Study of distribution of B and N using auger electron spectroscopy (AES) and atom probe tomography (APT) techniques, in modified 9Cr[sbnd]1Mo steels with controlled addition of these elements

Proof for distribution of boron in M₂₃C₆ precipitates and prior austenite grain boundaries, and role of boron in reducing the coarsening kinetics of the precipitates and thereby improving the creep strength, are established and quantified in modified 9Cr[sbnd]1Mo steels using AES, APT, and TEM. The average atomic concentration of B estimated using APT found to be ∼80 times more in the precipitate than its nominal composition in the 100 ppm B containing steel. The necessity to have a small quantity of nitrogen in the steel to take advantage of strengthening by carbo-nitrides along with boron-carbides is also proven from the characterization performed on three different steels after creep tests, one with 500 ppm N with no intentional addition of B (rupture life-44 h for creep test at 650 °C/120 MPa), another with 70 ppm B and 108 ppm N (rupture life-770 h) and the third steel with 100 ppm of B and 20 ppm of N (rupture life-282 h). Based on the results of characterization and the creep tests, maintaining an optimal concentration of around 70-120 ppm B and 90-120 ppm N are recommended for the maximum advantage of the presence of these two micro-alloying elements in this class of steel.