If you are designing a spring contact, a snap-action switch finger, or a small bushing that cycles thousands of times per day, C51900 phosphor bronze — the 6% tin version of the Cu-Sn-P family — deserves a place on your shortlist. Among all phosphor bronze strip grades, C51900 hits a sweet spot: it delivers roughly 20–30% higher tensile strength than the softer C51000 (CuSn5) without crossing into the stiffer but less formable territory of C52100 (CuSn8). That means better load capacity per unit thickness, yet still enough ductility to survive tight-radius forming, coining, and progressive die operations.
This article walks through what C51900 actually is, where its properties come from, how it behaves under machining conditions, and what you should request from a supplier to avoid unpleasant surprises at incoming inspection.
What C51900 Is — And What It Is Not
C51900 (UNS C51900) is a wrought phosphor bronze with a nominal tin content of 6% and a controlled phosphorus residual, typically 0.03–0.35%. The Chinese near-equivalent is QSn6.5-0.1, and in the old British system it corresponds roughly to PB103. In ISO terms, the composition aligns with CuSn6 under EN 1652.
The phosphorus does two things. First, it acts as a deoxidizer during casting, scavenging oxygen and preventing cuprous oxide embrittlement — the same role it plays in the DHP (C12000) and DLP grades. Second, and more importantly for finished-part performance, the residual phosphorus forms fine Cu₃P intermetallic particles dispersed through the alpha-copper matrix. These hard particles pin grain boundaries during annealing, refine the grain structure after cold work, and contribute to the alloy’s characteristic combination of high strength and good fatigue resistance.
It is often confused with C51000 (CuSn5, 5% Sn) and C52100 (CuSn8, 8% Sn). The difference matters. C51000 is slightly more formable and costs less, but gives up roughly 50–70 MPa in tensile strength. C52100 pushes strength higher but demands larger bend radii and heavier press tonnage. C51900 sits between them, and for many spring-contact designs it is the most cost-effective grade that still meets the required spring force.
Chemical Composition per ASTM B103
| Element | Content (wt%) | Role |
|---|---|---|
| Copper + named elements | ≥ 99.5 | Base metal, conductivity carrier |
| Tin (Sn) | 5.0 – 7.0 | Solid-solution strengthener; improves wear resistance |
| Phosphorus (P) | 0.03 – 0.35 | Deoxidizer + grain-refining precipitate former |
| Lead (Pb) | ≤ 0.05 | Incidental impurity |
| Iron (Fe) | ≤ 0.10 | Incidental impurity |
| Zinc (Zn) | ≤ 0.30 | Not intentionally added |
Composition limits per ASTM B103/B103M, the standard specification for phosphor bronze plate, sheet, strip, and rolled bar. Note that C51900 is strictly a wrought grade — it is not intended for sand or investment casting, where different solidification shrinkage behavior and gating requirements apply.
Temper Designations and Property Ranges
Like all phosphor bronzes, C51900 derives most of its mechanical properties from cold work, not from quench-and-age heat treatment. The temper designation — H01, H02, H04, H06, H08, H10 — indicates progressively increasing degrees of cold reduction after the last anneal. Each step raises strength and hardness while reducing elongation.
| Temper | Tensile Strength (MPa) | Yield Strength 0.2% offset (MPa) | Elongation in 50 mm (%) | Hardness (HRB / HV) |
|---|---|---|---|---|
| H01 (¼ hard) | 380 – 450 | 220 – 310 | ≥ 32 | 60 – 80 HRB |
| H02 (½ hard) | 430 – 510 | 310 – 400 | ≥ 22 | 75 – 88 HRB |
| H04 (hard) | 510 – 590 | 420 – 500 | ≥ 12 | 85 – 94 HRB |
| H06 (extra hard) | 570 – 650 | 490 – 570 | ≥ 5 | 90 – 98 HRB |
| H08 (spring) | 630 – 700 | 550 – 640 | ≥ 2 | 95 – 102 HRB |
| H10 (extra spring) | ≥ 690 | ≥ 620 | — | ≥ 98 HRB |
Typical ranges per ASTM B103 for strip up to 0.5 mm thickness. Thicker stock and different mill practices can shift these bands. When you specify C51900 on a print, you should always pair it with a temper designation and reference ASTM B103; writing “C51900” without temper is like ordering “steel” without telling the mill whether it is annealed, Q&T, or cold-drawn.
Electrical conductivity of C51900 in the annealed condition runs approximately 15% IACS at 20°C. This is markedly below pure copper (101% IACS for C11000) or even free-cutting brass (~26% IACS for C36000). If your application needs high current-carrying capacity, C51900 is the wrong choice — at 15% IACS, resistive heating becomes significant above a few amperes in thin cross sections.
Corrosion and Environmental Behavior
Phosphor bronze resists atmospheric corrosion better than most brasses. In rural and mild industrial atmospheres, C51900 forms a thin, adherent brown-green patina that slows further attack — similar in principle to the protective tarnish on architectural copper, though the tin content tends to darken the patina toward a deeper brown. In marine atmospheres it outperforms cartridge brass (C26000) and most leaded brasses, showing roughly half the weight-loss rate in synthetic seawater spray per ASTM B117 testing literature.
In contact with fresh water at ambient temperature, C51900 is generally satisfactory. It also withstands many non-oxidizing organic acids at low concentrations and temperatures. It is not, however, a substitute for aluminum bronze (C63000 series) in strongly oxidizing acids or high-flow seawater — the absence of aluminum and nickel in the alloy means it lacks the protective alumina-based film that distinguishes true marine-grade copper alloys.
Stress-corrosion cracking under ammonia or amine environments remains a risk, as it does for all copper alloys. Unless the part has been stress-relieved after the last forming operation (typically 200–260°C for 1 hour), residual tensile stress at the surface can combine with ammonia species to produce intergranular cracking. This is especially relevant for stamped-and-formed spring contacts used near cleaning agents or agricultural atmospheres.
CNC Machining C51900: What You Need to Know
Phosphor bronze is not a free-cutting alloy. The absence of lead (C51900 carries ≤ 0.05% Pb by specification) means there is no built-in chip-breaking mechanism. At the tool tip, the material tears rather than shears cleanly, producing long, stringy, ductile chips that wrap around the tool and score the finished surface. This is the single biggest complaint machinists raise about phosphor bronze: chip control, not tool wear.
For turning operations on C51900 round bar, use a sharp-ground carbide insert with a positive rake angle (6–10°) and a polished, uncoated or thin-TiN-coated rake face. Sharpness matters more than wear resistance — a slightly dull edge work-hardens the surface layer on the first pass and makes subsequent passes progressively harder to cut. Starting parameters for a rigid CNC lathe with 12–16 mm diameter bar stock, using a CNMG 120408-style insert in grade K10 or K20 uncoated carbide:
- Cutting speed: 180–280 m/min (higher end for finish passes with low DOC)
- Feed rate: 0.05–0.15 mm/rev (finish); 0.15–0.30 mm/rev (roughing)
- Depth of cut: 0.2–0.8 mm (finish); 1.0–3.0 mm (roughing)
These are starting points, not guarantees. Machine rigidity, coolant delivery, workpiece aspect ratio, and the specific temper of the bar stock all move the envelope. If you are parting off small-diameter bushings, reduce speed by 30–40% and use a narrow (1.5–2.0 mm) parting insert with positive geometry and generous coolant flow directed at the cutting zone.
For milling operations — especially slotting or profiling thin-walled spring components from strip — climb milling with a sharp carbide end mill (2- or 3-flute for aluminum-style geometries works well on phosphor bronze) at 120–180 m/min and 0.03–0.08 mm/tooth feed. Conventional milling tends to rub before cutting, which work-hardens the surface and shortens tool life. Use a generous coolant flow, not mist, to evacuate chips from the flute gullets.
Drilling and Burr Control
Small-diameter holes in C51900 strip — typical for connector pins and PCB terminals — are a burr hazard. The alloy’s ductility means the drill tends to push a collar of deformed material ahead of the cutting edge rather than shearing it. A 135° split-point carbide drill with polished flutes, run at 60–90 m/min and 0.02–0.06 mm/rev, reduces burr height compared to standard 118° HSS drills. Backing the strip with a sacrificial aluminum or phenolic plate — rather than drilling into air — also helps the drill exit cleanly and limits exit burrs.
Do not attempt to drill C51900 dry. Even a thin film of cutting oil improves surface finish and extends drill life by a factor of 2–3 compared to dry operation, because the oil reduces the tendency of the chip to adhere to the drill margin and gall the hole wall.
Surface Finish, Deburring, and Plating
As-machined surface finish on C51900 under good conditions lands in the Ra 0.4–1.6 µm range for turning and Ra 0.8–3.2 µm for milling. Achieving Ra 0.4 consistently requires sharp tools, rigid fixturing, and short tool-path engagement — chatter marks from slender workpieces or worn spindle bearings will dominate the finish before the alloy’s intrinsic roughness becomes the limiting factor.
For stamped-and-formed parts, vibratory tumbling with ceramic media (3–6 mm angle-cut cylinders) plus a mildly acidic compound removes burrs and breaks sharp edges without measurably altering the temper-inducing cold work near the surface. Electrolytic deburring is an option for high-volume connector strips but requires careful control of current density to avoid preferential etching at grain boundaries.
C51900 accepts electroplated tin, silver, nickel, and gold without unusual preparation beyond standard alkaline cleaning and acid activation. Tin plating over phosphor bronze is common for connector contacts; the intermetallic Cu₆Sn₅ layer that forms at the interface is more stable than the Cu₃Sn layer on pure copper, reducing the risk of brittle fracture in the intermetallic zone under thermal cycling. If the part will operate above 125°C, specify a nickel underplate (1–2 µm) as a diffusion barrier.
Comparing C51900 with Related Grades
| Property | C51000 (CuSn5) | C51900 (CuSn6) | C52100 (CuSn8) | C26000 (Cartridge Brass) |
|---|---|---|---|---|
| Tin content | 4.2 – 5.8% | 5.0 – 7.0% | 7.0 – 9.0% | — |
| Max tensile (spring temper) | ~620 MPa | ~700 MPa | ~770 MPa | ~580 MPa |
| Electrical conductivity | ~18% IACS | ~15% IACS | ~13% IACS | ~28% IACS |
| Corrosion in salt spray | Good | Good | Good | Fair (dezincification risk) |
| Stress-relaxation resistance at 105°C | Moderate | Good | Best of the family | Poor |
| Minimum bend radius (spring temper, 0.3 mm strip) | ~1.0t | ~1.5t | ~2.5t | ~1.0t |
| Approx. raw material cost index | 100 | 105 | 110 | 65 |
These comparisons assume the same temper (spring/hard) and comparable strip thickness per each alloy’s published mill data. Two critical takeaways: first, C51900 provides roughly 13% higher strength than C51000 for about 5% more cost, making it the better value when the C51000 spring force comes up short. Second, do not substitute brass for phosphor bronze where sustained spring force matters — cartridge brass loses 30–40% of its initial contact force after 1,000 hours at 105°C due to stress relaxation, while C51900 retains 75–85% under the same conditions (data from typical strip-mill relaxation curves).
Applications Where C51900 Excels
- Electrical connectors and spring contacts: The 6% tin grade’s balance of conductivity and spring force makes it a standard choice for DIN 41612-style connectors, relay contact springs, and battery tabs that must maintain contact pressure over years of service.
- Snap-action switch blades: High-cycle-count switches in appliances and industrial controls use C51900 strip because it survives millions of flexural cycles without losing snap force.
- Thrust washers and bushings: In small electric motors, gear pumps, and office-equipment mechanisms, C51900 bushings provide better embeddability for abrasive particles than hardened steel while outlasting brass bushings in unlubricated or marginally lubricated service.
- Diaphragms and bellows: The alloy’s combination of strength and formability, plus good fatigue resistance, makes it suitable for thin-walled pressure-responsive elements in regulators and sensors.
- Musical instrument springs and keys: The uniform spring rate and corrosion resistance of C51900 reduce the need for key-leveling adjustments over the instrument’s life.
What to Include in Your RFQ or Drawing
When you send a C51900 part out for CNC machining or stamping, a complete specification package saves weeks of back-and-forth. At minimum, include:
- Alloy and temper: “C51900 H06 per ASTM B103” — not just “phosphor bronze.”
- Dimensional tolerances: Critical-to-function dimensions with explicit tolerance bands. Do not rely on mill tolerances for machined features.
- Surface finish requirements: Ra in µm, with the measurement direction and cutoff wavelength if surface texture is functional (sealing surfaces, sliding contacts).
- Edge condition: “Break sharp edges 0.1–0.2 mm” or “Deburr, no visible burrs under 10× magnification” — be explicit.
- Plating specification: If plated, state the plating material, thickness range, underplate requirement, and any post-plate bake for hydrogen embrittlement relief (though phosphor bronze is low-risk, baking may still be required by end-use specifications).
- Quantity and delivery: Annual volume, first-article quantity, and required lead time. Small lots of custom-machined C51900 parts cost disproportionately more because of setup time.
- Incoming inspection criteria: Which dimensions will be checked, at what sampling plan (e.g., ANSI/ASQ Z1.4, AQL 1.0), and whether material certificates (mill test reports to ASTM B103) are required with each shipment.
For stamped strip parts, also specify the rolling direction relative to the bend axis. Bending across the grain (good way) versus parallel to the grain (bad way) can be the difference between a clean 90° bend and a cracked edge at the same radius — this is especially true in H08 and H10 tempers where elongation is already low.
This article draws on ASTM B103/B103M (Standard Specification for Phosphor Bronze Plate, Sheet, Strip, and Rolled Bar), published mill datasheets for C51900 strip products, and practical CNC machining experience with phosphor bronze grades. Published: 2026-07-31.
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