Phosphor Bronze
Phosphor bronze is copper with 1–10 % tin and a trace of phosphorus left from deoxidation. Tin in solution gives a hard, fatigue-resistant, low-friction alloy that still forms well: C51000 spring-temper strip yields about 550 MPa and survives tens of millions of flex cycles, which is why most stamped spring contacts, switch blades and bellows are made from it. Strength comes entirely from cold rolling, so the temper on the drawing (H04 hard, H08 spring, H10 extra spring) matters as much as the grade.
Popular phosphor bronze grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| C51000 | 140 | 340 | 8.86 | Poor | Fair | Stamped springs and contacts; the default |
| C51900 | 450 | 520 | 8.8 | Poor | Fair | Springs with a little more stress than C51000 |
| C52100 | 585 | 660 | 8.8 | Poor | Fair | High-stress springs, bellows, wear strip |
| C52400 | 585 | 690 | 8.78 | Poor | Fair | Strongest and hardest; heavy springs, bridge plates |
| C54400 | 393 | 470 | 8.89 | Very good | Poor | Free-machining bushings and valve stems |
| C50500 | 345 | 380 | 8.89 | Poor | Good | Conductive contacts and flexible tube |
| C50700 | 505 | 550 | 8.89 | Fair | Fair | Strongest phosphor bronze here |
Choose phosphor bronze for
Stamped spring contacts, connector terminals and switch blades, diaphragms and bellows, Belleville and lock washers, wear strip and thrust washers against steel, fasteners and cotter pins that must not corrode or spark, and machined bushings in C54400.
Look elsewhere when
The spring is highly stressed or miniature (C17200 beryllium copper doubles the yield strength), the contact needs conductivity above 50 % IACS (C19400 or C17510), the part is a heavy bearing (C93200 or C95400), or a deep-drawn shell with no spring function (C26000 brass draws deeper and costs less).
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Phosphor bronze by application
Working with phosphor bronze
- Forming
- Strip is stamped and bent in the rolling temper: H04 hard bends to about 1 t across the grain, H08 spring temper needs 2–3 t and prefers bends across, not along, the rolling direction. Stress-relieve formed springs at 175–260 °C for an hour to set them and stabilize contact force.
- Machining
- Unleaded grades machine at about 20 % of C36000 with long tough chips; use sharp positive tools and chip breakers, or specify C54400, which cuts at 80 % with short chips and holds ±0.02 mm on turned bushings.
- Joining and finishing
- Soft-solders and silver-brazes easily (braze before the final temper is set, since brazing anneals). Welding is rarely done; resistance and laser welds work on thin strip. Tin, nickel, silver or gold plate for contacts; unplated phosphor bronze tarnishes to a dull brown that does not affect function.
Other copper alloy series
Phosphor Bronze — frequently asked
What is phosphor bronze?
Phosphor bronze is copper with 1–10 % tin and a trace of phosphorus left from deoxidation. Tin in solution gives a hard, fatigue-resistant, low-friction alloy that still forms well: C51000 spring-temper strip yields about 550 MPa and survives tens of millions of flex cycles, which is why most stamped spring contacts, switch blades and bellows are made from it. Strength comes entirely from cold rolling, so the temper on the drawing (H04 hard, H08 spring, H10 extra spring) matters as much as the grade.
Which grade of phosphor bronze should I use?
C51000 (5 % tin) for the ordinary spring contact, connector, washer and bellows — the stocked default. C51900 (6 %) and C52100 (8 %) when the same spring must carry more stress or last longer; C52400 (10 %) is the strongest and hardest, for heavy springs, bridge plates and wear strip. C50500 (1.25 % tin) when conductivity matters more than strength, at 48 % IACS against 15 % for C51000. C54400 has 4 % each of tin, zinc and lead and is the free-machining member for bushings, valve stems and screw-machine parts.
Which grade of phosphor bronze is the strongest?
C52100 has the highest yield strength in the series at 585 MPa; C52400 follows at 585 MPa. Values are for each grade's default condition.
Which grade of phosphor bronze is easiest to machine?
C53800 (machinability very good, 80/100), then C54400 (80/100).
Is phosphor bronze heat-treatable?
No. These grades are strengthened by cold work and softened by annealing, not by heat treatment; the conditions are listed on each grade page.
C510 or C521 for a spring?
C51000 unless the design is stress-limited: it forms better, is stocked in more tempers and yields about 550 MPa in spring temper. C52100 has 8 % tin instead of 5, raising spring-temper yield to about 650 MPa and fatigue strength by roughly 15 %, at the cost of tighter bend limits and lower conductivity. If even C52100 is marginal, the next step is C17200 beryllium copper, not more tin.
Phosphor bronze or beryllium copper for a contact?
Phosphor bronze C51000 covers most contacts: half the yield strength of C17200 but a fraction of the cost, no beryllium handling, and it forms more freely. Beryllium copper earns its place when the contact is miniature, highly stressed, operates above 100 °C (phosphor bronze relaxes) or must survive hundreds of millions of cycles. Conductivity is similar, 15 % IACS for C51000 against 22 % for C17200.