Environmental Engineering Reference
In-Depth Information
8.4.4 Enhanced Plastic Flow Theory
Based on the fractographic evidence that HSC produced fracture surfaces show-
ing 100% microvoid coalescence (dimples) [15] and the observation of shallow
dimples (microdimpling) in the flat cleavage facets at magnification higher than
5000
in some steels [6], it has been proposed that hydrogen enhances the pro-
cesses of plastic flow associated with the propagation of fracture by making dis-
locations move at reduced stresses at the crack tips. The movement of screw
dislocations is generally considered to be favored. Further, it has been suggested
that hydrogen is subsequently transported by dislocations to the regions ahead
of cracks. If appreciable reductions or flow stress due to dissolved hydrogen
were localized to regions just ahead of crack tips, then embrittlement due to
strain localization would be expected.
In a modified version of the model [6], the adsorption of hydrogen has been
considered to ease the generation of dislocations at the crack tip which, in turn,
decreases the surface tension at the crack tip spreading apart the surface atoms.
The principal objection to this mechanism is that there has been evidence
that hydrogen may induce decreases or increases in flow stress according to the
circumstances. Furthermore, the plastic flow ahead of cracks is largely controlled
by the large number of solute atoms and precipitates in steel whether hydrogen
is present or not.
8.5 PRACTICAL EXAMPLES
In the petroleum refining industry, hydrogen blistering is a problem in the vapor
recovery section of catalytic cracking units and in the low-temperature areas of
the reaction effluent section of hydrotreating and hydrocracking units. Hydrogen
blistering has also been encountered in the overhead systems for sour water strip-
per towers and amine regenerator towers, as well as in the bottom of amine con-
tactor towers [17]. Corrosion-generated hydrogen is the cause for blistering of
steel in oil well equipment.
A case of hydrogen blistering produced on the interior surface of a low-carbon-
steel tank used to transport concentrated sulfuric acid has been reported [18].
Slight dilution of the acid had resulted in chemical attack on the 7/32-in.-thick
tank wall and the diffusion of atomic hydrogen into the steel. The blistering was
observed within 1/16 in. of the inside surface of the tank wall, which was attrib-
uted to the presence of internal discontinuities in an otherwise normal metal that
had partially spheroidized structure and was free from defects.
Cracks were observed during machining operations in a 6-in.-diameter alloy
steel shafting made by forging [19]. The radiography of a 1/2-in. disc represent-
ing the steel shaft revealed a large number of cracks (Fig. 8.16). The microstruc-
ture indicated that homogenization of the cast structure had not been attained and
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