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

Environmental influence on the stress corrosion cracking of rock bolts

As medium-strength steels are promising candidates for the hydrogen economy, it is important to understand their interaction with hydrogen. However, there are only a limited number of investigations on the behavior of medium-strength steels in hydrogen. The existing literature indicates that the influences of hydrogen on the tensile properties of medium-strength steels are mainly the following: (i) the steel can be hardened by hydrogen, as demonstrated by an increase in the yield stress or ultimate tensile stress; (ii) some steels can be embrittled by hydrogen, as revealed by lower yield stress or ultimate tensile stress; (iii) in most cases, these steels may experience hydrogen embrittlement (HE), as indicated by a reduction in ductility. The degree of HE mainly depends on the test conditions and the steel. The embrittlement can lead to catastrophic brittle fracture in service. The influence of hydrogen on the fatigue properties of medium-strength steels is dependent on many factors such as the stress ratio, temperature, yield stress of the steel, and test frequency. Generally, the hydrogen influence on fatigue limit is small, whereas hydrogen can accelerate the fatigue crack growth rate, leading to a shorter fatigue life. Inclusions are an important factor influencing the properties of medium-strength steels in the presence of hydrogen. However, it is not possible to predict the influence of hydrogen for any particular steel that has not been experimentally evaluated or to predict service performance. It is not known why similar steels can have different behavior, ranging from good resistance to significant embrittlement. A better understanding of the microstructural characteristics is needed.

/pdf/a-critical-review-of-the-influence-of-hydrogen-on-the-5g8oj9ffgm.pdf

A critical review of the influence of hydrogen on the mechanical properties of medium-strength steels

An in-depth analysis of the deterioration mechanisms in high-strength wires of suspension bridge cables is presented. Accelerated cyclic corrosion tests were conducted to assess the relative effect of corrosion on galvanized and ungalvanized wires. Samples were corroded under various levels of sustained loads in a cabinet that cyclically applied an acidic salt spray, dry conditions, and 100% relative humidity at elevated temperature, and mass loss, hydrogen concentration, ultimate load, and elongation at failure were measured. Elongation measurements indicated a significant embrittlement of the wires that could not be explained solely by the presence of absorbed hydrogen (hydrogen embrittlement). The main cause of reduction of wire elongation was found to be the surface irregularities induced by the corrosion process. The experimental results were validated through a numerical analysis using a finite-element method model of the corroded steel wire and through a series of scanning electron microscope analyses of the fracture surfaces.

Corrosion and embrittlement in high-strength wires of suspension bridge cables

The effect of grain size and aging on hydrogen embrittlement of a maraging steel

This article deals with a new nonconventional microscopic fracture mode with a characteristic feature: the tearing topography surface (TTS), associated with hydrogen embrittlement processes in pearlitic steel. The TTS mode appeared in fracture tests on precracked and notched specimens when tested under hydrogen charging. Experimental results showed phenomenological relations between the size of the TTS region and variables such as the electrochemical potential and the maximum stress intensity factor during fatigue precracking (for cracked samples) or the time to failure and the geometry (for notched samples). A hydrogen diffusion model is proposed which explains, from the theoretical point of view, the phenomenological relations between the TTS size and the test variables. According to this model, hydrogen diffuses not only toward the places of minimum concentration, but also to the sites of maximum hydrostatic stress.

The tearing topography surface as the zone associated with hydrogen embrittlement processes in pearlitic steel

The main macroscopic variables (in the continuum mechanics sense) which govern the microscopic fracture of notched samples of high strength steel are studied. The investigation, performed in inert and aggressive environments, offers therefore a quantitative relationship between the macroscopic and microscopic approaches to fracture, and allows a prediction of the size of the critical microstructural zone by numerical computation. The experimental programme included fracture tests on notched samples covering a broad range of geometries, in air and hydrogen environments. Fractographic analysis of the samples by means of scanning electron microscopy showed the microscopic topographies after failure. The numerical study consisted of elastic-plastic finite-element method computations to determine the distribution of macroscopic variables at the instant of fracture. The critical microstructural region for the tests in air develops by microvoid coalescence and its extension is governed by the stress triaxiality (the ratio of the hydrostatic to the equivalent stress). The tests in hydrogen show a specific microscopic fracture mode, called tearing topography surface whose asymptotic depth extends up to the location of maximum hydrostatic stress.

Macroscopic variables governing the microscopic fracture of pearlitic steels

Characteristics of the new tearing topography surface

In this paper a broad phenomenological study of the hydrogen embrittlement of high strength pearlitic steels is presented. The experimental study includes fracture tests under aggressive e...

A. M. Lancha

Papers

The tearing topography surface as the zone associated with hydrogen embrittlement processes in pearlitic steel

Macroscopic variables governing the microscopic fracture of pearlitic steels

Characteristics of the new tearing topography surface

Hydrogen Embrittlement of Pearlitic Steels: Phenomenological Study on Notched and Precracked Specimens

Factors influencing stress corrosion cracking of high strength pearlitic steels