Bainitic transformation in low carbon micro-alloyed hot forged steels for diesel engine components

Keskin, Burak
In recent years, the fuel injection pressures increased to very high level for more efficient internal combustion engines. With developing technology, it is expected that components must resist at least 2500 bar pressure working conditions. These high pressure levels must accompany with better properties such as mechanical, fatigue and corrosion. Automotive industry mostly use quenched and tempered steels for such critical applications. However, it is known that, bainitic steels have better mechanical properties in terms of UTS, toughness and fatigue. In recent years, bainitic forging steels are emerged as an alternative to Q&T steels. Furthermore, as bainite forms during continuous cooling from austenite region, the Q&T steps is omitted. However, there are only a few bainitic forging grade steels in the market. In this study, the bainite formation and its effect on mechanical properties of a continuous cooled forging grade experimental steel is investigated. By using the calculated CCT diagrams (Continuous Cooling Transformation), a basic alloy 0.2C-1.50Si-1.40Mn-1.45(Cr+Mo) is selected as an experimental steel. The formation of bainite and its morphology upon different cooling rates is observed. The effect of Ti, Nb and S addition on properties is also studied. The amount of retained austenite of the samples are analyzed using XRD. The DTA analysis is used to find the exact Bs and Ms temperature of the steel. The tensile strength and charpy impact toughness tests are carried out. It is found that the 0.01%Ti vi added experimental steel yields high toughness levels (around 20J) at a strength level of 1250 MPa and 16% elongation. Furthermore, it is found that the bainite formation is possible at a large interval of cooling rate which can be an advantage in industrial forging practice.