A stress-modified, critical-strain model of fracture-initiation toughness has been adapted to the case of hydrogen-affected pearlite shear cracking, which is a critical event in transverse fracture of cold-drawn, pearlitic steel wire. This shear cracking occurs via a process of cementite lamellae failure, followed by microvoid nucleation, growth, and linkage to create shear bands that form across pearlite colonies. The key model feature is that the intrinsic resistance to shear-band cracking at a transverse notch or crack is related to the effective fracture strain at the notch root. This fracture strain decreases with the logarithm of the diffusible hydrogen concentration (C H). Good agreement with experimental transverse fracture-initiation-toughness values was obtained when the sole adjustable parameter of the model, the critical microstructural dimension (l*), was set to the mean dimension of shearable pearlite colonies within this steel. The effect of hydrogen was incorporated through the relationship between local effective plastic strain (ɛ eff f ) and C H, obtained from sharply and bluntly notched tensile specimens analyzed by finite-element analysis (FEA) to define stress and strain fields. No transition in the transverse fracture-initiation morphology was observed with increasing constraint or hydrogen concentration. Instead, shear cracking from transverse notches and precracks was enabled at lower global applied stresses when C H increased. This shear-cracking process is assisted by absorbed and trapped hydrogen, which is rationalized to either reduce the cohesive strength of the Fe/Fe3C interface, localize slip in ferrite lamellae so as to more readily enable shearing of Fe3C by dislocation pileups, or assist subsequent void growth and link-up. The role of hydrogen at these sites is consistent with the detected hydrogen trapping. Large hydrogen-trap coverages at carbides can be demonstrated using trap-binding-energy analysis when hydrogen-assisted shear cracking is observed at low applied strains.

A critical-strain criterion for hydrogen embrittlement of cold-drawn, ultrafine pearlitic steel

This paper deals with the microstructure evolution in a pearlitic steel subjected to progressive plastic deformation produced by repetitive cold drawing. The study is focused on the evolution of the pearlite colony as a whole, so its changes in size and shape as the cold drawing proceeds are analyzed. Results demonstrate that progressive cold drawing slenderise the pearlite colony, this microstructural effect being associated with a clear enlargement of the main axis (that is quasi-parallel to the wire axis or cold drawing direction) and a shortening of the secondary axis (perpendicular to the former), thus producing a decrease of the ellipsoid aspect ratio.

Microstructure evolution in a pearlitic steel subjected to progressive plastic deformation

Failure analysis of cold drawn eutectoid steel wires for prestressed concrete

In this paper the fracture performance of axisymmetric notched samples taken from pearlitic steels with different levels of cold drawing is studied. To this end, a real manufacture chain was stopped in the course of the process, and samples of all intermediate stages were extracted. Thus the drawing intensity or straining level (represented by the yield strength) is treated as the fundamental variable to elucidate the consequences of the manufacturing route on the posterior fracture performance of the material. A materials science approach to the phenomenon is proposed, so that the strongly anisotropic fracture behaviour of the steels with high level of strain hardening (which exhibit a 90° step in the fracture surface) is rationalized on the basis of the markedly oriented pearlitic microstructure of the drawn steels which influences the operative micromechanism of fracture in this case.

Anisotropic fracture behaviour of cold drawn steel: a materials science approach

Fatigue and fracture performance of cold drawn wires for prestressed concrete

High-strength carbon steel patented wire is widely used in the skeleton of buildings and bridges, cemented in concrete in order to counterbalance severe compressive stresses being developed in the reinforced structures. For a number of reasons which are beyond the scope of this study, a reinforced wall of such a structure was severely cracked and the skeletal high-strength carbon steel wires were exposed to environmental corrosive conditions. Soon thereafter, a number of wires failed while being subjected to a tension stress lower than their strength

N. Sarafianos

Papers

Environmentally assisted stress-corrosion cracking of high-strength carbon steel patented wire