§1When plain carbon is not enough
Alloy spring steels buy three things carbon steel cannot deliver together: resistance to shock, a wider working-temperature window in both directions, and higher usable hardness.
They come at two costs. The first is money. The second is availability: spring makers do not stock these wires in depth, so material is ordered against the job, small quantities price poorly, and lead times run to weeks. Plan procurement early or the alloy advantage disappears into the schedule.
Supply spans wire from about 0.5 mm through 13 mm in the annealed, untempered (cold-drawn) or oil-tempered condition — round as standard, square and rectangular by arrangement — plus annealed bar to 50 mm and beyond for the hot-coiled and leaf springs of Sheet 4. Condition matters: annealed wire is for heavy sections and coil OD-to-wire ratios of 5 or less; untempered cold-drawn wire coils with more even pitch on automatic machines; both need hardening and tempering after forming. Oil-tempered wire arrives at final strength.
§2Chromium-vanadium — A231/A232, SAE 6150
The classic shock alloy: about one per cent chromium for depth of hardening and strength, a pinch of vanadium for fine grain and fatigue life.
ASTM A231 and SAE 6150 are the same composition — carbon 0.48–0.53 %, manganese 0.70–0.90 %, chromium 0.80–1.10 %, vanadium 0.15 % minimum (about 0.18 % is preferred), silicon 0.20–0.35 %. Electric-furnace heats hold phosphorus and sulphur to 0.025 % maximum. The vanadium earns its keep twice: it refines and pins the grain — insurance against accidental overheating in the hardening shop — and, working with chromium, measurably extends fatigue and endurance life.
This is the wire for impact duty — pneumatic hammers are the textbook case — for die springs in round, square and rounded-edge rectangular sections, and for moderately elevated temperatures up to about 218 °C. Sizes run 0.5–13 mm in wire, with bar to 50 mm and larger for hot-rolled work. Its valve-quality twin, ASTM A232, tightens phosphorus to 0.020 % and narrows silicon; it is supplied hardened and tempered like oil-tempered wire and serves aircraft, racing and marine engines where plain-carbon A230 runs out of fatigue headroom.
§3Chromium-silicon — ASTM A401
Born in England for anti-aircraft recoil and torpedo control springs, chromium-silicon holds more hardness without embrittling than any other spring steel — HRC 50–53 is routine.
The recipe reads like silico-manganese with the dials nudged — a little less silicon, a little more chromium: carbon 0.51–0.59 %, manganese 0.60–0.80 %, chromium 0.60–0.80 %, silicon 1.20–1.60 %. Those nudges produce deeper, more uniform hardening and distinctly higher mechanical properties. Service extends to about 246 °C, the highest of the ferrous spring wires short of stainless and high-speed steel.
Two product forms matter. Oil-tempered wire, 0.80–11 mm, looks and handles like chromium-vanadium and is clean enough for valve springs. A hard-drawn variant, 0.5–5 mm, behaves like music wire on the coiler: free of decarburisation and scale, smooth-surfaced, platable without an acid dip — at the price of a 5 mm size ceiling. Sections above about 9.5 mm are hot-rolled rather than cold-coiled.
§4Design properties
The headline advantage over carbon wire is not the modulus — it is how much of the tensile strength the elastic limit lets you use.
| Property | Cr-V · A231/6150 | Cr-V valve · A232 | Cr-Si · A401 |
|---|---|---|---|
| E, tension | 203.4 GPa | 203.4 GPa | 203.4 GPa |
| G, torsion | 77.2 GPa | 77.2 GPa | 77.2 GPa |
| Elastic limit, tension | 88–93 % of tensile | 88–93 % | 88–93 % |
| Elastic limit, torsion | 65–75 % | 65–75 % | 65–75 % |
| Hardness, HRC | 45–50 | 46–51 | 50–53 (oil-tempered) |
| Service ceiling | 218 °C | 218 °C | 246 °C |
| Wire sizes | 0.5–13 mm (+ bar) | 0.5–12 mm | 0.8–11 mm OT · 0.5–5 mm HD |
Compare the torsional elastic limit — 65–75 % of tensile against 45–50 % for music wire — and the alloy premium starts paying for itself: a compression spring can run markedly higher working stress before set. Density, wrap-test requirements and electrical conductivity match the carbon wires (7.85 g/cm³; wire to 4 mm winds on itself, larger to 8 mm on a two-diameter arbor).
§5Heat treatment — and the Cr-Si cracking rule
Schedules follow the carbon-wire pattern at higher temperatures — with one rule unique to chromium-silicon that is written in broken springs.
| Operation | Cr-V · A231 | Cr-V valve · A232 | Cr-Si · A401 |
|---|---|---|---|
| Harden (annealed stock) | 871–899, soak 8–10 min light / 15–20 min heavy, oil quench | 871–899, soak ≈15 min, oil | oil quench from above critical; mill schedule |
| Temper | 399–510 · ¾–1½ h | 399–482 · ¾–1 h | 371–454 · 30–60 min |
| Stress-equalise — general | 232–260 | 232–260 | 260–288 |
| — severe service | 288–316 | 288–316 | 316–343 |
| — high temperature | 343–371 | 343–371 | 371–399 |
Coiled chromium-silicon springs must go to the stress-equalising oven promptly — a delay of three to four hours can already be too long, and unheated springs must never sit overnight. The combination of very high hardness, high elastic limit and coiling residuals can crack or outright shatter springs left waiting. Some mills specify 371–399 °C for 30 minutes; the schedule above is common practice. Build the oven into the coiling cell, not the next shift.
Tempering-time tables mirror the oil-tempered wire pattern — roughly 15–20 minutes for fine wire in general service, stretching to 60–90 minutes for the heaviest sections in high-temperature service.
§6Choosing between the alloys
Both alloys out-tough carbon steel. The split between them is temperature and hardness against forgiveness in processing.
Shock, dies, valve fatigue
Impact and suddenly applied loads, die springs in shaped sections, aircraft and racing valve springs (A232). Forgiving to heat-treat, grain-safe when the furnace runs hot, proven to 218 °C.
Highest stress, hottest duty
Recoil-class energy storage, maximum working stress in minimum envelope, service to 246 °C. HRC 50–53 without brittleness — provided the prompt-heating rule is engineered into the process, not left to memory.
Where neither fits — bigger bars, leaf stacks, hot coiling — the hot-rolled alloy bars of the next sheet continue the same chemistry families at structural scale.
§7Quick reference
A231 / SAE 6150
C 0.48–0.53 · Cr 0.80–1.10 · V ≥0.15. HRC 45–50, 218 °C, shock-rated. Valve twin A232 with P ≤0.020 %.
ASTM A401
C 0.51–0.59 · Cr 0.60–0.80 · Si 1.20–1.60. HRC 50–53, 246 °C. Hard-drawn variant to 5 mm. Heat within hours of coiling.
Design numbers
E 203.4 GPa · G 77.2 GPa · elastic limit 88–93 % tension, 65–75 % torsion · density 7.85 g/cm³ · special-order material, weeks of lead time.