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Chromium Copper
Chromium copper alloys are high copper alloys, containing 0.6 to 1.2% Cr. The
chromium copper alloys are used for their high strength, corrosion resistance
and electrical conductivity. The chromium copper alloys are age hardenable,
which, in this case, means that a change in properties occurs at elevated
temperature due to the precipitation of chromium out of the solid solution. The
strength of fully aged chromium copper is nearly twice that of pure copper and
its conductivity remains high at 85% IACS, or 85% that of pure copper. These
high strength alloys retain their strength at elevated temperatures. The
corrosion resistance of chromium copper alloys is better than that of pure
copper because chromium improves the chemical properties of the protective oxide
film. Chromium copper has excellent cold formability and good hot workability.
It is used in applications such as resistance welding electrodes, seam welding
wheels, switch gears, cable connectors, circuit breaker parts, molds, spot
welding tips, and electrical and thermal conductors that require strength.
Chromium copper alloys are designated as UNS C18050 through C18600, the cast
alloys are C81400 through C81540.
The age hardening reaction occurs because the solid solubility of chromium in
copper decreases as the temperature decreases. The structure of slow cooled
chromium copper is a two phase mixture of chromium and alpha copper. Superior
mechanical properties are achieved by fast-cooling the chromium copper alloys
from the annealing temperature, so the chromium remains in a supersaturated
solid solution with the copper. Followed by an aging treatment where the
chromium precipitates from the solid solution forming a very fine dispersion of
precipitates in the matrix. The microstructure of a quenched or quickly cooled
chromium copper alloy appears similar to that of the unalloyed copper. A fast
cool prevents the chromium from precipitating out of the solid solution, so the
resulting cast structure consists of a single phase alpha copper structure. The
first material to solidify is pure copper, followed by a eutectic mixture of
alpha and chromium. The alpha and chromium eutectic material forms a lamellar
structure in the interdendritic regions. The microstructure of the wrought alloy
consists of equiaxed, twinned grains of alpha copper solid solution. Typically
the allow are cooled rapidly so the chromium remains in alpha copper solid
solution. The tempering treatment allows the chromium to precipitate out of
solution forming a dispersion of chromium precipitates throughout the matrix.
The chromium precipitates, or hardening precipitates, can be very fine and may
not be visible at low magnifications.
Copper Alloy No.
C18200
Chromium Copper , RWMA Class 2
Chrome Copper
Chemical Composition % by weight
| Copper (incl.
silver) |
Iron |
Chromium |
Silicon |
Lead |
| 99.1 Nominal |
.10 Maximum |
.60 Minimum |
.10 Maximum |
.05 Maximum |
Applications
Resistance welding machine electrodes, seam
welding wheels, electrical switch gear, electrode holder jaws, cable
connectors, current carrying arms and shafts, circuit breaker parts, arcing
and bridging parts, grid side rods in electron tubes, molds, spot welding
tips, flash welding electrodes, electrical and thermal conductors requiring
greater strength than copper, switch contacts.
Mechanical Properties
Typical for .500" rod solution heat treated
and aged (500C-3 hrs.)
| Hardness* |
Rockwell B Scales |
70 |
| Tensile Strength** |
KSI |
70 |
| Yield Strength** |
KSI |
55 |
| Elongation** |
% in 2 inch |
21 |
*Hardness conversions are approximate
**Test values are nominal approximations and
depend on specimen size and orientation.
Physical Properties
| Thermal Conductivity |
BTU/ (sq ft-ft-hr-F) |
187 |
| Specific Heat |
BTU/lb/ºF @ 68F |
.090 |
| Thermal Expansion |
Per °F from 68 F to 212 F |
.0000098 |
| Density |
lb/cu in @ 68 F |
.321 |
| Electrical Conductivity* (Annealed) |
% IACS @ 68 F |
80 |
| Modulus of Elasticity |
KSI |
17,000 |
**Volume basis
Fabrication Properties
| Capacity for
being cold worked |
Excellent |
| Capacity for
being hot formed |
Good |
| Hot
forgeability rating (forging brass=100) |
80 |
| Hot working
temperature |
1500-1700 F
or 800-925 C |
|
Machinability rating (Free Cutting Brass=100) |
20 |
| Suitability
for being joined by: |
Soldering/Good |
| |
Brazing/Good |
| |
Oxyacetylene
Welding/Not Recommended |
| |
Gas Shielded
Arc Welding/Good |
| |
Coated Metal
Arc Welding/Not Recommended |
| Resistance
Welding |
Spot/Not
Recommended |
| |
Seam/Not
Recommended |
| |
Butt/Fair |
The values listed above represent reasonable
approximations suitable for general engineering use. Due to commercial
variations in composition and to manufacturing limitations, they should not
be used for specification purposes. See applicable A.S.T.M. specification
references.
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