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

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tests, M S , A C1 , and A C3 temperatures, were used to design proper heat treatment cycles. In this regard, all samples were austenized at 900 °C for 30 min, followed by quenching into a salt bath at three different quenching temperatures below the Ms point. Then samples were partitioning treated at 340 °C in a molten salt bath for 4 hours, and finally, were water quenched to room temperature. Fig. 1 shows the applied Q-P treatment cycles obtained from a dilatometric study of phase transformations. The microstructural characterization of the Q-P treated specimens was done by optical metallography (OM), using an Olympus PMG3 TM optical metalscope and Tescan MRA3 TM field emission gun scanning electron microscope (SEM). The metallographic specimens were prepared by grinding and mechanically polishing and attacked with conventional Nital etching reagent. The bainite sheaves thickness was measured by metallography standard ASTM E1245. The volume fraction of retained austenite was measured by X-ray diffraction method using a GNR Explorer Diffractometer with monochromatic CuKα radiation at a voltage of 40 kV and a current of 30 mA. The data was collected over a 2θ range of 45 -105° with a scan step of 0.01 and dwell time 1 S. The XRD samples were etched and polished several times to remove the effect of the grinding stage. The volume fraction of retained austenite was measured using integrated intensities of (200), (220), and (311) peaks of austenite and the (200), (211), and (220) peaks of ferrite, according to Cullity et al. (2001).

Fig. 1. Illustration of the Q-P heat treatment process.

The tensile test specimens were machined in a rolling direction with a gauge length of 25mm. The tests were done on a Zwick Z250 25TN universal tensile testing machine equipped with an extensometer, according to the ASTM E8 09 standard, with a strain rate of 3×10 -4 s -1 at ambient temperature. Charpy impact tests were performed at room temperature using a Roell Amsler™ impact testing machine, according to ASTM E23 -07 standard. The 10×10×55 mm bars had the V-notch perpendicular to the rolling direction. At least five specimens were tested for each condition, and the average values were reported.