Heat Treatment (焼入れ・焼戻し)

Heat 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 single most important step in determining how a scissor performs — the same steel alloy can produce a mediocre or exceptional tool depending entirely on heat treatment.
• Cost position: Varies widely. Conveyor-oven batch processing is cheap; vacuum furnace + cryogenic treatment is expensive.
• Where used: Every professional scissor 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. Without this step, a $2,000 blank would cut no better than a butter knife.

The subtlety is in the details. Temperature accuracy within a few degrees, soak times measured in minutes, cooling rates that depend on the specific alloy — getting any of these wrong produces a blade that is too soft, too brittle, or unevenly hardened. This is why two scissors labelled “VG-10, HRC 60” can feel completely different in the hand: one was heat-treated by a specialist; the other ran through a conveyor furnace on a timer.

Stage 1: Quenching (焼入れ / yaki-ire)

Quenching is the hardening step. The blank is heated to its austenitising temperature — typically 1,020-1,080 °C for most scissor steels — and held until the internal structure fully transforms to austenite (a high-temperature crystal phase). The blank is then rapidly cooled (quenched) in oil, air, or inert gas, which traps the carbon in place and transforms the austenite into martensite — the hard, wear-resistant phase that holds an edge.

• Target hardness for professional scissors: HRC 58-62 for most alloys. Mizutani specifies HRC 59-62 across their range.
• Risk: If quenched too fast, internal stresses can cause micro-cracking. If quenched too slowly, not all austenite converts and the blade runs soft.

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

Fresh martensite is extremely hard but also brittle — a blade in this state would chip on coarse hair. Tempering reheats the hardened blank to a lower temperature (typically 150-250 °C) and holds it there, allowing the martensite to partially relax. This reduces peak hardness slightly but dramatically improves toughness and ductility.

• Low tempering (150-180 °C): Retains maximum hardness, accepts the brittleness trade-off. Used for ultra-hard blades (HRC 62+) on fine-hair scissors.
• High tempering (200-250 °C): Drops hardness a point or two but produces a much tougher blade. Common for all-purpose professional scissors expected to survive drops and barber work.
• Multiple temper cycles: Premium manufacturers temper two or three times, checking hardness between cycles, to dial in exact target properties.

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

After quenching, most scissor steels retain 10-20% untransformed austenite — soft spots that weaken the blade and cause dimensional instability over time. Cryogenic treatment addresses this by cooling the blade to extreme sub-zero temperatures, forcing the retained austenite to convert to martensite.

• Standard sub-zero: Cooling to -80 °C using dry ice or mechanical refrigeration.
• Deep cryogenic treatment (DCT): Cooling to -196 °C using liquid nitrogen immersion. Transforms virtually all retained austenite and precipitates ultra-fine eta-carbides throughout the matrix — a permanent structural improvement that cannot be achieved any other way.
• Mizutani’s Extramarise technology: A proprietary cryogenic process central to Mizutani’s performance claims. The exact parameters are not published, but the process is described as deep cryogenic treatment followed by controlled warm-up to lock in the refined structure.
• Sensei Duralite: Claims approximately 40% improved edge durability from their cryogenic processing cycle.

Stage 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 causes surface oxidation (scale) and decarburisation (carbon loss at the surface). Vacuum heat treatment eliminates both problems by running the entire hardening cycle in a sealed, evacuated chamber.

• Benefits: Cleaner surface (no scale to grind off), more uniform hardness (no decarburised soft skin), brighter finish, tighter dimensional control.
• Pioneered by: Yae Scissors (八栄シザー / Yae Shizā) is credited with advancing vacuum heat treatment for professional scissors in Japan.
• Adoption: Now standard at premium Japanese and German workshops. Economy manufacturers still use atmosphere furnaces due to equipment cost.

What 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.

Sources

1. Mizutani — About
2. sint.co.jp — Manufacturing Reference (Japanese)
3. Steel Types Reference

Related processes: Hot Forging

Two-Piece Welding

Quality Control