SPECIAL COVERAGE
METAL ASIA | 12/2017 13
high temperature. The addition of phosphorous produces grade
C12200 — the standard water-tube copper.
High-copper alloys contain small amounts of alloying
elements that improve strength with some loss in electrical
conductivity. In amounts of 1 per cent, for example, cadmium
increases strength by 50 per cent, with a loss in conductivity
to 85 per cent. Small amounts of cadmium raise the softening
temperature in alloy C11600, which is used widely for printed
circuits. Tellurium or sulphur, present in small amounts in Grades
C14500 and C14700, has been shown to increase machinability.
Copper alloys do not have a sharply defined yield point,
so yield strength is reported either as 0.5 per cent extension
under load or as 0.2 per cent offset. On the most common basis
(0.5 per cent extension), yield strength of annealed material is
approximately one-third the tensile strength. As the material
is cold worked or hardened, it becomes less ductile, and yield
strength approaches tensile strength.
Copper is specified according to temper, which is established
by cold working or annealing. Typical levels are: soft, half-hard,
hard, spring, and extra-spring. Yield strength of a hard-temper
copper is approximately two-thirds of tensile strength.
For brasses, phosphor bronzes, or other commonly cold-
worked grades, the hardest available tempers are also the
strongest and represent approximately 70 per cent reduction in
area. Ductility is sacrificed, of course, to gain strength. Copper-
beryllium alloys can be precipitation hardened to the highest
strength levels attainable in copper-base alloys.
The ASME Boiler and Pressure Vessel Code should be used
for designing critical copper-alloy parts for service at elevated
temperatures. The code recommends that, for a specific service
temperature, the maximum allowable design stress should be the
lowest of these values as tabulated by the code: one-fourth of the
ultimate tensile strength, two-thirds of the yield strength, and
two-thirds of the average creep strength or stress-rupture strength
under specified conditions. Silicon bronzes, aluminium brasses,
and copper nickels are widely used for elevated-temperature
applications.
All copper alloys resist corrosion by fresh water and steam.
Copper nickels, aluminium brass, and aluminium bronzes
provide superior resistance to saltwater corrosion. Copper alloys
have high resistance to alkalis and organic acids, but have poor
resistance to inorganic acids. One corrosive situation encountered,
particularly in the high-zinc alloy, is dezincification. The brass
dissolves as an alloy, but the copper constituent redeposits as
a porous, spongy metal. Meanwhile, the zinc component is
carried away by the atmosphere or deposited on the surface as
an insoluble compound.
Designating Alloys: Originally developed as a three-digit
system by the US copper and brass industry, the designation
system for copper-based alloys has been expanded to five digits
preceded by the letter C as part of the Unified Numbering System
for Metals and Alloys (UNS). The UNS designations are simply
an expansion of the former designation numbers. For example,Beryllium typically appears in military-aircraft and space-
shuttle brake systems, in missile re-entry body structures,
missile guidance systems, mirrors and optical systems, satellite
structures, and X-ray windows. The modulus-to-density ratio is
higher than that of unidirectionally reinforced, “high-modulus”
boron, carbon, and graphite-fiber composites. Beryllium has an
additional advantage because its modulus of elasticity is isotropic.
Copper
Copper conducts electricity at a rate 97 per cent that of
silver, and is the standard for electrical conductivity. Copper
provides a diverse range of properties: good thermal and electrical
conductivity, corrosion resistance, ease of forming, ease of
joining, and colour. In addition, however, copper and its alloys
have relatively low strength-to-weight ratios and low strengths
at elevated temperatures. Some copper alloys are also susceptible
to stress-corrosion cracking unless they are stress relieved.
Copper and its alloys — the brasses and bronzes — are
available in rod, plate, strip, sheet, tube shapes, forgings, wire,
and castings. These metals are grouped according to composition
into several general categories: coppers, high-copper alloys,
brasses, leaded brasses, bronzes, aluminium bronzes, silicon
bronzes, copper nickels, and nickel silvers.
Copper-based alloys form adherent films that are relatively
impervious to corrosion and that protect the base metal from
further attack. Certain alloy systems darken rapidly from brown
to black in air. Under most outdoor conditions, however, copper
surfaces develop a blue-green patina. Lacquer coatings can be
applied to retain the original alloy colour. An acrylic coating
with benzotriazole as an additive lasts several years under most
outdoor, abrasion-free conditions.
Although they work harden, copper and its alloys can be
hot or cold worked. Ductility can be restored by annealing or
heating incident to welding or brazing operations. For applications
requiring maximum electrical conductivity, the most widely used
copper is C11000, “tough pitch,” which contains approximately
0.03 per cent oxygen and a minimum of 99.0 per cent copper.
In addition to high electrical conductivity, oxygen-free grades
C10100 and C10200 provide immunity to embrittlement at
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