Effect of silicon and partitioning temperature on the microstructure and mechanical properties of high-carbon steel in a quenching and partitioning heat treatment

Quenching and partitioning (Q andP) heat treatments of high- and low-silicon hyper-eutectoid steels, 0.21% and 1.7% silicon grades, have been investigated using dilatometry. In the present work, the amount and stability of retained austenite were quantified by a magnetic measurement technique. Optical microscopy (OM), high-resolution scanning electron microscope techniques and electron backscattered diffraction (EBSD) were used to identify and characterise the constituent phases. The mechanical properties were evaluated by micro-Vickers hardness measurements and nano-indentation measurements and linked to microstructural features. The results illustrate that increasing the silicon content will not prohibit bainite formation. At partitioning temperatures of 300 °C and higher, most retained austenite (RA) transformed to bainite in the low-silicon steel, while carbon partitioning was the main phenomenon in the 1.7 silicon grade steel. However, 28% of the bainite still formed in the presence of 1.7% silicon. In the high-silicon steel, the hardness decreased by 120HV by a mere increase in partitioning temperature from 250 to 300 °C. The wear resistance of bainitic microstructures resulting from isothermal transformation at 200 °C was similar to those of martensite. These outcomes provide an improved understanding of microstructural development with a view to industrial applications. A combination of 20–30% pre-existing martensite with 20% stabilized retained austenite and untempered martensite or/and lower bainite is suggested as a means of achieving the required mechanical properties.