Al₂O₃-C refractories are used as essential structural materials in continuous casting processes, particularly in ladle shrouds that guide molten steel from the ladle to the tundish These components are subjected to extreme thermo-mechanical environments, including cyclic thermal shocks, severe oxidation, and aggressive slag/steel corrosion, inevitably leading to premature failure. Conventional studies mainly emphasized optimizing composition to improve oxidation resistance or thermal shock stability. The dynamic evolution of thermo-mechanical properties under realistic service conditions remains insufficiently understood. In specific, the time- and temperature-dependent performance degradation or strengthening, as well as fracture behaviours under repeated casting cycles, are rarely addressed systematically. Such a understanding is critical for extending shroud service life and improving casting stability and product quality. In this context, this study investigated the phase evolution, microstructural transformation, and fracture behaviours of Al₂O₃-C refractories extracted from post-mortem ladle shrouds that experienced different service cycles in both submerged and non-submerged regions. The work further validated the feasibility of using the single-edge notched bending (SENB) method for evaluating fracture toughness in refractories with limited sample sizes, in comparison with the wedge splitting test (WST) and conventional three-point bending test (3PBT).

Representative specimens were cut from both submerged (B) and non-submerged (N) regions of post-mortem ladle shrouds after 1, 5, and 10 service cycles. The compositions differed slightly, with Si powder incorporated into N regions to counter oxidation, while B regions contained trace silica additions to improve thermal shock resistance. The phase compositions of samples N and B were analyzed by X-ray diffraction (XRD). The microstructural features were determined by scanning electron microscopy equipped with an energy-dispersive spectroscopy (SEM-EDS). The mechanical performance was assessed through three complementary approaches, i.e., 1) three-point bending tests to evaluate flexural strength evolution; 2) wedge splitting tests for reference fracture parameters, and 3) single-edge notched bending tests with digital image correlation (DIC, a software named Match ID 2D) to monitor stable crack propagation and determine fracture energy, nominal tensile strength, and the critical stress intensity factor K_IC. The SENB geometry (125 mm×15 mm×15 mm, 3.5 mm notch, 80 mm span) was tailored for shroud walls with thickness limitations. The DIC provided full-field strain maps, enabling the visualization of crack initiation and propagation.

The results reveal a distinct dependence of phase evolution and fracture behaviours on service position and thermal cycles. In non-submerged specimens, the oxidation of metallic Si progressively generates dense SiC whiskers during thermal service, strengthening the microstructure through crack-bridging and densification. This process is accompanied via quartz crystallization at -1050 ℃, with further SiC formation at >1400 ℃. Consequently, the flexural strength nearly doubles, and SENB-derived fracture toughness (K_IC) increases from 8.48 to 15.04 MPa·mm½ with increasing service cycles (1T→10T). However, the ratio G/σ_t, representing brittleness, declines, indicating a trade-off between strength enhancement and increasing brittleness. In contrast, submerged specimens experience more severe thermal gradients (at 1500-1600 ℃) and cyclic shock from molten steel contact. A low silica content limits SiC formation, while the crystallization and phase transformation of fused silica (low- to high-cristobalite) induce microcracking and volume mismatch. This "roof-tile structure" generates stress concentrations and progressive damage. As a result, the flexural strength of submerged samples shows a non-linear trend. It initially increases after one thermal cycle due to sintering and slight densification, then declines during extended service. At the 10th cycle, the strength decreases below that of non-submerged specimens, while K_IC also decreases, highlighting a performance degradation under harsher conditions. The comparisons of fracture test methods indicate consistent fracture energy values between SENB and WST, confirming SENB as a reliable alternative when limiting the sample dimensions. Unlike 3PBT, which causes an abrupt brittle failure, SENB and WST both enable a stable post-peak crack propagation. The full-field DIC strain maps further confirm a gradual crack evolution, with non-submerged samples showing straighter, sharper cracks due to the higher brittleness, while submerged specimens display broader strain fields and more tortuous crack paths. This highlights the ability of SENB coupled with DIC to capture subtle fracture mechanisms in quasi-brittle refractories.

Overall, the findings indicate that thermo-mechanical property evolution is governed by a competition between strengthening mechanisms (i.e., SiC whisker formation, sintering densification) and damage mechanisms (i.e., silica crystallization, microcracking, oxidation). The relative dominance of these processes depends strongly on service position and thermal cycles. While non-submerged materials gain the strength over time, their rising brittleness concerns a catastrophic failure if used beyond safe limits. Submerged materials, despite initial improvements, exhibit a progressive weakening, underlining the necessity of microstructural design strategies tailored to each service environment.

The single-edge notched bending test, validated against wedge splitting and three-point bending methods, could be an effective approach for characterizing fracture toughness of post-mortem shrouds with a limited sample size, thus enabling stable crack growth monitoring. The thermo-mechanical performance of Al₂O₃-C refractories showed a pronounced dependence on service time, temperature, and position, with property evolution closely linked to oxidation-driven SiC formation and fused silica crystallization. In non-submerged regions, the flexural strength and fracture toughness increased with service cycles, and the brittleness increased as well, significantly elevating a risk of sudden fracture if service continued beyond the strength limit. In contrast, submerged regions exhibited a non-linear evolution of properties, i.e., the mechanical performance peaked during early service cycles due to densification, but then declined with increasing cycles, thus leading to silica crystallization and microcracking. These insights could provide a scientific basis for tailoring composition and microstructure to enhance the service life of Al₂O₃-C refractories in continuous casting operations.