What is Toughness?

Description

Toughness is the resistance of steel to fracture and chipping under impact. It has an inverse relationship with hardness — softer steels absorb more energy before breaking. Measured via Charpy impact test (ASTM E23), toughness is critical for scissors because blade-on-blade contact creates repeated micro-impacts.

What is Toughness?

Toughness is the resistance of steel to fracture and chipping under impact or sudden loading. It has a well-documented inverse relationship with hardness — as steel gets harder, it becomes more brittle. Toughness is measured using the Charpy impact test (ASTM E23), where VG-10 scores approximately 5.8 ft-lbs per Knife Steel Nerds data. For scissors, toughness is critical because blade-on-blade contact creates repeated micro-impacts during every cut.

Why It Matters for Scissors

Every time scissors close, the two blades make contact along the cutting edge. This blade-on-blade interaction produces micro-impacts that accumulate over thousands of cuts. A steel with insufficient toughness will develop micro-chips along the edge — visible under magnification as tiny scallops that degrade cutting performance.

The practical difference is visible when scissors are dropped on a hard salon floor. A 440C scissor at HRC 58-60 is relatively tough — the tip will bend on impact, which can be straightened by a sharpener. A ZDP-189 scissor at HRC 65+ has almost zero toughness at the edge — the tip may fracture completely, requiring removal of material and shortening the blade.

For everyday salon use, a moderate toughness level (VG-10 at HRC 60-62) provides enough impact resistance to survive normal blade-on-blade cycling while maintaining good edge retention. Ultra-hard steels demand more careful handling and cutting technique.

Technical Detail
The Charpy V-notch impact test (ASTM E23) is the standard method for quantifying toughness. A notched sample is struck by a pendulum, and the energy absorbed before fracture is recorded in foot-pounds (ft-lbs) or joules. Typical Charpy values for scissor steels at their working hardness: - **440C at HRC 58-60:** ~8-10 ft-lbs. Relatively tough for a stainless steel, which is why 440C scissors survive drops better than premium alternatives. - **VG-10 at HRC 60-62:** ~5.8 ft-lbs per Knife Steel Nerds (KSN) Thermo-Calc and physical testing. The vanadium carbides are fine-grained, which helps maintain reasonable toughness despite the higher hardness. - **ATS-314 at HRC 62-63:** ~4-5 ft-lbs. The increased carbide volume and hardness reduce impact resistance. - **ZDP-189 at HRC 65-67:** ~2-3 ft-lbs. Extremely brittle at working hardness. These steels demand careful handling. The inverse hardness-toughness relationship is governed by microstructure. Higher hardness requires more martensite and more carbide, both of which are brittle phases. The softer retained austenite and any ferrite provide ductility and energy absorption. As heat treatment pushes toward maximum hardness, these softer phases are minimized, and toughness drops. Carbide size and distribution also affect toughness. Powder metallurgy steels (like HAP40 or ZDP-189) have finer, more uniformly distributed carbides than conventionally cast steels. This means that at equal hardness, a powder steel is typically tougher than its conventional equivalent — the fine carbides are less likely to act as crack initiation sites. In scissor design, toughness is managed not just through steel selection but also through blade geometry. Thicker blade cross-sections and reinforced tips compensate for lower steel toughness at higher hardness levels.

Sources

Frequently Asked Questions

Hardness and toughness are inversely related. As steel gets harder (higher HRC), it becomes more brittle and less able to absorb impact energy. A dropped HRC 65 scissor tip may shatter, while an HRC 58 tip would bend.

The Charpy impact test (ASTM E23) measures the energy a notched steel sample absorbs before fracturing. VG-10 scores approximately 5.8 ft-lbs per Knife Steel Nerds testing. Higher values indicate tougher steel.

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