Spring Heat Treatment: Hardening and Tempering Process

Heat treatment is the step that decides whether a spring carries its rated load for millions of cycles or starts to sag after a few weeks in service. Two operations dominate the process: hardening, which turns the steel into a strong martensitic structure, and tempering, which trades a little of that strength back for the toughness a spring needs to survive real loads. This page explains the process in terms a buyer, designer or quality engineer can use when specifying or approving a spring.

Why springs are hardened and tempered

A coil spring stores energy by bending and twisting its wire. To do that without yielding, the wire must be strong enough that working stress stays inside the elastic range, yet tough enough that a stress concentration at a surface defect does not start a crack. Hardening alone produces very high hardness and strength, but as-quenched martensite is brittle and carries a lot of residual stress. Tempering reheats the part to a lower temperature so that some hardness is deliberately given back in exchange for ductility and a more stable structure. Quenched and tempered spring steels are typically used in the range 40–50 HRC — a balance point between load capacity and resistance to cracking.

Which springs need a full harden and temper

Not every spring follows the same route. The starting wire condition decides the process:

• Cold drawn patented wire — covered by EN 10270-1 (grades SL, SM, SH, DH). This wire is already strong from the drawing operation. Cold-coiled springs made from it normally only need a low-temperature stress-relief treatment, not a full re-hardening.
• Oil hardened and tempered wire — covered by EN 10270-2 (grades FDC, TDC, VDC). Also supplied in the finished condition and used widely for cold-coiled compression, extension and torsion springs.
• Stainless spring wire — covered by EN 10270-3, commonly grades 1.4310 and 1.4401.
• Hot-coiled springs — typically made from alloy spring steels such as 60Si2Mn and 51CrV4. Because the bar is coiled hot and then quenched, these parts require a full hardening and tempering cycle.

The hardening step

Hardening means heating the steel into its austenitizing range so the carbon dissolves into a uniform structure, then quenching fast enough that the structure transforms to martensite instead of softer pearlite. Our heat-treatment equipment reaches a maximum temperature of 1,500 °C, which covers the full range needed for spring steels. Two things a spring maker watches most closely here are:

• Uniformity — the whole part, including the heavy sections of a hot-coiled bar spring, must reach temperature evenly, or hardness will vary from coil to coil.
• Surface condition — if the surface loses carbon during heating (decarburisation), the outer skin ends up softer than the core and the spring fails there. Spring surfaces are usually checked for decarburisation, and cracks on safety-critical parts are checked with magnetic particle inspection.

The tempering step

Tempering is done immediately after quenching, before the part cools to room temperature and risks cracking. The part is reheated to a controlled lower temperature, held, and cooled. The tempering temperature is set to land the finished hardness inside the 40–50 HRC target window: a higher temperature gives a softer, tougher spring; a lower temperature gives a harder, stronger but more crack-sensitive spring. For springs that will be shot peened, tempering must be completed first, because the compressive surface layer produced by shot peening is a separate, deliberate treatment applied on top of the tempered structure.

What to put on the drawing

When buying or specifying a heat-treated spring, state the material and its standard, the finished hardness range, and any inspection requirements such as decarburisation limits or magnetic particle inspection. For springs made from pre-hardened wire, say so — that tells the supplier that stress relief, not re-hardening, is the correct process and avoids unnecessary rework and distortion.

Frequently asked questions

Q: What hardness should a spring steel spring be?
A: Quenched and tempered spring steels are typically used in the range 40–50 HRC.

Q: Can cold-coiled springs be used without a full heat treatment?
A: Springs coiled from EN 10270-1 cold drawn wire or EN 10270-2 oil tempered wire are usually only stress-relieved, because the wire is already in the hardened and tempered condition.

Q: Which steels need a full harden and temper?
A: Hot-coiled springs made from alloy spring steels such as 60Si2Mn and 51CrV4 are the usual candidates.

Q: Why is decarburisation a problem?
A: A carbon-depleted surface layer is softer and weaker than the core, so fatigue cracks initiate there. Spring surfaces are checked for it.

Q: Does shot peening replace heat treatment?
A: No. Shot peening is applied after tempering; it adds a compressive surface layer but does not harden the spring through its section.

Related resources

• Spring Steel Materials Guide
• Spring Steel Wire Grades: Music, Oil-Tempered & Chrome Silicon
• Shot Peening for Springs: Why It Extends Fatigue Life
• Spring Manufacturing Process: From Wire to Finished Product

Need a heat-treated spring made to your drawing? Send your specification to liu@chenjisprings.com or call / WhatsApp +86 158 5311 1612.