Thermodynamics of decarburization of chromium melt

In stainless steel making both chromium and carbon oxidizes when decarburization of melt is done. The Ellingham diagram for oxide formation indicates that carbon oxidation in preference to chromium oxidation can occur at temperatures greater than 1220^oC, when both elements are in pure state. Under all practical conditions carbon oxidation can occur at temperatures above 1800^oC in presence of chromium.

Chromium oxidizes to and C oxidizes to CO.

 

 

Equilibrium distribution between Cr and C is represented by considering

(1)

                                          
Assuming pure and Cr and using henry’s law for carbon, it follows

(2)

(3)

 and  are activity coefficient of carbon and chromium in liquid iron at 1wt % standard state. Hilly and Kaveney proposed the following equation for distribution of chromium and carbon. The equation 3 suggests that decrease in p_co can increase the ratio

log

(4)

The effect of Ni

log

(5)

Equations 4and 5 describes distribution ratio of Cr and c in Fe-Ni –Cr-C melt. Table shows the influence of temperature and pCO on the distribution ratio  without Ni and  in the presence of 10Wt % Ni

Table: chromium /carbon distribution ratio   

T(k) R R1 R R1 R R1 1873 25 36 89 129 207 301 1973 55 82 209 295 488 690 2073 128 173 460 619 1077 1477 2173 240 339 863 1220 2019 2852

We can infer the following form the table:

1. Increase in temperature increases R and  at all  values. High temperature is required to suppress chromium oxidation in favor of carbon oxidation.
   
   
2. Decrease in  increases both R and at all temperatures.
   
   
3. Nickel increases R and

The above observations suggest that high temperature is needed to remove carbon in presence of chromium, if stainless steel is produced at atmospheric pressure. If reduced pressures are used; lower temperature can cause oxidation of carbon.

Figure 18.1 shows carbon and chromium contents of simple Fe-Cr-C melt in equilibrium with pure solid Cr2O3 and CO gas at one atmospheric pressure at various temperatures. Carbon in the melt can be preferentially oxidized at much higher temperature without much loss of chromium. This can be done by oxygen lancing to molten bath. Any excess chromium would be oxidized and will have to be recovered from slag.

For a fix value of temperature T and partial pressure of CO, decreasing carbon content in the melt decreases the equilibrium chromium level. Hence, decarburization of the melt is not possible without chromium oxidation.

Therefore, thermodynamic alternative is to lower the equilibrium partial pressure of CO gas of the melt, so that further decarburization take place in absence of chromium oxidation at a specific temperature (as shown in Figure 18.2). With the increasing temperature and decreasing partial pressure of CO gad, equilibrium condition can be shifted to lower carbon levels. The injection of inert gas (argon or nitrogen) lowers the partial pressure of CO gas in the bath, thus allowing higher chromium contents to be in equilibrium with much lower carbon content.