Phosphor bronze is copper and tin — bronze in the original sense — with a residue of 0.03–0.35% phosphorus left behind from deoxidising the melt. The phosphorus is not incidental: it raises the elastic limit, hardness and wear resistance, and it is why the alloy is named for it. The six unleaded grades form a clean tin ladder from 1.25% to 10%, and reading it explains the family. As tin rises, strength climbs from 517 MPa to 1014 MPa in hard wire, conductivity collapses from 48% to 11% IACS, and the freezing range widens from 39 to 156 °C — which is why hot working is rated Poor on every grade but one. These are cold-worked spring alloys: high elastic limit, excellent fatigue strength, low stress relaxation, and no dezincification because there is no zinc. C51000 is ASTM F468 Cu 510. Two leaded grades, C53400 and C54400, exist because the unleaded ones rate just 20 for machinability.
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Tin decides everything, and the six unleaded grades form a ladder worth reading as one trend. As tin rises from 1.25% to 10%, strength climbs — C50500 reaches 517 MPa at spring temper, C52400 reaches 883 MPa in strip and 1014 MPa in hard wire. Conductivity collapses — from 48% IACS and 208 W/m·K down to 11% IACS and 50 W/m·K, a fourfold drop, because tin in solid solution scatters conduction electrons severely. The solidus falls and the freezing range widens — from 39 °C at C50500 to 156 °C at C52400. That wide mushy zone causes coring and inverse segregation and is the direct reason CDA rate hot forming Poor on every grade here except C50500. These are cold-worked alloys. Thermal expansion rises too, which matters where a phosphor bronze spring works against a steel or brass housing.
The phosphorus is there because of how the alloy is melted, and it stays because it is useful. Molten copper-tin readily forms tin oxide, which would leave the metal gassy and dirty. Phosphorus is added as a deoxidiser: it scavenges oxygen, the products float off, and the melt pours clean. What remains is 0.03–0.35% in every grade here — C54400 alone is registered at 0.01–0.50%. That residue increases strength and hardness, raises the elastic limit, improves wear resistance and improves melt fluidity. It is why a phosphor bronze outperforms a plain copper-tin alloy of the same tin content as both a spring and a bearing, and why the range is tight at both ends: too little and deoxidation is incomplete, too much and the low-melting phosphide network makes the alloy hot short.
Stress relief after forming is standard practice here, and CDA publish the numbers that prove it. There is no precipitation reaction and no quench and temper — every temper on this hub is a cold-work temper, from the OS grain-size anneals through H01, H02, H04, H06, H08 and H10. The HR tempers are those same conditions after a low-temperature stress-relief anneal, and the published pattern is consistent: the same tensile strength, a somewhat lower 0.2% offset yield, and markedly higher elongation. On C51900 at extra spring temper, elongation goes from 2% to 9% at an unchanged 765 MPa. The residual tension that drives distortion in service and ammonia stress-corrosion cracking is removed at essentially no cost in strength. Specify an HR temper for formed springs, contacts and clips.
Two grades carry lead, because the unleaded ones are genuinely difficult to machine. Every unleaded grade here rates 20 on the CDA machinability scale — the same as pure copper, and the lowest figure on any of this site's copper-alloy hubs. The tough, ductile copper-tin matrix smears rather than shearing. C53400 (B-1) at 0.8–1.2% lead rates 70; C54400 (B-2) at 3.0–4.0% lead rates 80 and is the standard free-cutting bar for machined bearings and bushings, where the free lead also acts as a solid lubricant. The cost is real: CDA rate C54400 Not Recommended for every arc and resistance welding process, and lead is restricted in drinking-water components (0.25% of wetted surfaces under the US Safe Drinking Water Act) and under RoHS. If the part must be welded or will contact potable water, specify an unleaded grade.
In service, phosphor bronze has one clear advantage over brass and one specific weakness. With little or no zinc, dezincification simply does not apply — the characteristic failure mode of the brasses is off the table. The alloys resist seawater, non-oxidising acids, neutral salt solutions and marine atmospheres, they are non-sparking and essentially non-magnetic, and unlike the aluminium bronzes they solder and braze excellently because there is no refractory oxide film. The weakness is stress-corrosion cracking in ammonia, amines and nitrites when the part carries residual tension — precisely the state a cold-formed spring is in, which is the practical reason stress relief matters. Note also that phosphor bronze relaxes under sustained load above about 100 °C; a preloaded spring will lose force over time, and the higher-tin grades do better than the lower-tin ones. C51000 is ASTM F468 and F467 Cu 510, specified at 414–621 MPa with 241 MPa minimum yield and 15% minimum elongation — chosen for fatigue resistance and resilience rather than raw load.
The unleaded phosphor bronzes are hard work, and it is worth being direct about it. All six rate 20 on the CDA machinability scale — the same as pure copper, and the lowest figure on any of this site's copper-alloy hubs. The tough, ductile copper-tin matrix smears rather than shearing cleanly, producing long continuous swarf and built-up edge. When a part is machining-intensive, specify a leaded grade: C53400 at 70 and C54400 at 80 cut three to four times more freely. Where an unleaded grade is required — for weldability, for potable-water or RoHS compliance, or for spring properties — use sharp positive-rake tooling with generous chip breakers, high surface speed with light to moderate feed, flood coolant, and plan for frequent tool changes; never let the tool dwell. On the forming side the picture inverts entirely. CDA rate cold working Excellent on C50500, C51000, C51100 and C51900 and Good on the rest — these alloys blank, draw, bend, stamp, coil, upset and roll-thread superbly annealed. Hot working is Poor on every grade but C50500, because of the wide freezing range. Form cold, then stress relieve.
All composition, conductivity, machinability and cold-working data from the Copper Development Association alloy records. Tin and lead figures are read directly from the individual grade pages; the freezing range is the liquidus minus the solidus. Machinability is on the CDA scale where C36000 free-cutting brass = 100.
| Grade | Designation | Tin % | Lead % | Conductivity | Freezing Range | Machinability | Cold Working | Chosen For |
|---|---|---|---|---|---|---|---|---|
| C50500 | Phosphor Bronze 1.25% E | 1.0–1.7 | 0.05 max | 48% IACS | 39 °C | 20 | Excellent | 48% IACS — the electrical grade |
| C51000 | Phosphor Bronze 5% A | 4.2–5.8 | 0.05 max | 15% IACS | 95 °C | 20 | Excellent | ASTM F468 Cu 510; the industry standard grade |
| C51100 | Phosphor Bronze 4% | 3.5–4.9 | 0.05 max | 20% IACS | 89 °C | 20 | Excellent | Springs and contacts with 20% IACS |
| C51900 | Phosphor Bronze 6% | 5.0–7.0 | 0.05 max | 14% IACS | 111 °C | 20 | Excellent | Every temper published with its stress-relieved HR pair |
| C52100 | Phosphor Bronze 8% C | 7.0–9.0 | 0.05 max | 13% IACS | 145 °C | 20 | Good | Heavy springs; 70% elongation annealed |
| C52400 | Phosphor Bronze 10% D | 9.0–11.0 | 0.05 max | 11% IACS | 156 °C | 20 | Good | 1014 MPa in hard wire — strongest here |
| C53400 | Phosphor Bronze B-1 | 3.5–5.8 | 0.8–1.2 | 15% IACS | 95 °C | 70 | Good | Low leaded — machinability 70 |
| C54400 | Phosphor Bronze B-2 | 3.5–4.5 | 3.0–4.0 | 19% IACS | 72 °C | 80 | Good | Free-cutting bearing bar — machinability 80 |
Read the table as a tin ladder and every column moves together. Tin rises down the first six rows; conductivity falls fourfold from 48% to 11% IACS; the freezing range widens from 39 to 156 °C, which is why cold working slips from Excellent to Good and hot working is Poor on everything but C50500 — the electrical grade, and the only one here with conductivity in brass territory. C51000 is the industry standard grade and the ASTM F468 Cu 510 fastener alloy, with fifteen listed specifications. C51900 is the best-documented, publishing every temper alongside its stress-relieved HR counterpart. C52400 tops the ladder at 1014 MPa in hard wire. The last two rows break the pattern: C53400 and C54400 are the leaded free-cutting grades, at machinability 70 and 80 against 20 for everything else — C54400 also carries a unique deliberate 1.5–4.5% zinc addition and is effectively unweldable, with lead restricted in potable-water and RoHS applications. Two data gaps are shown as blanks rather than filled: CDA publish no yield strength or elongation at all for C53400, and no yield for most C52400 and C50500 tempers; we have not substituted figures from neighbouring grades.
We are committed to delivering zero-defect phosphor bronze springs, fasteners and machined components. All products undergo rigorous testing including PMI (Positive Material Identification), chemical analysis with tin, phosphorus and lead certified explicitly — tin sets the strength, conductivity and freezing range, phosphorus confirms proper deoxidation, and lead governs both machinability and regulatory acceptability under the Safe Drinking Water Act and RoHS — mechanical and hardness verification against the specified temper and section, grain-size verification on annealed tempers, stress-relief treatment to the specified HR condition, electrical conductivity testing, dimensional inspection and full traceability with EN 10204 3.1 certification.
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