Low hardness after tempering
Bolts and screws that read under the drawing hardness: measurement error, temper temperature, hardenability limits and quench faults, ranked by how often each is the real cause.
| Defect family | Heat treatment |
|---|---|
| Related service | Heat Treatment |
| Reviewed |
Standards commonly cited
Grades this affects
What you see
The lot reads below the drawing minimum — for example a class 10.9 bolt at 28 HRC where 33–39 HRC is specified — and the readings are consistent rather than scattered. Proof load and tensile results follow the hardness down, threads deform at nominal torque, and the parts feel soft to a file. Because the readings are uniform, the cause is normally a whole-lot condition rather than a local quench fault.
How to confirm it
Prove the measurement first: Rockwell to ISO 6508-1 / ASTM E18 needs a flat, prepared, properly supported surface, the correct scale and anvil, and enough thickness for the load — a curved thread crest or a plated surface reads low. Confirm with Vickers to ISO 6507-1 on a mounted, polished cross-section and convert with care. Compare the result against the property class in ISO 898-1 rather than against a remembered number. Once the reading is trusted, section and etch: fully tempered martensite that is simply too soft means the temper was too hot or too long, while non-martensitic products mean the quench did not do its job.
Related calculator: Hardness converter (HRC / HV / HB)Root causes — most common first
- 1Temper temperature too high, or dwell too long, for the target class — the most common cause and the easiest to verify from the oven record.
- 2Measurement error: reading through a coating, on a curved crest, on a part too thin for the load, or with the wrong scale, which typically reads several points low.
- 3Insufficient hardenability for the section — the grade cannot reach the required hardness through that diameter however it is quenched.
- 4Incomplete austenitising: too cool, too short, or a cold load charged into a furnace whose recovery time was not counted.
- 5Quench fault — oil too hot or degraded, weak agitation, or a delay between furnace and quench.
- 6Wrong material: a lower-carbon or non-boron grade substituted at the supplier or mixed in stores.
Process controls that prevent it
- Verify the test method and surface preparation before questioning the furnace — re-test on a prepared flat and on a sectioned part.
- Control and record the temper per lot: set-point, load thermocouple, time at temperature and cooling.
- Select the grade against the section size and check carbon and boron on the incoming mill certificate.
- Charge to a stable furnace and count soak time from the load reaching temperature.
- Maintain the quench on a schedule — temperature, agitation, water content, oil condition.
- Segregate and identify wire rod in stores so grades cannot be mixed, and check hardness on a defined sample per lot after treatment.
What to send us for diagnosis
- Ten or more parts from the failing lot, plus parts from a lot that passed the same drawing.
- Your readings — instrument, scale, load, where on the part you measured, and whether the parts were plated at the time.
- The drawing, property class or hardness range, the grade, and the mill certificate if available.
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Part types this shows up on
Questions engineers ask
Can under-hardness be corrected by re-tempering?+
No — tempering only lowers hardness. If the parts were tempered too hot, the martensite is already over-tempered and the only route back is a full re-austenitise, quench and temper, agreed against the drawing and accepting the added distortion and decarburization risk.
The plated parts read low but bare ones are fine. Why?+
Because the indenter is pushing into a soft zinc layer. Hardness for acceptance is measured on a prepared bare surface or on a sectioned part, never through a coating.
Could the material be wrong rather than the process?+
It happens more often than people expect, especially with boron grades where a small change in boron or carbon moves the achievable hardness. The mill certificate plus a check analysis settles it quickly and stops the plant chasing furnace ghosts.
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.