Environmental Engineering Reference
In-Depth Information
and the absorption is accelerated at temperatures exceeding 70
C. Hydrogen is
readily picked up during melting or welding, and hydride formation takes place
during subsequent cooling. When sufficient hydrogen is present, the craking is
attributed to the strain-induced formation of hydrides. With lower concentrations
(
°
5 ppm), some other mechanism may be operative.
Both titanium and zirconium have two allotropic forms: a lower temperature
α
phase has a relatively low solubil-
ity for hydrogen and forms hydrides at low concentrations, whereas the
phase and a high-temperature
β
phase. The
α
phase
has a high hydrogen solubility and can form hydrides only at high concentrations.
The embrittlement of
β
-titanium alloys is observed when tested at high strain
rates and low temperatures. In contrast, the embrittlement in
α
titanium alloys
is more pronounced when tests are conducted at low strain rates. In
α
-
β
alloys,
hydrides form at the phase interfaces and cracking occurs by stepwise brittle
cleavage of the thin hydride film, resulting in an integranular fracture at the
α
-
β
α
-
β
boundaries.
In the refractory metals, i.e., tungsten, vanadium, tantalum, and niobium, hy-
drides formed are not stable; nevertheless, embrittlement and cracking are en-
countered.
8.3.2 Hydrogen Blistering
This type of damage is prevalent in low-strength unhardened steels and is caused
by the pressure generated by the process of combination of atomic hydrogen
into molecular hydrogen. The diffusing hydrogen atoms accumulate at internal
macrodefects such as voids, laminations, or inclusion-matrix interfaces already
present in the steel. At sufficiently high concentrations they tend to combine into
molecular hydrogen, exerting an estimated pressure of several thousand atmo-
spheres, which brings about the damage.
Hydrogen blistering literally means the formation of surface bulgings resem-
bling a blister (Fig. 8.11). The generation of gas in voids or other defect sites
situated near the surface can lead to such a situation. The blisters often rupture
producing surface crackings. Internal hydrogen blistering on a microscopic scale
along grain boundaries (fissures) can lead to hydrogen-induced stepwise cracking.
The interaction of accumulated hydrogen at an elongated inclusion-matrix inter-
face may lead to delamination of a steel sheet or plate (Fig. 8.12). In ordinary
rolled sheet or plate, banded structures containing elongated and flattened inclu-
sions are common. Killed steels are more susceptible to blistering than semikilled
steels because of greater hydrogen intake after deoxidation, but the nature and
size of inclusions are overriding factors. Rimmed steels show a high susceptibility
because of inherent presence of voids. Sulfur-bearing free-cutting steels are also
specially prone because sulfur favors the hydrogen entry by acting as a cathodic
poison. Hydrogen blistering is encountered mostly during acid pickling opera-
