Chemo-mechanical effects on the cutting-induced mixed-mode II-III fracture of martensitic stainless steels: An in-situ investigation

Abstract

Coated carbide-rich martensitic stainless steels are typically used for sharp edges because of their high wear and corrosion resistance. Yet they fail under the combined chemo-mechanical actions of the harsh environment they operate in, and under the mixed mode II-III stress they are subjected to. To investigate this complex failure process, we exposed sharp edges to increasing corrosion severity and carried out in-situ electron microscopy cutting experiments with two micro-mechanical testing setups: a Deben Gatan micro-tensile deformation stage and a Hysitron PI 88 Picoindenter. At low corrosion levels, byproducts formed on the surface are removed during cutting, and cracks propagate at an angle with respect to the sharp edge to form a chip. As corrosion severity increases, a percolated void structure develops in the substrate and cracks propagate perpendicularly to the sharp edge, with portions of the material bending out-of-plane. At high corrosion, a chromium-rich oxide forms in place of the percolated void structure and the material becomes brittle and unable to withstand any applied stress. Although increasing corrosion severity increases the cutting force, our numerical investigation suggests that the formation of the percolated porous structure, which in turn increases substructural heterogeneity in the material, is responsible for the change in crack propagation direction and failure mechanism.