Fatigue failure mechanisms in a nitrided En 41B steel

Abstract

S-N curves for direct stress fatigue have been established for a ‘Nitralloy’ type En 41B steel in the un-nitrided condition, and after nitriding at 525°C for 1,2, and 24 h. A metallographic study of the fracture surfaces has also been made. Significant improvements in fatigue limit have been obtained, in contrast to previously published reports. Failure in the low-cycle region (≯ 10³ cycles) is due to cracking of the brittle nitride case and propagation of the surface cracks into the core. This can cause a reduction infatigue life compared with the un-nitrided condition. At lives > 10⁴ cycles, failure is controlled by cracks which propagate from non-metallic subsurface inclusions, and prolific cracking may be observed. The sum of the longitudinal core, residual stress, and the maximum tensile stress of the applied load at the fatigue limit is shown to increase linearly with the case/core cross-sectional area ratio of the specimens.