Influence of boron and nitrogen on microstructure and hardness of heat-affected zone of modified 9Cr-1Mo steel-Gleeble simulation study

Microstructural evolution in three different boron-added modified 9Cr-1Mo (P91) steels has been studied after subjecting them to in different peak simulation conditions. Optical microscopy, scanning electron microscopy, and Xray diffraction tools have been used to characterize the microstructure. Prior austenite grain boundaries are found to be present after heat treatments at 950-1200 degrees C, suggesting thereby that these boundaries are stable and that the "memory effect" exists in these steels. Analysis based on the modified Williamson-Hall plot shows that while the crystallite size remains similar irrespective of the peak simulation temperature, the highest peak simulation temperature results in lower dislocation density. The measured hardness values remain similar irrespective of the peak simulation temperature, except for marginal variations among the three different heats.