# UNS C51000 bronze

Specify 5% tin phosphor bronze by form and temper; interpret spring, fatigue, conductivity and fabrication data with their stated conditions.

Reviewed: 2026-11-01

Tags: materials, bronze, copper-alloys, c51000, phosphor-bronze

## Identity and purchasing scope

Concast lists **15% IACS at 20°C** for **C51000** containing **5% tin and 0.2% phosphorus**. KME gives **10 MS/m (about 17% IACS)** as **typical at 20°C in the annealed condition**. Keep the composition and temper bases with these figures and agree the conductivity test for the delivered stock. [Concast physical-property footnote](https://www.concast.com/files/C51000.pdf), [KME typical annealed values](https://www.kme.com/fileadmin/DOWNLOADCENTER/COPPER%20DIVISON/4%20Industrial%20Rolled/3_Datasheets/Datasheets_NEW_2021/kme_CuSn5_en.pdf)

**UNS C51000** is commonly identified as **phosphor bronze 5% A**. Concast's ASTM B139/B139M-12(2017)-based sheet lists **Sn 4.20–5.80%, P 0.03–0.35%, Pb maximum 0.05%, Zn maximum 0.30%, Fe maximum 0.10%, copper remainder**, with copper plus named elements at least **99.5%**. “5% tin” is nominal shorthand rather than an exact composition requirement. [Concast C51000 chemistry, p. 1](https://www.concast.com/files/C51000.pdf)

KME cross-references **CuSn5, CW451K and C51000** for its rolled material. Use such references to locate candidate products, then confirm the required standard, form and delivered condition. A cross-reference does not establish that every heat or product meets all requirements of a particular purchase specification. [KME CuSn5 sheet](https://www.kme.com/fileadmin/DOWNLOADCENTER/COPPER%20DIVISON/4%20Industrial%20Rolled/3_Datasheets/Datasheets_NEW_2021/kme_CuSn5_en.pdf)

## Temper controls the strength/forming compromise

KME's **EN 1652 rolled-product** table lists these tensile intervals:

| Condition | Tensile strength Rm |
| --- | --- |
| R400 | 400–500 MPa |
| R490 | 490–580 MPa |
| R630 | 630–720 MPa |

Its table separately distinguishes standard and bending-optimized properties and gives bend guidance for strip thickness **≤0.50 mm**. Preserve those qualifiers rather than treating a radius or elongation entry as universal. [KME mechanical and bend table](https://www.kme.com/fileadmin/DOWNLOADCENTER/COPPER%20DIVISON/4%20Industrial%20Rolled/3_Datasheets/Datasheets_NEW_2021/kme_CuSn5_en.pdf)

Select the delivered condition around the actual forming sequence and required finished force. Establish bend direction, radii, springback, burr direction and residual set in representative parts. Do not translate an R-condition to an H-condition using only a similar strength number. The applicable standard, product form and supplier agreement determine acceptance.

For mechanical design, keep tensile strength, proof/yield strength, hardness and elongation separate. A hardness check can support a defined acceptance plan; it does not automatically establish all mechanical properties or a finished spring's load-deflection curve. Define the completed part's allowable deflection, load, permanent set and dimensional requirements.

## Fatigue and stress relaxation answer different questions

Wieland's **B15 CuSn5/C51000** sheet describes fatigue strength in terms of **10⁷ cycles under symmetrical alternating bending**, dependent on temper, and gives an approximate relationship to tensile strength. This is a bounded material description, not an infinite-life limit or an arbitrary spring's fatigue qualification. [Wieland B15 fatigue definition, p. 2](https://www.wieland.com/en/content/download/9884/file/B15_global.pdf?inLanguage=eng-GB)

For a cyclic component, establish stress amplitude, mean stress, geometry, finish, environment, temperature and target life. Evaluate the actual feature, including bends, edges and attachment regions. A catalog tensile minimum cannot determine fatigue life without an appropriate design basis. Do not turn a symmetric bending result into a contact-force or vibration guarantee for a different load history.

The same Wieland sheet presents thermal stress relaxation measured on **stress-relief-annealed specimens parallel to rolling direction**. It states that relaxation depends on applied stress and is increased to some extent by cold deformation. [Wieland relaxation conditions](https://www.wieland.com/en/content/download/9884/file/B15_global.pdf?inLanguage=eng-GB)

Keep this time-at-temperature behavior separate from cyclic failure. For a contact spring, define required retained force after the intended dwell, thermal cycles and assembly deflection. Test representative formed and finished parts when their processing differs from the supplier's specimen condition. A favorable short test or room-temperature spring force does not establish long-term contact performance.

## Conductivity: retain composition and condition

Concast lists **15% IACS at 20°C**, explicitly measured on an alloy containing **5% tin and 0.2% phosphorus**, and warns that conductivity varies with composition. [Concast physical-property footnote, p. 4](https://www.concast.com/files/C51000.pdf)

KME instead gives **10 MS/m, approximately 17% IACS**, as typical **annealed values at 20°C**. Neither entry should be silently converted into a guaranteed minimum for every delivered C51000 temper. [KME physical-property conditions](https://www.kme.com/fileadmin/DOWNLOADCENTER/COPPER%20DIVISON/4%20Industrial%20Rolled/3_Datasheets/Datasheets_NEW_2021/kme_CuSn5_en.pdf)

As an original calculation using KME's rounded conductivity, a uniform conductor **100 mm long** with **1 mm² cross-section** has `R = L/(σA) = 0.1/(10 × 10⁶ × 10⁻⁶) = 0.01 Ω`, or **10 mΩ**, at the assumed material condition and temperature. This excludes contacts, plating and temperature rise. It is a bulk-resistance illustration, not a current rating or predicted assembled connector resistance.

## Fabrication, joining and release

Concast rates C51000 cold-working capacity **excellent**, hot-forming capacity **poor**, and machining at **20**; its soldering and brazing ratings are **excellent**. These are comparative fabrication descriptions, not process parameters or joint-strength guarantees. [Concast fabrication table, p. 4](https://www.concast.com/files/C51000.pdf)

Qualify machining, stamping and deburring on the actual stock condition. Define edge quality and surface damage limits at highly stressed features. For joining, specify cleaning, filler/interlayer where applicable, thermal history and inspection. Determine whether heating alters the condition or force of nearby spring regions. Parent-stock properties do not establish finished-joint capacity.

Define corrosion exposure by substance, concentration, temperature, duration and contact conditions; “bronze” is not a blanket compatibility approval. For plated contacts, qualify the coating and substrate together. Retain grade, supplier/lot, form, temper, drawing revision and required inspection or test results. Review substitutions against mechanical, electrical and processing requirements together.

Sources reviewed November 1, 2026. Wieland's PDF prints **06.21**. The cited KME and Concast PDFs provide no explicit printed publication/revision date; their filenames and website upload paths are not assigned as dates. Concast identifies its specific **ASTM B139/B139M-12(2017)** table basis; select the contract-required edition for actual procurement.

## Related reference

[Grades, conditions and material selection](../grades-tempers-and-conditions.md)