PSI - Issue 68

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S. Ghosh et al. / Procedia Structural Integrity 68 (2025) 1329–1336 S. Ghosh et al. / Structural Integrity Procedia 00 (2025) 000–000

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ΔK test was completed, the increasing ΔK test was started with the positive normalized K gradient as 0.08 mm -1 . The time taken to complete the decreasing ΔK test and obtain the threshold was around 35 h per sample. The increasing ΔK test took around 1h, and hence the total time elapsed was around 36 h to complete one full test. 3. Results and discussion 3.1 Microstructures Figure 1a-d shows the initial microstructures of H-Si and M-Si DQP specimens before the FCG tests, illustrated via both bright field (BF) and dark field (DF) images recorded using transmission electron microscopy (TEM) to discern the presence of RA and its morphology. TEM BF images of the DQP specimen reveal a highly dislocated martensitic lath structure (Figure 1a,c) together with a small fraction of twinned martensite (Figure 1a,c). Though existence of fine film-like RA was obvious in the BF (Figure 1a,c) images, DF images (Figure1b,d) clearly reveal the fine distribution of interlath RA, corroborating our previously published results (Ghosh et al. (2022), Ghosh et al. (2023)). The thickness of RA films is estimated to be in the range 25–100 nm, as estimated from the magnified BF images. Based on XRD results, the volume fractions of different phases were estimated to be ~15% RA, 85% martensite in H-Si DQP steel, as compared to ~13% RA, 8% bainite and 79% martensite in M-Si DQP steel.

(a)

(b)

1000 1200 1400 1600 1800 2000 2200

(e)

RA

Twinned martensite

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H-Si M-Si

Martensite laths

0 Engineering stress (MPa) 2 4 6

8

10 12 14

Engineering strain (%)

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(c)

RA

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(f)

Twinned martensite

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H-Si M-Si

0

2

4

6

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True strain (%)

Strain hardening rate (MPa)

Fig. 1. TEM bright (a,c) and dark field images (b,d) of H-Si and M-Si DQP steels, respectively, e) corresponding engineering stress-strain and f) strain hardening curves.

Figure 1e depicts the tensile strain-strain curve of H-Si and M-Si DQP steels. The corresponding tensile properties are summarized in Table 3, as also reported earlier (Ghosh et al. (2022)). Accordingly, the average yield strengths