Oil-Tempered vs Hard-Drawn Spring Wire: Two Ways to Make Steel Springy
Carbon steel spring wire reaches spring hardness by two different roads, and the road matters to price, consistency, and size range.
Hard-drawn (HD) wire
Strength comes from cold drawing alone (JIS SW-B/SW-C, ASTM A227; music wire is its premium cousin).
- Cheapest spring wire made
- Strength falls as diameter grows — cold work can only do so much in thick sections
- Residual stresses higher; cast and straightness livelier
- Right choice: cost-driven springs under ~2 mm, static or low-cycle duty
Oil-tempered (OT) wire
Wire is austenitized, quenched in oil, and tempered continuously (JIS SWO/SWOSC, ASTM A229/A401 for the Cr-Si variant).
- Uniform through-section properties even at 10+ mm
- Straighter, rounder, more consistent coiling behavior
- Better relaxation resistance at temperature
- Right choice: wire above ~2 mm, higher duty cycles, dimensional consistency demands
The practical decision table
| Situation | Choose |
|---|---|
| < 1 mm, cost focus | Hard-drawn |
| < 2 mm, fatigue focus | Music wire |
| 2–10 mm, general duty | Oil-tempered SWO |
| 2–10 mm, high fatigue/heat | Oil-tempered Cr-Si (SWOSC-V) |
| Corrosive environment | None of these — go stainless |
Buying advice
Let the factory pick between equivalent grades unless your standard mandates one; regional availability shifts prices week to week. What matters is stating duty, temperature, and environment on the RFQ — grade selection inside the carbon-steel family is exactly the kind of decision a manufacturing partner should carry for you.
Frequently Asked Questions
Is music wire hard-drawn or oil-tempered?
Hard-drawn — but drawn from premium rod under tighter control, which is why it out-performs generic HD wire and costs more.
Why does my thick hard-drawn spring take a set?
Above ~2 mm, hard-drawn strength and residual stress control degrade; the yield margin thins out. This is the textbook trigger to move to oil-tempered wire.
Do OT and HD wires coil differently?
Yes — OT wire runs noticeably more consistent in free length and diameter because its properties and straightness are more uniform. High-volume precision work favors OT partly for this reason.
What does "spring temper" wire mean?
It means wire supplied already at spring hardness, ready to coil without further hardening — reached either by cold work (hard-drawn) or by quench-and-temper (oil-tempered), the two routes compared above. "Spring temper" on a spec sheet just tells you the wire arrives pre-hardened, not which of the two routes produced it.