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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growth rate test under a constant load range of 2.5 kN at a frequency of 10 Hz and a load ratio of 0.1 was conducted to investigate the fatigue crack growth rate (d a /d N ) behaviour with change in stress intensity factor range ( ∆ K ) till specimen fractured due to overload. Profilometry using Zeta Instruments surface profilometer was done to measure the mean surface roughness near the threshold region of crack growth. An area of 496 µm x 372 µm was scanned at a step size of 0.4-0.5 μm. In order to see the TRIP effect, Hysitron Inc nano-indenter was used to measure the hardness in plastically deformed zone corresponding to stage II of the fatigue crack growth. One half of the fatigue tested specimen was cut at mid-thickness and hot mounted in Bakelite in such a way that the hardness as a function of distance below the crack surface could be measured. Indentations were made under displacement controlled mode (depth=180 nm) using a Berkovich tip. 3. Results and discussion SEM micrographs (Fig. 1) clearly show that the microstructure gets refined with a decrease in austempering temperature. The true mean lath thickness of BF and RA are stated in Table 1. The lath of BF and RA are below 50 nm in case of NB250 and NB300 steels. Although, NB350 steel shows thicker films of RA, they are not blocky type of RA and seems intact between nano-structured BF laths. The carbon concentration in RA, volume percentage of phases formed and dislocation density in BF of the three steels obtained by X-ray diffraction study are given in Table 1. A lower transformation temperature leads to increase in carbon content in RA, increase in volume fraction of BF at the expense of RA and increase in dislocation density as well. Moreover, the micrographs also shows higher fraction of BF and lower fraction of RA at lower transformation temperatures.

Fig. 1. SEM micrographs of nano-structured bainitic steels (a) NB250; (b) NB 300 and (c) NB350 (Kumar and Singh, 2018b)

Table 1. Quantitative results obtained from SEM micrographs and XRD studies of produced steels, where T , C , V and ρ represent plate thickness, carbon content, volume percentage and dislocation density respectively. The subscripts α and γ stand for BF and RA respectively (Kumar and Singh, 2020)

T α , nm

T γ , nm

Specimen

V α

V γ

C γ (wt.%)

ρ α m

-2

NB250 NB300 NB350

37±9

22±7

81.6 70.6 51.1

18.4 29.4 48.9

1.87 1.66 1.52

11.1±4.6×10 15 6.1±2.8×10 15 3.5±1.9×10 15

47±12 101±44

45±12 177±31

Table 2 summarizes the mechanical properties obtained in our previous work on the same heat-treated steel blocks (Kumar and Singh, 2018b, 2018a). That prior study on improvement of strength-toughness combination of nano structured bainitic steels showed that impact as well as plane-strain fracture toughness increases with an increase in austempering temperature. However, it comes with a concomitant loss of tensile strength and hardness. The decrease in strength with an increase in austempering temperature has been attributed to formation of coarse BF laths and decrease in its volume fraction. Moreover, greater volume fraction of RA in spite of its coarser morphology at higher austempering temperature was found responsible for improving ductility and toughness of nano-bainitic steels.