PREDICTION OF THE SOLUBILITY OF NITROGEN IN MOLTEN DUPLEX STAINLESS-STEEL

The influence of temperature and effects of alloying elements on the s olubility of nitrogen in pure iron and iron alloys have been investiga ted in a pure nitrogen gas atmosphere. Both decrease in temperature an d additions of Cr, Mn and Mo are found to increase the solubility of n itrogen in Fe-Cr alloys, whilst increase in temperature and additions of Ni and C are found to decrease the solubility of nitrogen. The firs t and second order interaction parameters e(NFe)(j) and r(NFe)(j) have been calculated, with reference to the infinitely dilute solution of nitrogen in liquid pure iron, giving good agreement with the recent re sults of other workers. A more relevant reference stare to the duplex phase field has been selected to be an Fe-20Cr alloy and further inter action parameters, e(NFe20Cr)(j) measured. Equations have been derived for the prediction of nitrogen solubility in molten duplex stainless steels containing 20-30 mass% Or, which obey Sieverts law at pressures of nitrogen up to 1 atm. log%N-T,PN2(alloy)=log root P-N2+log%N-T,PN2 (Fe20Cr)-logf(T,PN2)(alloy) where, (T,PN2)(alloy)=[e(NF20Cr)(Cr)x(%Cr- 20)+Sigma(j=1,j not equal Cr)e(NFe20Cr)(j)x%j](T,PN2) Where values of e(NFe20Cr)(j) are given in the text and are applicable between 1.550 a nd 1650 degrees C. Large-scale experimental work is also shown to conf irm the laboratory results and the practical implications, including t he effects of oxygen on the rate of nitrogen absorption, of these equa tions is given.