PSI - Issue 13

Avanish Kumar et al. / Procedia Structural Integrity 13 (2018) 548–553 Avanish et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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dimensions were 3 mm thickness, 6 mm width and 25 mm gauge length machined along the transverse direction of rolling. Plane strain fracture toughness tests were performed on Instron 250 kN servo hydraulic machine while following the ASTM E399-12 Ɛ 3 standard (ASTM 2013). Compact tension specimens in longitudinal-transverse (L-T) orientation were machined out of heat treated block using electric discharge machining and specimens’ dimensions were thickness ( B ) = 12.5 mm and width ( W ) = 25 mm. Chevron notch was machined in each specimen to act as a starter notch. Fatigue pre-cracking was done in order to make a crack with size, a , nominally between 0.45 W to 0.55 W . Subsequently, fracture toughness test on fatigue pre-cracked specimens was conducted at a strain rate of 0.5 mm/minute. Charpy impact test was also performed for all the conditions with U-notch machined in the specimens as per ASTM E 23-88 standard (ASTM Int. 2016). It is to be noted that duplicate mechanical tests were performed for each heat treatment condition. The X-ray diffraction patterns of NB250, NB300 and NB350 are shown in Fig. 1. It is very clear that only BCC phase ferrite and FCC phase austenite is present and no diffraction peaks of carbides are seen. The volume percentage of phases formed and dislocation density in the three different bainitic steels are given in Table 1. It is found that with a decrease in austempering temperature there is significant increase in formation of bainitic ferrite at the expense of retained austenite. This can be explained by the fact that total chemical free energy available for transformation increases with decrease in austempering temperature (Bhadeshia 2001). 3. Results and discussion

Fig. 1. XRD patterns of produced steel

Fig. 2 shows the SEM micrographs of produced steels. The microstructure consists of two phases: bainitic ferrite and retained austenite. The thickness of bainite plate measured by linear intercept method is given in Table 1. It can be seen that when austempering temperature is increased to 350°C, the microstructure becomes coarse with plentiful of retained austenite.

Fig. 2. SEM micrographs of produced steels (a) NB250; (b) NB 300 and (c) NB350