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/F @ 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.