Prediction of blowhole formation in iron-carbon-nitrogen alloys

Abstract

An equilibrium model is developed which can satisfactorily he used to predict the required initial nitrogen content for blowhole formation to occur, for Fe–C–N alloys with carbon contents up to about 1 wt-% For the most part, the accepted criteria of solidified fraction g_s = 0·9 and a gas pressure in the blowhole of 1 atm are satisfactory, Only for alloys with compositions within the peritectic region does it appear that lower g_s-values may be appropriate in equilibrium with the δ- phase. A rapid increase in the nitrogen available for blowhole formation occurs in the 0·25-0·51 Wt-% C range; this increase derives mainly from the large value of the liquid/austenite nitrogen-distribution coefficient k^γ _N, compared to that of the liquid/δ-ferrite distribution coefficient k^δ _N. For alloys containing more than 1·0 wt-% C, the distribution coefficient k^γ _N attains unity during freezing and here the criterion for blowhole formation is that a nitrogen pressure of 1 atm must be attained before k^γ _N actually equals unity. This criterion ensures that sufficient gas is available for blowhole growth and the results obtained using it are supported by the experimental evidence. In general, the g_s = 0·9, 1 atm criteria apply, provided sufficient gas is available. In order to comply with the latter condition, it is sometimes necessary to use lower values of g_s to ensure adequate solute supply to the blowhole.