PSI - Issue 68

Bhawesh Chhajed et al. / Procedia Structural Integrity 68 (2025) 708–714 Bhawesh Chhajed et al. / Structural Integrity Procedia 00 (2025) 000–000

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transform into brittle martensite during early stages of loading. FA morphology, however, contributes positively to the mechanical strength and does not lead to detrimental effects like BA. Owing to the superior properties of NSB steel, it has been widely used in structural applications and rails. This led to a wide interest in the fatigue properties of NSB steel which are crucial for such applications. It is widely established that the fatigue crack propagation occurs in three stages where stage I is crack initiation, stage II is stable and long crack propagation; stage III is the instability stage or final rupture stage. Kumar and Singh (2019) conducted a fatigue crack growth (FCG) study at a load ratio of 0.1 on Fe-0.85C-1.30Si 1.92Mn-0.44Al-2.05Co-0.29Mo (wt%) steel. It was austenitised at 950°C for 40 min and isothermally transformed at 250°C, 300°C and 350°C for 40, 30 and 20 hrs respectively. ΔK Th calculated from FCG test was found to be highest for specimen isothermally transformed at 350°C and lowest for specimen isothermally transformed at 250°C. Upon closer inspection of da/dN - ΔK curve for all three heat-treated specimens, it was observed that the Paris law regime for specimen isothermally transformed at 250°C was smaller than both 300°C and 350°C specimen, which showed a larger region of stable crack propagation. Fractography revealed that the crack followed a straight path in the specimen isothermally transformed at 250°C contrary to the zigzag trajectory having a rough crack surface followed in the specimen isothermally transformed at 350°C. Kumar and Singh (2020)b also performed nano-indentation experiments on FCGR tested specimens in the region corresponding to Stage 2 of crack growth, Kumar and Singh (2019). It was observed that the hardness profile did not show any significant variation in the case of specimen isothermally transformed at 250°C while specimen isothermally transformed at 350°C displayed high hardness near the crack surface. The current study focuses on revealing the evolutions in microstructure in the Paris law regime because of FCG test of NSB steel using TEM. Kumar and Singh (2019) has prepared NSB steel which is used in the current study and was austenitised at 950°C for 40 min followed by isothermal transformation at 250°C and 350°C for 40 and 20 hrs respectively. TEM results for samples extracted from the Paris law regime of fractured specimens reveal that a higher proportion of RA in the specimen austempered at 350°C significantly absorbs the input mechanical energy responsible for crack growth. This is attributed to the transformation of RA into martensite and formation of deformation twins in RA. Specimen austempered at 250°C shows remarkably higher crack growth rates due to lower RA content as compared to the specimen austempered at 350°C. It was further observed that, unlike the deformation twins in specimen austempered at 350°C, specimen austempered at 250°C containing finer distribution of RA, display the formation of microtwins. 2. Experimental procedure Two separate blocks of steel containing Fe-0.85C-1.30Si-1.92Mn-0.44Al-2.05Co-0.29Mo (wt%) were austenitised at 950°C for 40 min and isothermally transformed at 250°C and 350°C for 40 and 20 hrs respectively, Kumar and Singh (2019). The steel block transformed at 250°C and 350°C will be referred to as NSB_250 and NSB_350 respectively in the current study. Auriga Zeiss dual beam FIB-SEM has been used to capture multiple images of the microstructure in both heat treated specimens. TEM specimen in shape of discs (3 mm diameter) were punched out from thin sheets of both heat-treated undeformed specimens. Twin-jet electro-polishing had been used to perforate the discs at the centre at an operating voltage of 16 V by maintaining a temperature of -25°C using liquid nitrogen. While in the case of the deformed specimen, FIB lamella extraction technique was used to obtain the specimen from a region below the fracture surface where the crack length was measured to be ~ 17 mm. This region corresponds to the Paris law regime or stage II in the da/dN v/s ΔK plots for both the heat-treated steel blocks. The lamella and discs were observed using Thermo scientific Themis 300 G3 TEM operating at 300 kV. FCG rate tests were performed in the study by Kumar and Singh (2019) using Instron 25 kN machine as per instruction given in ASTM E647 – 15 (2015) at a load ratio R = 0.1 and load 2.5 kN while maintaining a frequency of 10 Hz. The values obtained using FCG rate tests have been reported in Table 2, Kumar and Singh (2019) and will be used as a reference in the current study. 3. Results and discussion 3.1 SEM imaging Figure 1 shows the representative SEM micrographs for NSB_250 and NSB_350 specimens. White arrows