can be done in the factory or on the site when the plant is erected, which is indicated in the drawing by a flag.