Industrial Ovens for Spring Manufacturing: Box Type vs. RJC Conveyor — Which Heat Treatment Setup Fits Your Line?

Heat treatment is the least visible operation in spring manufacture and one of the most consequential. Skip or under-treat it and parts measure correctly on the day, then drift in service — which is among the hardest failure modes to diagnose, because the inspection record looks fine.

Two oven types cover the work, and they suit different production patterns.

WHAT STRESS RELIEF ACTUALLY DOES

Coiling deforms wire plastically, and that leaves residual stress locked into the part. Left in place, that stress relaxes over time and the spring changes dimension — typically losing free length — without any load being applied.

A stress relief cycle holds the spring at temperature long enough for those stresses to redistribute. The part comes out dimensionally stable. This is not optional on a coiled spring; it is the step that makes the measurement taken at inspection mean something six months later.

One consequence worth planning for: stress relief moves dimensions. How much depends on material, section, and geometry, but never zero. Final dimensional acceptance belongs after heat treatment, not before it.

BOX OVENS

Our box oven runs three chambers, each with two 450 W heaters, 2.75 kW total on a 220 V supply, reaching 350 °C. Constant temperature time is adjustable from 60 to 1,800 seconds.

The character of a box oven is batch processing. Load, run the cycle, unload. That gives excellent control over time at temperature and suits varied work, small batches, and situations where different parts need different cycles.

Its limitation is flow. Parts accumulate before it and release in a group afterwards, which introduces a queue into an otherwise continuous line.

CONVEYOR OVENS

The RJC series moves parts through on a conveyor, reaching up to 500 °C, with models from the RJC206 through to the RJC420 covering different lengths and widths.

Time at temperature is set by belt speed and heated length rather than by a timer. That converts heat treatment from a batch step into something closer to continuous flow, which helps line balance considerably at volume.

The trade is flexibility. Changing the cycle means changing belt speed, which affects everything on the belt. A conveyor oven suits steady, high-volume work with a consistent recipe.

CHOOSING BETWEEN THEM

• Box oven for varied work, small to moderate batches, several different cycles, and where 350 °C is sufficient.
• Conveyor oven for steady high volume on a consistent recipe, where continuous flow matters and higher temperature is needed.
• Both is common at scale — conveyor for the runners, box for everything else.

STRESS RELIEF AND AGE HARDENING ARE NOT THE SAME OPERATION

Both happen in an oven and they are frequently confused, which leads to the wrong cycle being applied.

Stress relief applies to steel springs after coiling. It relaxes residual stress from forming so the part is dimensionally stable. It does not substantially change hardness or strength; the material arrived at its properties before coiling.

Age hardening applies to precipitation-hardening alloys such as beryllium copper and 17-7 PH stainless. Here the heat treatment develops the mechanical properties. The part is formed in a softer condition and hardened afterwards, which is why forming is possible at all.

The distinction matters because the cycles differ in temperature and duration, and because age hardening moves dimensions more than stress relief does. A beryllium copper contact formed to nominal and then aged will not be at nominal afterwards. That movement is predictable and compensated in the tooling, but only if the process is planned as age hardening rather than treated as a stress relief step.

WHAT DETERMINES THE CYCLE

Temperature and time depend on material and section. Heavier sections need longer for the core to reach temperature, and a cycle validated on a 2 mm spring does not transfer to a 6 mm one.

Loading matters as much as the recipe. An overloaded oven heats unevenly, and parts in the middle of a dense load see a different cycle from those at the edge. If dimensional consistency after treatment is poor, loading practice is worth checking before the oven is blamed.

A final point on verification. Oven temperature should be checked against an independent probe rather than trusted to the controller display alone, and the survey should cover the working volume rather than a single point. Ovens develop hot and cold zones as elements age, and a load positioned in a cold corner receives a different cycle from the one specified.

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Published by the Seyunda Team · 2026