ASTRID & HANNS-FRIEDER MICHLER / SCIENCE PHOTO LIBRARY ASTRID & HANNS-FRIEDER MICHLER / SCIENCE PHOTO LIBRARY
Case-hardened steel. Light micrograph of a section through a case-hardened steel tool. The brown area (right) is the original low-carbon (soft) steel, the orange area (left) is case-hardened. Case-har- dening is performed when soft steel has been used to manufacture a tool, which allows easy finishing of the piece. In order to harden the surface, the tool is either heated near a source of carbon or dipped into liquid sodium cyanide. Carbon atoms dissolve through the metal, altering its crystal structure and increasing mechanical strength. Magnification unknown.
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