Spall properties and damage mechanisms of a low-alloy steel fabricated via laser powder bed fusion

Abstract

Plate impact experiments are conducted on a representative, low-alloy, 24CrNiMo steel (containing lath martensite substructures) fabricated via laser powder bed fusion and tempering, to investigate its spall strength and damage mechanism within a peak stress range of 5–11 GPa. The Hugoniot elastic limit (2.4 GPa) and spall strengths (3.9–4.2 GPa) are obtained from free surface velocities, and show negligible anisotropy. The microstructure before and after impact loading are characterized by electron backscatter diffraction and scanning electron microscopy. The length, width and orientation angle of cracks are quantified. Spall cracks are large in quantity, but small in mean propagation distance and opening displacement, especially at high peak stress. Cracks develop mainly along the {100} cleavage planes of laths of the same variant rather than along melt pool boundaries. The mismatch between the {100} cleavage planes across the ∼60°〈111〉 lath boundaries leads to high resistance of fine laths against crack propagation. The fine lath substructure is beneficial to the dynamic mechanical properties of low-alloy steels.