Welding Inspection & Metallurgy API ICP Self Study Notes

10.2.1 The Structure of Castings
The overall arrangement of the grains, grain boundaries and phases present
in the casting is called the microstructure of the metal. Microstructure is a
significant area that inspectors should understand, as it is largely responsible
for the physical and mechanical properties of the metal. Because castings
used in the refinery industry are typically alloyed, they will contain two or
more microstructural phases. A phase is any structure that is physically and
compositionally distinct. As the chemical composition is altered or
temperature changed, new phases may form or existing phases may
disappear. Cast structures, depending on their chemical composition can
exhibit a wide range of mechanical properties for several reasons.
In general, it is desirable to keep the size of grains small, which improves
strength and toughness. This can be achieved by maximizing the rate of
cooling or minimizing the heat input (in the case of welding). This increases
the rate of crystal formation and decreases the time available for crystal
growth, which has a net effect of reducing crystalline grain size.