Hydrogen embrittlement of steels

7.4.5 Hydrogen embrittlement of steels

It is well known that ferritic and martensitic steels are severely embrittled by small amounts of hydrogen. The hydrogen may be introduced during melting and retained during the solidification of massive steel castings. Plating operations (e.g. Cd plating of steel for aircraft parts) may also lead to hydrogen embrittlement. Hydrogen can also be introduced during acid pickling or welding, or by

exposure to H 2 S atmospheres. The chief characteristics of hydrogen embrittlement are its (1) strain-rate sensitivity, (2) temperature dependence and (3) susceptibility to produce delayed fracture (see Figure 7.34). Unlike normal brittle fracture, hydrogen embrittlement is enhanced by slow strain rates and, consequently, notched impact tests have little significance in detecting this type of embrittlement. Moreover, the phenomenon is not more common at low temperatures, but is most severe in some intermediate temperature range around

Mechanical properties II – Strengthening and toughening 433 Uncharged

25 Stress (MN m 50 H 2 Charged

% Reduction in area

H 2 Charged

T Time to fracture (h)

(b) Figure 7.34 Influence of hydrogen on fracture behavior, showing time dependence (a) and

(a)

temperature dependence (b).

room temperature (i.e. −100 to 100 ◦ C). These effects have been taken to indicate that hydrogen must be present in the material and must have a high mobility in order to cause embrittlement in polycrystalline aggregates.

A commonly held concept of hydrogen embrittlement is that monatomic hydrogen precipitates at internal voids or cracks as molecular hydrogen, so that as the pressure builds up it produces fracture. Alternatively, it has been proposed that the critical factor is the segregation of hydrogen, under applied

stress, to regions of triaxial stress just ahead of the tip of the crack, and when a critical hydrogen concentration is obtained, a small crack grows and links up with the main crack. Hydrogen may also exist in the void or crack but it is considered that this has little effect on the fracture behavior, and it is only the hydrogen in the stressed region that causes embrittlement. Neither model considers the Griffith criterion, which must be satisfied if cracks are to continue spreading.

An application of the fracture theory may be made to this problem. Thus, if hydrogen collects in microcracks and exerts internal pressure P, the pressure may be directly added to the external stress to produce a total stress (P + p) for propagation. Thus, the crack will spread when

(P + p)na = (γ s +γ p ), (7.29) where the surface energy is made up from a true surface energy γ s and a plastic work term γ p . The

possibility that hydrogen causes embrittlement by becoming adsorbed on the crack surfaces thereby lowering γ is thought to be small, since the plastic work term γ p is the major term controlling γ, whereas adsorption would mainly affect γ s .

Supersaturated hydrogen atoms precipitate as molecular hydrogen gas at a crack nucleus, or the interface between non-metallic inclusions and the matrix. The stresses from the build-up of hydrogen pressure are then relieved by the formation of small cleavage cracks. Clearly, while the crack is propagating, an insignificant amount of hydrogen will diffuse to the crack and, as a consequence, the pressure inside the crack will drop. However, because the length of the crack has increased, if a sufficiently large and constant stress is applied, the Griffith criterion will still be satisfied and completely brittle fracture can, in theory, occur. Thus, in iron single crystals, the presence or absence of hydrogen appears to have little effect during crack propagation because the crack has little difficulty

434 Physical Metallurgy and Advanced Materials spreading through the crystal. In polycrystalline material, however, the hydrogen must be both present

and mobile, since propagation occurs during tensile straining. When a sufficiently large tensile stress is applied such that ( p + P) is greater than that required by the Griffith criterion, the largest and sharpest crack will start to propagate, but will eventually be stopped at a microstructural feature, such as a grain boundary, as previously discussed. The pressure in the crack will then be less than in adjacent cracks which have not been able to propagate. A concentration gradient will then exist between such cracks (since the concentration is proportional to the square root of the pressure of hydrogen), which provides a driving force for diffusion, so that the hydrogen pressure in the enlarged crack begins to increase again. The stress to propagate the crack decreases with increase in length of crack, and since p is increased by straining, a smaller increment

P of pressure may be sufficient to get the crack restarted. The process of crack propagation followed by a delay time for pressure build-up continues with straining until the specimen fails when the area between the cracks can no longer support the applied load. In higher strain-rate tests, the hydrogen is unable to diffuse from one stopped crack to another to help the larger crack get started before it becomes blunted by plastic deformation at the tip. The decrease in the susceptibility to hydrogen embrittlement in specimens tested at low temperatures results from the lower pressure build-up at these temperatures since PV = 3nRT , and also because hydrogen has a lower mobility.