ASTRID & HANNS-FRIEDER MICHLER / SCIENCE PHOTO LIBRARY ASTRID & HANNS-FRIEDER MICHLER / SCIENCE PHOTO LIBRARY
Light micrograph of a section through a case- hardened steel tool, photographed using differential interference contrast (DIC). The brown area shows the original low-carbon (soft) steel, the orange area is case-hardened. Case hardening is performed when soft steel has been used to manufacture a tool, allowing easy finising of the piece. In order to harden the surface, the tool is either heated in a hydrocarbon atmosphere, or dipped into liquid sodium cyanide. Carbon atoms diffuse into the surface, providing a layer of hardened (high- carbon) metal.
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