Machining C75200 Nickel Silver: Composition, Work Hardening, and What Engineers Must Specify
If your part needs the tarnish-resistant silver look of plated brass but must stay one solid alloy that can be cold-formed and lightly machined, C75200 nickel silver (UNS C75200, SAE CA752, ~65Cu-18Ni-17Zn) is usually the lower-cost, more durable pick than plating. It is a copper-nickel-zinc solid-solution alloy: it cannot be hardened by heat treatment, only by cold work, and its machinability is roughly 20% of the C36000 free-machining brass benchmark. The practical read is that C75200 is a forming-first, machining-second material — ideal for springs, clips, nameplates, jewelry, and optical or musical components where a white “silver” look and good ductility matter more than raw strength. Engineers who choose it for turned or milled parts must specify temper, form, and finish on the drawing, because nearly every “typical hardness” for nickel silver is temper-dependent.
What “Nickel Silver” Is (and Is Not)
C75200 belongs to the nickel silver group of copper alloys, also called German silver. It contains no elemental silver — it is a ternary Cu-Ni-Zn solid-solution alloy. Nickel provides the whitish color and improves corrosion and tarnish resistance; zinc lowers cost and raises strength relative to binary copper-nickel alloys such as C70600 90/10 and C71500 70/30 cupronickel.
This distinction matters because Cu-Ni and Cu-Zn alloys behave very differently in the machine. A machinist who has run free-cutting brass all day then gets a C75200 job will notice nickel silver does not form the brittle short chips of leaded brass. It is tougher and more ductile, work-hardens during cutting, and throws stringy chips unless geometry and feed are adjusted. In difficulty it sits between C26000 cartridge brass (gummy, very formable) and the harder cupronickels.
Grades, Designations, and Applicable Standards
C75200 is covered by several real specification systems and appears under multiple designations depending on product form:
| Designation | System / Scope | Typical Product Forms |
|---|---|---|
| UNS C75200 | Unified Numbering System | Sheet, strip, plate, bar, rod, wire |
| SAE / ASTM CA752 | ASTM B122/B122M (sheet, strip, plate, rolled bar) | Flat rolled product |
| UNS C75200 | ASTM B151/B151M (bar and rod) | Round and rectangular bar |
| EN CW404J (CuNi18Zn20) | European EN material designation | Strip, sheet, wire |
| GB/T BZn18-26 (approximate) | Chinese standard designation (compositionally close, not identical) | Strip, wire |
A caution for specification writers: BZn18-26 and CW404J are “approximate” near-equivalents, not drop-in substitutes — zinc and impurity limits differ between national systems. If the drawing calls out ASTM B122/B122M with UNS C75200, quote and supply to that standard.
Chemical Composition
Nickel content in particular drives both color and tarnish resistance. The nominal composition is 65% Cu, 18% Ni, 17% Zn, with these limit ranges:
| Element | Content (wt %) | Role in the Alloy |
|---|---|---|
| Copper (Cu) | 63.5 – 66.5 | Base metal; ductility and electrical/thermal behavior |
| Nickel (Ni) | 16.5 – 19.5 | Whitens color, improves strength and tarnish resistance |
| Zinc (Zn) | Remainder (≈12 – 20) | Solid-solution strengthener; lowers cost |
| Iron (Fe) | ≤ 0.25 | Residual impurity |
| Manganese (Mn) | ≤ 0.50 | Residual / deoxidizing addition |
| Lead (Pb) | ≤ 0.05 | Kept low — this is essentially lead-free |
| Other (total) | ≤ 0.10 | Total named/unnamed residuals |
The near-zero lead content explains why C75200 machines slower than leaded brass and why it is chosen when lead must be avoided (jewelry, food-contact, some consumer goods). For free-cutting speed, a leaded nickel silver such as C79200 or C79800 exists — but those are different grades with different color and corrosion behavior.
Mechanical Properties and Temper
C75200 gains strength only through cold work; annealing softens it between forming operations and final temper is set by the amount of cold reduction. For this reason the temper designation — not a bare “C75200” — is the most important line on the drawing. Values below are typical ranges for the annealed (O) and cold-worked (H) tempers, and are representative, not a single authoritative table:
| Temper | Tensile Strength (MPa) | Yield Strength (0.5% ext, MPa) | Elongation (%) | Approx. Hardness (HV) |
|---|---|---|---|---|
| Annealed (O / soft) | 400 – 480 | ≈ 170 – 220 | 35 – 45 | ≈ 90 – 120 |
| 1/4 Hard (H01) | 490 – 560 | ≈ 330 – 400 | 15 – 25 | ≈ 140 – 170 |
| 1/2 Hard (H02) | 550 – 640 | ≈ 430 – 520 | 8 – 15 | ≈ 165 – 190 |
| Hard (H04) | 650 – 760 | ≈ 550 – 680 | 3 – 8 | ≈ 190 – 220 |
| Spring (H08) | 760 – 860 | ≈ 690 – 800 | 1 – 3 | ≈ 220 – 250 |
Treat these as starting points, not purchase-spec guarantees: actual values depend on section size, cold-reduction history, and the test standard (tensile per ASTM E8/E8M; Vickers hardness per ASTM E92). Density is about 8.73 g/cm³ and the melting range roughly 1070–1110°C. A thick bar cannot be cold-worked to H08 the way a 0.5 mm strip can, so verify hardness on the actual section.
Corrosion and Tarnish Behavior
Nickel silver’s best-known property is tarnish resistance: it holds a bright, silver-like surface far longer than ordinary brass because nickel slows the sulfide/oxide films that discolor copper-zinc alloys. It is not “stainless” — it dulls gradually in sulfur-bearing air and can patina in chloride-rich environments. For most indoor, non-marine uses (nameplates, instruments, flatware, jewelry, optical frames, keyboard springs) it works uncoated. It is lead-free, but make any food-grade claim only against a specific published regulation. Where abrasion or seawater is the driver, C70600 cupronickel is more robust.
Machining Behavior and Practical Parameters
This is where C75200 surprises people, with a machinability index near 20% (relative to C36000 free-cutting brass = 100%). It is ductile, produces no short free-machining chip, and work-hardens in the cut. Typical consequences:
- Stringy, tough chips that wrap around tools and smear surfaces if feed is too low or the edge dull.
- Built-up edge (BUE) on the rake face, degrading finish and dimensional control.
- Dimensional drift as internal stress from prior cold work relieves during machining — thin-wall parts can warp.
The parameters below are conditional starting points for a rigid CNC lathe or mill with sharp carbide and adequate coolant — not guaranteed recipes. Values shift with machine stiffness, tool grade and coating, workpiece rigidity, chip evacuation, and starting temper (H04 hard cuts cleaner than gummier annealed O).
| Operation | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Tooling Note |
|---|---|---|---|---|
| Turning | 70 – 130 | 0.08 – 0.20 | 0.5 – 3 | Sharp uncoated or TiN carbide, positive rake |
| Milling | 50 – 100 | 0.03 – 0.10 (per tooth) | 0.3 – 2 (radial) | 2- or 3-flute solid carbide or HSS, high helix |
| Drilling | 35 – 70 | 0.05 – 0.15 | Full diameter | Polished-flute drill, peck deep holes |
| Threading / Tapping | 10 – 25 | Per pitch | — | Sharp taps, spiral-point for through holes |
A few rules specific to C75200:
- Keep the tool sharp — a dull edge smears the surface; change inserts earlier than on brass.
- Use positive rake and polished, low-friction surfaces to discourage built-up edge; a TiN or CrN coating helps.
- Run adequate coolant (water-soluble or synthetic flood) to flush stringy chips; through-tool high pressure helps deep features.
- Don’t under-feed — too-low a feed rubs instead of cutting, accelerating work hardening.
- Account for spring-back — a low-temperature stress-relief anneal (roughly 250–300°C) before finishing reduces drift on thin or long parts; confirm it against the temper you must preserve.
I do not state a fixed surface roughness or tolerance here: achievable Ra must be validated against your fixture, stock temper, and tooling.
Surface Finish and Plating Options
C75200 is often supplied uncoated because its tarnish resistance is the finish. When more protection or a specific look is demanded:
- Bright dipping / mechanical polishing to lift the natural white-silver luster.
- Clear lacquer to lock the bright surface against handling and tarnish.
- Silver, gold, or nickel plating where a specific color or harder surface is required (silver plating on nickel silver is common for cutlery and instruments).
If the part will be plated, say so on the drawing and keep the machined surface free of embedded chips and smeared metal, because contamination from a dull tool shows up directly as plating defects.
Where C75200 Fits (and Where It Doesn’t)
Typical C75200 applications cluster around appearance plus moderate strength plus formability:
- Nameplates, badges, and dial plates;
- Jewelry findings, zippers, and flatware; musical instrument keys and frets;
- Optical and instrument frames and fittings;
- Electrical springs, contacts, and shielding;
- Decorative turned components where a lead-free “silver” metal is wanted.
It is the wrong choice for load-bearing structural parts, high-wear bearings, deep seawater service, or high-speed production where cycle time dominates. For those, consider C46400 naval brass for marine load parts, the cupronickels above, or a plated free-cutting brass when maximum machinability outweighs a lead-free solid-alloy surface.
Specifying C75200 on the RFQ: What to Send
Because nearly every property of C75200 is temper- and form-dependent, a bare “C75200” callout forces the shop to guess. Include on every request:
- Exact alloy and standard — e.g. “UNS C75200 per ASTM B122/B122M” (and product form: strip, bar, wire).
- Temper — annealed (O), half-hard (H02), hard (H04), or spring (H08).
- Product form and section size — gauge or bar diameter.
- Surface and finish — bright-annealed, polished, lacquered, or to be plated.
- Tolerances and critical dimensions — especially on thin or springy features.
- Quantity and any certification — and, only if genuinely required, a specific regulatory conformance statement with its published reference.
Send the drawing, material spec, temper, quantity, tolerances, and surface requirement, and a competent shop can quote it accurately. If unsure whether C75200 is right — or a plated brass or cupronickel would serve better at lower cost — ask for a materials comparison before committing tooling. Submit your drawing, material specification, temper, quantity, tolerance, and finish requirements for a quote and a recommendation on whether C75200 or a near-equivalent fits the part better.
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