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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The average bainitic lath thickness of NB350 steel is 100 nm with large deviation. It shows irregular morphology when compared with NB300 and NB250 that are transformed at lower temperatures. It is also noticeable that the volume fraction and thickness of retained austenite films increases with an increase in austempering temperature. However, they still appear continuous for large lengths. The retained austenite plates in NB350 steel are very thick, not nanostructured but the typical island type morphology of blocky type austenite observed in most studies is absent. The average bainitic lath thickness in NB250 and NB300 steels is below 50 nm with large fraction of film type retained austenite. Table 1. Quantitative results obtained f rom XRD studies and SEM micrographs of produced steels. Vα is the percentage of bainitic ferrite; Vγ = (100 - Vα) is volume p ercentage of retained austenite; and T is bainitic ferrite lath thickness Specimen V α V γ T, nm NB250 81.6 18.4 37±9 NB300 70.6 29.4 47±12 NB350 51.1 48.9 101±44 Fig. 3 shows the engineering stress-engineering strain plots of produced nanostructured bainitic steels. The yield strength(YS), ultimate tensile strength(UTS) and total elongation in percentage (TE) obtained from the tensile tests of the specimens transformed at different conditions are summarized in Table 2. The YS increases from 1.02 GPa to 1.56 GPa, UTS increases from 1.3 GPa to 1.8 GPa, and TE decreases from 25% to 7% when the austempering temperature is decreased from 350°C to 250°C (Table 2). This enhancement of strength with a decrease in austempering temperature is attributed to reduction in the mean free path for dislocation glide owing to the refinement of bainitic plates. Moreover, higher volume fraction of nano-bainite at the expense of softer retained austenite also favors strengthening (Bhadeshia 2010). On the other hand, ductility is mainly attributed to volume fraction and stability of retained austenite which provides transformation induced plasticity to the steel (Bhadeshia 2001). Thus, the ductility improves at higher austempering temperature due to an increase in volume fraction of retained austenite from 18 % at 250°C to 49 % at 350°C.

Fig. 3. Engineering stress-engineering strain plots for tensile tests of produced steels Table 2. Summary of mechanical properties of produced steels obtained from various mechanical tests performed at room temperature. YS – Yield strength, UTS – Ultimate tensile strength, TE – Total % elongation

YS (MPa)

UTS (MPa)

TE (%)

K 1C (MPam -1/2 )

Impact Energy (J)

Specimen

NB250 NB300 NB350

1560±32 1382±20 1028±52

1807±156 1676±7 1285±27

7.2±0.16

29.1±1.2 37.1±2.8 45.6±1.8

6.5±0.7 11±1.4

14.1±2

25.7±3.65

14.75±0.35