In order to explain discontinuous brittle cracking in materials subjected to hydrogen, single-crystal and crack/dislocation simulation studies have been made. The computational scheme allows the local stress tensor near a crack in an anisotropic elastic solid with shielding dislocations to be determined. The model system is iron with a load axis perpendicular to the {100} cleavage plane, growth in the macroscopic 〈010〉 direction, and an applied stress intensity of 16 M Pa m1/2. The result is that the elastic stress distribution is translated out from the crack tip by about 23 nm and that large stresses, although not of a singular nature, still exist. These stresses near 20 000 M Pa are sufficient to concentrate dilute solutions of hydrogen to nearly one hydrogen for every iron atom. Such stresses, combined with these extremely high hydrogen concentrations are proposed for the initiation of decohesion which grows locally over 1 μm size areas, as observed experimentally.

The necessity of both plasticity and brittleness in the fracture thresholds of iron

Formulating stress corrosion cracking growth rates by combination of crack tip mechanics and crack tip oxidation kinetics

A novel procedure for hydrogen charging and studying the Internal Hydrogen Embrittlement (IHE) of steels is described here. A cylindrical notched tensile sample with an extended end is employed for hydrogen charging. The extended portion of the sample forms the cathode in an alkaline bath and a constant uni-axial tensile load is applied during hydrogen charging. The stress gradient set up by the notch, which is not in contact with the electrolyte, enhances the hydrogen concentration at various trapping sites of the matrix beyond the solubility limit. Subsequent to charging, the specimen is kept under the same load as that during charging, for another 24 h to stabilize the population of hydrogen within the specimen matrix. At the end of this stage, the specimen is tensile tested to failure at room temperature. Two different steels namely maraging and mild steels have been chosen to study the effect of hydrogen ingress on mechanical properties. While an increase in tangent modulus (linear portion of the stress–strain diagram), yield strength, work hardening rate and ultimate tensile stress (UTS) has been observed on hydrogenation, a decrease in total elongation has been noticed for both the steels studied. Fractographic investigation has revealed that the fracture mode is predominantly ductile dimple (failure by micro-void coalescence) in both the materials and that the hydrogen reduces the size of the dimples. The observations of this investigation are significant in two respects: firstly, it demonstrates the efficacy of a hydrogen charging method for steels which can introduce hydrogen to a level much higher than its solubility limit and secondly, it reports for the first time enhancement of modulus and work hardening by hydrogen charging. These observations have been rationalized on the basis of current understanding on the effect of hydrogen on plastic properties and hypothesis of the models of IHE. It is suggested that the trapping of hydrogen by dislocations and other structural features of the matrix and the mutual interactions of their strain fields can account for the observed effects on yield strength, tangent modulus, work hardening rate, UTS and ductility.

A study of internal hydrogen embrittlement of steels

Absorption of hydrogen in tensile strained iron and high-carbon steel studied by electrochemical permeation and desorption techniques

Internal hydrogen effects on stage II crack growth rates in AISI 4340 steel have been studied as a function of test temperature. A model is developed that is physically based in that classical thermodynamics relates to solubility and trapping and Fick’s second law controls hydrogen transport. Both of these are microstructurally related to how trapping affects both the crack initiation site and diffusion to it. For two tempered conditions of 4340 steel, it is shown that there is a test temperature,T
 0, for stage II crack growth, above which the crack does not grow. The fractography associated with test temperatures approachingT
 0 tends toward 100 pct intergranular for both 1340 MPa and 1620 MPa strength levels. At lower test temperatures, there is as much as 50 pct microvoid coalescence or 30 pct quasi-cleavage. In the lower strength condition, hydrogen traps at oxysulfide particles with a binding energy near 75 kJ/mol. Where these intersect the prior austenite grain boundaries, this promotes fingers of intergranular fracture which later triggers tearing of 100 μm size ligaments by microvoid coalescence. For the higher strength material, it is proposed that hydrogen traps along martensite lath intersections with prior austenite grain boundaries, the binding energy being near 27 kJ/mol. This promotes 1 μm size striations along intergranular facets. In both cases the fractography is consistent with a proposed model of stress field concentration of hydrogen, further concentration along trap sites, fracture nucleation at trap sites, and local, discontinuous fracture instabilities.

R. W. Staehle

Papers

Stress corrosion cracking and hydrogen embrittlement of iron base alloys, June 12-16, 1973, Unieux, Firminy, France : [proceedings]