What Is C93700 Copper Alloy?
UNS C93700 is a high-leaded tin bronze, often described in older trade literature as an 80-10-10 bronze because it contains roughly ten per cent tin and ten per cent lead. It is a cast grade rather than a wrought one, ordered against ASTM B584 for sand-cast and permanent-mould parts or ASTM B505 for continuously cast and centrifugally cast stock. Properties quoted for C93700 are always tied to the casting route, section size and temper that produced the component, so the standard designation and the product form must be stated together on any enquiry.
The characteristic microstructure is a copper-rich, tin-strengthened matrix with soft lead particles distributed as discrete islands. That two-phase structure, rather than a single alloying addition, is what gives the grade its unusual combination of load-bearing capacity and sliding tolerance.
Chemical Composition and Microstructure
| Element | Composition (% by weight) | Function in the alloy |
|---|---|---|
| Copper (Cu) | Remainder | Matrix, thermal and electrical conduction path |
| Tin (Sn) | 9.0 - 11.0 | Solid-solution strengthening and stiffness |
| Lead (Pb) | 8.0 - 11.0 | Boundary lubricant, chip breaker when machining |
| Zinc (Zn) | 0.8 max | Residual, from melting stock |
| Nickel (Ni) | 0.8 max | Residual |
| Antimony (Sb) | 0.25 max | Residual |
| Iron (Fe) | 0.08 max | Residual, controlled to limit hard particles |
| Phosphorus (P) | 0.15 max | Deoxidation residual |
Tin dissolves in the copper lattice and raises strength, hardness and fatigue resistance without making the alloy brittle. Lead is essentially insoluble in copper, so it solidifies as finely dispersed particles that remain soft even when the matrix around them is strengthened. Iron, antimony and zinc are treated as residuals and are kept low because coarse intermetallics or excess zinc would reduce ductility and, in the case of zinc, increase sensitivity to dezincification in moist service.
Why C93700 Resists Wear So Well
The wear behaviour of C93700 comes from the interaction of its two phases rather than from bulk hardness alone.
Lead as a built-in lubricant. Under sliding contact the soft lead particles smear across the mating surface and form a thin transfer film that separates the two metal bodies and lowers friction even when external lubrication is marginal.
Conformability and embeddability. The relatively soft matrix allows the bearing surface to conform to small misalignments, and foreign particles such as grit or wear debris are pressed into the surface instead of scoring the shaft.
Strength behind the sliding surface. The tin-bearing copper matrix supports the compressive load, so the bearing does not collapse even under shock or oscillating duty.
Stable performance without heat treatment. C93700 is used as cast; there is no hardening step to lose during service, and its properties are essentially uniform through the section.
Because these mechanisms depend on the lead dispersion, a coarse or segregated casting performs noticeably worse than a sound one. Consistent melting practice and controlled solidification are therefore as important as chemical composition.
Electrical and Thermal Conductivity of C93700
It is important to be realistic about conductivity in this grade. C93700 conducts on the order of ten per cent of the International Annealed Copper Standard, so its resistivity is roughly ten times that of electrical-grade copper. Thermal conductivity is likewise modest in comparison with pure copper, and both values fall as the tin and lead content rises.
The value of C93700 in electrical equipment is therefore not that it conducts well in absolute terms, but that a single part can carry current and simultaneously withstand rubbing contact. Current-collector shoes, contact segments and sliding bushes in switchgear are typical examples: the alloy tolerates continuous friction and intermittent arcing damage that would quickly destroy a hard, brittle material, while still passing the modest current required. Where high conductivity is the primary requirement, a wrought high-conductivity copper such as C11000 is the correct choice and C93700 should not be substituted.
Typical Applications
Plain and flanged bearings, bushings and thrust washers for moderate loads at low to medium sliding speed
Pump impellers, wear rings, casings and gland parts handling water, oil and mildly corrosive fluids
Valve bodies, stem guides and trunnion bushings in general industrial service
Worm-wheel blanks, gear segments and oscillating linkages where shock loading is present
Electrical contact shoes, current collectors and sliding contacts
General fittings and hardware requiring pressure tightness and corrosion resistance
Casting, Machining and Selection Guidance
C93700 is normally supplied as sand castings, permanent-mould castings, continuous cast bar and centrifugally cast tube or bushing stock. Machine stock allowance should account for the fact that the alloy is not weld-repairable in the conventional sense: lead-bearing bronzes are unsuitable for fusion welding because the lead volatilises and produces porosity, so repair is limited to brazing or soldering where the design permits.
Machining is straightforward. The dispersed lead acts as an internal lubricant and chip breaker, so free-cutting conditions can be used, although tooling should be sharp and well supported to avoid smearing the surface. No heat treatment is required or normally applied. When selecting the grade, confirm the casting method and the applicable standard on the drawing, specify soundness requirements for pressure-retaining parts, and check that the intended duty keeps the bearing pressure and sliding speed within the limits for a leaded tin bronze. For heavily loaded high-speed bearings, a higher-tin or continuous-cast bearing bronze may be a better fit.
FAQ
Q: What does the grade number C93700 mean?
C93700 is the Unified Numbering System designation for a cast high-leaded tin bronze containing about ten per cent tin and ten per cent lead in a copper matrix. The number is a composition identifier, not a strength class, so mechanical requirements come from the casting specification.
Q: Which standards cover C93700 castings?
Sand-cast and permanent-mould parts are ordered against ASTM B584, while continuously cast and centrifugally cast forms are covered by ASTM B505. Both standards define the chemical limits and the mechanical property minima for the grade in each product form.
Q: Is C93700 a good conductor of electricity?
No. Its electrical conductivity is only about a tenth of that of electrical-grade copper. It is chosen for current-carrying parts that must also resist sliding wear, not for power transmission.
Q: Can C93700 be welded?
Fusion welding is not recommended because the lead in the alloy volatilises and causes porosity and fume. Brazing and soldering are the accepted joining methods, and mechanical fixing is often preferred for repair.
Q: Does C93700 need heat treatment after casting?
The grade is used in the as-cast condition. No hardening or solution treatment is applied, which keeps the property profile stable through the section and simplifies production.
Q: How does C93700 compare with a phosphor bronze bearing grade?
Phosphor bronzes offer higher strength and better fatigue performance but less inherent sliding tolerance. C93700 is preferred where marginal lubrication, misalignment or embedded debris are expected, and phosphor bronze where higher loads and speeds dominate.
