Red-short

Red-short, hot-short or Sulfur embrittlement is the quality possessed by carbon steel that suffers from having too much sulfur as an impurity.

Iron or steel, when heated to above 900 °F (480 °C), glows with a red color. The color of any heated object changes predictably (due to black body radiation) from dull red through orange and yellow to white, and can be a useful indicator of its temperature. Good quality iron or steel at and above this temperature becomes increasingly malleable and plastic. Iron or steel having too much sulfur, on the other hand, becomes crumbly and brittle. This is due to the sulfur forming iron sulfide/iron mixtures in the grain boundaries of the metal which have a lower melting point than the steel.