CU · copper alloy
Phosphor Bronze C51000 (5% tin)
The spring bronze: good machinability with enough tin to give real strength and fatigue resistance. Chips are short and the finish is clean.
Quoting this material
- Typical lead time
- 10–15 working days
- Condition
- Annealed through to spring temper (cold worked)
- Achievable finish
- Ra 1.6 µm typical · Ra 0.4 µm best
Machined in-house to your drawing: see the precision CNC machining service and the machine and inspection capability it runs on.
Machinability
Our own 1–10 shop ranking, not a published index: 10 is free-cutting brass C360, 1 is Inconel 718. It ranks how a material behaves on the floor - cycle time, chip control and tool life together. Where a published index exists it is quoted in the note.
The spring bronze: good machinability with enough tin to give real strength and fatigue resistance. Chips are short and the finish is clean.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength | 300–600 | MPa |
| Yield strength (0.2%) | 190–450 | MPa |
| Elongation | 10–50 | % |
| Hardness | 100 | HB |
| Thermal conductivity | 62 | W/m·K |
| Modulus of elasticity | 110 | GPa |
Hardness, tensile and thermal figures are typical published values for the condition stated. Actual mill certificates govern; send the specification with the drawing and we confirm against the certificate before the material is cut.
Condition
Annealed through to spring temper (cold worked)
Heat treatment
Not heat hardenable; the temper comes from cold work. Stress relief at 200–260 °C removes residual stress from forming and machining without destroying the temper.
Achievable surface finish
Ra 1.6 µm
Ra 0.4 µm
Takes a fine finish and is often used bare where conductivity and appearance both matter.
Tooling and cutting parameters
Cutting speed
- 120–250 m/min (carbide)
Feed
- 0.10–0.30 mm/rev turning · 0.08–0.20 mm/tooth milling
Tooling
- Uncoated carbide, high positive rake
Coolant
- Flood or mist
Starting points for carbide on a rigid setup, not a guarantee. The right numbers depend on the machine, the toolholder and the fixturing - ask and we will say what we would actually run.
Typical applications
- Electrical springs and contacts
- Bushings and thrust washers
- Bellows and diaphragms
- Marine and pump hardware
- Instrument springs and flexures
DFM notes for this grade
- The spring temper is a material selection, not something achieved by machining - order the temper the part needs, and design the machining so it does not stress-relieve it away
- Stress relief is often specified after forming or heavy machining to stop a spring part from relaxing in service
- Lower conductivity than pure copper but far better strength: it is the electrical spring material rather than the conductor
Questions buyers ask
- Why is it called phosphor bronze?
- The phosphorus is added as a deoxidiser in the melt and a small residue stays in the alloy, improving casting quality and the mechanical properties. The tin, not the phosphorus, is what gives the strength and the fatigue resistance.
- Can a machined phosphor bronze part hold a spring temper?
- Yes, if the stock was ordered in that temper and the machining does not impose enough heat to relieve it. Where a part is both machined and formed, the sequence and an explicit stress relief determine whether the spring property survives.
Grades we would quote instead
C93200 Bearing Bronze
6/10The standard bearing bronze: it machines well, it is soft enough to embed grit, and it runs against a steel shaft without galling.
C17200 Beryllium Copper
5/10Machines like a hard brass in the annealed state and like a tool steel after ageing.
C65500 Silicon Bronze
6/10A tough, corrosion-resistant bronze that machines reasonably in the annealed condition and work-hardens quickly.