ROSENFELD IMAGES LTD / SCIENCE PHOTO LIBRARY ROSENFELD IMAGES LTD / SCIENCE PHOTO LIBRARY
Hardening of cogs. Red-hot steel cogs being lifted out of a furnace (at lower centre) during case- hardening heat treatment. Case-hardening is done after a component has been made. The cogs are heated with a substance which can donate either carbon or nitrogen. During the heating process, which can last from several hours to several days, carbon and nitrogen atoms dissolve into the outer 0.3-3 millimetres of the steel. The glowing components are then rapidly cooled. This makes the outside of the component harder and more wear- resistant, whilst leaving the interior softer and so more shock-resistant. Photograph of cogs being made for rolling mill machinery.
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