Niobium Carbo-Nitride Precipitation Behavior in a High Nitrogen 15Cr-15Ni Heat Resistant Austenitic Stainless Steel

A high nitrogen 15Cr-15Ni niobium-stabilized austenitic alloy has been produced and subjected to a special heat treatment consisting of 5 hours of solution treatment at 1270 degrees C followed by hot rolling, quenching and subsequent aging at temperatures of 700 degrees C to 800 degrees C. It was found that fine dispersion of nano-sized thermally stable primary Nb(C,N) precipitates had already formed in the as-cast condition. The particles were presented at all examined stages of the TMT process (as-homogenized, as-solution treated and as-aged conditions). Secondary precipitates Nb(C,N) were densely formed during subsequent aging; these precipitates had sizes of 4 nm to 5 nm. Both the primary and secondary Nb(C,N) particles showed excellent thermal stability within the temperature range of 700 degrees C to 800 degrees C. The creep properties of the studied alloy at 750 degrees C were superior when compared to those of commercial type 347 stainless steel.