PSI - Issue 69

Pekka Kantanen et al. / Procedia Structural Integrity 69 (2025) 53–60

56

anticipated the higher RA fraction was achieved at the higher IAT temperature. Notably, even a modest temperature difference of 50 °C had significant effect on the RA fraction. The morphology of martensite and RA was further examined using EBSD as shown in Figs. 2b–c (RA 1.1% IAT at 650 °C) and Figs. 2e–f (RA 31.1% IAT at 700 °C). Local RA fractions detected through EBSD were considerably lower than those measured by XRD, likely due to the fine size of RA in the studied samples. These fine interlath RA films are usually undetected by EBSD due to its resolution limit (0.12 µm) but can be precisely detected via XRD. Furthermore, the polishing process for EBSD sample preparation might have induced partial transformation of RA to martensite, contributing to the lower RA fractions in EBSD measurements. In contrast, XRD provided averaged results over a larger rough surface (1–3 mm²) [7].

Fig. 2. FESEM images of (a) IAT 650 °C and (d) IAT 700 °C steels. EBSD measurement maps showing (b, e, 650 and 700 °C, respectively) lath shaped structures and (c, f, 650 and 700 °C, respectively) phase fractions of RA (blue is austenite and red is ferrite/martensite).

Tensile testing results, including mean values (Table 1), engineering stress-strain curves (Fig. 3a), and strain hardening rate curves (Fig. 3b), were obtained for the IAT steels treated at 650 °C and 700 °C in the longitudinal direction. The IAT at 650 °C material exhibited a yield point plateau before increased strain-hardening, whereas the IAT 700 °C material showed continuous yielding behaviour. This difference in the strain-hardening behavior correlated with the transformation rate and fraction of RA. The slower RA transformation rate observed in the IAT 700 °C material enabled prolonged strain hardening and a sustained TRIP effect [8]. Higher IAT temperature could reduce the average C and Mn content per volume RA and influence the mechanical stability of RA [9].

Table 1. Tensile test results tested in the longitudinal direction. Standard deviations of the measurements are given.

Material

R p0.2 [MPa] R m [MPa]

R p0.2 /R m

n 4–6

A g [%]

A 50 [%]

R m ´ A 50 [GPa%]

IAT 650 °C 841±8

1027±13

0.82±0.00 0.18±0.00 10.4±0.6

15.8±2.2

16.3±2.4

IAT 700 °C 774±17

911±10

0.85±0.01 0.11±0.00

16.7±0.4

27.1±0.8

24.7±0.9