Corrosion-assisted cracking in progressively drawn pearlitic steels: A materials science approach

Abstract

This is a summary of the achievements of a major research line at the Materials Science Department of the University of La Coruna (Spain) in the field of environmentally assisted cracking in general and hydrogen degradation in particular. It deals with a materials science approach to the study of corrosion-assisted cracking of progressively drawn pearlitic steels for use in civil engineering. The approach is based on the fundamental idea of materials science of linking the microstructure of different steels (progressively oriented as a consequence of the process of manufacturing by cold drawing) with their macroscopic stress-corrosion behavior (increasingly anisotropic as the degree of cold drawing increases). In the first part of the research, we performed metallographic analysis and gave special attention to the evolution with cold drawing of the two basic microstructural levels: pearlite colonies (first level) and pearlitic lamellae (second level). For the first microstructural level, we observed progressive eleongation and orientation of the pearlite colonies in the direction of cold drawing (axis of the wire). For the second microstructural level, the analysis revealed an increase in the closeness of packing (with a decrease in the interlamellar spacing) and progressive orientation of the pearlitic lamellar microstructure in the direction of cold drawing. Therefore, in the process of cold drawing, both the pearlite colonies and pearlitic lamellar microstructure tend to align in a direction quasiparallel to the axis of the wire. In the second part of the research, we carried out a program of stress-corrosion-cracking tests under both cathodic and anodic conditions to promote two very different mechanisms of cracking: hydrogen-assisted cracking (HAC) and localized anodic dissolution (LAD). Both types of stress corrosion tests confirm that steels subjected to cold drawing exhibit anisotropic behavior connected with clear changes in the direction of crack propagation, which approaches the axis of the wire or the direction of cold drawing. For both mechanisms (HAC and LAD), there is a strong correlation between the angles of microstructural orientation (on the levels of pearlitic colonies and lamellae) and the angles of propagation of macroscopic cracks, which clearly demonstrates the influence of the oriented microstructure (and, thus, of the process of manufacturing by increasing cold drawing) on the macroscopic corrosion-assisted behavior of steel wires.