2. Further treatments
page 10

Photo of heat treatment
Picture 2.6 Heat treating a steel sample.
Quenched steel is hard and brittle. Often it is just too brittle and must be made more malleable, even if this means that some of the hardness has to be sacrificed. This is achieved by a process known as tempering.

The quenched steel is heated again but this time to a temperature between 200 °C and 300 °C. The colour of the steel gives an indication of its temperature. It ranges from straw yellow at 200 °C to dark blue at 300 °C. This is due to an increasingly thick film of iron oxide on the surface of the metal.

When the metal reaches the tempering temperature, it is quenched again in cold water or oil. The result is a steel that is still hard but is more malleable and ductile. The only drawback to this procedure is that the metal must not be worked further above its tempering temperature.

Question 2-4.
At which of the following temperatures will tempered steel appear dark blue?

(A) 200 °C         (B) 300 °C          (C) 400 °C

Summary                   Close
  • a quenched metal can be tempered to restore its malleability and ductility