PSI - Issue 37

E. Entezari et al. / Procedia Structural Integrity 37 (2022) 145–152 E.Entezari et al. / Structural Integrity Procedia 00 (2021) 000 – 000

148 4

3. Results and discussion Fig. 2 (a) represents the time-temperature-transformation (TTT) diagram of the studied steel designed based on thermodynamic theories, using the MUCG83 thermodynamic model. TTT diagram composed of two upper and lower C-shape curves predicting, respectively, reconstructive solid-state transformation kinetics and the kinetics of the shear transformations. Further, the dilatometry test results showed that the practical M S , A C1 , and A C3 temperatures were 321 °C, 736 °C, and 841 °C, respectively (Fig. 2 (b)).

Fig. 2. (a) TTT diagram obtained from MUCG83™ thermodynamic model, (b) dilatometric curve to measure the ”‹–‹…ƒŽ temperatures. Fig. 3 illustrates the OM and SEM pictures of studied samples after the Q-P treatment at three different quenching temperatures. The microstructure contains bainitic sheaves (dark regions) separated by austenite/martensite microblocks (light regions). It can be seen that the higher volume fraction of austenite/ martensite microblocks (light regions) resulted from a lower quenching temperature (260 °C). High magnification SEM pictures were used to observe the austenite/ martensite microblocks (A/M), tempered martensite (TM), and the alternative layer of bainitic ferrites (B) and austenite films (A). When the quenching process was applied below the martensite start temperature, martensite microblocks were formed, and the retained austenite was stabilized, whereas the martensite was tempered (Figs 3 (b), 3 (d), and 3 (f)). Moreover, bainite was formed below the martensite start temperature in the martensitic matrix during Q-P treatment, as shown by arrows in Fig. 3. These observations coincide with the results of Wang et al. (2013). They showed that bainitic ferrite formed below the martensite start temperature can grow slowly, similar to Widmanstatten ferrite growth above the martensite start temperature, confirming the observations made here. It was observed that decreasing the quenching temperature increased the volume fraction of tempered martensite and austenite/martensite microblocks as shown in Fig. 3 (b). The austenite phase can be classified into two morphologies at the end of bainitic and martensitic transformations, as reported by Xiong et al. (2013). First, filmy austenite appears within the bainitic plate, which can enhance mechanical properties by a phase mixture effect, and the second is austenite microblocks, which separate bainitic sheaves and introduce an additional strengthening by the same mechanism. Decreasing the quenching temperature from 300 °C to 260 °C reduced the average size of bainitic plates from 0.253 µm to 0.162 µm, as shown in table 1. Consequently, decreasing the quenching temperature made it possible to decrease the average size of bainitic plates. A decrease in quenching temperature caused bainitic plates to consume more primary austenite and reduce their average size. Mousalou et al. (2018) reported that slender bainitic sheaves are gained at lower transformation temperatures, and these sheaves absorb primary austenite.