WELDING THEORY - OD1650 - LESSON 1/TASK 1
Brazing, the only welding process in which the melting of the base metal is not necessary for
coalescence, is similar to soldering, except that higher temperatures are used. The term soldering
is used to describe a joining process using nonferrous filler alloys melting below 800F (427C).
Soldering is not considered a welding process. Brazing is a welding process using nonferrous
filler alloys that have a melting point above 800F (427C) but below that of the base metal.
Intimacy of Contact. The second basic requirement for coalescence, intimacy of contact,
may be divided into two groups: pressure processes and nonpressure processes. In pressure
processes, intimacy of contact is achieved by applying pressure while the contact surfaces are at
a high enough temperature to allow plastic flow of the metal. In nonpressure processes, a space
remains between the surfaces to be joined. This space is then filled, either progressively or all at
once, with molten metal. The molten metal may be obtained from a filler metal (welding rod or
electrode) by melting the surfaces to be joined, or by combining a filler metal and melted base
All nonpressure processes involve fusion, and they are often referred to as fusion processes.
However, this term is somewhat misleading since some pressure processes also involve fusion.
The various welding processes differ not only in the way coalescence is achieved, but also in
their ability to produce a satisfactory joint in a given kind of metal under the conditions in which
the weld must be made. Many factors influence the selection of a welding process for a
particular application. These factors include the relative cost, the amount of welding required,
the location and position of welds, the service conditions the welded structure must withstand,
and the qualifications of the person who does the welding. Probably the most important single
factor, however, is the weldability of the metal.
Weldability. The term weldability means the capacity of a metal to be fabricated by a
welding process into a structure that will perform its purpose satisfactorily. Weldability also
means the degree of simplicity or complexity of the procedures and techniques necessary to