Investigates impact of microstructural features on fatigue performance in niobium microalloyed steels, suggesting critical insights for automotive reliability.
This study investigates the effect of microstructural heterogeneities on the high‐cycle fatigue (HCF) performance of conventional and Nb‐microalloyed SAE 4320 steels processed industrially. Fully reversed fatigue tests ( R = −1) using the staircase method were conducted. Although Nb microalloying increased tensile strength, it did not significantly improve the fatigue limit (681 MPa) compared to the conventional steel (718 MPa). Notably, the Nb‐microalloyed steel exhibited markedly higher scatter (79 MPa vs. 24 MPa). Fractographic analysis revealed subsurface crack initiation in both alloys, indicating that internal features govern failure. TEM characterization identified fine TiN precipitates (~20 nm) in the conventional steel, whereas the Nb‐variant contained coarser NbC precipitates (125–175 nm). These larger particles increased local stress concentrations and statistical dispersion in fatigue life. The findings demonstrate a decoupling between static strength and HCF resistance, emphasizing that microstructural uniformity and precipitate size are critical for fatigue reliability in automotive components.
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Campanelli et al. (2026) studied this question.
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