The effects of hydrogen on the physical and mechanical properties of iron and steel are reviewed. A new mechanism for the cold work peak for hydrogen in iron is considered. Together, internal friction and mechanical properties indicate that hydrogen softens iron by enhancing screw dislocation mobility at room temperature but hardens iron by core interactions at low temperatures. No single mechanism exists for the degradation of the properties of steel by hydrogen. Instead a complex process involving many of the proposed mechanisms as contributing factors is shown to account for most degradation phenomena.

Effects of hydrogen on the properties of iron and steel

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

Crack tip shielding--an elastic theory of dislocations and dislocation arrays near a sharp crack

The characteristics of fatigue-crack propagation in metals and alloys have been the subject of several extensive reviews in recent years, but in very few instances have the details of ultralow growth rate, nearthreshold fatigue-crack propagation been similarly discussed. In this review the effects are examined of various mechanical, microstructural, and environmental factors which influence fatigue-crack propagation in steels at growth rates less than 10-6 mm/cycle, where the alternating stress intensity ∆K approaches the so-called threshold stress intensity ∆K0 below which crack growth cannot be experimentally detected. The marked influences of load ratio, material strength, and microstructure on such near-threshold growth are analysed in detail and rationalized in terms of possible environmental contributions and crackclosure concepts. These effects are contrasted with crack-propagation behaviour in other engineering materials and at higher growth rates.

Near-threshold fatigue-crack propagation in steels

: Two important advances over the past 40 years enable the optimization and management of the structural integrity of components in high performance applications. First, the solid mechanics conununity established linear elastic fracture mechanics as the premier framework for modeling the damage tolerance of fracture critical components (Irwin and Wells, 1997; Paris, 1998). Second, materials scientists developed metals with outstanding balances of high tensile strength and high fracture toughness (Garrison, 1990; Wells, 1993; Boyer, 1993; Starke and Staley, 1995; Olson, 1997; Kolts, 1996). An example of achievable strength-toughness properties is provided in Fig. 1, a plot of plane strain fracture toughness vs. tensile yield strength (sigma-YS) for ultra-high strength steels (UHSS) and beta-Ti alloys precipitation hardened with a phase (Gangloff 2001). New nano-scale characterization and high performance computational methods provide for additional advances in the mechanical performance properties of structural metals. These modem alloys and analysis tools satisfy technological needs for optimization and management of component performance in demanding fatigue and fracture critical applications in the aerospace, marine, energy, transportation, and defense sectors.

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Hydrogen Assisted Cracking of High Strength Alloys

Coordinated fracture mechanics and surface chemistry experiments were carried out to develop further understanding of environment enhanced subcritical crack growth in high strength steels. The kinetics of crack growth were determined for an AISI 4340 steel (tempered at 204°C) in hydrogen and in water, and the kinetics for the reactions of water with the same steel were also determined. A regime of rate limited (Stage II) crack growth was observed in each of the environments. Stage II crack growth was found to be thermally activated, with an apparent activation energy of 14.7 ±2.9 kJ/mole for crack growth in hydrogen, and 33.5 ± 5.0 kJ/mole in water. Fractographic evidence indicated that the fracture path through the microstructure was the same for these environments, and suggested hydrogen to be the embrittling species for environment enhanced crack growth in hydrogen and in water/water vapor. A slow step in the surface reaction of water vapor with steel was identified, and exhibited an activation energy of 36 ± 14 kJ/ mole. This reaction step was identified to be that for the nucleation and growth of oxide. The hydrogen responsible for embrittlement was presumed to be produced during this reaction. On the basis of a comparison of the activation energies, in conjunction with other supporting data, this slow step in the water/metal surface reaction was unambiguously identified as the rate controlling process for crack growth in water/water vapor. The inhibiting effect of oxygen and the influence of water vapor pressure on environment enhanced subcritical crack growth were considered. The influence of segregation of alloying and residual impurity elements on crack growth was also considered.

G. W. Simmons

Papers

Fracture Mechanics and Surface Chemistry Studies of Subcritical Crack Growth in AISI 4340 Steel.