Microstructural evolutions of low carbon Nb/Mo-containing bainitic steels during high-temperature tempering
Journal
Materials Characterization
Journal Volume
131
Pages
298-305
Date Issued
2017
Author(s)
Abstract
In this work, the effects of tempering at three high temperatures (660, 680 and 700 °C) for different holding times (5 min to 16 h) on the secondary hardening in Nb-bearing and Nb-Mo-bearing bainitic steel strips were investigated. It was found that with 0.3 wt% Mo addition in Nb-Mo-bearing bainitic steel strip, the peak hardening occurred with a significant increment in hardness (up to 40 HV) after approximately 10 min of tempering at all temperatures (660, 680 and 700 °C). In contrast, for the Nb-bearing steel strip, the corresponding increment in hardness was small (about 15 HV). The microstructural evolutions of Nb-bearing and Nb-Mo-bearing steel strips during tempering at 700 °C were investigated with scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. It was found that during tempering, bainitic ferrite platelets in Nb-Mo-bearing steel strips remained much more stable and had a higher dislocation density than those in Nb-bearing steel strips. High-resolution transmission electron microscopy provided strong evidence that the Mo addition had significant effects on suppressing the annihilation rate of dislocations and retarding the coarsening rate of nanometer-sized carbides at higher temperatures, leading to a remarkable resistance to softening after secondary hardening.
Type
journal article