§1Why copper, and which three
Pure copper is too soft to hold a spring. Alloying fixes that — at a price paid in electrical conductivity — and three alloy families dominate spring practice.
Metallurgists spent a long time looking for something to add to copper that would give both strength and resilience. The additions always cost conductivity: take pure copper as 100 %, and a mere 5 % of phosphor-tin — the addition that makes phosphor-bronze — drops it to about 18 %. Even ordinary spring brass, at 70 % copper and 30 % zinc, keeps only about 27 %. That is still two to three times the conductivity of steel, which is the whole reason these alloys exist: a spring that must also carry current, resist corrosion or stay non-magnetic is a job steel cannot do.
Brass is copper and zinc; bronze is copper and tin. Spring practice narrows to three: spring brass, phosphor-bronze and beryllium-copper. All three are corrosion-resistant, non-magnetic, readily plated, welded, brazed and soldered, and — unusually among spring materials — excellent below freezing. All are drawn and rolled to the American Wire Gauge (identical to Brown & Sharpe); the popular round-wire range is 0.78–4.75 mm, with sizes down to 0.13 mm and up to about 13 mm available. Only one of the three, beryllium-copper, can be hardened by heat treatment; the other two reach spring temper by cold work alone.
| Property | Spring brass | Phosphor-bronze | Beryllium-copper |
|---|---|---|---|
| Modulus E (tension) | 103 400 MPa | 103 400 MPa (wire) 110 300 (strip) | 117 200 MPa (hard) 131 000 (aged) |
| Modulus G (torsion) | 34 500 MPa | 41 370 MPa (wire) 43 100 (strip) | 44 815 MPa (hard) 50 330 (aged) |
| Elastic limit (% of TS) | 75–80 tension 45–50 torsion | 75–80 tension 45–50 torsion | 10–15 tension 50–55 torsion |
| Conductivity (% of Cu) | 27–28 | 13–17 | 15–27 (heat-dependent) |
| Temperature ceiling | ≤80 °C | ≤105 °C | ≤150 °C |
| Density | 8.53 g/cm³ | 8.80–8.86 g/cm³ | 8.22 g/cm³ |
| Heat-hardenable? | No — cold work only | No — cold work only | Yes — age hardening |
§2Spring brass — cartridge brass in spring temper
The cheapest copper-base spring material, and the least demanding to make: a 70/30 brass hardened entirely by cold drawing or rolling.
Spring brass is a yellow (high) brass of roughly 70 % copper, 30 % zinc — the same alloy that, annealed, is drawn into rifle cartridge cases, which is why it is often called cartridge brass. It is cast to bars, then drawn to wire or rolled to strip; the cold work is what gives it spring temper, so reduction is deliberate: about 75–85 % reduction of area for wire, 50–60 % for strip. Because the grains elongate in the drawing direction, strip takes on strong directional properties, and bends across the grain behave differently from bends along it — a real constraint when stamping flat springs.
Ordering is by temper number, which is simply the count of American Wire Gauge sizes the metal is drawn through: brass springs are usually specified extra hard (6 numbers) or spring hard (8 numbers). It is the weakest of the three in spring quality, so it is reserved for severely cold-formed parts — wire forms and flat stampings that will not see high operating stress — and for jobs where its colour harmonises with surrounding parts, or where cost decides. It should not be used much above 66 °C (90 °C absolute maximum), but it is excellent below freezing.
| Temper | Numbers hard | Reduction, % | Wire (0.50–6.35 mm) | Strip |
|---|---|---|---|---|
| Hard | 4 | 60.5 wire / 37.1 strip | 700 MPa | 490 MPa |
| Extra hard | 6 | 75.0 / 50.2 | 790 MPa | 570 MPa |
| Spring hard | 8 | 84.4 / 60.5 | 830 MPa | 625 MPa |
| Extra spring | 10 | — / 68.7 | — | 655 MPa |
Composition is tightly held: copper 68.5–71.5 %, iron 0.05 % max, lead 0.07 % max, the remainder (about 30 %) zinc. Maximum tensile strength runs roughly 70–100 MPa above the minimums tabled. Brass cannot be hardened by heat; annealing above 260–300 °C rapidly destroys spring properties, so it is never annealed for spring use.
§3Phosphor-bronze — the workhorse
By a wide margin the most-used non-ferrous spring alloy: good conductivity, real fatigue life, and forgiving to form.
Phosphor-bronze differs from brass in carrying tin and phosphorus and little or no zinc. It is a high-copper alloy — 90–96 % — with about 4–10 % tin; the phosphorus arrives as a "phosphor-tin" deoxidiser that scavenges oxides from the melt, with a slight residue left behind as proof that deoxidation is complete. Like brass it hardens by cold work only: 75–85 % reduction for wire, 50–60 % for strip. It costs a little more than brass but may be stressed 30–50 % higher and lasts far longer in fatigue, which is why it dominates contact fingers, switches, circuit breakers and any stamped part that flexes. It is good to 66 °C at average stress and 100 °C at low stress, and — like all these alloys — excellent below zero.
Three grades cover the range, distinguished by tin content: grade A (alloy 510, 4.2–5.8 % Sn) is the standard spring wire; grade C (alloy 521, 7–9 % Sn) and grade D (alloy 524, 9–11 % Sn) are strip grades. All carry 0.03–0.35 % phosphorus and hold copper-plus-tin-plus-phosphorus to 99.5 % minimum. Wire is specified extra hard or spring hard. Modulus is 103 400 MPa in tension (wire) and 41 370 MPa in torsion; strip runs slightly stiffer. A newer superfine-grain phosphor-bronze, made by special heat treatment at the same chemistry, claims 30 % better endurance and — measured on the author's own fatigue tests — can nearly double it: 2.36 mm grade-510 wire at 884 MPa tensile survived a million deflections cycling between 187 and 444 MPa with no appreciable set.
§4Beryllium-copper — the heat-treatable one
The only copper-base spring alloy that hardens in a furnace, and the strongest by a distance — at the highest price.
Add about 2 % beryllium to copper and it behaves rather as carbon does to steel: the alloy becomes hardenable by heat treatment, reaching hardness and tensile strength the cold-worked alloys cannot approach. Beryllium-copper is roughly 98 % copper, 2 % beryllium (alloy 172, formerly grade 25); a leaner alloy 170 (1.6–1.79 % Be) is strip-only and cheaper, and a low-beryllium alloy 10 (0.4–0.7 % Be, with cobalt) trades strength for much higher conductivity. Its allowable stress and conductivity are nearly twice those of phosphor-bronze, it shows very low drift and hysteresis — prized for instruments, diaphragms and bellows — and, having little directional character, it bends to any angle regardless of grain. It keeps good spring properties to 150 °C and works below freezing.
The two-stage heat treatment is the key. The mill supplies the alloy solution-treated (supersaturated) at about 760 °C and water-quenched, in which soft state it is drawn, rolled or formed. The spring maker then precipitation-hardens the finished part — the standard cycle is 315 °C for two hours, air-cooled — throwing a fine gamma-phase precipitate that keys the slip planes and lifts strength while relieving forming stress. Distortion during ageing is real: coil springs shrink in diameter and twist, so fixturing on rods is common, and the diameter change must be allowed for in the design.
| Condition | Processing | Hardness | Tensile (min.) |
|---|---|---|---|
| Pretempered | Mill cold-drawn & aged; no further heat needed if lightly stressed | Rc 36–40 | 1207 MPa |
| ¼ hard, drawn | Coil sharply, then age 315 °C for 1–2 h | Rc 39–43 | 1310 MPa |
| Annealed | Severe forming, then age 315 °C for 2–3 h | Rc 36–40 | 1138–1240 MPa |
Strip is available in four tempers (¼, ½, ¾, full hard) in alloys 172, 170 and 10; the ½-hard temper suits diaphragms and deep drawing, ¾ hard is the general spring choice provided bend radii are three times stock thickness or more. Peak hardness varies between suppliers and demands trial heats, but for most work a close approach — the standard 315 °C cycle — is enough.
§5Temper, cold work and heat treatment
Two of the three harden only by cold work; all three benefit from a low-temperature stress relief after forming.
For spring brass and phosphor-bronze the rule is simple: hardness and tensile strength come solely from cold drawing (wire) or cold rolling (strip), quantified by the temper number — the count of gauge sizes drawn through without intermediate annealing. Neither can be hardened by heat, and annealing destroys spring properties. But after coiling or forming, a low-temperature stress relief repays itself in fatigue life and reduced season-cracking: immersion in boiling water for an hour suffices for lightly stressed parts, while highly stressed springs are held at 165–190 °C for 30–60 minutes. On phosphor-bronze this can raise a faint silvery tin bloom on the surface, which is harmless.
Beryllium-copper is the exception that proves the rule: it is solution-treated at the mill and precipitation-hardened by the spring maker at 315 °C. An overaged-and-drawn "mill-hardened" condition reaches about 90 % of the fully aged strength and needs no hardening heat, but is best kept to large-index springs (D/d over 8). Where maximum conductivity matters more than peak strength, longer ageing — up to several hours — is used, lifting conductivity toward 30–34 % of copper.
Temper numbers are not tensile grades. A "6 numbers hard" brass and a "6 numbers hard" phosphor-bronze have been through the same number of dies, but reach different strengths because they start from different alloys. Always pair the temper number with the alloy and the finished tensile requirement.
§6Joining, cleaning and corrosion
All three take common joining methods, but welding heat destroys spring temper — so on springs, joining is kept to a minimum.
Brass welds best by oxyacetylene; phosphor-bronze by carbon- or metal-arc (not oxyacetylene); beryllium-copper by carbon-arc, though no reliable technique exists for brazing thin beryllium-copper. Whatever the method, the weld heat destroys spring temper locally, so on a spring the most that is normally done is an occasional spot or tacking an end coil to a plate — and a broken spring is never welded, because it will not survive repeated deflection. Brazing uses silver alloys with borax or boracic-acid flux; all three soft-solder well, since the low, brief soldering heat does not spoil temper, and a 90 % tin / 10 % zinc solder gives strong joints.
Copper-base alloys resist atmosphere, fresh water and sea water well, and for practical purposes phosphor-bronze and beryllium-copper corrode like pure copper. Grease and dirt come off in hot alkaline solutions or solvents; tarnish is removed by a 1–7 minute sulphuric pickle (four parts water to about half-to-one part acid, 38–70 °C), and a glossy finish by a bright dip. What they must not touch: ammonia, ammonium hydroxide, cyanide, ferric or mercuric salts, oxidising acids, and sulphur or chlorine gas. Ammonia is the specific enemy of high-zinc brass, which is prone to a stress-corrosion failure called season cracking — the reason a low-temperature stress relief after forming matters. Phosphor-bronze and beryllium-copper are practically immune to it.
§7Choosing between the three
Cost rises with capability. Match the alloy to the demand and no further.
Reach for spring brass when the part is a severely formed stamping or wire form at modest stress, when colour matters, or when cost governs and neither high stress nor long fatigue life is asked for. Reach for phosphor-bronze — the default — when the part flexes repeatedly, carries current, and needs real fatigue life: it is the most versatile and most available. Reach for beryllium-copper when the job demands the highest stress and longest life a copper alloy can give, or the low drift and hysteresis of a precision instrument spring, and the budget allows the highest-cost material and a hardening heat treatment.
A handful of further copper-base alloys — nickel-silver (a copper-nickel-zinc alloy that whitens to a silver colour, good for switch contact fingers at 18 % nickel), cupro-nickel, silicon-bronze, aluminium-bronze and manganese-bronze — serve occasional or special-purpose roles. None is as readily available as the main three, and their properties, cost and stock sizes should be checked before they are specified.
§8Quick reference
Cheapest, weakest
70Cu · 30Zn, cold-worked to extra/spring hard. TS to 830 MPa (wire). Conductivity ≈28 %, ≤80 °C. Season-crack risk near ammonia — stress-relieve after forming. Formed stampings at modest stress only.
The default
90–96Cu with 4–10Sn, grades A/C/D. TS to 1000 MPa, may be stressed 30–50 % above brass with far longer life. ≤105 °C. Not heat-hardenable; stress-relieve 165–190 °C.
Strongest, dearest
≈98Cu · 2Be, alloy 172. Solution-treat at mill, age 315 °C · 2 h. TS to ≈1585 MPa, low drift/hysteresis, ≤150 °C. Allow for shrink and twist during ageing.