Heat Treatment (焼入れ・焼戻し)

Reference sectionHeat Treatment (焼入れ・焼戻し / yaki-ire / yaki-modoshi)

Quick look

• Process: Controlled heating, cooling, and reheating cycles that transform the crystal structure of steel to set hardness, toughness, and edge stability.
• Key benefit: The step that turns an alloy into a cutting tool. Two scissors with the same steel and the same nominal HRC can perform differently because their schedules differ.
• Cost position: Varies widely. Conveyor-oven batch processing is cheap; vacuum furnace plus sub-zero treatment costs more.
• Where used: Every hardenable martensitic scissor steel undergoes some form of heat treatment. The method and precision vary enormously.

Why it matters

Heat treatment is where steel becomes a cutting tool. The raw alloy—whether VG-10, SUS440C, ATS-314, or cobalt—is relatively soft after forging or stamping. It cannot hold an edge. The hardening cycle rearranges the atomic structure of the metal, locking carbon atoms into a rigid lattice (martensite) that resists deformation.

The subtlety is in the details. Furnace atmosphere, section size, hold, quench medium, cooling endpoint, tempering and any sub-zero step all matter—getting any of them wrong produces a blade that is too soft, too brittle, or unevenly hardened. This is why two scissors labeled “VG-10, HRC 60” can feel completely different in the hand: one was heat-treated to a controlled schedule; the other ran through a conveyor furnace on a timer. Cobalt-base and ceramic scissor materials such as Stellite do not follow this sequence at all.

DetailStage 1: Quenching (焼入れ / yaki-ire)

Quenching is the hardening step. The blank is heated to the range prescribed for its grade, held as specified, then cooled at a rate that produces the intended hard structure while controlling distortion and cracking. The austenite transforms into martensite, the hard, wear-resistant phase that holds an edge.

There is no generic scissor-steel schedule. Published mill examples show the spread: Proterial lists ATS34 at a 1050 °C quench, air or oil cool, with a 100–180 °C temper and a hardening hardness of at least 59 HRC; Alleima lists 14C28N at 1050 °C with a five-minute hold and an oil quench, followed by selection from that grade’s tempering-response data. Proterial tells users to follow each grade’s standard conditions and varies holding time with section thickness, so the two examples cannot be generalized into one band.

• Hardness depends on the grade and the schedule, not on a category. Published finished-blade values include Jaguar SOLINOX54 at 54 HRC and SOLINOX58 at 58 HRC, Kasho’s SG2 SlideCut at 61–62 HRC, and Hayashi’s HYS-MAX67 at 67 HRC. Mill quench-hardness figures are a different kind of number: Takefu publishes VG-10 at ≥60 HRC and SPG2 at ≥62 HRC as hardening response, not as a finished blade.
• Risk: Quench too fast and internal stresses can cause micro-cracking; quench too slowly and non-martensitic transformation products can form, leaving the blade soft.

DetailStage 2: Tempering (焼戻し / yaki-modoshi)

Fresh martensite is extremely hard but also brittle, so tempering reheats the hardened steel to reach the grade-specific balance of hardness, toughness, retained-austenite stability and dimensional control. Peak hardness usually drops; toughness and ductility improve.

• The schedule is grade-specific. Proterial’s published ATS34 example uses a 100–180 °C temper. Its high-speed-steel guidance uses high-temperature tempering and requires repeated tempers—at least two, or at least three for cobalt-bearing high-speed steel. There is no universal 150–250 °C band.
• Repeated tempers are a grade-dependent practice, not a premium-maker signature: some grades require two or three cycles to reach their intended properties, and others do not.
• Hardness is a product measurement, not a reconstruction of the schedule. Use the maker’s published finished-blade figure as one product parameter and pair it with the named mill grade, geometry and maker service guidance.

DetailStage 3: Cryogenic / Sub-Zero Treatment (サブゼロ処理 / sabu-zero shori)

After quenching, some austenite can stay untransformed. How much depends on the alloy, the austenitizing condition, the quench path and the final temperature, and it can affect dimensional stability. A sub-zero step can convert some of that retained austenite, but its value and safe schedule depend on the exact steel, the prior austenitizing and quench, the blade section, timing, soak, warming and the tempering that follows.

• It is not automatically required, and colder is not automatically better. A maker’s cryogenic temperature cannot be copied to another grade or used by itself as proof of hardness, toughness or edge retention.
• Named implementations differ. ZWILLING’s Japanese explanation and MIYABI catalog describe FRIODUR at −70 °C or below and CRYODUR at −196 °C; JAGUAR’s 2025 hair-scissor catalog gives its FRIODUR blade treatment at −180 °C. These are named proprietary processes, not one transferable recipe, so quote the product documentation rather than a generic number.
• Mizutani’s Extramarise: Mizutani defines this as its proprietary combination of hardening, tempering, sub-zero processing and temperature management. Its public explanation places the sub-zero step between hardening and tempering and describes it as −100 °C or below. The complete time and temperature schedule is not published, and Extramarise I and II are also current cobalt-alloy material labels.
• Sensei Duralite: Sensei says the cryogenic tempering in its Duralite (440C) steel improves durability by about 40%. That is the maker’s own claim for its own product.

DetailStage 4: Vacuum Heat Treatment (完全真空焼き入れ / kanzen shinkū yaki-ire)

Conventional furnaces expose steel to air during heating, which can cause surface oxidation (scale) and decarburization, the loss of carbon at the surface. Vacuum heat treatment limits both by running the hardening cycle in a sealed, evacuated chamber.

• Benefits: Cleaner surface with less scale to grind off, more uniform hardness without a decarburized soft skin, brighter finish, tighter dimensional control.
• Still governed by the process, not the label: furnace pressure, gas choice, quench capacity, load and the steel’s hardenability control the result, so a vacuum label alone does not establish a better blade.
• Maker claim: Yasaka (Yasaka Seiki, 八栄精機) of Nara states that it pioneered complete vacuum hardening through the moving and fixed blade contact point and uses sub-zero treatment. Record that as Yasaka’s own claim, not as proof that vacuum hardening is standard practice across premium Japanese and German workshops.

DetailWhat to ask a manufacturer

The most revealing question is not “what hardness?” but “how do you achieve it?” A manufacturer that can describe their austenitising temperature, quench medium, number of temper cycles, and whether they use cryogenic or vacuum processing is one that controls their own heat treatment. A manufacturer that only quotes an HRC number may be outsourcing the process — or guessing.

Source trailSources

1. Mizutani — About
2. sint.co.jp — Manufacturing Reference (Japanese)
3. Steel Types Reference
4. Yakiire Netsushori—Sub-Zero Treatment Technical Details (Japanese—retained-austenite conversion)
5. Damascus Houchou — VG-10 Steel Properties (Japanese — cobalt amakire property)
6. Takefu Special Steel (Japanese—mill quench-hardness figures for VG-10 and SPG2)
7. Proterial / Yasugi Specialty Steel (mill-published ATS34 and high-speed-steel hardening, tempering and repeated-temper conditions)
8. Alleima—14C28N material datasheet (mill-published hardening and tempering data)
9. Hayashi Scissors—Official Japan (maker-published HYS-MAX67 blade hardness)
10. Kasho—Blue Series specifications (maker-published SG2 SlideCut hardness)

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