What Is Gunmetal? Composition, Grades and Bearing Applications
Gunmetal is a cast copper-tin-zinc alloy. The classical composition is 88 % copper, 10 % tin and 2 % zinc — the ratio still recorded as UNS C90500 and sold as 88-10-2. Modern grades widen that band with lead, so today “gunmetal” describes a family of pressure-tight, free-machining casting alloys used for pump bodies, valve bodies, marine fittings and bearing bushings.
This guide sets out where gunmetal sits among the copper alloys, cross-references the three grades ordered most often — C90500, C83600 and C93200 — and gives the practical limits that decide whether gunmetal is the right bushing material for a given duty cycle.
What Is Gunmetal?
Gunmetal belongs to the bronze family. Bronze is broadly copper alloyed with tin; gunmetal adds zinc to that base, and most commercial grades add lead as well. The zinc improves castability and deoxidises the melt, while the lead improves machinability and, in bearing grades, provides a soft phase that holds lubricant and absorbs debris.
The name dates from the eighteenth century, when the alloy was cast into cannon barrels. The 88-10-2 ratio was chosen because it cast pressure-tight, machined cleanly and survived repeated firing without cracking. That mix of castability, pressure tightness and machinability is why the alloy family still carries the name, long after the foundries stopped making ordnance. The colour term gunmetal grey comes from the dark blue-grey patina the surface develops in service.
Three properties keep gunmetal in current production: it casts into dense, pressure-tight sections that pass hydrostatic testing; it machines with short, broken chips at high metal-removal rates; and in bearing service it tolerates misalignment and dirty lubrication better than harder alloys. Where load and temperature stay moderate, those three properties usually outweigh the higher strength available from aluminium bronze.
How Gunmetal Compares with Bronze and Brass
The boundaries between these alloy families are set by which element comes second. Copper with tin is bronze, copper with zinc is brass, and gunmetal sits between them with tin dominant and zinc present in a smaller amount. The table gives nominal compositions for the alloys most often compared against gunmetal during material selection.
| Alloy family | Cu | Sn | Zn | Pb | Other elements |
|---|---|---|---|---|---|
| Gunmetal 88-10-2 (C90500) | 86 – 89 | 9 – 11 | 1 – 3 | ≤ 0.3 | Ni ≤ 1, P ≤ 0.05 |
| Leaded gunmetal 85-5-5-5 (C83600) | 84 – 86 | 4.5 – 6 | 4 – 6 | 4 – 6 | Ni ≤ 1, Fe ≤ 0.3 |
| High-lead bearing bronze (C93200) | 81 – 85 | 6.3 – 7.5 | 1 – 4 | 6 – 7 | Ni ≤ 1, P ≤ 0.15 |
| Phosphor bronze (C51000) | 94 – 96 | 7 – 9 | ≤ 0.3 | — | P 0.03 – 0.35 |
| Naval brass (C46400) | 59 – 62 | 0.5 – 1 | 36 – 39 | ≤ 0.2 | Fe ≤ 0.1 |
| Aluminium bronze (C95400) | ≥ 83 | — | — | — | Al 10 – 11.5, Fe 3 – 5, Ni ≤ 1.5 |
Two consequences follow from those numbers. First, gunmetal holds far less zinc than brass, so it does not suffer the dezincification that limits high-zinc brasses in warm seawater. Second, leaded gunmetal contains enough lead to machine freely and to run against a shaft under boundary lubrication, which is why grades like C93200 dominate general bearing stock. Where a drawing calls for a bronze bushing material without naming a grade, C93200 is usually the default starting point.
Suggested Searches: Copper， Brass， Bronze， Copper Nickel， Manganese， Aluminum
Gunmetal Grades and Specification Cross-Reference
The same alloy is sold under a UNS number in North America, an EN material number in Europe and a DIN symbol in German-speaking markets. The table below aligns the three systems with the mechanical properties measured on cast test bars.
| Common name | UNS | EN / DIN | Tensile (MPa) | Hardness (HB) | Elongation (%) | Typical form |
|---|---|---|---|---|---|---|
| Gunmetal 88-10-2 | C90500 | CC480K / G-CuSn10Zn | 250 – 310 | 75 – 95 | 12 – 20 | Sand and continuous cast bushings, valve and pump bodies |
| Leaded gunmetal 85-5-5-5 | C83600 | CC491K / G-CuSn5Zn5Pb5 | 220 – 255 | 60 – 70 | 15 – 25 | General bearings, pump bodies, plumbing fittings |
| High-lead tin bronze | C93200 | CC493K / CuSn7Zn4Pb7 | 240 – 275 | 65 – 80 | 10 – 20 | Continuous cast bearing stock, thrust washers, wear plates |
| Nickel gunmetal | C92200 | CC490K / G-CuSn6ZnPb | 235 – 275 | 65 – 80 | 14 – 22 | Marine castings, pressure-tight valve bodies |
Casting method shifts these values as much as composition does. Continuous cast bar cools faster than sand castings, producing a finer grain structure and tensile strength at the upper end of the ranges shown. A continuous cast C93200 bushing blank will typically test 10 to 20 HB harder than the same alloy poured in sand.
Selecting Gunmetal for Bearing and Bushing Service
Gunmetal earns its place in bearings through three behaviours rather than through strength. The lead phase embeds hard particles so they do not score the shaft, it conforms to slight shaft deflection or housing misalignment, and it holds a lubricating film during start-stop cycles when the oil film has not yet formed. The table maps these behaviours onto common duty cycles.
| Operating condition | Suggested grade | Why it fits | Where another alloy fits better |
|---|---|---|---|
| Grease-lubricated sleeve bearing, moderate load, below 100 °C | C93200 | Lead phase gives embeddability and conformability; machines cleanly from continuous cast bar | — |
| Oscillating pivot with boundary lubrication and slow speed | C83600 | Softer matrix tolerates dirt and intermittent motion | Graphite-plugged CuSn10 where re-lubrication is impossible |
| Pressure-tight pump or valve body in seawater | C90500 | Low lead and high tin; passes hydrostatic test and resists cavitation | C95800 nickel-aluminium bronze at higher flow velocity |
| High-volume machined bushings from bar stock | C93200 | Consistent microstructure, short chips, stable tool life | — |
| Potable-water or food-contact equipment | Low-lead gunmetal, Pb ≤ 0.2 % | Meets lead-leaching limits for wetted surfaces | Bismuth-tin bronze where lead must be absent entirely |
Where a duty cycle runs dry or dry-ish, gunmetal on its own is usually not the answer. Dry running removes the oil film that the lead phase is there to support, and the material then wears faster than a self-lubricating bronze bushing with graphite plugs embedded in the matrix. The same reasoning applies in submerged marine duty, where oilless bushings in mooring equipment are normally specified with solid lubricant rather than relying on grease that water will wash out.
Practical Limits and Compliance
Three limits decide whether a gunmetal bushing will hold up. Each should be checked against the drawing before the alloy is released:
- Temperature. Leaded gunmetal is generally held below about 150 °C in continuous bearing duty. Above that the lead phase softens, load capacity drops and smearing becomes likely. Low-lead grades such as C90500 reach roughly 200 °C before the tin bronze matrix itself becomes the limiting factor.
- PV value. For lubricated C93200, published PV limits sit near 2.6 MPa·m/s. Treat that as a screening figure only: it assumes adequate lubrication, a hardened shaft and good heat dissipation. PV is load × sliding velocity, so raising speed cuts the allowable load by the same factor.
- Lead content. C83600 and C93200 exceed 4 % lead, the ceiling in the RoHS copper-alloy exemption, and that exemption is reviewed periodically. NSF/ANSI 372 sets a weighted average of 0.25 % lead for wetted surfaces in potable water. Where either applies, specify a low-lead grade or a bismuth-tin bronze from the start rather than substituting later.
Shaft condition matters as much as alloy choice. Gunmetal is the softer member in the pair, so a rough or soft shaft will wear the lead phase away quickly; a shaft at 55 HRC or harder with a surface finish of 0.4 µm Ra or better is the usual recommendation for continuous duty.
Frequently Asked Questions (FAQs)
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