Strain-path-controlled strengthening in pearlitic steel: mechanical fragmentation and chemical stability of cementite under multidirectional forging

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The microstructural mechanisms governing the mechanical response of pearlitic SAE 1080 steel processed by multidirectional forging (MDF) at room temperature (strain amplitude Δε ≈ 0.30, cumulative strain ε t ≤ 10.8) were investigated using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and energy-filtered TEM (EFTEM) carbon mapping. EBSD revealed ferritic grain refinement from ∼33 μm to ∼3 μm through continuous dynamic recrystallization (cDRX). The high-angle grain boundary (HAGB) fraction decreased to 26% after one cycle and increased to 56.7% after 12 cycles, reflecting cDRX-driven boundary evolution. TEM observations showed cementite lamellae undergoing bending, kinking, and mechanical fragmentation into submicrometric particles without preferential alignment. Interlamellar spacing evolved non-monotonically (0.06–0.10 μm), rendering the Embury–Fisher strengthening model physically inapplicable. The abrupt decrease in interlamellar spacing and ferritic subgrain size between cycles 9 and 12 marked the transition to a regime in which collapse of long-range lamellar continuity promoted ferritic refinement through cDRX. EFTEM carbon mapping confirmed that carbon remained confined to fragmented but chemically intact cementite, with no detectable redistribution into ferrite, indicating suppression of strain-induced cementite decomposition under MDF. Despite grain refinement, yield strength peaked at ∼785 MPa (ε t ≈ 5.4) and decreased to ∼455 MPa at ε t ≈ 10.8, whereas hardness increased monotonically. This divergence reflects strain-path-dependent evolution of the dislocation substructure and its reduced effectiveness during yielding under loading reorientations. These results demonstrate that the deformation path governs the preservation or collapse of colony-scale geometric coherence, controlling ferritic refinement, phase stability, and strengthening in severely deformed pearlitic steel.