Characteristics and mechanisms of hydrogen-induced quasi-cleavage fracture of lath martensitic steel

This study presents an in-depth characterization of the microstructures, crystallographic orientations, and dislocation characteristics beneath hydrogen-induced quasi-cleavage fracture features of an as-quenched, lath martensitic (alpha') 22MnB5 steel. The fracture surfaces of gaseous hydrogen-embrittled martensitic specimens were analyzed by a combination of multiple analytical tools: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). The dominant fracture mode in the hydrogen-affected zones was quasi-cleavage fracture, which involved significant plasticity, evidenced by plastic zones near tear ridges and a high density of dislocations beneath the quasi-cleavage facets. The martensite constituent sizes, variant orientations, and boundaries influenced the quasi-cleavage surface morphologies. Fractography revealed the occurrence of {100}(alpha')-type cleavage across martensitic laths, developing relatively flat quasi-cleavage surfaces, in addition to {110}(alpha')-type cracking likely along lath and block boundaries, and fracture along noncleavage planes. The likelihood of the formation of the relatively flat quasi-cleavage surfaces increased with increasing martensitic constituent sizes. Substantial dislocation bands formed on intersecting {112}(alpha'), slip planes within a martensite block below the hydrogen-induced cleavage fractures on {100}(alpha') planes. River markings on the quasi-cleavage surfaces were found to originate from the complex, hierarchical lath martensitic microstructure. Steps and ridges on the quasi-cleavage facets generally linked with the various martensitic boundaries, suggesting that they were produced as a result of crack deviations at those boundaries. The fracture paths and martensite quasi-cleavage mechanisms are discussed in the context of the role of hydrogen and Cottrell cleavage model. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study provides a detailed characterization of the microstructures and dislocation characteristics beneath hydrogen-induced quasi-cleavage fracture features of an as-quenched martensitic steel. The dominant fracture mode in the hydrogen-affected zones was found to be quasi-cleavage fracture, involving significant plasticity. The sizes, orientations, and boundaries of the martensite constituents influenced the surface morphologies of the quasi-cleavage fractures.