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BASICS:
THE A TO Z OF
VALVE
MATERIALS
BY GREG JOHNSON
If you walk through any large PVF
warehouse and look around, it seems
that valves come in three flavors:
black, silver and gold. These “flavors”
are really iron, steel and bronze, which
constitute the lion’s share of valve body
materials used in today’s valve industry.
Valve designers also utilize dozens
of other metals and alloys to accomplish their goal of effective valve
design and function. So why do we
need a huge variety of materials for
valve construction? Why can’t that
golden valve in the big box store work
for everything? The answer is that
valve materials, particularly valve
body materials, are chosen primarily
for two reasons—strength and corrosion resistance. And in valve material
selection, one type does not fit all.
Withstanding Stresses
Strength in a valve is its ability to
withstand the internal stresses generated by containing and controlling the
fluid under pressure. Strength can be
measured in several ways, but the
most common measure is by quantifying the metal’s tensile strength. Tensile
strength is the resistance of the metal
to stretch or break when pulled. The
ability of the metal to stretch slightly
is called “ductility,” and some ductility is generally useful in valve applications. But not all metals have good
ductility. For example, cast iron is not
ductile at all and it bends very little
before it breaks. This lack of ductility
is called “brittleness.” While brittleness is expected in cast iron, it is not
expected and definitely not wanted in
most valve metals such as cast steel.
