Reply To: Carbon into cementite – a question

[Rchard Williams]

Yes, Keith, in a normally cooled plain carbon steel, the pearlite content at room temperature is a function of carbon content, since all the carbon has to be redirected when austenite changes to ferrite, (the solubility of carbon in ferrite is very low). However, a microstructure which is 100% pearlite only exists at a carbon content around 0.8%, the eutectoid value. Up to that point ferrite occurs in the microstructure roughly pro rata with the carbon content, whilst beyond 0.8% carbon, cementite is increasingly the phase in equilibrium with the pearlite. You presumably mean that your sword contained 0.6% carbon. (6% is not possible, even in saturated cast irons.) Very crudely, the sword’s microstructure would contain a weight ratio of 2/8 ferrite to 6/8 pearlite, but of course it is a volume ratio which you observe..

If it had been cooled more quickly, perhaps quenched in water, then all sorts of tortured intermediate structures could result, of which martensite is the most famous. Surely, by the era that you are talking about, iron smiths would have known that to get the best performance out of of a steel sword, some sort of heat treatment was necessary and the material would not have been left in what is known as the normalised state?