PSI - Issue 69

72 R. Surki Aliabad et al. / Procedia Structural Integrity 69 (2025) 69–75 [110] " orientation relationship with the matrix martensite phase [19]. Fig. 1e displays a dark field (DF) image using the 2 2' 0 η reflection from the selected area electron diffraction (SAED) pattern shown in Fig. 1d. The heterogeneous distribution of carbides observed in Fig. 1a is attributed to martensite formation occurring at varying temperatures. Auto-tempering of martensite was active at temperatures just below the martensitic start (Ms) temperature, gradually slowing as the temperature decreased further [2]. The presence of these plate-like carbides in the studied temperature range has also been documented for other MMnS [6,20,21].

Fig. 2. (a) Carbide peaks evolution tracked by HEXRD at different temperatures; (b) The volume fraction and the lattice parameters of RA across different heating temperatures.

Fig. 2a illustrates HEXRD patterns of the sample at different temperatures. Before reaching 375°C, the evolution of carbide peaks intensity only shows a slight increase in the background, adaptable with 1 2 1 and 2 2 0 reflections of η-Fe 2 C carbide. Up to 375°C, only a faint bump appears in the background, likely due to an increase in the tempering level. Comparing the observed XRD pattern at 375°C with Fe 2 C, Fe 2.5 C, Fe 5 C 2 and Fe 3 C patterns reveals that this bump could be more related to η-Fe 2 C carbide that eventually gives way to cementite at higher temperatures as cementite peaks become distinctly visible in the patterns above 530°C. Fig. 2b displays both the volume fraction and the lattice parameter of RA across different heating temperatures. Additionally, it shows the various phenomena occurring in the different regions. The HEXRD results reveal that the initial RA volume fraction increases from 7.5% in the as-rolled specimen to 10.5% in the specimen heated to 375°C. This increase is likely related to the movement of the γ/α′ interface caused by carbon partitioning [22]. The carbon content of RA, calculated using Equation 1 [23], initially increases from 0.4 wt.% in the as-annealed specimen to 1 wt.% between ~ 375°C to 530°C. Beyond this temperature range, RA volume fraction exhibits only a slight fluctuation while its lattice parameter changed linearly with temperature; showing the end of C partitioning at ~ 375°C. On the other hand, Fig. 2a indicates that between 375°C and 530°C, η carbide transitions to cementite. However, the kinetics of this transition appear to be very slow, possibly due to the nucleation stage of cementite. a (Å) = 3.572 + 0.033%C + 0.0012%Mn + 0.00157%Si + 0.0056%Al (1) Where a is austenite lattice parameter in Å, and the elements composition in mass weight.