Environmental Engineering Reference
In-Depth Information
tions. Corrosion-generated hydrogen causes blistering of steel in oil well equip-
ment and petroleum storage and refinery equipment.
8.3.3 Flakes, Fish-eyes, and Shatter Cracks
Flaking refers to small internal fissures that occur in steels when cooled from
temperatures of the order of 1100
C in hydrogen atmospheres. These are also
described by the terms fish-eyes, shatter cracks, or snowflakes and are common
features of hydrogen damage in forgings, weldments, and castings. At higher
temperatures of melting or welding or heat treatment in the austenite range, the
solubility of hydrogen in steel is higher than in the solidified or low-temperature
bcc state. The excess released hydrogen atoms accumulate at internal defects,
combine to form hydrogen gas, and cause these types of damage. The mechanism
of formation of these damages as well as many features are similar to those of
hydrogen blistering and as such may be considered as a special class of hydrogen
blistering.
The extent of damage is dependent on the time of exposure to a hydrogen-
containing environment. The cracks produced are readily detectable by radio-
graphic or ultrasonic inspection, or by visual and microscopic observation of
transverse sections (Fig. 8.13). On a fracture surface, the flakes appear as bright,
highly reflective spots, which at higher magnifications (more than 6000
°
) often
reveal striation patterns.
(a)
(b)
Figure 8.13
Hydrogen flake patterns in SAE 5145 steel (a) on unetched transverse
section, 1
. (b) Microscopic features in eched condition, 240
[23].
