What Is C18200 Copper Alloy?
C18200 is the UNS designation for chromium copper, usually written CuCr. It is a precipitation-hardenable copper alloy containing chromium as the principal alloying addition, and it is one of the most widely used electrode materials in resistance welding. The alloy is valued because it delivers a combination that pure copper cannot: electrical conductivity in the region of 80% IACS together with hardness and strength high enough to resist the pressure and thermal cycling of a welding electrode.
Unlike beryllium-copper grades, C18200 contains no beryllium. That matters for fabricators, because it avoids the beryllium-bearing dust and fume controls that apply when beryllium-copper electrodes or parts are machined, ground or welded. For many resistance welding duties the chromium copper grade therefore provides a simpler route to a compliant workshop while still meeting the electrical and mechanical requirement.
Composition and Typical Property Profile
| Item | Typical requirement | Notes |
|---|---|---|
| UNS number | C18200 | Chromium copper, CuCr |
| Chromium | 0.6-1.2% | Principle hardening element, present as fine precipitates after aging |
| Copper | Remainder | Matrix, provides the conductivity |
| Electrical conductivity | Approximately 80% IACS typical | Measured on the finished temper; verified per ASTM B193 practice for conductor materials |
| Thermal conductivity | Of the order of 320 W/(m·K) | Consistent with the electrical conductivity of the aged alloy |
| Strength and hardness | Above ETP copper C11000 | Level depends strongly on cold work and aging; confirmed by tensile and hardness testing |
| Softening resistance | Better than pure copper | Retains hardness through repeated welding thermal cycles |
The property combination is achieved by heat treatment rather than by alloy content alone, which is why the material is normally ordered by temper and by the required conductivity and hardness rather than by composition only. Wrought product is supplied as rod, bar, plate and forgings in tempers designated according to ASTM B601, and the mechanical property values to be met are taken from the applicable product specification for copper and copper alloy rod, bar and shapes.
Heat Treatment and Hardening Behaviour
Chromium copper is hardened in two stages. First the material is solution annealed and quenched so that the chromium is held in supersaturated solid solution; the alloy is then relatively soft and easy to form. The second stage is an aging (precipitation) treatment that brings out the fine chromium-rich precipitates responsible for the strength increase, followed in many electrode products by cold working to the finished size. The exact solution temperature, quench rate, aging time and aging temperature are set by the section size and the target temper, and they should be confirmed by measuring hardness and electrical conductivity on samples from the actual lot rather than assumed from a handbook cycle.
Two practical consequences follow. Over-aging cannot be reversed by a second thermal cycle at the same temperature, so furnace control and batch traceability matter. And any welding, brazing or hot forming operation performed after the final aging treatment will change the properties in the affected zone, which means that finished electrodes and current-carrying components should be ordered to final shape wherever possible.
Applications
Resistance welding: spot welding electrode caps, tips and adaptors, seam welding wheels and electrode holders for coated and uncoated sheet steel.
Electrical equipment: contact tips, current-carrying clamps and bolted connections, and switchgear components where high conductivity and resistance to softening are both required.
Rotating machinery: commutator segments, rotor bars and other current-carrying parts in motors and generators.
General engineering: bushings, wear pads and mould components where a conductive, hard copper alloy is needed but the electrical duty is moderate.
Fabricated assemblies: brackets, terminals and busbar fittings produced from plate, bar or forged blanks.
Selection and Inspection
Selection normally starts from the welding schedule. Heavier gauges, longer weld times and higher electrode force need a harder electrode material with a compromise on conductivity; lighter, faster schedules can use a more conductive grade. Chromium copper is the general-purpose choice for uncoated low-carbon and galvanised steel, while beryllium-containing grades such as C17510 are selected for high-current, high-force duties and for welding aluminium and coated steels, at the cost of additional safety controls during machining.
Acceptance testing for a delivery of chromium copper bar or electrode material should include: verification of the grade and heat number on the mill certificate; electrical conductivity measured on the finished temper; hardness testing on the end faces of the bar; tensile testing where the order requires it; and a metallographic check for a uniform, precipitate-bearing structure without coarse chromium stringers. For finished electrodes, check the tip geometry, the cooling-hole concentricity and the seating taper against the holder drawing.
FAQ
Q: Is C18200 the same as beryllium copper?
No. C18200 is chromium copper and contains no beryllium, so it is outside the beryllium dust and fume control regime that applies to beryllium-copper grades. C17510 is the beryllium-bearing grade most often used for the same resistance welding duties.
Q: What conductivity does C18200 have?
Chromium copper is normally quoted at approximately 80% IACS in the aged, finished condition. Conductivity is measured on the finished temper per the standard test practice for electrical conductor materials, and it should be certified on the mill test report for electrode applications.
Q: How hard is chromium copper after aging?
Hardness depends on the amount of cold work and the aging practice, so it is specified with the temper rather than quoted as a single figure. The values to be met are those of the applicable rod, bar and shapes specification for the ordered temper, confirmed by hardness testing on the delivered lot.
Q: Why does chromium copper soften during welding service?
Electrode tips are heated by the welding current and then quenched by the cooling water, and repeated cycles cause gradual over-aging of the precipitates. Chromium copper resists this softening better than pure copper, but electrodes still require periodic dressing and eventual replacement based on tip diameter and weld quality monitoring.
Q: Can C18200 be machined and formed easily?
Yes. The alloy machines in the same way as other copper-rich alloys, though it is gummier than free-cutting brass and benefits from sharp tooling, positive rake angles and adequate coolant. It cold forms readily but work hardens, so intermediate annealing is needed for severe forming operations.
