Relative effect of B and N concentrations on the microstructural stability and mechanical properties of modified 9Cr-1Mo steel

The present study systematically varied the concentrations of B (0–100 ppm) and N (20–500 ppm) in modified 9Cr-1Mo steel to understand the combined effect of B and N addition on the microstructural stability and the creep resistance (650 °C, 120 MPa). The Auger Electron Spectroscopic analysis reveals the enrichment of B within the M₂₃C₆ precipitates at the vicinity of prior-austenite grain boundaries in B added steels both in normalized and tempered specimens and also in creep tested specimens. The 70 ppm B steel with 108 ppm N showed the best creep resistance (rupture time as high as 1536 h and minimum creep-rate as low as 1 × 10⁻⁵/h), followed by 90 ppm B, 90 ppm N steel. Too low B content (25 ppm or less) or too high N content (500 ppm) affected precipitate stability and consequently the creep resistance of the steel. Finally, it can be recommended that it is necessary to maintain optimum concentrations of B (70–100 ppm) and N (90–110 ppm) to improve the creep resistance of modified 9Cr-1Mo steel.