Beryllium Copper Spring Design Guide: C17200 Properties, Stress and Heat Treatment
Beryllium copper (BeCu) is used for spring contacts where high strength, good conductivity and fatigue resistance are needed in the same part. C17200 is one of the most common grades for stamped contacts and springs.
This guide covers the properties that matter for design, the basic spring equations and the practical limits to discuss with a stamper.
Why C17200
C17200 contains about 1.8-2.0% beryllium and is age-hardenable. In the aged condition it combines high tensile strength with conductivity typically in the range of about 22-28% IACS, depending on temper and heat treatment. It also has good fatigue resistance and resistance to stress relaxation at moderate temperatures.
Property comparison
| Property | C17200 BeCu | C5191 phosphor bronze | 301 stainless |
|---|---|---|---|
| Conductivity (typical) | ~22-28% IACS | ~13% IACS | ~2-3% IACS |
| Strength | High | Moderate | High (work-hardened) |
| Fatigue resistance | Good | Good | Moderate |
| Springback | Moderate | Moderate | High |
| Relative cost | Higher | Lower | Low |
Basic spring calculations
For a rectangular cantilever spring with an end load, the maximum bending stress and deflection can be estimated as:
- Stress: σ = 6PL / (b t²)
- Deflection: δ = 4PL³ / (E b t³)
Where P is the load, L the cantilever length, b the width, t the thickness and E the elastic modulus (about 131 GPa for C17200). Use these as starting estimates and confirm with the material supplier's data and a prototype test.
Stress and safety factor
For static contacts, a common approach is to keep the working stress below roughly 60-70% of the yield strength. For fatigue-critical springs, the allowable stress is lower and depends on the number of cycles and the stress ratio. A fatigue test at the actual working stroke is the most reliable check.
Heat treatment
C17200 is usually age-hardened after forming. Typical aging is in the range of about 300-345°C for 2-3 hours, depending on temper and the required balance of strength and conductivity. Confirm the exact cycle with the material supplier and the heat treater.
Forming and springback
BeCu springback is lower than stainless but still significant. Die compensation should be based on the actual temper and bend direction. Sharp bends and bends across the grain increase cracking risk, so a generous bend radius and a review of the bend line are recommended.
Plating and contact resistance
Gold, silver, nickel and tin plating are common. Plating affects contact resistance, solderability and corrosion. For low-resistance contacts, gold or silver over nickel is often used; the final choice depends on the mating surface and the environment.
FAQ
Is BeCu still necessary?
Not always. Phosphor bronze or beryllium-free high-conductivity alloys may be sufficient for lower-stress contacts. BeCu is chosen when strength, conductivity and fatigue life must be combined.
Can C17200 be stamped after aging?
It is usually formed in the mill-hardened or annealed condition and aged after forming. Forming fully aged material increases cracking risk.
What affects contact resistance most?
Contact force, surface finish, plating and contamination. The material conductivity matters, but the contact interface often dominates.
Related: precision metal springs and discuss a spring design.
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