Over-tempering — strength lost in the tempering stage
Uniformly low hardness and strength after a correct quench: how over-tempering happens, how a section distinguishes it from a quench fault or decarburization, and the oven controls that prevent it.
| Defect family | Heat treatment |
|---|---|
| Related service | Heat Treatment |
| Reviewed |
Standards commonly cited
Grades this affects
What you see
Hardness and tensile strength are uniformly below the class while the parts are otherwise sound — no scatter, no cracks, no soft patches. Bolts stretch and yield rather than break, joints lose preload, and a tensile test gives a fully ductile fracture with heavy necking. A deep blue-grey oxide film on bare parts can be a hint of a hot temper, but colour is never evidence on its own.
How to confirm it
Compare hardness to ISO 6508-1 / ISO 6507-1 against the class in ISO 898-1, then section, polish and etch. Over-tempered material originated as martensite but shows coarsened, spheroidised carbides and a soft, featureless etch response that is uniform right through the section — and that uniformity is what separates it from a quench problem (mixed structures, position-dependent) and from decarburization (a soft rim over a hard core, so a microhardness traverse falls only near the surface). Then read the oven record: set-point, load thermocouple trace and time at temperature.
Related calculator: Hardness by steel grade — reference tableRoot causes — most common first
- 1Temper set-point too high for the target class — often chosen to guarantee toughness and then never revisited.
- 2Time at temperature far longer than intended: a slow belt, an overnight hold or a stalled load.
- 3Furnace control fault — a drifting or badly placed thermocouple, so the load runs hotter than the display.
- 4An uncontrolled second thermal exposure downstream: a hot paint cure, a hot phosphating stage or a de-embrittlement bake run above the original tempering temperature.
- 5Hot spots and poor uniformity in the oven, so part of the load is tempered far above the mean.
- 6A temper recipe applied to the wrong grade — an alloy-steel curve used on a plain-carbon or boron grade with a different response.
Process controls that prevent it
- Set the temper from the grade tempering curve and the required class, and record the load temperature rather than only the controller set-point.
- Survey the tempering oven for uniformity, and re-survey after any element, fan or controller work.
- Keep every downstream thermal step — bake, phosphate, paint cure — below the tempering temperature, and check the numbers whenever a new coating or bake is introduced.
- Log time at temperature per lot against a defined maximum, and treat a stalled belt as a deviation that triggers a hardness re-check.
- Calibrate thermocouples and place them where the load actually sits, verifying periodically with a load thermocouple.
- Re-check hardness after the last thermal operation in the route, not only after quenching.
What to send us for diagnosis
- Parts from the failing lot together with the process or oven record for the tempering step if you have it.
- The full route after quenching, including any bake, phosphating or paint cure, and the temperature of each.
- The drawing class and hardness range, plus your measured values with method and location.
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Part types this shows up on
Questions engineers ask
Can we simply harden them again?+
Only by re-austenitising, quenching and tempering — tempering alone cannot raise hardness. Re-treatment has to be agreed against the drawing because it adds distortion, decarburization and grain-growth risk to parts that are already at finished size.
Our de-embrittlement bake runs at 210 °C. Can that over-temper a class 10.9 bolt?+
Normally no. Class 10.9 fasteners are tempered well above that, so a bake at 190–230 °C sits far below the tempering temperature and does not measurably change hardness. The risk appears where a part was deliberately tempered low to sit at the top of a hardness band, and on springs and high-carbon parts with a low temper.
How do we tell over-tempering from soft material?+
Section and etch. Over-tempered parts came from a correct quench, so the structure is uniform tempered martensite throughout. A hardenability or quench failure leaves non-martensitic products whose distribution depends on position in the section.
Guidance based on published standards and general fastener metallurgy. Values are typical ranges that depend on the part, the material and the governing specification — the customer drawing and the applicable standard always prevail. This page does not replace a qualified failure analysis of your own parts.