Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 10⁷ cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.