Generally, the brittle cast irons are
only used for lower pressures, particularly below 300 psi and in situations
where water hammer (sudden pressure
spikes) is not an issue. Higher pressures
are reserved for the stronger and more
ductile steel and high alloy valves.
Table 1. Strength Comparison of Common Valve Casting Materials
Corrosion Resistance
The second major consideration in choosing a valve material is its corrosion
resistance. Corrosion is the breakdown of
a metal due to attack by various chemical reactions. We all have seen corroded
bolts or rusted out fenders on a car. This
Description Minimum tensile strength (ksi) Notes
B61 Bronze 30,000 Cast bronze
A126 (class B) Cast Iron 31,000 Cast iron
A494 grade M35-1 65,000 Monel 400
A216 grade WCB 70,000 Plain cast carbon steel
A351 grade CF8M 70,000 316ss
A217 grade C12 90,000 9 Cr- 1 Mo
rust and corrosion is a result of a chemi- process. In valve materials, basic exte-cal oxidation of the steel caused by a rior rusting of the valve is usually sec-combination of oxygen and iron, with ondary to the corrosion going on within
moisture helping to accelerate the the valve due to the unique characteris-
VALVE MATERIALS: A SHORT HISTORY
More than 2,000 years ago, the Romans made bronze plug
valves for use in their fresh water distribution systems. As
advanced as the Romans were they couldn’t keep their technological advances from disappearing under the cloak of the dark
ages. It wasn’t until James Watt and others began their experiments into steam power in the late 1700s that valve technology began to surface again.
During this period, most valves were made out of the same
materials as the pipe and boilers of the time, and that was
iron. Iron was relatively easy to cast and was used for numerous piping components. In the mid 1800s, brass foundry productivity improved to the point where most of the small valves
(1/2 to 2 inches) were made of bronze.
Bronze and Iron Rule
The 19th century came to a close with bronze and iron as the
materials of choice for valve construction. These materials
could even handle the “extra heavy” 250 psi steam working
pressures of the period. Since the Bessemer Converter jump-started the steel age in the 1860’s, steel castings had been
working their way into American industry. The turn of the century saw steady increases in power plant steam pressures as
superheated steam came into vogue and the capabilities of iron
valves and fittings were approaching their practical design limits. Initial attempts to solve this materials issue resulted in the
development of a much stronger cast iron called ferro-steel,
also called semi-steel. This material was a cast iron that had
been mixed with steel scrap during the melting process.
The semi-steel was only a stopgap as cast steel valves began
to appear in all the major manufacturer’s catalogs during the
first decade of the 20th century. By 1950, cast steel would
become the primary valve construction material for the steam
generation industry.
Materials for Valve Trim
The material of choice for valve trim was bronze until the
introduction of Monel in 1906. Within a decade or two, Monel
became the severe service trim material of the valve industry.
Monel held that position until air-hardenable, martensitic
stainless steels (400 series) became popular just prior to World
War II. Following the war, Stellite, a cobalt alloy, took the
position of the best severe service valve trim material.
Demand for Alloys
Meeting the production needs of World War II fostered much
technological advancement in American industry, including
valve design. The race for synthetic rubber, 100 octane gasoline and other valuable products needed for the war effort created a demand for alloys that could handle the pressures,
temperatures and corrosion created by these processes. Valves
of austenitic stainless steel (300 series) helped handle production in these plants, and these materials are still a staple today.
As pressures and temperatures continued to rise in steam
power plants and refinery process equipment, the plain carbon
steels were not hearty enough, so alloys containing chrome and
molybdenum were developed, such as the 1-1/4, 2-1/4, 5 & 9
chrome/moly alloys. Today the ultimate metal for super-heated,
power generation valves is C12A, an alloy of 9% chrome,
molybdenum, vanadium and other elements.
Importance of Elastomers
Probably the most important valve material to come out of the
20th century was not a metal at all, but an elastomer called
Teflon. Created by DuPont in 1938 and perfected in the late
1940s, this material gave life to the soft-seated ball valve
industry. It is safe to say that without Teflon, there would not
be the huge ball valve industry that exists today.
The development of nickel alloy and superalloy castings
such as Hastelloy and Inconel during the past 40 years have
helped valve manufacturers meet the pressure temperature rating and corrosion resistance requirements found in many of
today’s critical process environments. Metallurgists are continuing to improve these unique materials to meet tomorrow’s
fluid handling challenges. And tomorrow’s ultimate valve
material might possibly be a graphite composite, containing no
metal at all.
