What is Tempering?

Description

Tempering is a heat treatment process where hardened steel is reheated to 100-250°C to reduce brittleness while retaining most of its hardness. Double tempering with two cycles is standard practice for professional scissor blades to ensure toughness during blade-on-blade contact.

What is Tempering?

Tempering is a heat treatment process where hardened steel is reheated to a controlled temperature (typically 100-250°C for scissors) to reduce brittleness while retaining most of its hardness. After quenching, steel is extremely hard but also dangerously brittle — tempering restores enough toughness for reliable blade-on-blade cutting without excessive hardness loss.

Why It Matters for Scissors

Untempered scissor blades would shatter or chip catastrophically during use. The blade-on-blade contact in scissors creates stresses that single-blade cutting tools never experience, making proper tempering essential.

Double tempering — two full heating and cooling cycles — is the industry standard for professional scissors. The first cycle converts retained austenite and relieves the most severe internal stresses. The second cycle tempers any fresh martensite that formed from the retained austenite converted during the first cycle. Without the second cycle, pockets of untempered martensite remain, creating hard-brittle zones that chip preferentially.

Typical tempering temperatures for scissor steels range from 150-200°C, reducing the as-quenched hardness by 1-2 HRC points. For VG-10, Takefu recommends tempering that achieves a final working hardness of 59-61 HRC, down from an as-quenched potential of 62-63 HRC. This controlled sacrifice provides dramatically better service life.

Technical Detail
Tempering of scissor steels proceeds through several metallurgical stages depending on temperature: **Stage 1 (100-200°C):** Carbon atoms migrate within the martensite lattice, forming transition carbides (epsilon carbide, eta carbide). The martensite partially decomposes from a body-centered tetragonal (BCT) structure toward body-centered cubic (BCC). Internal stresses are relieved, and toughness improves significantly with minimal hardness loss. This is the primary tempering range for scissor steels. **Stage 2 (200-350°C):** Retained austenite decomposes to ferrite and cementite. This is one reason double tempering is important — the first cycle converts retained austenite, and the second cycle tempers the resulting fresh martensite. **Stage 3 (350-500°C):** Transition carbides transform to stable cementite (Fe3C). In most scissor steels, this range is avoided because it causes excessive softening. However, VG-10 exhibits an unusual secondary hardening response in this range due to its molybdenum content, making it suitable for blades that will receive titanium or ceramic coatings applied at elevated temperatures. The tempering parameter (Hollomon-Jaffe parameter) combines time and temperature into a single value, meaning a lower temperature held for a longer time can achieve the same result as a higher temperature for a shorter time. Industry practice for scissor steels typically uses 1-2 hours per tempering cycle. Proterial publishes specific tempering curves for their GIN-series steels showing the hardness response at each temperature. GIN-3, for example, maintains HRC 58-60 when tempered at 150-200°C, dropping to HRC 54-56 at 300°C.

Sources

Frequently Asked Questions

Scissors experience blade-on-blade impact with every cut, unlike knives that only contact food. Double tempering (two heating cycles) ensures maximum conversion of brittle retained austenite and produces a more uniform, tougher microstructure that resists chipping.

Yes, but deliberately. Tempering at 150-200°C typically reduces HRC by 1-2 points compared to the as-quenched state. This small hardness sacrifice dramatically improves toughness and prevents the catastrophic chipping that would occur with fully hard, untempered steel.

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