The relative effect of boron and nitrogen concentrations on microstructure and creep resistance of weld simulated heat-affected zones of modified 9Cr-1Mo steel

Abstract

To comprehend the combined impact of B and N concentrations on modified 9Cr-1Mo steel's heat affected zone (HAZ) creep resistance, five different steels with varying B (0-100 ppm) and N (20-500 ppm) concentrations were subjected to two types of weld thermal cycles (WTC), namely coarse-grained HAZ simulation and inter-critical HAZ simulation with peak temperatures of 1175 °C and 850 °C, respectively. Prior to and after creep testing (at 650 °C, 120 MPa stress), the microstructures and precipitates in the WTC simulated samples were studied using optical and electron microscopy, electron backscattered diffraction, and auger electron spectroscopy techniques. As expected, simulated HAZs of high B-containing steels (70-100 ppm B) showed superior creep resistance compared to low B steels (0-25 ppm B) due to the B stabilizing effect of M23C6 precipitates (primarily (Fe,Cr)23(B,C)6) though the rupture time for the simulated HAZs are lower than that of the base metals tested at the same test temperatures and stress levels. An interesting finding is that, despite the fact that 70 ppm B steel (with 108 ppm N) demonstrated the best creep resistance in normalized and tempered condition, 100 ppm B steel (with just 20 ppm N) obtained the best creep resistance of simulated HAZ. Large difference in the rupture lives between simulated ICHAZ and simulated CGHAZ specimens reveals the effect of triaxility in reducing the rupture lives in simulated CGHAZ specimens. This is also applicable to actual weld joints.