**4. Conclusions**

*Engineering Steels and High Entropy-Alloys*

mechanisms, however martensite formed by shear process. The contribution of

The lower (Ac1) and upper (Ac3) critical temperatures during heating at 5°C/s were obtained 734 and 912°C respectively from the present study. The ITT diagram's purpose is not only to collects the statistics about the nature of phases however, it also finds out the kinetics of phase transformation for the holding period at constant (isothermal) temperature. This is a very useful set of heat treatment processes to obtain the desired properties by moderation of microstructure. This ITT diagram is also known as time temperature transformation (TTT) diagram. An ITT diagram plotted based on isothermal temperature heat treatment is displayed in **Figure 6**. The initial microstructure was consisting of proeutectoid ferrite and pearlite which transforms into proeutectoid ferrite, pearlite, and other new phases such as widmanstätten ferrite, upper/lower bainite and transformed martensite (**Figures 2** and **3**). It is worth noticing from **Figure 6** that the time of start transformation decreases with decreasing isothermal temperature until 250°C, and further starts increasing. The start transformation time is delayed by around 12 s and takes longer time to finish (end) transformation when the temperature is close to the lower critical temperature as can see in **Figure 6**. As decreasing in isothermal temperature, both Ts and Tf phase transformation times are observed decreasing. The nose of ITT is found to be at 500°C, where there was the shorter delay (less than 1 s) between the start and end transformation of phases. An analysis of this ITT diagram is divided into five regions which show a combination of phase's microstructure (**Figure 6**) as

shear process increases as the temperature decreases [11].

confirmed by the microstructural evolution (**Figures 2** and **3**).

IT750°C (>Ac1), the start transformation occurred after 12 s (incubation period) and the end transformation (100% transformation of phases) after 1080 s, which is predicted in the form of diametric changes in mm during experimental studies. The microstructure consists of proeutectoid ferrite, widmanstätten ferrite, pearlite and martensite at high temperature isothermal holding. Small volume fraction of

**34**

**Figure 6.**

*Isothermal transformation diagram of micro-alloyed steel.*

The following outputs are constructed from the present work on micro-alloyed steel.


transforms into predominantly lower bainite with increment in martensite. At less than 300°C, the microstructure was observed to be a martensite structure and acicular martensitic dendrite structure at austenite grain boundary.
