Intergranular cracking associated with hydrogen embrittlement represents a particularly severe degradation mechanism in metallic structures which can lead to sudden and unexpected catastrophic fractures. As a basis for a strategy for the prognosis of such failures, here we present a comprehensive physical-based statistical micro-mechanical model of such embrittlement which we use to quantitatively predict the degradation in fracture strength of a high-strength steel with increasing hydrogen concentration, with the predictions verified by experiment. The mechanistic role of dissolved hydrogen is identified by the transition to a locally stress-controlled fracture, which is modeled as being initiated by a dislocation pile-up against a grain-boundary carbide which in turn leads to interface decohesion and intergranular fracture. Akin to cleavage fracture in steel, the “strength” of these carbides is modeled using weakest-link statistics. We associate the dominant role of hydrogen with trapping at dislocations; this trapped hydrogen reduces the stress that impedes dislocation motion and also lowers the reversible work of decohesion at the tip of dislocation pile-up at the carbide/matrix interface. Mechanistically, the model advocates the synergistic action of both the hydrogen-enhanced local plasticity and decohesion mechanisms in dictating failure.

https://www2.lbl.gov/ritchie/Library/PDF/JMPS-Novak2-10.pdf

A statistical, physical-based, micro-mechanical model of hydrogen-induced intergranular fracture in steel

Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

We present a model of hydrogen embrittlement based upon: (i) a cohesive law dependent on impurity coverage that is calculated from first principles; (ii) a stress-assisted diffusion equation with appropriate boundary conditions accounting for the environment; (iii) a static continuum analysis of crack growth including plasticity; and (iv) the Langmuir relation determining the impurity coverage from its bulk concentration. We consider the effect of the following parameters: yield strength, stress intensity factor, hydrogen concentration in the environment, and temperature. The calculations reproduce the following experimental trends: (i) time to initiation and its dependence on yield strength and stress intensity factor; (ii) finite crack jump at initiation; (iii) intermittent crack growth; (iv) stages I and II of crack growth and their dependence on yield strength; (v) the effect of the environmental impurity concentration on the threshold stress intensity factor; and (vi) the effect of temperature on stage II crack velocity in the low-temperature range. In addition, the theoretically and experimentally observed intermittent cracking may be understood as being due to a time lag in the diffusion of hydrogen towards the cohesive zone, since a buildup of hydrogen is necessary in order for the crack to advance. The predictions of the model are in good quantitative agreement with available measurements, suggesting that hydrogen-induced degradation of cohesion is a likely mechanism for hydrogen-assisted cracking.

A quantum-mechanically informed continuum model of hydrogen embrittlement

Hydrogen embrittlement is a complex phenomenon, involving several length- and timescales, that affects a large class of metals. It can significantly reduce the ductility and load-bearing capacity and cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Despite a large research effort in attempting to understand the mechanisms of failure and in developing potential mitigating solutions, hydrogen embrittlement mechanisms are still not completely understood. There are controversial opinions in the literature regarding the underlying mechanisms and related experimental evidence supporting each of these theories. The aim of this paper is to provide a detailed review up to the current state of the art on the effect of hydrogen on the degradation of metals, with a particular focus on steels. Here, we describe the effect of hydrogen in steels from the atomistic to the continuum scale by reporting theoretical evidence supported by quantum calculation and modern experimental characterisation methods, macroscopic effects that influence the mechanical properties of steels and established damaging mechanisms for the embrittlement of steels. Furthermore, we give an insight into current approaches and new mitigation strategies used to design new steels resistant to hydrogen embrittlement.

/pdf/understanding-and-mitigating-hydrogen-embrittlement-of-27qkbb7dok.pdf

Understanding and mitigating hydrogen embrittlement of steels: a review of experimental, modelling and design progress from atomistic to continuum.

The quantitative relationship between notch tensile strength and diffusible hydrogen content has been investigated for the AISI 4135 steel at 1320 MPa. The notch tensile strength was obtained by means of a slow strain rate test on circumferentially notched round bar specimens with stress concentration factors of 2.1, 3.3 and 4.9 after hydrogen charging, and the diffusible hydrogen content was then measured by thermal desorption spectrometry analysis. The diffusible hydrogen has been found to decrease the notch tensile strength in a power law manner, and the decrease is more prominent at a higher stress concentration factor. The finite element analysis results of stress and hydrogen distributions in the vicinity of the notch root have shown that the local fracture stress decreases with increasing local hydrogen concentration as the diffusible hydrogen content or stress concentration factor increases, resulting in the decrease in the notch tensile strength.

Effect of hydrogen and stress concentration on the notch tensile strength of AISI 4135 steel

The crack growth rates and threshold stress intensities,K
 TH, for a 3 1/2 NiCrMoV steel (0.2 pct proof stress 1200 MPa) have been measured in a hydrogen environment at various temperatures and hydrogen pressures. Fractographic evidence and the observation of alternating fast and slow crack growth nearK
 TH suggests that the crack advances by the repeated nucleation of microcracks at microstructural features ahead of the main crack. Transient crack growth is observed following load increases just belowK
 TH. Using the idea, from unstable cleavage fracture theory, that for fracture a critical stress must be exceeded over a critical distance ahead of the crack, and assuming that this critical stress is reduced in proportion to the local hydrogen concentration (in equilibrium with the external hydrogen atK
 TH), a theoretical dependence ofK
 TH on hydrogen pressure is derived which compares well with the experimental evidence.

The Threshold Stress Intensity for Hydrogen- Induced Crack Growth

Enhanced fracture resistance of high strength steel produced by extrusion of nickel coated steel powder

A high strength steel with an austenite-martensite duplex microstructure has been produced by extruding nickel coated steel powder. The austenite is stable and is present as a continuous network surrounding a high strength martensite. The dual-phase steel exhibits superior resistance to stress corrosion cracking in 3.5 pct NaCl solution, compared with steels of similar strength having conventional microstructures. Also, the effectiveness of the austenite in improving stress corrosion cracking resistance increases as the yield strength increases. The austenite, because of its inherent toughness, reduces the effective stress intensity at the advancing crack tip, and at the same time shields the crack tip from the corrosive environment.

/pdf/enhanced-stress-corrosion-resistance-from-steels-having-a-zkm194a2cd.pdf

T. J. Baker

Papers

The Threshold Stress Intensity for Hydrogen- Induced Crack Growth

Enhanced fracture resistance of high strength steel produced by extrusion of nickel coated steel powder

Enhanced stress corrosion resistance from steels having a dual-phase austenite-martensite microstructure

Enhanced stress corrosion resistance from steels having a dual-phase austenite-martensite microstructure