PSI - Issue 28

Avanish Kumar et al. / Procedia Structural Integrity 28 (2020) 93–100 Avanish et al. / Structural Integrity Procedia 00 (2019) 000–000

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effect (Bhadeshia, 2010). The combination of high strength and ductility has made these steels suitable for several engineering applications. In prior works, authors have tried to establish the correlation between microstructure and mechanical response under static loading conditions (Kumar and Singh, 2018b, 2018a; Singh, Kumar and Singh, 2018). Selection of these steels based only on static load design may not suffice and can lead to immature failure under cyclic loading in presence of flaws. Therefore, it is important to study the response of these steels under fatigue loading for safer design. There are multiple factors like prior austenite grain size, morphology of BF and RA as well as content of phases which can influence the residual strength and fatigue life. In a study on high cycle fatigue behaviour of Q&P steel (Diego-Calderón et al. , 2015), it was reported that higher phase fraction of RA led to higher fatigue crack resistance by delaying crack propagation due to austenite to martensite phase-transformation. It was also concluded that size and crystallographic orientation of RA governs its stability towards martensitic phase transformation. Finer size RA grains were found to be more stable against cyclic deformation than larger ones. In summary, many prior works on steels containing RA have shown that the TRIP effect improves fracture toughness (Antolovich and Singh, 1970, 1971; Rementeria et al. , 2015; Kumar and Singh, 2018a). However, a thorough look at the fatigue studies on nano-structured bainitic steels shows that they are very few in number and do not capture the processing microstructure-properties relationship very systematically (Mueller et al. , 2016; Peet et al. , 2017). In a recent study, we investigated the role of microstructural variants in affecting the strength-toughness combination in nano-structured bainitic steels (Kumar and Singh, 2018b, 2018a). In that study a range of microstructures was developed by changing the austempering temperature. Austempering at lower temperature decreases the BF lath thickness, lowers the RA volume fraction but increases the content of film type RA as compared to blocky RA (Yang, 2011; Kong, Liu and Yuan, 2014; Soliman and Palkowski, 2016; Kumar, Singh and Singh, 2019). The refinement of microstructure by lowering the austempering temperature leads to enhancement of strength while retaining significant amount of ductility due to the presence of RA. However, a reduction in impact as well as fracture toughness has been observed with a decrease in austempering temperature (Kumar and Singh, 2018b). Thus, it was found that the increase in strength comes with concomitant loss in toughness. Therefore, it becomes difficult to predict the fatigue life of these steels in presence of a sub-critical defect/flaw. In our prior work, we postulated the role of microstructural constituents on sub-critical fatigue crack growth behaviour in nano-structured bainitic steels prepared by induction melting in an inert atmosphere followed by heat treatment involving austenitization followed by austempering at 250, 300 and 350°C respectively (Kumar and Singh, 2019). In the present study, we tried to verify those envisaged postulates through results obtained from experiments like surface profilometry and nano-indentation. 2. Experimental details A clean steel of chemical composition Fe-0.85C-1.30Si-1.92Mn-0.44Al-2.05Co-0.29Mo (in wt.%) has been produced under inert atmosphere. Subsequently, the cast ingot was homogenized at 1000°C for 48 hours and then hot rolled from a thickness of 34 mm to 14 mm at 1000°C and air cooled to room temperature. In order to produce bainitic microstructure, rolled plates were austenitized at 950°C for 40 minutes followed by austempering in a salt bath at three different temperatures 250, 300 and 350°C for 40, 30 and 20 h respectively, and their name were given as NB250, NB300 and NB350 respectively. The mean lath thickness of BF and RA was measured by linear intercept method using SEMmicrographs and stereological corrections were made to obtain the true mean lath thickness (Garcia-Mateo et al. , 2016). X-ray diffraction patterns of all the steels were obtained using PANalytical X’pert Pro MPD system with Cu-Kα radiation. Volume fraction of BF and RA were calculated from integrated intensities of (110), (200), (211) ferrite peaks and (111), (200), (220) austenite peaks respectively. Details of tensile, plane-strain fracture toughness and Charpy impact tests which were performed as per ASTM standards can be seen in prior work (Kumar and Singh, 2018b). In order to conduct fatigue crack growth rate test, chevron notched compact tension specimens in longitudinal transverse (L-T) orientation were machined out of heat treated block using electric discharge machining and specimens’ dimensions are shown in prior work (Kumar and Singh, 2019). Fatigue crack growth rate tests were conducted as per ASTM E647-15Ɛ1 standard (Conshohocken, 2016) and the measurement of crack length was done through the compliance method using a crack opening displacement (COD) gage. Fatigue pre-cracking under ∆ K decreasing (force shedding) mode was performed in order to obtain a nominal crack with size, a = 10.5 mm. Subsequently, ∆ K decreasing test procedure to determine the threshold cyclic stress intensity factor range ( ∆ K th ) was started at ∆ K and K max value greater than the terminal pre-cracking values of (Conshohocken, 2016). The ∆ K th for all the specimens was obtained from the terminal data of d a /d N versus ∆ K plot obtained from this test. 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