Oxygen is an element that is first purposely brought into the steel melt to remove some unwanted elements or to reduce their concentration

(oxidation). In the made cast steel there is on the contrary necessary to reduce the oxygen content with the use of deoxidation to such a

level in order to avoid a reaction with carbon with the formation of CO bubbles. Concentration of oxygen in steel before casting is given,

in particular, by the manner of metallurgical processing and the used deoxidation process. Oxygen is found in molten steels both as

chemically bound in the form of oxides and in the form of oxygen dissolved in the solution – the melt. Chemical composition

of the melt strongly influences the activity of oxygen dissolved in the melt and further on the composition of oxidic inclusions forming in

the melt during the reaction with oxygen. In the Fe-C-Cr-Ni based alloys in the reaction with oxygen greatly participates also chrome,

whose products are often in solid state and they are the cause of forming such defects as e.g. oxidic films.

Deoxidation of steel with carbon under reduced pressure is often used for increasing the steel purity. Suitable units for this purpose in

foundries are vacuum induction furnaces. Possibilities of increasing the steel purity by deoxidation with carbon in the vacuum induction

furnace were studied for the steel for the petrochemistry of specific composition 25Cr/35Ni. The charge composed of the return material

only was melted in the air. During melting the charge oxidized and the oxidizing slag formed. Chemical composition of steel, morphology,

chemical composition of inclusions in the steel and chemical composition of slag after vacuuming were studied on the basis of samples

taken before and after vacuuming. Temperature and oxygen activity were measured before and after vacuuming. Globular inclusions with

dominant content of silicon and manganese were observed in steel before and after vacuuming. Contents of total oxygen in steel didn’t

change significantly during vacuuming. On the basis of composition of inclusions and measured oxygen activity the activity of Cr2O3

in

inclusions was calculated. A slag sample was taken after vacuuming and equilibrium oxygen activity in steel with regard to the Cr2O3

content in the slag was estimated from the slag composition. Equilibrium oxygen activity in relation to the Cr2O3 content in the slag was

higher than equilibrium activity measured in the steel. For this reason it is not possible, under the studied conditions, to decrease oxygen

content in steel during vacuuming.

The contribution summarises the results of oxygen activity determinations, which were measured and registered continuously in castings from cast irons with various types of graphite. The results were used to find the relationship between two variables: natural logarithm of oxygen activities and reverse value of thermodynamic temperature 1 /T. Obtained regression lines were used to calculate oxygen activity at different temperatures, to calculate Gibbs free energy ΔG at the different temperatures and to calculate the single ΔG value for significant temperature of the graphite solidification. The results were processed by a statistical analysis of data files for the different types of graphite with flake, vermicular and spheroidal graphite. Each material has its proper typical oxygen activities range and individual temperature function of Gibbs free energy for analysing and governing casting quality.