Welding Inspection & Metallurgy API ICP Self Study Notes

The properties of the cast structure can also be impaired by compositional
variations in the microstructure called segregation. Because of the solubility
of trace and alloying elements, such elements as carbon, sulfur, and
phosphorous, can vary in a pure metal, these elements can cause variations
in the solidification temperature of different microstructural phases within the
melt. As the melt cools, these elements are eventually contained in the micro
structural phases that solidify last in spaces between the grains. These grain
boundary regions can have a much higher percentage of trace elements that
the grains themselves, which may lead to reductions in ductility and strength
properties. This effect can be minimized by using high purity melting stocks,
by special melting practices (melting under vacuum or inert gas, for example)
to minimize contamination and/or subsequent heat treatment to homogenize
the structure. In many carbon steels this is achieved using oxygen
scavengers such as aluminum and the steels are often described as “killed”
or “fully killed” steels. Minimizing trace elements or “inclusions” at this stage is
often important as they can provide sites for formation of in-service defects
such as hydrogen induced cracking (HIC).