What is Martensite?

Martensite forms during quenching when austenite is cooled too rapidly for the carbon atoms to diffuse out of the iron lattice. The carbon atoms become trapped in the lattice positions they occupied in the austenite phase, distorting the crystal structure from cubic (FCC) to tetragonal (BCT). This lattice distortion is what creates the extreme hardness — the trapped carbon atoms resist dislocation movement, which is the mechanism of plastic deformation. The transformation from austenite to martensite is diffusionless and nearly instantaneous — it occurs at the speed of sound in the metal. However, the transformation is never 100% complete. Residual austenite always remains in the microstructure (see retained austenite). In scissor steels, retained austenite levels of 10-25% are common after quenching, depending on alloy composition and quenching conditions. Tempering after quenching is essential for scissor blades. As-quenched martensite is extremely hard but also extremely brittle — too fragile for the repeated blade-on-blade impact that scissors endure. Tempering slightly reduces martensite hardness but dramatically improves toughness by allowing some of the trapped carbon to precipitate as fine carbide particles, relieving internal stresses in the lattice. Double tempering (two cycles at 100-250°C) is standard practice for scissor steels. The first temper cycle relieves the most severe internal stresses and converts any unstable retained austenite. The second cycle ensures completeness and further stabilizes the microstructure. Takefu recommends double tempering at 150-200°C for VG-10, while Proterial specifies 180-250°C for their GIN series steels. The optimal temper temperature is a balance: lower temperatures preserve more hardness (higher HRC) while higher temperatures provide more toughness. For scissors, the tempering sweet spot is typically lower than for knives, because the finer edge geometry of scissor blades benefits from maximum hardness — provided the martensite is properly stress-relieved.