§1The family at a glance
Plain high-carbon wire is the default answer to most spring problems — stocked everywhere, drawn to gauge, decimal and metric sizes, and (music wire aside) the cheapest spring material there is.
All four families below are cold-coiled only and share the same iron-carbon backbone; they differ in how their strength is put in. The hard-process wires — music and hard-drawn — arrive already at full strength from cold work and are merely stress-relieved after coiling. The oil-tempered and valve wires get their strength from a mill heat treatment, so their properties are more uniform along the coil and less sensitive to size. Economy favours this family whenever it can do the job; the alloy steels of Sheet 3 exist for the cases where it cannot.
§2Music wire — ASTM A228
The aristocrat of spring wire: the highest tensile strength and elastic limit of the carbon grades, in a bright tin-flashed finish inherited from the piano trade.
Music wire is made by the hard process — patented rod, cold-drawn through progressively finer carbide (and for the finest sizes, diamond) dies — with a thin tin coating picked up before the last pass. The coating is cosmetic and a plating aid rather than corrosion protection. The tradition is Swedish: exceptionally pure ores and small, careful mills set the quality benchmark that domestic producers matched only after wartime necessity forced the issue. It is supplied round only, essentially never annealed, in every three-place decimal size from about 0.10 mm (a violin E string) up to 5 mm in 0.025 mm steps; the popular band runs 0.10–3.17 mm.
Nearly all heats carry 0.80–0.90 % carbon (the specification band is wider — order a ten-point range for consistency) with manganese held low at 0.20–0.60 %. The ceiling on service is thermal: keep springs at or below 121 °C, where a load loss of about 5 % already appears at 621 MPa of stress, worsening rapidly above.
Drawing to an exact tensile value is genuinely hard — holding ±69 MPa in fine sizes is exceptional work — so a 0.76 mm wire aimed at 2482 MPa may legitimately arrive anywhere from 2344 to 2620 MPa. Treat that spread as three usable grades: the low range bends best (torsion and extension springs with sharp hooks, coil OD-to-wire ratios under 5), the middle range suits general work, and the high range gives the longest fatigue life in heavily stressed compression springs with index above 7. Specify which — or state the tensile window on the drawing and require torsional testing to ASTM E558 for important springs.
One quiet bonus: all hard-drawn steels, music wire included, pick up 2–3 points HRC of "secondary hardness" during the post-coiling bake, lifting tensile strength and elastic limit slightly rather than softening the spring.
§3Hard-drawn grades — A227, A679, upholstery
Where cost rules and life requirements are modest, hard-drawn wire is the least expensive spring material made.
MB hard-drawn
The general-purpose grade — usually the same chemistry as oil-tempered A229 but finished by cold drawing. Class II serves ordinary compression and extension springs in appliances, hardware, automotive fittings and toys; Class I is the choice for torsion springs and sharp bends. Improving uniformity keeps widening its territory, but where load accuracy and long fatigue life matter, step up.
HB high-tensile
Higher carbon, drawn to tensile levels close to commercial music wire — think of it as budget music wire at roughly half the price. Few mills roll it, so specify it only when the order justifies several hundred kilograms. Sits neatly between oil-tempered MB and music wire in stress capability.
Upholstery — knotting
Ductile enough to knot without fracture; carbon 0.45–0.70 %, six tensile grades, diameters 0.884–4.115 mm only. For furniture springs, hooks and wire forms — never for machine springs needing accurate loads.
Upholstery — zig-zag
For zig-zag, square-formed and no-sag furniture and vehicle-seat springs; carbon 0.50–0.75 %, five tensile grades, 2.324–4.877 mm. Not a mechanical-spring material.
An ordering discipline worth copying from the wire mills: hold any one purchase to a 13-point carbon window — say 0.50–0.63 % for wire under 2.5 mm and 0.60–0.73 % for larger — rather than accepting the full specification spread. Uniform carbon is uniform springs.
§4Oil-tempered grades — A229, SAE 1065/1080
Mill-hardened and mill-tempered in one continuous pass, oil-tempered wire is the standard choice from about 3 mm to 13 mm — the dull grey workhorse of machine springs.
The wire is cold-drawn to size from annealed rod, then pulled through a lead or furnace line: heat above the critical range, quench in oil, re-draw through a tempering bath. Because strength comes from heat treatment rather than accumulated cold work, properties are consistent and shape options broaden — square and rectangular sections are routine. The MB grade (ASTM A229) covers over 90 % of use; the HB grade (SAE 1080) buys more tensile strength when stresses climb, though by that point the alloy steels often earn their premium. Annealed stock exists for sharp bends and coil OD-to-wire ratios of 5 or less — remembering that extension springs coiled from annealed wire cannot hold initial tension.
Limits to respect: no impact or shock duty, service at or below 177 °C, and caution below freezing. The light heat-treating scale should be blasted or tumbled off before electroplating.
| Element | MB · ASTM A229 | SAE 1065 | HB · SAE 1080 | Valve · ASTM A230 |
|---|---|---|---|---|
| Carbon | 0.55–0.85 | 0.60–0.70 | 0.75–0.88 | 0.60–0.75 |
| Manganese | 0.60–1.20 | 0.60–0.90 | 0.60–0.90 | 0.60–0.90 |
| Silicon | 0.10–0.35 | 0.15–0.30 | 0.15–0.30 | 0.15–0.35 |
| Phosphorus max | 0.040 | 0.040 | 0.040 | 0.025 |
| Sulphur max | 0.050 | 0.050 | 0.050 | 0.030 |
Note the pattern in the last column: valve quality is largely a matter of what is kept out. Within any one lot, carbon should not spread more than 0.13 %; manganese runs 0.60–0.90 % up to about 5 mm wire and 0.80–1.20 % above.
§5Valve-spring quality — ASTM A230
Engine valve springs live with surge, vibration and heat, and a single surface flaw ends them. Valve-quality wire is ordinary chemistry produced to extraordinary cleanliness.
A230 is the premium plain-carbon round wire: smooth, scale-free, tightly inspected, supplied hardened and tempered in 1.57–6.25 mm diameters (other sizes by mill agreement). It is the least expensive material fit for engine valve duty, good to 177 °C, and the default in automotive practice; the chromium-vanadium A232 of Sheet 3 takes over in aircraft and racing service. Design detail does part of the job too — most valve springs close the pitch of the last two or three coils at one end so the natural frequency ramps, damping surge. The valve-spring failures endemic to 1920s motoring were engineered away by exactly three things: better steel selection, better design, and shot-peening.
The English school reached similar quality by the hard process: spheroidise the rod near 750 °C, patent from 1000 °C into a bath near 540 °C (or air-jet quench at 124–241 kPa), pickle, then draw down about 60 % in area, belt-polish and copper-flash before a final sizing pass. Either route, the specification intent is identical — a fatigue surface without excuses.
§6Properties, moduli and elastic limits
Two numbers drive every spring calculation — E for torsion springs, G for compression and extension — and for hard-drawn products they change with wire size.
| Property | Music A228 | Oil-tempered A229/1080 | Valve A230 |
|---|---|---|---|
| E, tension | 193.1–203.4 GPa by band (chart) | 196.5 GPa | 203.4 GPa |
| G, torsion | 80.0–82.7 GPa by band | 77.2 GPa | 77.2 GPa |
| Elastic limit, tension | 65–75 % of tensile | 85–90 % (≤6.35 mm); 80–85 % above | 85–90 % |
| Elastic limit, torsion | 45–50 % | 45–50 % (≤6.35 mm); 40–45 % above | 50–60 % |
| Hardness, HRC | 42–46 | 45–50 (≤3.18 mm) grading to 40–45 (>6.35 mm) | 44–48 |
| Service ceiling | 121 °C | 177 °C | 177 °C |
| Density | 7.85 g/cm³ (0.284 lb/in³) — all grades; electrical conductivity 8–12 % of copper | ||
Ductility is checked the mill's way: wire to 4 mm must wind on itself as its own arbor without surface cracking (larger sizes, an arbor of twice the diameter), and music wire must additionally survive a close-wound extension coil being yanked from 127 mm out to a 380 mm set with clean, even pitch.
§7Heat treatment after coiling
Every cold-coiled spring goes through a low-temperature bake — stress-equalising — before it ships. The band chosen sets the spring's temperature credentials.
| Service class | Music A228 | Oil-tempered A229 · Valve A230 | Typical time |
|---|---|---|---|
| General | 204–216 | 216–232 | 10–30 min by size |
| Severe | 232–260 | 260–288 | 20–50 min |
| High-temperature | 274–288 | 316–343 | up to 60–90 min |
Times scale with wire size — roughly 10–15 minutes for wire under 0.38 mm out to 60–90 minutes for the heaviest oil-tempered sections. Springs coiled from annealed stock are a different matter: they must be through-hardened after forming — oil-tempered grades at 802–843 °C into oil (soak 4–8 minutes under 6.35 mm, 8–12 above) then tempered at 316–371 °C for 30–60 minutes; valve A230 at 802–829 °C, tempered 288–371 °C.
A 1.2 mm compression spring cycling fast at 90 °C in a latch: music wire, high-tensile range, severe-service bake — the secondary-hardness bonus is free margin. The same duty at 4.8 mm and 150 °C: oil-tempered A229 MB, high-temperature bake. Add engine surge and 170 °C: A230, shot-peened, close-pitched end coils.
§8Quick reference
Which wire
Music A228 — small, highly stressed, best fatigue. A227/A679 — cost first. A229 — 3–13 mm mainstream. A230 — valve duty and surge.
Carry these
E 193–203 GPa (by band) · G 77–83 GPa · density 7.85 g/cm³ · music ceiling 121 °C, oil-tempered 177 °C · HRC 42–50.
Do not forget
Annealed-wire extension springs hold no initial tension. Oil-tempered wire dislikes shock and freezing. Order 13-point carbon and, for critical work, a stated tensile window with E558 torsion tests.