Microstructure of 20MnTiB Steel After 870 ℃ Quenching
20MnTiB steel is a high carbon type steel with an alloying element of manganese, titanium, and boron. It is a highly suitable steel for undergoing quenching process, wherein the steel is exposed to a high temperature and rapidly cooled to attain a uniform microstructure. In this study, it is observed that when the 20MnTiB steel is heat treated at a temperature of 870oC and then quenched, the microstructure of the steel consists of bainite and martensite phases. Moreover, there is also some amount of ferrite present in the microstructure due to the effects of localized cooling.
At a metallurgical level, the entire microstructure of the 20MnTiB steel after quenching is relatively uniform and homogenized. Upon careful microscopy, it was noted that there were large areas of bainite present that were shaped like octahedral flakes. The bainite was characterized by hard and wear resistant particles that created an overall increase in the mechanical strength of the steel. The presence of bainite was further attributed to the localized cooling effect, as the frozen bainite structure offers a strong resistance to wear and tear. Moreover, the high hardness and wear resistance of the bainite allowed for a better wear capability when compared to ferrite and martensite.
It was also established that the presence of martensite phase in the microstructure was due to the high temperature quenching process that was used to heat treat the steel. The martensite was characterized by a hexagonal packed lath-like structure that created a higher hardness and strength when compared to the ferrite phase. Furthermore, the presence of martensite also offered the 20MnTiB steel greater wear and tear resistance, as well as better fatigue load management.
The third phase that was found within the microstructure of the 20MnTiB steel was a small amount of ferrite. The ferrite phase was characterized by a more elongated structure in comparison to the octahedral shaped bainite phase, and existed in relatively small amounts. Although the presence of ferrite phase offered the steel greater hardness and strength, it also made it vulnerable to corrosion due to the concentration of chromium at the grain boundaries.
Overall, the quenching process that was used to heat treat the 20MnTiB steel greatly improved its microstructure. The bainite phase, due to its excellent wear and tear resistance, was responsible for the strengthening of the steel, whereas the martensite phase contributed to the wear capability of the steel. Finally, the ferrite phase offered the steel additional hardness and strength, albeit with a greater risk of corrosion due to the presence of chromium at the grain boundaries.
