Quenched steel is often too brittle to be useful. Tempering involves reheating the hardened steel to a lower temperature to sacrifice a small amount of hardness in exchange for a massive boost in toughness and impact resistance. 6. Manufacturing and Shaping Methods
Metallurgy for the Non-Metallurgist " (often associated with author Arthur C. Reardon and ASM International metallurgy for the non-metallurgist pdf
A metal part is not a single perfect crystal but a collection of millions of small crystals (grains). The boundaries where grains meet impede dislocation motion, making fine‑grained metals stronger and tougher at room temperature. Quenched steel is often too brittle to be useful