During welding all three states of matter are present.
Metals to be welded are in the solid state. The heat
generated by the welding arc will melt the metal and
gases will be produced. Metallurgy is a science that
studies these changes of states in metals and the
compounds they form.
Since welding is concerned with solid matter, we will
concentrate our efforts on elements that can be
worked within the solid state. Changes to a material
can happen in the solid state; metals are particularly
useful in this regard. For example, the properties of
steel can be changed while it is solid. One of the best
ways to change properties is to heat and cool the
material. Welding metallurgy not only studies the
weld metal, but it is also used to predict changes in
the base metal that happened due to the welding
heat. Welding locally “heat treats” the parts being
joined.
Welding metallurgy attempts to predict the effect of
this heat treatment on the structure and properties of
the material.