What is Toughness?

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.