Abstract: 

For melting and refining of stainless steel grades there are many different process routes. The individual choice is influenced by raw material availability, desired final product, existing shop logistics and capital economics. In general the stainless steel making process can be classified as a Duplex or Triplex type. The Duplex refining process which is composed of electric arc furnace melting, followed by converter refining, can be extended to a Triplex refining route by installation of a VOD plant. Today, about 90% of all stainless steel is produced according to the duplex or triplex route. 

Slag Metal Reactions

For CaO-MgO-SiO2-(Fe, Cr) oxide slags with lime/silica ratios between 1.2 and 2.0, Pathy and Ward found the experimental solubility limit of Cr2O3 in the slag to vary from 3% in contact with low-chromium alloys (less than 2% Cr) to about 9% when the alloy was in the range 14-20% Cr. These data agree with the phase diagram Cr2O3-CaO-SiO2 as determined by Glasser and Osborn. These observations may have somewhat limited validity since it has been found that the primary oxide phase is not Cr2O3, but, either chromite FeCr2O4 or Cr3O4.

(Cr)SLAG / [Cr]METAL = 0.3 (%FeO)  ..........  (1)

Will give close representation of the chromium partition ratio. By measuring the iron oxide concentration of the slag, the extent of chromium oxidation can be determined. The degree of oxidation of the slag depends on practice and must be determined experimentally.

In Triplex process, after melting in primary unit, refining is carried out in two different vessels. In the first vessel, decarburization and major refining take place and final stages of desulphurization and degassing take place in second vessel.

Compare to duplex, triplex process has following merits:
Increase productivity

Increase number of heats per day

Increase scrap to liquid metal yield

Improved quality of metal

High operational flexibility

Comparatively lower cost of production.