Heat Treating Springs and Spring Steel — How It Works

A spring does not just need to be "hard" — it needs a high elastic limit: it must return fully to shape after being compressed or stretched, over millions of cycles, without fatiguing or taking a set (staying deformed).

Harden it too much, like a screw, and the spring becomes brittle and snaps; leave it too soft and it takes a set instead of springing back. The whole job is finding the right balance of hardness and elasticity.

That is why spring steels and spring heat-treat processes are purpose-designed and not the same as ordinary screw, nut and bolt work.

Spring steels are high-carbon and often alloyed to resist fatigue:

• SUP9 / SUP10 (Cr / Cr-V family) — standard JIS spring steels for coil and leaf springs, good fatigue life.
• SUP6 / SUP7 (Si-Mn) — heavy-duty, high-load springs.
• SK5 / SK85 (carbon steel) — flat springs, clips, spring washers.
• 65Mn — an economical, widely used spring steel.
• Oil-tempered wire / Music wire — supplied already hardened by the mill; formed, then only stress-relieved.

Spring work splits into two routes depending on the raw material:

• Route 1 — form soft, then harden: used for SUP, SK5, 65Mn formed while annealed, then hardened (austenitize → quench) and tempered to reach the required elasticity.
• Route 2 — form pre-hardened wire, then stress-relieve: oil-tempered or music wire is already hard, so after coiling it only gets a stress-relief at ~230–400°C to hold shape and remove forming stresses.

The route depends on the material and spring design, so please tell us the wire grade and how it was formed, so we set the right process.

For fully hardened springs the temper is usually around 350–450°C — higher than for general wear parts — because the goal is not maximum hardness but maximum elastic limit.

Temper too low → hard but brittle, risk of fracture; temper too high → tough but takes a set (loses springback). Either way the spring is out of spec.

Typical working hardness for springs is around 42–52 HRC depending on type and duty.

The most common cause of early spring failure is decarburization — if the furnace atmosphere is not controlled during heating, the surface loses carbon and softens, creating a crack starter at the surface. The spring fatigues and breaks early even though the core is fine.

So springs must be heat treated in an atmosphere-controlled furnace to prevent decarb.

Other key factors: a smooth surface free of deep scratches (fatigue starters), and controlled distortion so the spring meets its rate and load per the drawing.

V.S. Heat Treatment processes springs and spring-steel parts in volume in atmosphere-controlled furnaces:

• Hardening + tempering of spring steels (SUP/SK5/65Mn) to the drawing’s hardness and elasticity.
• Stress-relief of springs formed from oil-tempered / music wire.
• Controlled atmosphere to prevent decarburization, plus hardness testing and a test certificate on every lot.

Send the spring drawing, material grade and target hardness/load, and we will recommend the right process with a price within 24 business hours.