Fatigue Crack Propagation Behaviours of Carburized Steel

Abstract

The rotating bending fatigue tests were performed on the notched carburized steel specimen at room temperature, and their properties of fatigue crack propagation were investigated. Furthermore, the relation between fatigue crack behaviours and microscopic structure was discussed.
All the SCM21 specimens were carburized at 930°C, under 0.9% carbon potential atmosphere for 4hrs, and oil-quenched from 830°C. Tempering was carried out at 180°C for 2hrs.
The results obtained are as follows.
(1) The non-propagating crack was observed at 10⁷ cycles in the case of SCM21 carburized steel. The maximum notch radius of ρ=0.1mm to 0.08mm was essential for the existence of non-propagating cracks at 10⁷ cycles.
(2) In the case of ρ=0.08mm, the crack propagation rate was almost constant or slightly decreasing during a considerable part of its life. That tendency could be understood by considering the distribution of compressive residual stress in the surface layer.
(3) In the case of ρ=0.5mm, the crack propagation rate increased proportionally to the crack length.
(4) In the case of ρ=0.08mm, the fatigue crack initiated at the grain boundary of prior austenite. It propagated along the grain boundaries of prior austenite at a high stress amplitude level and through the grains of prior austenite at a low stress amplitude level.