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In
the forty some years since the invention of Ductile Iron, virtually
every manufacturer of pressure pipe has switched from gray iron to this
superior metal. Why? Because Ductile Iron's exceptional characteristics
enabled improved performance with a reduction in thickness and weight.
Widespread
evaluation of Ductile Iron in the 1950s demonstrated its superior
strength and impact toughness, prompting more producers to switch from
gray iron. By the mid 1970s, all major producers of iron water main pipe
had converted entirely to Ductile Iron.
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| Small diameter pipe
of the American Cast Iron Pipe Company with 6 machines
centrifigally casting Ductile Iron pipe of 4 to 16 inch diameter
in 20 foot length. |
The
United States currently sees approximately 1 1/4 million tons of Ductile
Iron pipe produced annually. And an estimated 1 1/2 million miles of
pipe have already been installed to distribute the several hundred
gallons of water used each day by every family.
History of Pipe Materials
Water
has always been a precious commodity, and its transmission has
facilitated the advancement civilization enabled the comforts of our
modern living standard.
While
the Romans were acclaimed for their transmission of water, their
aqueducts provided only downhill passage, by gravity flow. Complete
distribution required pressurization in pipe. Earliest pipe materials
were predominantly bored logs, with some use of lead pipe and brittle
baked clay pipe.
Iron
pipes were first used in the 15th century in Germany and France. The
most notable installation as the Palace of Versailles built by King
Louis XIV in 1664. These cast iron pipes are still in use today, over
300 years later. Gray cast iron pipe progressively became the preferred
material for water.
The
first iron pipe in the United States was imported from England and used
by the city of Philadelphia in 1804. Early Iron in the United States was
made in charcoal furnaces, which were concentrated mostly in the New
Jersey area. Some of the earliest pipe foundries were also located
there.
With
the use of coke in blast furnaces, then cupolas, foundry growth expanded
westward into Pennsylvania and Ohio, then southward toward the
Birmingham area, where all the materials for iron were available. Soon,
various accessories such as fittings, valves and fire hydrants joined
with pipe as parts of the total industry, and were cast in many
foundries.
Sanitary
or soil pipe also grew into a considerable industry, but distinct from
pressure pipe with less demanding iron specifications, shorter lengths
and thinner walls.
Since
1913, Alabama has been the leading state, and Birmingham the leading
city, for iron pipe production. back
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Evolution
of Casting Methods
The
first iron pipes were cast statically in horizontal modules in lengths
limited to three to six feet because of suspension of the massive center
cores. Many of the early foundries produced pipe along with many other
types of casting. As demand for pipe increased, the casting equipment
became more specialized, casting vertically in pits with lengths
increased first to 12 feet, then 16 feet.
In
1918, a Brazilian, DeLavaud, presented a centrifugal casting process
which revolutionized pipe production. The use of the centrifugal force
of a rotating mold eliminated need for the center core. And the
water-cooled metal mold permitted repetitive casting at high production
rates. Long heat treating ovens were necessary to anneal some chill
obtained against the metal mold.
A
centrifugal process using sand rammed molds was developed to avoid the
chilled structure and necessity for annealing. Later, a resin bonded
thin sand process was invented by ACIPCO and used by a few plants for
several years.
With
increase labor cost and the advent of Ductile Iron, which required
annealing on all processes, sand lined processes have been abandoned.
All pressure pipe is now cast in water cooled metal molds using highly
specialized equipment with automation and computerized controls.
Gray
Iron Pipe Quality Advances
With
advances in metallurgy and melting controls, and through the use of
chemical, and spectrometer laboratories, gray iron pipe progressively
improved and its strength increased. Quality was closely controlled to
specified levels of strength, and the moduli of rupture and modulus of
elasticity determined on test specimens from the wall of the pipe. Each
pipe was hydrostatically tested to 5O0 psi.
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| Twin 30 inch lines
of Ductile Iron pipe being installed in one of our large cities
to expand and improve water service. |
Guided
by a strong pipe association, thickness classifications were established
for various operating pressures and depths of cover with allowances for
pressure surges from valve closing and from super imposed truck loads.
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Ductile
Iron Evaluated and Adopted
When
the discovery of Ductile Iron was announced, some pipe producers saw no
need for it, since gray iron had served well and was stronger that other
competitive pipe materials, primarily plastic in small diameters and
reinforced concrete in large diameters. But others recognized the
potential for Ductile Iron in pressure pipe service. Several companies
experimented with Ductile Iron, and some trial orders were produced for
special applications and evaluation. Performance tests and experiences
were very favorable.
Again,
the pipe association guided evaluation of Ductile Iron on test pipe of
various thicknesses, outfitted with strain gages, installed in various
type trenches and depths of cover and subjected to various pressures and
external loads simulating passage of trucks.
These
tests verified the superior properties of Ductile Iron, which permitted
reduction in thickness. The established thickness reductions more than
compensated for the increased cost of Ductile Iron and actually reduced
the pipe cost per foot.
Through
a period of transition, most companies produced both gray iron and
Ductile Iron pipe to suit customer preference. But with good experience
from Ductile Pipe in service, preference for Ductile Iron progressively
increased. In the mid seventies, all major producers went totally to
Ductile Iron and abandoned gray iron pipe. Those producers who resisted
Ductile Iron were eventually closed or purchased.
Processes
and Properties
With
conversion to Ductile Iron, pressure pipe production became an even more
specialized process.
Pipe
producers generally melt in large water-cooled cupolas with the latest
accessory equipment. Some producers duplex in electric furnaces for more
positive supply and control. In the early years, some used a basic slag
to obtain low sulfur from the cupola. In recent years, general practice
has been to melt with an acid to neutral slag and desulfurize externally
using on of several available agitation methods. Low sulfur iron is then
treated with magnesium by several of the available treatment processes.
Magnesium
treated iron with closely controlled chemistry and temperature is poured
at a controlled rate into molds rotation at proper centrifugal speed
with post inoculant added in the stream and mold.
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After
stripping, the pipes are fully annealed through long annealing furnaces,
some as long as 240 feet. As-cast pipe has a slight depth of chilled
iron which requires some time in a 1700°F zone. Due to the late post
inoculation, the carbides break down fast to fine nodular graphite with
a high nodule count. Then a lower temperature zone breaks down pearlite
to a ferritic matrix for highest ductility and impact toughness.
Guided
by tests the pipe association, specifications established by A.N.S.I.
and A.W.W.A require tensile tests from the wall of the pipe to meet
60,000 psi tensile, 42,000 psi yield, and 10 percent elongation. Impact
strength is specified both at room temperature and at -40°F on a
modified Charpy-V specimen testing the full wall of the pipe.
Conclusion
The
pressure pipe industry has taken advantage of the superior properties of
Ductile Iron, totally converting all iron pipe to Ductile Iron.
Ductile
Iron pipe has established an outstanding performance record against
increasing stresses, superimposed loads and earthquakes. Ductile Iron
has also enhanced the reliability of water transmission, thereby
providing comfort and sanitation and contributing to our high standard
of living.
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