Tin Bronze

What Tin Bronze Is and Why It Matters

Tin bronze is a copper-base alloy in which the principal addition is tin. In the Unified Numbering System it occupies the C90000 to C94999 band for cast tin and leaded-tin bronzes, and parts of the C50000 to C52999 band for wrought phosphor bronzes. The defining metallurgical event is that tin dissolves into copper to form the alpha solid solution, a face-centred-cubic structure that is substantially harder and more wear resistant than pure copper while keeping copper's corrosion resistance. A cast tin bronze such as C90500 reaches roughly 303 megapascals (44,000 pounds per square inch) minimum tensile strength and about 75 Brinell hardness, against the soft, ductile, low-strength behaviour of pure annealed copper.

Tin bronze is most often the answer to a tribological problem: a surface that must slide, carry load, and resist seizure against a steel shaft. Tin bronze bearings and bushings tolerate poor lubrication far better than steel on steel, because the soft copper matrix conforms and embeds debris while the harder phases carry the load. The same combination of corrosion resistance and strength makes tin bronze and its zinc-bearing cousin, gunmetal, the traditional material for marine and steam valve bodies, pump impellers, and fittings that must stay pressure tight in water or steam for decades.

The industrial history of tin bronze is the longest of any structural alloy. Humans have cast copper-tin bronze for more than five thousand years, and the Bronze Age is named for it. The modern distinction between cast and wrought grades dates to the nineteenth and early twentieth centuries, when phosphorus deoxidation produced clean, sound metal that could be cold rolled into spring strip without oxide cracking, creating the wrought phosphor bronze family. Standardisation followed: the ASTM B-series casting and wrought specifications and, in Europe, EN 1982 for cast and the EN 12163 series for wrought now fix chemistry, form, and properties.

Engineering tin bronzes span a deliberately chosen range of tin content because tin trades ductility for hardness. Wrought spring grades sit near 4 to 10 percent tin, where the metal remains workable; cast bearing grades sit near 8 to 12 percent tin, where wear resistance peaks and the alloy is poured rather than formed; bell metal and speculum reach 20 percent tin and beyond, where the alloy is hard, sonorous, and brittle. There is no single best tin bronze. The discipline of selection is matching the tin content, the casting or wrought route, and the standard to the service.

Four engineering attributes decide a tin bronze purchase: tin content (which sets hardness, wear resistance, and ductility), the product route (cast shape versus wrought mill form), the corrosion environment, and the governing standard. These determine whether a bushing lasts ten thousand hours or scores a shaft in a season, and whether a valve body passes a hydrostatic test. The remaining chapters take each in turn.

Cast and Wrought Grade Families

Tin bronze grades split first into cast and wrought families, then by tin content and by whether zinc and lead are added. The cast tin bronzes (C903xx to C907xx) and leaded gunmetals (C922xx and the leaded red brasses) are poured to shape; the wrought phosphor bronzes (C510xx to C544xx) are rolled and drawn. The table below compares the most-specified grades by nominal chemistry, governing standard, and the property that drives their selection. Cast tensile values are the standard minimums for continuous or sand cast bar; wrought values vary with temper and are detailed in Chapter 5.

C90300 and C90500 are the workhorse cast bearing bronzes. C90300 (nominal 88 percent copper, 8 percent tin, 4 percent zinc) and C90500 (88 copper, 10 tin, 2 zinc) both deliver about 303 MPa minimum tensile strength, with C90300 the more ductile of the two at 18 percent minimum elongation against C90500's 10 percent. The small zinc addition acts as a deoxidizer and improves castability. These are the default grades for general machinery bushings, gear blanks, pump impellers, piston rings, and valve bodies under moderate load and speed.

C90700 is the high-tin bearing grade, nominal 89 percent copper and 11 percent tin with no intentional zinc. The extra tin raises Brinell hardness to about 102 and wear resistance well above C90500, but minimum tensile strength is slightly lower at 276 MPa and ductility falls, so C90700 is reserved for heavy loads at low or intermittent sliding speed: worm wheels, gear sets, and heavily loaded slow bushings. Its European equivalent is CuSn12-C (CC483K).

C92200, the leaded gunmetal known as Navy M, is nominally 88 copper, 6 tin, 1.5 lead, 4.5 zinc, and is the alloy named in ASTM B61 for steam and valve castings. The zinc improves castability and pressure tightness while economising on tin, and the lead improves machinability and helps seal porosity, which is why this family dominates pressure-containing valve and fitting castings rather than the highest-wear bearing duty.

The wrought phosphor bronzes C51000, C52100, and C54400 are tin bronzes deoxidized with phosphorus and then cold worked. C51000 (5 percent tin, often called phosphor bronze A) is the connector-spring standard; C52100 (8 percent tin, phosphor bronze C) is the higher-strength spring grade; and C54400 (4 percent tin plus 4 percent zinc and 4 percent lead, phosphor bronze B-2) is a free-cutting grade for machined bearings and thrust washers. Their properties depend on temper, covered in Chapter 5.

Copper-Tin Metallurgy

To choose a tin bronze intelligently, a buyer needs to understand what tin does to copper, because every property in the spec sheet traces back to the copper-tin phase behaviour. The copper-tin system was mapped experimentally by Heycock and Neville at the turn of the twentieth century and remains a classic reference. Two microstructural constituents dominate engineering bronzes: the soft, tough alpha solid solution and the hard, brittle delta intermetallic. The table below summarises the constituents that matter for selection.

The alpha solid solution is the foundation of every engineering tin bronze. Tin atoms substitute into the copper face-centred-cubic lattice, distorting it and impeding dislocation motion, which raises strength and hardness while the metal stays ductile and tough. Under true equilibrium, copper dissolves a large amount of tin, but commercial castings cool too quickly for full diffusion, so the practical solubility limit of the alpha phase falls below what the equilibrium diagram suggests. This is why a cast 10 percent tin bronze already shows some of the second phase even though equilibrium says it should be single phase.

The delta phase is the hard intermetallic compound of approximate formula Cu₃₁Sn₈. It forms on cooling when the high-temperature beta phase decomposes, with the eutectoid reactions occurring near 586 and 520 degrees Celsius. Like most intermetallics, delta is extremely hard and brittle. In small, discrete amounts dispersed in the tough alpha matrix it is exactly what a bearing needs: hard load-carrying islands in a conformable matrix. But if tin content is pushed too high and the delta network becomes continuous, the casting turns brittle and impact-sensitive, which is why bearing bronzes stop around 11 to 12 percent tin and bell metal at 20-plus percent tin is deliberately brittle and sonorous.

Phosphorus enters the picture as a deoxidizer. Molten copper-tin readily absorbs oxygen, which would otherwise form brittle tin oxide inclusions, so a controlled addition of phosphorus, typically 0.03 to 0.35 percent, ties up oxygen as slag and leaves clean metal. Residual phosphorus above the deoxidizing requirement forms a hard copper phosphide that slightly increases wear resistance, and gives the wrought spring family its phosphor bronze name. The cost is conductivity: residual phosphorus scatters conduction electrons, helping drop C51000 to roughly 15 to 20 percent IACS.

Zinc and lead are the two common further additions. Zinc, the defining element of the gunmetals, acts as a deoxidizer without changing the microstructure appearance and improves castability and pressure tightness while economising on expensive tin. Lead is nearly insoluble and freezes as soft free globules dispersed through the structure; it improves machinability and helps seal microporosity, which is why leaded gunmetals are favoured for machined, pressure-tight valve bodies. Because tin bronzes solidify over a wide freezing range, cast structures are strongly cored and can show inverse segregation; annealing or controlled slow cooling homogenises the casting.

Product Forms and Standards

Tin bronze reaches the buyer in two fundamentally different routes, and the routes carry different standards. Cast grades arrive as poured shapes, with the casting method (sand, continuous, centrifugal) affecting density, grain size, and achievable properties. Wrought phosphor bronze arrives as mill product, rod, bar, sheet, strip, and wire, in a specified temper. A purchase order must name both the product-form standard and the grade, because the same chemistry behaves differently as a sand casting, a continuous casting, and a cold-rolled strip.

Continuous cast bar (ASTM B505) is the dominant supply form for bearing and bushing stock. Continuous casting pulls solidified bar from a water-cooled die, producing a fine, dense, uniform grain structure with low porosity, ideal for machining into bushings, sleeves, and gear blanks. Most stock-bar suppliers of C90300, C90500, and C90700 quote to ASTM B505, and minimum properties such as the 303 MPa tensile of C90500 are defined there.

Sand and centrifugal castings (ASTM B584, B61, B62) cover near-net-shape parts: valve bodies, pump housings, large gear rings, and bearing shells too large or complex for bar. Sand casting gives design freedom but coarser grain and more porosity than continuous casting, so for the same grade the guaranteed minimums can differ by casting method. Centrifugal casting, which spins the mould, produces dense, sound cylindrical parts and is common for large bushings and bearing rings. ASTM B61 and B62 specifically cover steam, valve, and composition bronze castings such as C92200 and C83600.

Wrought rod, bar, and shapes (ASTM B139) and plate, sheet, strip, and rolled bar (ASTM B103) govern the phosphor bronzes. These products are supplied to a temper from the ASTM B601 code: O60 soft annealed, the H0x cold-worked tempers (H01 quarter hard, H02 half hard, H04 hard), and H08 spring. The temper, not just the grade, fixes the strength and formability, so a connector designer specifies, for example, C51000 strip in H08 spring temper for maximum spring force.

The table below cross-references the leading UNS grades to their European EN designations. The mapping is approximate, since national chemistry windows differ slightly, but it is the practical starting point for sourcing equivalents across regions. Always confirm against the supplier certificate, which should cite both the grade and the governing standard.

Key Specification Parameters

Reading a bronze certificate is a fundamental skill for a purchasing engineer. A cast-bronze datasheet may list a dozen lines, but only a few truly drive selection: tin content, tensile and yield strength, elongation, hardness, density, melting range, and for electrical use, conductivity. Each is explained below, with the verified numbers for the leading grades.

Tensile and yield strength for cast tin bronzes are quoted as minimums. C90300 and C90500 both guarantee about 303 MPa (44 ksi) minimum tensile with yield (at 0.5 percent extension) of 152 and 172 MPa respectively; C90700 guarantees 276 MPa (40 ksi) tensile, 172 MPa yield. These are modest absolute numbers, but tin bronze is rarely chosen for tensile strength: it is chosen for compressive load capacity and wear, where the hard delta phase carries far more than the tensile figure implies.

Elongation is the ductility indicator and falls as tin rises. C90300 reaches 18 percent minimum elongation in 50 millimetres, C90500 only 10 percent, and C90700 likewise about 10 percent. For a bushing that must be press-fit and may see shock, the higher elongation of C90300 is a real advantage. Hardness moves the opposite way: C90300 is about 70 Brinell, C90500 about 75 Brinell, and high-tin C90700 about 102 Brinell, which is the figure that predicts wear life.

Density and melting range are needed for casting and weight calculations. Cast tin bronzes run about 8.7 to 8.8 grams per cubic centimetre (0.315 to 0.318 pounds per cubic inch), slightly less than pure copper because tin is lighter. The freezing range is wide, with C90500 melting between a solidus near 854 and a liquidus near 999 degrees Celsius (1570 to 1830 degrees Fahrenheit); this wide range is the source of the coring and porosity that casting practice must manage.

Wrought temper is the parameter unique to phosphor bronze, and it changes the numbers dramatically. The table below shows how C51000 strength climbs with cold work. The same grade ranges from a soft, formable annealed condition to a hard spring temper, so the temper code is as load-bearing as the grade itself.

Electrical and thermal conductivity matter when tin bronze is used for current-carrying springs and contacts. Tin in solid solution scatters electrons strongly, so C51000 conducts only about 15 to 20 percent IACS and roughly 77 watts per metre-kelvin of heat, far below pure copper's 100-plus percent IACS. This is acceptable for connector springs, where mechanical resilience matters more than resistance, but it rules tin bronze out of busbar duty. Wear and bearing capacity are not a single spec-sheet number; they depend on grade, lubrication, sliding speed, and contact pressure. As a practical rule, plain tin bronze bushings run unlubricated only at low speed, and above roughly 0.5 metres per second sliding speed they need forced lubrication or a graphite-plugged construction.

Selection Decision Factors

To turn the previous five chapters into a specific grade, form, and temper, follow the decision sequence below. Most selection mistakes are not a single wrong number but a decision taken at the wrong level, for example choosing a grade before deciding cast versus wrought. These seven steps can serve as a fixed RFQ template for any tin bronze part.

1. Cast or wrought route: Decide first whether the part is a poured shape (bearing, bushing, gear, valve body, pump part) or a mill form worked into shape (spring strip, connector, fastener, bellows). Cast points to the C9xxxx grades under ASTM B505 or B584; wrought points to the C5xxxx phosphor bronzes under ASTM B139 or B103.
2. Tin content versus duty: For cast bearings, pick tin by load and speed. Moderate load and speed favours C90300 or C90500 (8 to 10 percent tin) for a balance of strength and conformability; heavy load at low speed favours high-tin C90700 (11 percent) for maximum hardness and wear, accepting lower ductility.
3. Pressure tightness and machinability: If the casting must hold pressure and be heavily machined, such as a valve body or fitting, choose a leaded gunmetal such as C92200 (ASTM B61) or C83600, where zinc aids casting soundness and lead aids sealing and machining, rather than a straight high-tin bronze.
4. Temper, for wrought parts: Specify the ASTM B601 temper alongside the grade. Choose soft O60 when the part is formed after delivery, and H04 hard or H08 spring when it must hold shape and deliver spring force, such as a connector contact in C51000 or C52100.
5. Corrosion environment: Confirm compatibility with the service fluid. Tin bronzes resist fresh water, seawater, steam, and many organics well, which is why they dominate marine and steam valves, but verify against the maker's corrosion data for acids, ammonia, and oxidising media before committing.
6. Lubrication and sliding speed: For bearings, fix the lubrication regime. Below roughly 0.5 metres per second a plain tin bronze bushing can run with minimal lubrication; above that, plan forced lubrication or specify a graphite-plugged or oil-impregnated construction to keep within the alloy's pressure-velocity limit.
7. Standard, certification, and total cost: Cite the exact product-form standard (B505, B584, B61, B139, B103) and the UNS or EN grade on the order, and require a mill or foundry certificate. Then weigh total cost: tin is expensive, so a leaded gunmetal or a lower-tin grade that still meets the duty is often the right economic answer, while under-specifying tin shortens bearing life and costs more over the service life.

One last commonly overlooked dimension is supplier serviceability and traceability: whether the foundry or mill can supply a certificate of conformance to the named ASTM or EN standard, whether continuous-cast bar is available in the diameters that minimise machining scrap, and whether the supplier holds stock of the exact grade rather than substituting a near equivalent. For long-production-run parts these factors decide repeatability and re-order lead time more than the headline price does, so they belong in the selection, not just the purchasing, stage.