Fatigue crack propagation in some PM steels

Mechanisms of fatigue crack growth have been studied for a range of PM steels at relative densities of 0-90 and 1-0, for which strength, fracture toughness, and microstructural information was also available. It is shown that the Paris exponents for steady state crack growth are between 8 and 18 when Pr is ~ 0.9 but when Pr is ~ 1.0 the exponents are between 2.6 and 4.0, i.e in the range typical of wrought steels (2-4). At both densities, threshold stress intensities are between 5.5 and 10.8 MPa m^(1/2) when R= 0.1. Combinations of these thresholds and yield strengths are comparable with those for wrought steels. When R = 0.8, reductions in threshold to between 2.7 and 5 MPa m^(1/2) are attributed to crack closure effects. At Pr = 0.90, Fe-0.5C fails by progressive rupture of sinter necks. Astaloy A, with 0.2%C and 0.6%C, and Distaloy AB-0.6C have smaller plastic zone sizes and the cracks follow more difficult paths through particles as well as necks. When Pr is ~ 1.0, fracture is partially by true fatigue modes and partly by cleavage, the bursts of cleavage being more noticeable when K(max) is high.