4 General
welding and brazing information Workpiece
Preparation The
removal of dirt, oil, oxides, and other contaminants allows a sound bond to the
base metal, free of porosity, and
minimizes risk of spalling. Where appropriate, the removal of fatigued, work-hardened,
or otherwise damaged metal
is recommended. Preheat
Preheating is sometimes necessary to
reduce the risk of distortion due to excessive heat input, to prevent
spalling or cracking due to underbead
embrittlement, and to avoid thermal shock in metals susceptible to crack
propagation. Preheating
temperatures are controlled by two major factors •
the carbon content of the base metal, and •
the alloy content of the base metal. Basically,
the higher the carbon content of the base metal, the higher the preheat temperature
required. To a slightly
lesser degree, this is also true of the alloy content. When the correct preheat
temperature has been determined,
it is essential that this temperature be achieved and maintained throughout the
entire mass of the workpiece.
All preheated welding applications should be slow-cooled —
in still air if possible. *
