Martensitic variant connectivity governs surface fatigue cracking in a motorcycle valve spring

Premature fatigue failure of shot-peened high-strength springs is usually attributed to surface defects, corrosion, inclusions, decarburization, or insufficient compressive residual stress. Here, a motorcycle valve spring fractured from the outer surface after 6.21 × 10 6 cycles, far below the required life of 2.30 × 10 7 cycles, although no conventional crack-origin defects or residual-stress deficiency were detected. The crack-origin region retained a compressive residual stress of approximately −400 MPa and contained relatively coarse prior-austenite grains, but showed neither pronounced coarse martensitic blocks nor obvious soft/hard mechanical contrast. Instead, EBSD-based crystallographic reconstruction reveals a previously overlooked role of connected martensitic close-packed and Bain-group domains can provide continuous slip pathways for cyclic slip localization and defect-free surface crack initiation. PAG refinement fragmented this variant network and introduced dense crystallographic barriers, thereby suppressing this failure mode. These findings identify martensitic variant connectivity as an intrinsic crystallographic origin of premature fatigue cracking in shot-peened spring steels and provide a practical microstructural strategy for improving fatigue reliability.