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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ductility of nanostructured metals and alloys decreases significantly with a decrease in microstructural length scale (Yakubtsov et al. 2008; Yang et al. 2012). However, the presence of film-type retained austenite in between the bainitic subunits provides adequate ductility to nanostructured bainitic steels (García-Mateo & Caballero 2005; Bhadeshia 2010). The other type of retained austenite present in the microstructure is blocky austenite which is less stable than film-type retained austenite enriched with higher carbon content. During the tensile tests of nanostructured bainitic steels, retained austenite transforms into the harder phase martensite which gives transformation strain (TRIP effect) and enhances the work hardening capacity (Bhadeshia 2010; Sourmail et al. 2017). Hence, the morphology and volume percentage of phases present in the microstructure control the strength ductility combination. However, the possible incorporation of nanostructured bainitic steels in engineering applications will be determined by the resistance to impact loading as well as tolerance to the presence of flaws. Thus, it is critical to investigate the role of morphology and volume fraction of phases on the impact and plain strain fracture toughness of nano-bainitic steels. B. Avishan et al.(Avishan et al. 2012) reported the impact energy trend with respect to austempering temperature and showed that the maximum in toughness is obtained at intermediate temperatures. The lower toughness at higher austempering temperature was linked to the coarse morphology of retained austenite while the degraded toughness at the lowest temperature was ascribed to insufficient volume fraction of ductile austenite phase. In a recent work on clean super bainitic steel, M. J. Peet et al. (Peet et al. 2017) have improved the toughness – strength combinations via reduction of prior austenite grain size. Although, they could not perform valid K IC test, an ultimate tensile strength of 2.5 GPa with impact energy of 5 J was obtained for 20 µm prior austenite grain size. Plain strain fracture toughness values for nanostructured bainitic steels have been reported to be in the range of 28-55 MPa m 1/2 in isolated studies (Bhadeshia 2010; Fielding et al. 2016). However, none of these studies have systematically correlated toughness to morphology and volume fraction of bainite and austenite. A reduction in austempering temperature of bainite results in a reduction in bainitic lath thickness, increase in volume fraction of bainite, reduction in volume fraction of austenite and increase in the volume fraction of film-like retained austenite vs. blocky austenite. Thus, it is really difficult to predict the effect of all these changes in morphology on the fracture toughness and impact toughness without methodical experimental studies. In the current work, specimens have been prepared by isothermal holding at 250, 300 and 350°C to get a range of microstructures with different bainitic lath thicknesses and volume fraction. Tensile tests have been conducted on the specimens to determine the strength and ductility. Plain strain fracture toughness as well as impact toughness testing has been done on all the specimens. Volume fraction of retained austenite has been found to play a critical role in enhancing the toughness properties of nanostructured bainite. The cast steel of composition Fe-0.85C-1.30Si-1.92Mn-0.44Al-2.05Co-0.29Mo (in wt.%) has been produced using vacuum induction melting under argon atmosphere. Subsequently, the cast ingot was homogenized at 1000°C for 48 hours in muffle furnace. The homogenized ingot was hot rolled from a thickness of 34 mm to 14 mm at 1000°C and air cooled to room temperature. Rolled plates were transformed to bainite via austenitization at 950°C for 40 minutes followed by isothermal transformation in a salt bath at three different temperatures 250, 300 and 350°C for 40, 30 and 20 h respectively. The steels transformed by austempering at the three different temperature conditions of 250, 300 and 350°C are named as NB250, NB300 and NB350 respectively. Specimens were cut out of transformed steel and ground on emery papers followed by final fine polishing using diamond paste of size 0.5 µm. Nital was used as the etching reagent for detailed characterization using AURIGA ZEISS dual beam scanning electron microscope (SEM). The average bainitic lath thickness was measured by drawing perpendicular line to the long edges of bainitic plates in SEM micrographs. At least 100 measurements were made on multiple SEM micrographs for each specimen, on random sections, so that stereological corrections were made to obtain the true bainitic lath thickness. True lath thickness, T is calculated by applying the relation T = (2×average thickness)/ (Garcia-Mateo et al. 2016). The X-Ray diffraction (XRD) patterns of all the alloys were obtained using PANalytical X’pert Pro MPD system with Cu- Kα radiation. The XRD patterns were analyzed using X’Pert Highscore software and volume fraction of bainite and retained austenite were calculated from integrated intensities of (110), (200), (211) ferrite peaks and (111), (200), (220) austenite peaks respectively. Tensile tests were performed for all the conditions as per ASTM E8 (ASTM Int. 2016) on Instron 250 kN servo hydraulic machine at a cross head velocity of 0.5 mm/minute. The specimens’ 2. Experimental details