The effect of niobium carbide precipitation on the density changes and creep properties of Type 347 austenitic stainless steels

Density changes due to cavitation in Type 347 and Type 316 austenitic stainless steels, where precipitation occurs during creep, have been determined by calculating the density changes due to precipitation and subtracting them from accurately measured density changes. These determinations of creep damage throughout the creep process indicate small amounts of cavitation during the primary and early secondary stages of creep and large cavitation effects in tertiary creep. Calculations of the amount of niobium carbide available for precipitation during creep in Type 347 alloys have emphasized the importance of obtaining the exact stoichiometric ratio of 10.3:1 Nb:C to give the optimum strength and rupture life. Small deviations from stoichiometry produce a rapid decrease in the amount of precipitation available and a deterioration in the creep properties. The data on creep damage, although limited, suggest that small amounts of cavitation are associated with increased rupture life but that creep ductility is not necessarily related to cavitation. The influence of niobium in the correct stoichiometric ratio with carbon appears to be (i) to provide creep strength by the precipitation of Nb₄C₃ in the matrix, and (ii) to extend creep life by the precipitation of Nb₄C₃ at grain boundaries, reducing grain-boundary sliding and cavitation, and delaying fracture.