Micro-axial cracking in unnotched, cold-drawn pearlitic steel wire: Mechanism and beneficial effect on the resistance to hydrogen embrittlement

Effects of cold drawing ratio (CDR) on the microstructure, tensile properties, H absorption and hydrogen embrittlement (HE) of pearlitic steel wire were investigated by means of thermal desorption analysis, slow strain rate tensile testing, fractography and silver decoration. The ultimate tensile strength (UTS) and diffusible H content of unnotched tensile specimens were raised with increasing CDR due to the reduction of interlamellar spacing and the increase in dislocation density. Nevertheless, the resistance to HE was improved as the CDR became above 30% due to micro-axial cracking. Elongations of H-uncharged and charged specimens decreased until the CDR of 30%, and then were maintained despite of a continuous rise of UTS with increasing CDR further. This is because micro-axial cracking occurred in the specimens with CDRs above 30% so that tensile fracture was retarded. As the CDR was over ∼30%, a fracture mode changed from brittle fracture featured by tearing topography surface and cleavage to ductile-like fracture characterized by many dimples and micro-axial cracks. Micro-axial cracking occurred as follows: Some micro-cracks preexisted inside well-aligned pearlite colonies of specimens with CDRs above 30%. While an unnotched, H-uncharged specimen was strained, micro-cracks additionally formed to be coalesced. The merged cracks traversed the pearlite colonies to be propagated into neighboring misaligned colonies and grew along the direction of cold drawing and tensile deformation, resulting in micro-axial cracking. In H-charged specimens, H atoms near micro-cracks clustered into micro-cracks so that micro-axial cracking was accelerated and the regions near micro-cracks underwent ductile fracture, leaving many dimples.