What is Tempering?

Tempering of scissor steels proceeds through several metallurgical stages depending on temperature: **Stage 1 (100-200°C):** Carbon atoms migrate within the martensite lattice, forming transition carbides (epsilon carbide, eta carbide). The martensite partially decomposes from a body-centered tetragonal (BCT) structure toward body-centered cubic (BCC). Internal stresses are relieved, and toughness improves significantly with minimal hardness loss. This is the primary tempering range for scissor steels. **Stage 2 (200-350°C):** Retained austenite decomposes to ferrite and cementite. This is one reason double tempering is important—the first cycle converts retained austenite, and the second cycle tempers the resulting fresh martensite. **Stage 3 (350-500°C):** Transition carbides transform to stable cementite (Fe3C). In most scissor steels, this range is avoided because it causes excessive softening. However, VG-10 exhibits an unusual secondary hardening response in this range due to its molybdenum content, making it suitable for blades that will receive titanium or ceramic coatings applied at elevated temperatures. The tempering parameter (Hollomon-Jaffe parameter) combines time and temperature into a single value, meaning a lower temperature held for a longer time can achieve the same result as a higher temperature for a shorter time. Industry practice for scissor steels typically uses 1-2 hours per tempering cycle. Proterial publishes specific tempering curves for their GIN-series steels showing the hardness response at each temperature. GIN-3, for example, maintains HRC 58-60 when tempered at 150-200°C, dropping to HRC 54-56 at 300°C.