Spring Heat Treatment: Hardening and Tempering Wire
Short answer: heat treatment makes a spring a spring — but the process differs completely by how the spring was formed. Cold-wound springs are made from wire that is already hardened, so they need only a low-temperature stress relief, typically 200-400°C depending on grade, to remove coiling stresses and stabilize the load curve. Hot-wound springs are coiled from soft bar or wire heated to roughly 850-950°C, then oil-quenched and tempered to build their strength from scratch. Get the temperature wrong in either direction and the spring is either brittle, soft, or decarburized — so time and temperature control is the heart of spring manufacturing.
Every spring failure that is not a design failure is usually a heat treatment failure in disguise: a music wire spring that snapped because it was stress-relieved too hot, a chrome-vanadium spring that relaxed because it was tempered too low, a hot-wound spring that cracked because it was quenched from the wrong temperature. Heat treatment is where raw wire becomes a reliable component, and understanding the two regimes — stress relief for cold-wound work and full quench-and-temper for hot-wound work — is what separates a spring that holds its curve from one that drifts after a few thousand cycles. This guide walks through both regimes, the temperature bands by wire grade, and the defects that wrong heat treatment leaves behind.
Cold-Wound Springs: Stress Relief, Not Hardening
Most springs under roughly 8-10 mm wire diameter are cold-wound: the wire is drawn, hardened, and tempered by the wire mill, and the spring maker coils it at room temperature. The coiling operation bends the wire past its yield point, which leaves residual stresses locked into the spring and makes it brittle in the as-coiled condition. The low-temperature stress relief that follows — sometimes called drawing or tempering in the shop — relaxes those stresses without dropping the wire's hardness, and it does three jobs: it stabilizes the free height and load curve so the spring does not drift in service, it restores ductility so the spring does not crack at the heavily bent inner fibers, and it sets the spring for the presetting or scragging operation that follows. Typical soak times run 20-60 minutes at temperature, in air or a protective atmosphere depending on the grade.
| Wire grade | Typical stress-relief range | Notes |
|---|---|---|
| Music wire (A228) | ~200-260°C | Keep low; over-tempering cuts tensile |
| Hard-drawn carbon (A227) | ~200-260°C | Similar behavior to music wire |
| Oil-tempered carbon (A229) | ~260-320°C | Higher range safe after oil tempering |
| Chrome-vanadium (A232) | ~350-430°C | Alloy grades tolerate hotter relief |
| Chrome-silicon (A401) | ~370-450°C | High-temperature cold-wound alloy |
| 302/304 stainless | ~260-400°C | Improves fatigue and corrosion resistance |
The temperature bands above are typical production guidance, and the right point within each band depends on the exact wire lot and the load tolerance required. The governing rule is simple: high enough to relieve stress, low enough to keep strength. Music wire is the sensitive case — it is already at maximum strength from drawing, so stress relief above roughly 300°C starts to lower its tensile, and a spring maker that runs a generic 400°C oven for all wires will quietly weaken every music wire spring it makes. The deeper selection logic between drawn, oil-tempered, and alloy wires belongs to material choice; our spring material selection guide covers which wire starts life with which strength so the heat treatment question has a sensible baseline.
Hot-Wound Springs: Quench and Temper
Above roughly 8-10 mm wire diameter, and for many heavy-duty springs, cold coiling is not practical: the wire is too stiff to wind accurately. Hot-wound springs are coiled from material heated to around 850-950°C, where the steel is soft enough to form, then the spring is quenched — typically in oil — to harden it, and finally tempered to set the strength and toughness balance. This is full heat treatment in the classic sense: austenitize by heating, quench to form martensite, temper to remove brittleness. The tempering range for common spring steels generally falls around 350-550°C depending on the grade and the hardness target, and the exact schedule is chosen to land the spring in its specified tensile band with enough toughness for fatigue.
Hot winding changes the rules of the game. The spring is formed hot, so there is no "coiling stress" to relieve — instead, everything depends on the uniformity of the quench and the accuracy of the temper. Decarburization becomes the enemy: at 850-950°C the surface of the steel reacts with oxygen, and losing carbon from the surface layer leaves soft, weak skin that starts fatigue cracks. Hot-wound springs therefore demand controlled atmospheres, and critical springs are ground or machined after heat treatment to remove any decarburized layer. If your application is in this size range, the choice between hot and cold winding changes the whole manufacturing conversation — our hot-wound vs cold-wound springs guide lays out the crossover point and cost logic in detail.
Process Order: Where Heat Treatment Fits
Heat treatment is not one step; it is several steps placed at specific points in the route, and the order protects the work done by earlier operations.
| Step | Purpose | Typical position |
|---|---|---|
| Coil (cold) or wind (hot) | Form geometry | Start |
| Stress relief (cold-wound) | Stabilize, restore ductility | After coiling |
| Quench + temper (hot-wound) | Develop strength | After hot winding |
| End grinding | Bearing faces | After stress relief / temper |
| Preset (scragging) | Set the working curve | After grinding |
| Shot peening (if specified) | Fatigue strength | After preset |
| Final low-temperature relief | Relax peening stress | After peening |
For a cold-wound compression spring the skeleton of the route is: coil, stress relieve, grind ends if specified, preset to solid height, optionally shot peen, then a final low-temperature relief. Each step leaves residual stress that the next step manages; skipping the stress relief before grinding, for example, lets the ground spring relax into distortion afterwards. This sequence, with the temperature bands applied per grade, is exactly what runs on a spring line making parts from 0.1 mm to 8 mm wire, and it is why a spring drawing should state the wire grade and any heat treatment requirement rather than leaving the factory to guess. For high-temperature and corrosion service the picture extends beyond plain carbon and alloy steel — stainless and nickel grades follow their own rules, summarized in our high-temperature spring materials guide.
Defects When Heat Treatment Goes Wrong
Wrong time-temperature combinations leave recognizable damage, and each defect has a mechanical consequence. Under-tempering (or skipping stress relief) leaves the spring hard but brittle with high residual stress, so it cracks early, often at the inner coil surface where bending stress concentrates. Over-tempering softens the wire, so the spring takes a permanent set, relaxes under load, and loses its specified force. Decarburization — from heating without atmosphere control — removes carbon from the surface and destroys fatigue life even when the core looks perfect, because cracks start in the weak surface skin. Overheating in grinding after heat treatment produces the same local burn damage at the end coils, which is why grinding burns and heat treatment defects show up in the same failure mode: early cracking at a specific location. Each of these is detectable: hardness testing, microstructure checks, and load testing after presetting all catch bad heat treatment before it ships, which is why a spring supplier's quality system — and its willingness to share process records — is part of the buying decision.
Specifying Heat Treatment on the Drawing
A spring drawing should not leave heat treatment to chance. State the wire grade and its standard — music wire per ASTM A228, oil-tempered per A229, chrome-vanadium per A232 — because the grade sets the stress-relief window, and add any special requirement such as presetting, shot peening, or passivation for stainless. If the application is temperature-sensitive or fatigue-critical, say so and name the service temperature and cycle count; that information lets the factory pick the safe end of the temperature band and add the right surface treatment instead of shipping a generic schedule. For hot-wound springs, specify hardness or tensile range, and ask for confirmation that decarburization is controlled — for critical parts, that means a controlled atmosphere and a post-treatment grind or a surface check.
Before production, ask the supplier three questions that separate a real process from a best-guess oven: what temperature and soak time will run for your grade, how the oven temperature is controlled and recorded, and what checks confirm the result — hardness testing, load testing after presetting, or both. A supplier that answers with numbers and offers batch records is treating heat treatment as an engineering step; one that answers with vague assurances is a reason to look further. On delivery, the load test at your working height is the final proof that the heat treatment did its job, because a correctly treated spring holds its specified force, while an over-tempered or under-relieved one drifts on the tester bench before it ever reaches your assembly.
Frequently Asked Questions
Q: What temperature are springs tempered at?
A: Cold-wound springs get a low-temperature stress relief, typically 200-260°C for music wire, 260-320°C for oil-tempered carbon wire, and roughly 350-450°C for chrome-vanadium and chrome-silicon alloys. Hot-wound springs are quenched and tempered as a full heat treatment, with tempering typically in the 350-550°C band depending on grade and target hardness.
Q: What is the difference between stress relief and tempering for springs?
A: Stress relief is the low-temperature treatment given to cold-wound springs made from already-hardened wire; it relaxes coiling stresses and stabilizes the spring without reducing strength much. Tempering is part of full quench-and-temper heat treatment used on hot-wound springs, where it sets the final strength and toughness after hardening by quenching.
Q: Why do music wire springs need a lower heat treatment temperature?
A: Music wire is cold-drawn to its maximum strength by the wire mill, so it is already "hardened." Stress relief above roughly 300°C starts lowering its tensile strength, so music wire springs are treated low, typically 200-260°C — just enough to relax coiling stress without giving back the drawing strength.
Q: What is decarburization and why does it hurt springs?
A: Decarburization is the loss of carbon from the steel surface during high-temperature heating, which leaves a soft, weak surface layer. Fatigue cracks start in that weak skin, so a decarburized spring fails early even though its core looks sound — which is why hot-wound springs need controlled atmospheres and critical parts are ground after heat treatment.
Q: Can heat treatment fix a spring that has the wrong load?
A: No — heat treatment develops and stabilizes the properties of the wire grade that was chosen, but it cannot turn the wrong wire or wrong geometry into the right spring. The load is set by wire diameter, coil count, and material modulus; heat treatment's job is to lock those properties in and prevent drift, so load problems are solved at design and coiling, not in the oven.
Related Resources
- Spring Material Selection Guide — wire grades and their as-supplied strength, the baseline for any heat treatment decision.
- Hot-Wound vs Cold-Wound Springs — which forming route your spring size should use and why.
- Custom spring products — heat-treated springs from 0.1 mm to 8 mm wire at a Dongguan source factory.
- Contact us — send your drawing and wire grade and get a spring quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
