Mohammad Ali Mohtadi Bonab

In the current research, a complex microstructure and crystallographic data were developed through quenching and partitioning (Q&P) process to improve tensile properties of commercial pearlitic carbon-silicon steel. Two-stage Q&P process, including full austenitization, quenching at 220
C, followed by two different partitioning temperatures, was applied to the as-received specimen to generate a complex microstructure composed of tempered martensite, bainite, ultrafine carbides/martensite-austenite/retained austenite particles. Microstructure and crystallographic data were investigated by scanning electron microscopy, electron backscattered diffraction (EBSD), and X-ray diffraction techniques. Then, hardness and tensile properties were evaluated to confirm the improvement of mechanical properties. Dilatation-temperature curves exhibited the kinetics of martensitic and bainitic transformation during quenching and isothermal partitioning stages. The presence of nano-carbide particles inside athermal martensite was confirmed by electron microscopy due to the pre-formed martensite carbon depletion during the partitioning stage coupled with bainitic transformation. The formation of preferential atomic-compact <111> direction in BCC (martensite/bainite) plates characterized by EBSD, could enhance ductility by providing adequate slip systems. Point-to-point misorientation analyses demonstrated a slight dominance of low angle boundaries proportion in bainitic dominance structure in Q&P-220-375 specimen, which could be used in phase characterization. Results revealed that the development of nanoscale carbide dispersed in refined bainite/ martensite matrix boosted the yield and ultimate tensile strength by over 100% and 110% compared to the initial pearlitic microstructure. However, ductility reduced to half value in Q&P-220-325 and Q&P-220-375 specimens.