Calorised lance pipes are prepared by rendering calorising
finish to carbon steel in a thickness of 100-150 microns on both inner and
outer surfaces, and further coating with refractory materials in 200-300 microns
thickness on both inner and outer surfaces. This metal diffusion treatment
promotes the fireproof properties of steel pipes. The diffusion is an inter
metallic bond, which doesn’t get damaged either by mechanical working
like bending or straightening or by high temperatures. In the case of general
fireproof pipes, oxidation takes place at that part of surface in contact
with the flame. In the case of these metal diffused pipes, the metal existing
at the surface of the diffused zone is oxidised to its respective oxide, which
prevents the further progress of oxidation and also melting. For instance,
in calorsing, aluminum is oxidized to alumina, which has a very high melting
point such as 2050oC compared to the melting point of aluminum, which is 658oC.
As shown in the following sketch, the splashing action of high-pressure oxygen
on molten steel is most severe at the upper inside part of the pipe (‘A’
point in the sketch). At this point the following reaction takes place.
