Environmental Engineering Reference
In-Depth Information
the crack growth rate for specific stress intensity value, as can be seen in Fig.
8.4 for AISI 4340 steel. The threshold stress intensity and crack growth rate are
a function of the specific hydrogen environment. The threshold stress intensity
is represented by
K
1H
, which is lower than that measured in inert environments
and is a measure of susceptibility to HSC. HSC is observed in the temperature
range of
C, the most severe being at about room temperature.
At temperatures well below room temperature, the hydrogen diffusion rate is
slow and the necessary hydrogen buildup at the crack tip for its propagation is
also slow. At temperatures in excess of 120
100
°
Cto
100
°
C, the hydrogen in solid solution
tends to be homogenized and the local buildup of hydrogen concentration needed
to cause embrittlement may not occur. The deleterious effect of hydrogen on
delayed failure is increased by the triaxiality of the stress state, i.e., with an
increase in notch acuity, as shown in Fig. 8.5.
Hydrogen concentration in the alloy is a function of the fugacity or the approx-
imate concentration of hydrogen at the surface exposed to the environment.
Therefore, hydrogen gas pressure and pH of the environment are controlling fac-
tors for the embrittlement. Certain constituents of the environment also may play
a significant role. Hydrogen evolution poisons contained in the aqueous environ-
ments favor more hydrogen entry in the metal. On the other hand, traces of oxy-
gen in hydrogen gas environment inhibit HSC in steels. The susceptibility of an
alloy to hydrogen embrittlement is strongly dependent on hydrogen concentration
in the metal (Fig. 8.4). The effect is reversible to the extent that if hydrogen is
removed from the metal, brittle fracture is avoided. Figure 8.6 represents static
fatigue curves for a 4340 steel, showing the reversibility of embrittlement with
systematic baking out of the changed hydrogen. The term
internal reversible
hydrogen embrittlement
is applied to describe this behavior. The reversibility is
absent in case the hydrogen undergoes any type of chemical reaction after it has
been absorbed within the lattice, or if any immediate and resolvable damage has
taken place in the material structure.
Hydrogen-induced cracking has also been observed as a consequence of me-
chanical testing at slow-strain rates in hydrogen environments. Like delayed fail-
ure, slow-strain rate embrittlement also requires a minimal level of applied stress
for the failure to occur. The susceptibility is greater in stronger alloys and exhibits
the similar temperature dependence. Additionally, it is strongly dependent on the
strain rate, the embrittlement increasing with decreasing strain rate. At relatively
high strain rates (
°
10
4
in./in./min) there may be little or no decrease in macro-
scopic plasticity because of hydrogen. Apparently, at these high strain rates, the
alloy may fracture before any significant diffusion of hydrogen can take place.
Figure 8.7 shows the variation in notch tensile strength of a high-strength steel
as a function of strain rate and testing temperature.
The susceptibility to embrittlement of steels depends to a large extent on their
microstructure. Highly tempered martensitic structure with equiaxed ferrite grains
