Effect of normalization temperatures on ductile-brittle transition temperature of a modified 9Cr-1Mo steel

A modified 9Cr-1Mo steel has been exposed to three separate normalization treatments i.e. at 950 °C, 1025 °C and 1100 °C from As-received (normalized at 1050 °C and tempered at 750 °C) condition. After subsequent tempering treatment the impact toughness in terms of upper shelf energy (USE) and ductile-to-brittle transition temperature (DBTT) has been evaluated using Charpy impact testing (with 10 mm×10 mm×55 mm specimens) following ASTM E23 procedures. A substantial improvement in USE (~20 J) and reduction in DBTT (~20 °C) has been noticed for the 1025 °C treatment. The result has been analysed in terms of change in yield strength and ductility evaluated with tensile tests. Further, effects of microstructure, precipitate and texture on the change of DBTT have been studied with TEM and EBSD analyses. The results have been attributed to the smaller 'effective grain size', higher fraction of high-angle boundaries, presence of beneficial γ-fibre texture and dissolution of the pre-existing coarse precipitates in the reheated sample as compared to As-received material.