Ferrochrome, which contains about 55 to 70% chromium is the principal source of
chromium. This ferroalloy can be classified into various grades, based primarily on their carbon
content, such as:
1)Low carbon ferrochrome (about 0.1% C).
2) Intermediate carbon ferrochrome (about 2% C).
3)High carbon ferrochrome (around 7% C).
Amongst these grades, the high carbon variety has the drawback that though it is the least expensive, it raises the carbon content of the melt. This is undesirable, since all SS grades demand carbon contents less than 0.03%. As per Ellingham diagram , chromium forms stable oxides. Hence, the removal of carbon from the bath by oxidation to CO is associated with the problem of simultaneous oxidation of chromium in molten steel. The higher the temperature, the greater is the tendency for preferential oxidation of carbon rather than chromium. From this point
of view, higher bath temperatures are desirable; however, too high a temperature in the bath gives rise to other process problems.
The dilution of oxygen with argon lowers the partial pressure of CO, which helps in
preferential removal of CO without oxidising bath chromium. Attempts were made to use this in the EAF, but the efforts did not succeed. Hence, as is the case with the production of plain carbon steels, the EAF is now basically a melting unit for stainless steel production as well.
Decarburisation is carried out partially in the EAF, and the rest of the carbon is removed in a separate refining vessel. In this context, the development of the AOD process was a major
breakthrough in stainless steelmaking.