Effect of highly stressed volume and non-metallic inclusions on the fatigue strength of a steel for heavy forgings

This study investigates the influence of highly stressed volume (HSV) and non-metallic inclusions on the fatigue strength of 27NiCrMoV15-6 steel used in heavy forgings. Six specimen series with progressively increasing HSV were tested under load ratios of R = − 1 and R = 0 . Fatigue testing revealed a reduction in fatigue strength of up to 4.5% for a six-times increase in reference volume and up to 30.2% for an HSV 27 times the reference volume. To quantitatively assess the influence of HSV, an extension to the traditional volumetric approach is proposed. Fractographic examination identifies three primary failure mechanisms: surface-initiated fatigue, inclusion-initiated surface fatigue, and subsurface inclusion-initiated fatigue with characteristic fish-eye formation. The fish-eye failures typically occurred at high cycle counts, beyond one million load cycles. Statistical analysis of inclusion and fish-eye dimensions provides quantitative relationships between specimen size and the corresponding fish-eye and inclusion sizes. Application of Murakami’s model demonstrates good agreement between statistically characterized inclusion sizes and fatigue performance. The comprehensive fatigue dataset and the proposed volumetric approach extend the existing database for 27NiCrMoV15-6 steel and enable more accurate fatigue strength calculations for heavy steel forgings.