Prediction of the solubility of nitrogen in molten duplex stainless steel

Abstract

The influence of temperature and effects of alloying elements on the solubility of nitrogen in pure iron and iron alloys have been investigated in a pure nitrogen gas atmosphere. Both decrease in temperature and additions of Cr, Mn and Mo are found to increase the solubility of nitrogen in Fe-Cr alloys, whilst increase in temperature and additions of Ni and C are found to decrease the solubility of nitrogen. The first and second order interaction parameters eNFej and rNFej have been calculated, with reference to the infinitely dilute solution of nitrogen in liquid pure iron, giving good agreement with the recent results of other workers. A more relevant reference state to the duplex phase field has been selected to be an Fe-20Cr alloy and further interaction parameters, eNFe20Crj, measured. Equations have been derived for the prediction of nitrogen solubility in molten duplex stainless steels containing 20-30 mass% Cr, which obey Sieverts law at pressures of nitrogen up to 1 atm. log %NT,PN2alloy = log √PN2 + log %NT,PN2Fe20Cr-log fT,N2alloy log fT,PN2alloy = [ eNFe20CrCr × (%Cr-20) + Σj=1.j ≠ Cr eNFe20Crj × % j]T.PN2 Where values of eNFe20Crj are given in the text and are applicable between 1 550 and 1 650°C. Large-scale experimental work is also shown to confirm the laboratory results and the practical implications, including the effects of oxygen on the rate of nitrogen absorption, of these equations is given.