WELDING THEORY - OD1650 - LESSON 1/TASK 1
welds with properties that are equal to or better than the properties of the base material. For
example, mild steel can be welded by most welding processes, but the welds produced are not
equally satisfactory, and one method may be more complicated or more expensive than another.
While it is possible to weld mild steel through the use of a variety of welding processes, some
metals such as aluminum and its alloys can be welded satisfactorily through only a few welding
processes. Mild steel does not require elaborate preparations, fluxes, and special techniques
because its characteristics are such that the welding operation can be easily performed. Other
metals require special preparatory steps, complex welding sequences, skillful use of a specific
welding technique, and extensive heat treatments after welding.
Many factors influence the weldability of a metal. Some important ones that must be taken into
account and, so far as possible, controlled are:
(1) the chemical composition of the metals involved (that is, the kind and percentage of
elements present) and the effect of radical temperature changes on the various elements;
(2) the expansion and contraction characteristics of the base metals;
(3) the filler metal (welding rod or electrode);
(4) the joint design; and
(5) the welding procedure.
In steel, carbon is probably the most important element that limits weldability. Carbon gives
steel hardenability; that is, when certain carbon steels are heated above a critical temperature and
then cooled rapidly, they become much harder. At the same time, they lose ductility. In fact, the
metal may become extremely brittle. With few exceptions, the temperatures used in welding
exceed the critical temperature of carbon steels. Further, more hardening may occur when the
mass of relatively cold metal surrounding the weld area conducts heat away so fast that rapid
cooling occurs. Thus, certain steels may become hardened by many of the welding processes.