PSI - Issue 69

Zeynab Aalipour et al. / Procedia Structural Integrity 69 (2025) 105–112

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was determined using the Rietveld refinement method. To quantify the amount of carbon in RA, the lattice parameters derived from XRD measurements were utilized in accordance with the approach and Equation (1) [10,11]:

a γ =3.556+0.0453xC+0.00095xMn+0.0056xAl+0.0006xCr

(1)

where a γ denotes the lattice parameter (in Å) of RA, andxC, xMn, xAl, xC, xMn, xAl,andxCrrepresent the respective mass concentrations (wt.%) of carbon, manganese, aluminum, and chromium. The retained austenite content determined by XRD with an accuracy of approximately ±1%, and the fitting error for the austenite lattice parameters is significantly smaller than the precision reported to two decimal places.

Figure 1-Schematic representation of the DQ&P process.

Table 1. Chemical compositions of the experimental steels (wt.%)

Steel

C

Si

Mn

Al

Cr

Ni

L-Si H-Si

0.40 0.40

0.25 1.51

2.02 2.05

0.02 0.02

1.00 1.00

0.49 0.49

3. Results and Discussion 3.1. Phase Fraction Based on XRD Results

Figure 2 (a) illustrates the evolution of the RA fraction at different partitioning time (10 s, 100 s, 1000 s and 10000 s) for both H-Si and L-Si steels. In the L-Si steel, the RA fraction begins at a relatively low level at 10 s and 100 s, then rises sharply at 1000 s. This rapid increase suggests that, prior to 1000 s, carbon does not have sufficient time to diffuse from martensite to austenite. As time progresses beyond 1000 s, the rate of increase diminishes; however, the RA fraction continues to grow steadily, indicating that the mechanisms driving austenite stabilization remain active over a longer duration. In contrast, the H-Si steel requires a shorter partitioning time to achieve a stable RA fraction. As shown in Figure 2(a), at 100 s, approximately 17% of the RA is already stabilized, and by 1000 s, the 19% RA is stabilized and remains so after the final quench. Figure 2(b) illustrates the evolution of secondary martensite content over time in the H-Si and L-Si steels, covering time intervals up to 10000 s. The two steels exhibit markedly different behaviors. In the L-Si steel, the secondary martensite fraction begins at a relatively high level of ~ 20%. A rapid decrease in secondary martensite fraction (from 20% to 5%) occurs after 1000 s, indicating a substantial increase in austenite stability. After this significant reduction, the rate of decline gradually slows down, leading to a gradual reduction that stabilizes around 5% more by 10000 s and in the H-Si steel starts with a lower secondary martensite fraction, approximately 10%. A steep reduction occurs within the first 1000 s, driving the secondary martensite content close to 0%. Beyond this rapid decline, the percentage remains constant, indicating that secondary martensite is